For the grammatical concept, see Grammatical gender. For other uses, see Gender (disambiguation).
Gender is the range of characteristics pertaining to, and differentiating between, masculinity and femininity. Depending on the context, these characteristics may include biological sex (i.e., the state of being male, female, or an intersex variation), sex-based social structures (i.e., gender roles), or gender identity. People who do not identify as men or women or with masculine or feminine gender pronouns are often grouped under the umbrella terms non-binary or genderqueer. Some cultures have specific gender roles that are distinct from "man" and "woman," such as the hijras of South Asia. These are often referred to as third genders.
SexologistJohn Money introduced the terminological distinction between biological sex and gender as a role in 1955. Before his work, it was uncommon to use the word gender to refer to anything but grammatical categories. However, Money's meaning of the word did not become widespread until the 1970s, when feminist theory embraced the concept of a distinction between biological sex and the social construct of gender. Today the distinction is strictly followed in some contexts, especially the social sciences and documents written by the World Health Organization (WHO).
In other contexts, including some areas of social sciences, gender includes sex or replaces it. For instance, in non-human animal research, gender is commonly used to refer to the biological sex of the animals. This change in the meaning of gender can be traced to the 1980s. In 1993, the US Food and Drug Administration (FDA) started to use gender instead of sex. Later, in 2011, the FDA reversed its position and began using sex as the biological classification and gender as "a person's self representation as male or female, or how that person is responded to by social institutions based on the individual's gender presentation."
The social sciences have a branch devoted to gender studies. Other sciences, such as sexology and neuroscience, are also interested in the subject. While the social sciences sometimes approach gender as a social construct, and gender studies particularly do, research in the natural sciences investigates whether biological differences in males and females influence the development of gender in humans; both inform debate about how far biological differences influence the formation of gender identity. In the English literature, there is also a trichotomy between biological sex, psychological gender, and social gender role. This framework first appeared in a feminist paper on transsexualism in 1978.
Etymology and usage
The modern English word gender comes from the Middle Englishgender, gendre, a loanword from Anglo-Norman and Middle Frenchgendre. This, in turn, came from Latingenus. Both words mean "kind", "type", or "sort". They derive ultimately from a widely attested Proto-Indo-European (PIE) rootgen-, which is also the source of kin, kind, king, and many other English words. It appears in Modern French in the word genre (type, kind, also genre sexuel) and is related to the Greek root gen- (to produce), appearing in gene, genesis, and oxygen. The first edition of the Oxford English Dictionary (OED1, Volume 4, 1900) notes the original meaning of gender as "kind" had already become obsolete.
The word was still widely attested, however, in the specific sense of grammatical gender (the assignment of nouns to categories such as masculine, feminine and neuter). According to Aristotle, this concept was introduced by the Greek philosopher Protagoras.
In 1926, Henry Watson Fowler stated that the definition of the word pertained to this grammar-related meaning:
"Gender...is a grammatical term only. To talk of persons...of the masculine or feminine g[ender], meaning of the male or female sex, is either a jocularity (permissible or not according to context) or a blunder."
The modern academic sense of the word, in the context of social roles of men and women, dates at least back to 1945, and was popularized and developed by the feminist movement from the 1970s onwards (see § Feminism theory and gender studies below). The theory was that human nature is essentially epicene and social distinctions based on sex are arbitrarily constructed. Matters pertaining to this theoretical process of social construction were labelled matters of gender.
The popular use of gender simply as an alternative to sex (as a biological category) is also widespread, although attempts are still made to preserve the distinction. The American Heritage Dictionary (2000) uses the following two sentences to illustrate the difference, noting that the distinction "is useful in principle, but it is by no means widely observed, and considerable variation in usage occurs at all levels."
The effectiveness of the medication appears to depend on the sex (not gender) of the patient.
In peasant societies, gender (not sex) roles are likely to be more clearly defined.
In the last two decades of the 20th century, the use of gender in academia has increased greatly, outnumbering uses of sex in the social sciences. While the spread of the word in science publications can be attributed to the influence of feminism, its use as a synonym for sex is attributed to the failure to grasp the distinction made in feminist theory, and the distinction has sometimes become blurred with the theory itself; David Haig stated, "Among the reasons that working scientists have given me for choosing gender rather than sex in biological contexts are desires to signal sympathy with feminist goals, to use a more academic term, or to avoid the connotation of copulation."
In legal cases alleging discrimination, sex is usually preferred as the determining factor rather than gender as it refers to biology rather than socially constructed norms which are more open to interpretation and dispute. Julie Greenberg writes that although gender and sex are separate concepts, they are interlinked in that gender discrimination often results from stereotypes based on what is expected of members of each sex. In J.E.B. v. Alabama ex rel. T.B., United States Supreme Court Justice Antonin Scalia wrote:
The word ‘gender’ has acquired the new and useful connotation of cultural or attitudinal characteristics (as opposed to physical characteristics) distinctive to the sexes. That is to say, gender is to sex as feminine is to female and masculine is to male.
Gender identity and gender roles
Main articles: Gender identity and Gender role
Gender identity refers to a personal identification with a particular gender and gender role in society. The term woman has historically been used interchangeably with reference to the female body, though more recently this usage has been viewed as controversial by some feminists.
There are qualitative analyses that explore and present the representations of gender; however, feminists challenge these dominant ideologies concerning gender roles and biological sex. One's biological sex is directly tied to specific social roles and the expectations. Judith Butler considers the concept of being a woman to have more challenges, owing not only to society's viewing women as a social category but also as a felt sense of self, a culturally conditioned or constructed subjective identity.Social identity refers to the common identification with a collectivity or social category that creates a common culture among participants concerned. According to social identity theory, an important component of the self-concept is derived from memberships in social groups and categories; this is demonstrated by group processes and how inter-group relationships impact significantly on individuals' self perception and behaviors. The groups people belong to therefore provide members with the definition of who they are and how they should behave within their social sphere.
Categorizing males and females into social roles creates a problem, because individuals feel they have to be at one end of a linear spectrum and must identify themselves as man or woman, rather than being allowed to choose a section in between. Globally, communities interpret biological differences between men and women to create a set of social expectations that define the behaviors that are "appropriate" for men and women and determine women’s and men’s different access to rights, resources, power in society and health behaviors. Although the specific nature and degree of these differences vary from one society to the next, they still tend to typically favor men, creating an imbalance in power and gender inequalities within most societies. Many cultures have different systems of norms and beliefs based on gender, but there is no universal standard to a masculine or feminine role across all cultures. Social roles of men and women in relation to each other is based on the cultural norms of that society, which lead to the creation of gender systems. The gender system is the basis of social patterns in many societies, which include the separation of sexes, and the primacy of masculine norms.
Philosopher Michel Foucault said that as sexual subjects, humans are the object of power, which is not an institution or structure, rather it is a signifier or name attributed to "complex strategical situation". Because of this, "power" is what determines individual attributes, behaviors, etc. and people are a part of an ontologically and epistemologically constructed set of names and labels. Such as, being female characterizes one as a woman, and being a woman signifies one as weak, emotional, and irrational, and is incapable of actions attributed to a "man". Butler said that gender and sex are more like verbs than nouns. She reasoned that her actions are limited because she is female. "I am not permitted to construct my gender and sex willy-nilly," she said. "[This] is so because gender is politically and therefore socially controlled. Rather than 'woman' being something one is, it is something one does." More recent criticisms of Judith Butler's theories critique her writing for reinforcing the very conventional dichotomies of gender.
Social assignment and gender fluidity
See also: Sex assignment
According to gender theoristKate Bornstein, gender can have ambiguity and fluidity. There are two contrasting ideas regarding the definition of gender, and the intersection of both of them is definable as below:
The World Health Organization defines gender as the result of socially constructed ideas about the behavior, actions, and roles a particular sex performs. The beliefs, values and attitude taken up and exhibited by them is as per the agreeable norms of the society and the personal opinions of the person is not taken into the primary consideration of assignment of gender and imposition of gender roles as per the assigned gender. Intersections and crossing of the prescribed boundaries have no place in the arena of the social construct of the term "gender".
The assignment of gender involves taking into account the physiological and biological attributes assigned by nature followed by the imposition of the socially constructed conduct. The social label of being classified into one or the other sex is necessary for the medical stamp on birth certificates. Gender is a term used to exemplify the attributes that a society or culture constitutes as "masculine" or "feminine". Although a person's sex as male or female stands as a biological fact that is identical in any culture, what that specific sex means in reference to a person's gender role as a woman or a man in society varies cross culturally according to what things are considered to be masculine or feminine. These roles are learned from various, intersecting sources such as parental influences, the socialization a child receives in school, and what is portrayed in the local media. It is also important to note that learning gender roles starts from birth and includes seemingly simple things like what color outfits a baby is clothed in or what toys they are given to play with. The cultural traits typically coupled to a particular sex finalize the assignment of gender and the biological differences which play a role in classifying either sex as interchangeable with the definition of gender within the social context.
In this context, the socially constructed rules are at a cross road with the assignment of a particular gender to a person. Gender ambiguity deals with having the freedom to choose, manipulate and create a personal niche within any defined socially constructed code of conduct while gender fluidity is outlawing all the rules of cultural gender assignment. It does not accept the prevalence of the two rigidly defined genders "man" and "woman" and believes in freedom to choose any kind of gender with no rules, no defined boundaries and no fulfilling of expectations associated with any particular gender.
Both these definitions are facing opposite directions with their own defined set of rules and criteria on which the said systems are based.
Sexologist John Money coined the termgender role in 1955. The term gender role is defined as the actions or responses that may reveal their status as boy, man, girl or woman, respectively. Elements surrounding gender roles include clothing, speech patterns, movement, occupations, and other factors not limited to biological sex. In contrast to taxonomic approaches, some feminist philosophers have argued that gender "is a vast orchestration of subtle mediations between oneself and others", rather than a "private cause behind manifest behaviours".
Because social aspects of gender can normally be presumed to be the ones of interest in sociology and closely related disciplines, gender role is often abbreviated to gender in their literature.
Non-binary and third genders
Main articles: Genderqueer and Third gender
Traditionally, most societies have only recognized two distinct, broad classes of gender roles, masculine and feminine, that correspond with the biological sexes of male and female. When a baby is born, society allocates the child to one gender or the other, on the basis of what their genitals resemble. However, some societies explicitly incorporate people who adopt the gender role opposite to their biological sex; for example, the two-spirit people of some indigenous American peoples. Other societies include well-developed roles that are explicitly considered more or less distinct from archetypal female and male roles in those societies. In the language of the sociology of gender, they comprise a third gender, more or less distinct from biological sex (sometimes the basis for the role does include intersexuality or incorporates eunuchs). One such gender role is that adopted by the hijras of India and Pakistan. Another example may be the muxe (pronounced [ˈmuʃe]), found in the state of Oaxaca, in southern Mexico. The Bugis people of Sulawesi, Indonesia have a tradition that incorporates all the features above.
In addition to these traditionally recognized third genders, many cultures now recognize, to differing degrees, various non-binary gender identities. People who are non-binary (or genderqueer) have gender identities that are not exclusively masculine or feminine. They may identify as having an overlap of gender identities, having two or more genders, having no gender, having a fluctuating gender identity, or being third gender or other-gendered. Recognition of non-binary genders is still somewhat new to mainstream Western culture, and non-binary people may face increased risk of assault, harassment, and discrimination.
Joan Roughgarden argues that some non-human animal species also have more than two genders, in that there might be multiple templates for behavior available to individual organisms with a given biological sex.
Measurement of gender identity
Early gender identity research hypothesized a single bipolar dimension of masculinity-femininity, with masculinity and femininity being opposites on one continuum. Assumptions of the unidimensional model were challenged as societal stereotypes changed, which led to the development of a two-dimensional gender identity model. In the model, masculinity and femininity were conceptualized as two separate and orthogonal dimensions, coexisting in varying degrees within an individual. This conceptualization on femininity and masculinity remains the accepted standard today.
Two instruments incorporating the multidimensional nature of masculinity and femininity have dominated gender identity research: The Bem Sex Role Inventory (BSRI) and the Personal Attributes Questionnaire (PAQ). Both instruments categorize individuals as either being sex typed (males report themselves as identifying primarily with masculine traits, females report themselves as identifying primarily with feminine traits), cross sex-typed (males report themselves as identifying primarily with feminine traits, females report themselves as identifying primarily with masculine traits), androgynous (either males or females who report themselves as high on both masculine and feminine traits) or undifferentiated (either males or females who report themselves as low on both masculine and feminine traits). Twenge (1997) noted that men are generally more masculine than women and women generally more feminine than men, but the association between biological sex and masculinity/femininity is waning.
Feminist theory and gender studies
Biologist and feminist academic Anne Fausto-Sterling rejects the discourse of biological versus social determinism and advocates a deeper analysis of how interactions between the biological being and the social environment influence individuals' capacities. The philosopher and feminist Simone de Beauvoir applied existentialism to women's experience of life: "One is not born a woman, one becomes one." In context, this is a philosophical statement. However, it may be analyzed in terms of biology—a girl must pass puberty to become a woman—and sociology, as a great deal of mature relating in social contexts is learned rather than instinctive.
Within feminist theory, terminology for gender issues developed over the 1970s. In the 1974 edition of Masculine/Feminine or Human, the author uses "innate gender" and "learned sex roles", but in the 1978 edition, the use of sex and gender is reversed. By 1980, most feminist writings had agreed on using gender only for socioculturally adapted traits.
In gender studies the term gender refers to proposed social and cultural constructions of masculinities and femininities. In this context, gender explicitly excludes reference to biological differences, to focus on cultural differences. This emerged from a number of different areas: in sociology during the 1950s; from the theories of the psychoanalyst Jacques Lacan; and in the work of French psychoanalysts like Julia Kristeva, Luce Irigaray, and American feminists such as Judith Butler. Those who followed Butler came to regard gender roles as a practice, sometimes referred to as "performative".
Charles E. Hurst states that some people think sex will, "...automatically determine one's gender demeanor and role (social) as well as one's sexual orientation (sexual attractions and behavior). Gender sociologists believe that people have cultural origins and habits for dealing with gender. For example, Michael Schwalbe believes that humans must be taught how to act appropriately in their designated gender to fill the role properly, and that the way people behave as masculine or feminine interacts with social expectations. Schwalbe comments that humans "are the results of many people embracing and acting on similar ideas". People do this through everything from clothing and hairstyle to relationship and employment choices. Schwalbe believes that these distinctions are important, because society wants to identify and categorize people as soon as we see them. They need to place people into distinct categories to know how we should feel about them.
Hurst comments that in a society where we present our genders so distinctly, there can often be severe consequences for breaking these cultural norms. Many of these consequences are rooted in discrimination based on sexual orientation. Gays and lesbians are often discriminated against in our legal system because of societal prejudices. Hurst describes how this discrimination works against people for breaking gender norms, no matter what their sexual orientation is. He says that "courts often confuse sex, gender, and sexual orientation, and confuse them in a way that results in denying the rights not only of gays and lesbians, but also of those who do not present themselves or act in a manner traditionally expected of their sex". This prejudice plays out in our legal system when a person is judged differently because they do not present themselves as the "correct" gender.
Andrea Dworkin stated her "commitment to destroying male dominance and gender itself" while stating her belief in radical feminism.
Political scientist Mary Hawkesworth addresses gender and feminist theory, stating that since the 1970s the concept of gender has transformed and been used in significantly different ways within feminist scholarship. She notes that a transition occurred when several feminist scholars, such as Sandra Harding and Joan Scott, began to conceive of gender "as an analytic category within which humans think about and organize their social activity". Feminist scholars in Political Science began employing gender as an analytical category, which highlighted "social and political relations neglected by mainstream accounts". However, Hawkesworth states "feminist political science has not become a dominant paradigm within the discipline".
American political scientist Karen Beckwith addresses the concept of gender within political science arguing that a "common language of gender" exists and that it must be explicitly articulated in order to build upon it within the political science discipline. Beckwith describes two ways in which the political scientist may employ 'gender' when conducting empirical research: "gender as a category and as a process." Employing gender as a category allows for political scientists "to delineate specific contexts where behaviours, actions, attitudes and preferences considered masculine or feminine result in particular" political outcomes. It may also demonstrate how gender differences, not necessarily corresponding precisely with sex, may "constrain or facilitate political" actors. Gender as a process has two central manifestations in political science research, firstly in determining "the differential effects of structures and policies upon men and women," and secondly, the ways in which masculine and feminine political actors "actively work to produce favorable gendered outcomes".
With regard to gender studies, Jacquetta Newman states that although sex is determined biologically, the ways in which people express gender is not. Gendering is a socially constructed process based on culture, though often cultural expectations around women and men have a direct relationship to their biology. Because of this, Newman argues, many privilege sex as being a cause of oppression and ignore other issues like race, ability, poverty, etc. Current gender studies classes seek to move away from that and examine the intersectionality of these factors in determining people's lives. She also points out that other non-Western cultures do not necessarily have the same views of gender and gender roles. Newman also debates the meaning of equality, which is often considered the goal of feminism; she believes that equality is a problematic term because it can mean many different things, such as people being treated identically, differently, or fairly based on their gender. Newman believes this is problematic because there is no unified definition as to what equality means or looks like, and that this can be significantly important in areas like public policy.
Social construction of sex hypotheses
See also: Sex and gender distinction
Sociologists generally regard gender as a social construct, and various researchers, including many feminists, consider sex to only be a matter of biology and something that is not about social or cultural construction. For instance, sexologistJohn Money suggests the distinction between biological sex and gender as a role. Moreover, Ann Oakley, a professor of sociology and social policy, says "the constancy of sex must be admitted, but so also must the variability of gender." The World Health Organization states, "'[s]ex' refers to the biological and physiological characteristics that define men and women," and "'gender' refers to the socially constructed roles, behaviours, activities, and attributes that a given society considers appropriate for men and women." Thus, sex is regarded as a category studied in biology (natural sciences), while gender is studied in humanities and social sciences. Lynda Birke, a feminist biologist, maintains "'biology' is not seen as something which might change." Therefore, it is stated that sex is something that does not change, while gender can change according to social structure.
However, there are scholars who argue that sex is also socially constructed. For example, gender theorist Judith Butler states that "perhaps this construct called 'sex' is as culturally constructed as gender; indeed, perhaps it was always already gender, with the consequence that the distinction between sex and gender turns out to be no distinction at all."
It would make no sense, then, to define gender as the cultural interpretation of sex, if sex is itself a gender-centered category. Gender should not be conceived merely as the cultural inscription of meaning based on a given sex (a juridical conception); gender must also designate the very apparatus of production whereby the sexes themselves are established. [...] This production of sex as the pre-discursive should be understood as the effect of the apparatus of cultural construction designated by gender.
Butler argues that "bodies only appear, only endure, only live within the productive constraints of certain highly gendered regulatory schemas," and sex is "no longer as a bodily given on which the construct of gender is artificially imposed, but as a cultural norm which governs the materialization of bodies." Marria Lugones states that, among the Yoruba people, there was no concept of gender and no gender system at all before colonialism. She argues that colonial powers used a gender system as a tool for domination and fundamentally changing social relations among the indigenous.
With regard to history, Linda Nicholson, a professor of history and women's studies, says that the notion of human bodies being separated into two sexes is not historically consistent. She argues that male and female genitals were considered inherently the same in Western society until the 18th century. At that time, female genitals were regarded as incomplete male genitals, and the difference between the two was conceived as a matter of degree. In other words, there was a gradation of physical forms, or a spectrum. Therefore, the current perspective toward sex, which is to consider women and men and their typical genitalia as the only possible natural options, came into existence through historical, not biological roots.
In addition, drawing from the empirical research of intersex children, Anne Fausto-Sterling, a professor of biology and gender studies, describes how the doctors address the issues of intersexuality. She starts her argument with an example of the birth of an intersexual individual and maintains "our conceptions of the nature of gender difference shape, even as they reflect, the ways we structure our social system and polity; they also shape and reflect our understanding of our physical bodies." Then she adds how gender assumptions affects the scientific study of sex by presenting the research of intersexuals by John Money et al., and she concludes that "they never questioned the fundamental assumption that there are only two sexes, because their goal in studying intersexuals was to find out more about 'normal' development." She also mentions the language the doctors use when they talk with the parents of the intersexuals. After describing how the doctors inform parents about the intersexuality, she asserts that because the doctors believe that the intersexuals are actually male or female, they tell the parents of the intersexuals that it will take a little bit more time for the doctors to determine whether the infant is a boy or a girl. That is to say, the doctors' behavior is formulated by the cultural gender assumption that there are only two sexes. Lastly, she maintains that the differences in the ways in which the medical professionals in different regions treat intersexual people also give us a good example of how sex is socially constructed. In her Sexing the body: gender politics and the construction of sexuality, she introduces the following example:
A group of physicians from Saudi Arabia recently reported on several cases of XX intersex children with congenital adrenal hyperplasia (CAH), a genetically inherited malfunction of the enzymes that aid in making steroid hormones. [...] In the United States and Europe, such children, because they have the potential to bear children later in life, are usually raised as girls. Saudi doctors trained in this European tradition recommended such a course of action to the Saudi parents of CAH XX children. A number of parents, however, refused to accept the recommendation that their child, initially identified as a son, be raised instead as a daughter. Nor would they accept feminizing surgery for their child. [...] This was essentially an expression of local community attitudes with [...] the preference for male offspring.
Thus it may be said that determining the sex of children is actually a cultural act, and the sex of children is in fact socially constructed. Therefore, it is possible that although sex seems fixed and only related to biology, it may be actually deeply related to historical and social factors as well as biology and other natural sciences.
Another work of Ann Fausto-Sterling’s in which she discusses gender is The Five Sexes: Why Male and Female Are Not Enough. In this article, Fausto-Sterling states that Western culture has only two sexes and that even their language restricts the presence of more than two sexes. She argues that instead of having a binomial nomenclature for organizing humans into two distinct sexes (male and female), there are at least five sexes in the broad spectrum of gender. These five sexes include male, female, hermaphrodite, female pseudohermaphrodites (individuals who have ovaries and some male genitalia but lack testes), and male pseudohermaphrodites (individuals who have testes and some female genitalia but lack ovaries). Fausto-Sterling additionally adds that in the category of hermaphrodites, there are additional degrees and levels in which the genitalia are developed; this means that there may be more intersexes that exist in this continuum of gender.
Fausto-Sterling argues that sex has been gradually institutionally disciplined into a binary system through medical advances. She brings up multiple instances where gender in history was not split into strictly male or female, Fausto-Sterling mentioned that by the end of the Middle Age, intersex individuals were forced to pick a side in the binary gender code and to adhere by it. She then adds on that "hermaphrodites have unruly bodies" and they need to fit into society's definition of gender. Thus, modern-day parents have been urged by medical doctors to decide the sex for their hermaphroditic child immediately after childbirth. She emphasizes that the role of the medical community is that of an institutionalized discipline on society that there can only be two sexes: male and female and only the two listed are considered "normal." Lastly, Fausto-Sterling argues that modern laws require humans to be labelled either as male or female and that "ironically, a more sophisticated knowledge of the complexity of sexual systems has led to the repression of such intricacy." She mentions this quote to inform the prevailing thought that hermaphrodites, without medical intervention, are assumed to live a life full of psychological pain when in fact, there is no evidence in which that is the case. She finishes up her argument asking what would happen if society started accepting intersex individuals.
The article Adolescent Gender-Role Identity and Mental Health: Gender Intensification Revisited focuses on the work of Heather A. Priess, Sara M. Lindberg, and Janet Shibley Hyde on whether or not girls and boys diverge in their gender identities during adolescent years. The researchers based their work on ideas previously mentioned by Hill and Lynch in their gender intensification hypothesis in that signals and messages from parents determine and affect their children’s gender role identities. This hypothesis argues that parents affect their children's gender role identities and that different interactions spent with either parents will affect gender intensification. Priess and among other’s study did not support the hypothesis of Hill and Lynch which stated "that as adolescents experience these and other socializing influences, they will become more stereotypical in their gender-role identities and gendered attitudes and behaviors." However, the researchers did state that perhaps the hypothesis Hill and Lynch proposed was true in the past but is not true now due to changes in the population of teens in respect to their gender-role identities.
Authors of Unpacking the Gender System: A Theoretical Perspective on Gender Belief’s and Social Relations, Cecilia Ridgeway and Shelley Correll, argue that gender is more than an identity or role but is something that is institutionalized through "social relational contexts." Ridgeway and Correll define "social relational contexts" as "any situation in which individuals define themselves in relation to others in order to act." They also point out that in addition to social relational contexts, cultural beliefs plays a role in the gender system. The coauthors argue that daily people are forced to acknowledge and interact with others in ways that are related to gender. Every day, individuals are interacting with each other and comply with society's set standard of hegemonic beliefs, which includes gender roles. They state that society's hegemonic cultural beliefs sets the rules which in turn create the setting for which social relational contexts are to take place. Ridgeway and Correll then shift their topic towards sex categorization. The authors define sex categorization as "the sociocognitive process by which we label another as male or female."
Biological factors and views
See also: Sexual differentiation and Sexual differentiation in humans
Gender, as identified through gender normative play, self-identification with a gender, and tendency to engage in aggressive behavior is influenced by prenatal hormone exposure. Studies on other gendered behavior are inconsistent, however some evidence indicates other gendered behavior is influenced by prenatal and early life androgen exposure. Males of most mammals, including humans, exhibit more rough and tumble play behavior, which is influenced by maternal testosterone levels. These levels may also influence sexuality, with non-heterosexual persons exhibiting sex atypical behavior in childhood.
The biology of gender became the subject of an expanding number of studies over the course of the late 20th century. One of the earliest areas of interest was what became known as "gender identity disorder" (GID) and which is now also described as gender dysphoria. Studies in this, and related areas, inform the following summary of the subject by John Money. He stated:
The term "gender role" appeared in print first in 1955. The term gender identity was used in a press release, November 21, 1966, to announce the new clinic for transsexuals at The Johns Hopkins Hospital. It was disseminated in the media worldwide, and soon entered the vernacular. The definitions of gender and gender identity vary on a doctrinal basis. In popularized and scientifically debased usage, sex is what you are biologically; gender is what you become socially; gender identity is your own sense or conviction of maleness or femaleness; and gender role is the cultural stereotype of what is masculine and feminine. Causality with respect to gender identity disorder is sub-divisible into genetic, prenatal hormonal, postnatal social, and post-pubertal hormonal determinants, but there is, as yet, no comprehensive and detailed theory of causality. Gender coding in the brain is bipolar. In gender identity disorder, there is discordance between the natal sex of one's external genitalia and the brain coding of one's gender as masculine or feminine.
Money refers to attempts to distinguish a difference between biological sex and social gender as "scientifically debased", because of our increased knowledge of a continuum of dimorphic features (Money's word is "dipolar") that link biological and behavioral differences. These extend from the exclusively biological "genetic" and "prenatal hormonal" differences between men and women, to "postnatal" features, some of which are social, but others have been shown to result from "post-pubertal hormonal" effects.
Although causation from the biological—genetic and hormonal—to the behavioral has been broadly demonstrated and accepted, Money is careful to also note that understanding of the causal chains from biology to behavior in sex and gender issues is very far from complete. For example, the existence of a "gay gene" has not been proven, but such a gene remains an acknowledged possibility.
There are studies concerning women who have a condition called congenital adrenal hyperplasia, which leads to the overproduction of the masculine sex hormone, androgen. These women usually have ordinary female appearances (though nearly all girls with congenital adrenal hyperplasia (CAH) have corrective surgery performed on their genitals). However, despite taking hormone-balancing medication given to them at birth, these females are statistically more likely to be interested in activities traditionally linked to males than female activities. Psychology professor and CAH researcher Dr. Sheri Berenbaum attributes these differences to an exposure of higher levels of male sex hormones in utero.
Main article: Sexual reproduction
Sexual differentiation demands the fusion of gametes that are morphologically different.
— Cyril Dean Darlington, Recent Advances in Cytology, 1937.
Sexual reproduction is a common method of producing a new individual within various species. In sexually reproducing species, individuals produce special kinds of cells (called gametes) whose function is specifically to fuse with one unlike gamete and thereby to form a new individual. This fusion of two unlike gametes is called fertilization. By convention, where one type of gamete cell is physically larger than the other, it is associated with female sex. Thus an individual that produces exclusively large gametes (ova in humans) is called female, and one that produces exclusively small gametes (spermatozoa in humans) is called male.
An individual that produces both types of gametes is called hermaphrodite (a name applicable also to people with one testis and one ovary). In some species hermaphrodites can self-fertilize (see Selfing), in others they can achieve fertilization with females, males or both. Some species, like the Japanese Ash, Fraxinus lanuginosa, only have males and hermaphrodites, a rare reproductive system called androdioecy. Gynodioecy is also found in several species. Human hermaphrodites are typically, but not always, infertile.
What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, of more than 1.5 million living species, recorded up to about the year 2000, "no third sex cell—and so no third sex—has appeared in multi-cellular animals." Why sexual reproduction has an exclusively binary gamete system is not yet known. A few rare species that push the boundaries of the definitions are the subject of active research for light they may shed on the mechanisms of the evolution of sex. For example, the most toxic insect, the harvester ant Pogonomyrmex, has two kinds of female and two kinds of male. One hypothesis is that the species is a hybrid, evolved from two closely related preceding species.
Fossil records indicate that sexual reproduction has been occurring for at least one billion years. However, the reason for the initial evolution of sex, and the reason it has survived to the present are still matters of debate, there are many plausible theories. It appears that the ability to reproduce sexually has evolved independently in various species on many occasions. There are cases where it has also been lost, notably among the Fungi Imperfecti. The blacktip shark (Carcharhinus limbatus), flatworm (Dugesia tigrina) and some other species can reproduce either sexually or asexually depending on various conditions.
The following systematic list gender taxonomy illustrates the kinds of diversity that have been studied and reported in medical literature. It is placed in roughly chronological order of biological and social development in the human life cycle. The earlier stages are more purely biological and the latter are more dominantly social. Causation is known to operate from chromosome to gonads, and from gonads to hormones. It is also significant from brain structure to gender identity (see Money quote above). Brain structure and processing (biological) that may explain erotic preference (social), however, is an area of ongoing research. Terminology in some areas changes quite rapidly as knowledge grows.
- 46,XX (genetic female); 46,XY (genetic male) ;45,X (Turner's syndrome); 47,XXY (Klinefelter syndrome
Split sex ratios, when some colonies produce only male and others only female reproductives, is a common feature of social insects, especially ants. The most widely accepted explanation for split sex ratios was proposed by Boomsma and Grafen, and is driven by conflicts of interest among colonies that vary in relatedness. The predictions of the Boomsma–Grafen model have been confirmed in many cases, but contradicted in several others. We adapt a model for the evolution of dioecy in plants to make predictions about the evolution of split sex ratios in social insects. Reproductive specialization results from the instability of the evolutionarily stable strategy (ESS) sex ratio, and is independent of variation in relatedness. We test predictions of the model with data from a long-term study of harvester ants, and show that it correctly predicts the intermediate sex ratios we observe in our study species. The dioecy model provides a comprehensive framework for sex allocation that is based on the pay-offs to the colony via production of males and females, and is independent of the genetic variation among colonies. However, in populations where the conditions for the Boomsma–Grafen model hold, kin selection will still lead to an association between sex ratio and relatedness.
Keywords: split sex ratios, dioecy, ants, sex allocation
Sex ratios in social insects have been the subject of intense study since Trivers & Hare (1976) first modified Fisher's (1930) sex ratio theory to include kin selection, particularly in species with haplodiploid sex determination. Their argument made specific quantitative predictions about the investment sex ratio and predicted that it would be affected by several aspects of colony structure (mating frequency, worker reproduction, queen number, etc.). In the simplest case, a single queen that mates once, workers are predicted to bias investment in a ratio of 3 : 1 in favour of their sisters, with whom they share higher relatedness. Female-biased sex ratios have subsequently been confirmed in many social Hymenoptera (e.g. Queller & Strassmann 1998; Ratnieks et al. 2006).
When colonies are founded by multiple queens, by a multiply mated queen or when workers reproduce, the predictions about sex ratio also change. Many studies have explored the effect of mating frequency (e.g. Starr 1979; Nonacs 1986; Roisin & Aron 2003), queen number (Nonacs 1986; Kümmerli et al. 2005; Bargum et al. 2007) and worker reproduction (Benford 1978; Bourke 1988; Pamilo 1991) on the sex ratio. Characteristics of the breeding system, such as the amount of inbreeding, were among the first factors shown to affect sex ratios (Hamilton 1967; Charnov 1982). A number of investigators have predicted that there should be a correlation between relatedness and colony-level sex ratio. Colonies in which the queen mates a greater number of times, or which have more queens, typically have lower relatedness. In polyandrous colonies, or polygynous ones where the queens are relatives, workers will be more equally related to males and females. If workers control sexual investment, they should favour the production of more males than that will occur in colonies with a higher relatedness. Reviews of the evidence across many species largely support this generalization, and the interpretation that sex ratios are modified adaptively by workers (Bourke & Franks 1995; Bourke 1997, 2001; Queller & Strassmann 1998; Meunier et al. 2008).
A logical extension of these ideas is split sex ratio theory. While adaptive modification of sex ratios can result in differences among colonies that have different genetic structure, it does not require sex ratios to fall into two discrete categories. However, a frequent observation in ant species is that some colonies specialize on producing gynes (new reproductive females) while others produce males (e.g. Talbot 1945; Scherba 1961; Pamilo & Rosengren 1983; synthesized by Nonacs 1986). Regardless of the average sex ratio in the population, the frequency distribution for single colonies is U-shaped. Boomsma & Grafen (1990, 1991) explained this phenomenon as a consequence of how selection on sex ratio is altered by the variation in relatedness among colonies. If colonies vary in relatedness, and the workers can control the colony sex ratio, then colonies will differ in their ESS sex ratio. Workers from colonies that have a relatively high relatedness will prefer a more female-biased sex ratio while those from colonies with lower relatedness will prefer sex ratios that are biased towards males. These conflicts of interest shift the colony sex ratios until the population reaches an equilibrium where colonies specialize in the production of one sex or the other.
The Boomsma–Grafen model predicts that sex ratios will be split when the following three conditions hold: (i) relatedness varies among colonies, (ii) workers control investment, and (iii) workers can determine whether they are in a high or low relatedness colony. If these conditions are not met, colony sex ratios should approach the average ESS for the population. Queens lay eggs with a particular primary sex ratio, and workers may modify the secondary sex ratio by either killing male eggs or increasing the proportion of female eggs that become gynes. A queen can respond to worker actions by altering the primary sex ratio, leading to a coevolutionary arms race (e.g. Helms et al. 2005). Models that incorporate costs for modifying the primary or secondary sex ratio, make the detection of colony state very expensive or assume constraints on egg availability can make it uneconomical for workers to split the sex ratio (Ratnieks & Boomsma 1996; Reuter & Keller 2001; Helms et al. 2005).
The pattern of sex allocation is consistent with split sex ratio predictions in many species. The test of this theory by Sundström (1994) in Formica ants made split sex ratio theory literally a textbook case of the ability of evolutionary theories in general, and ESS theories in particular, to make quantitative predictions (e.g. Alcock (2005)). The ability of the Boomsma–Grafen model to explain the variation of sex ratios in ant species is considered a strong validation of both kin selection and the frequent resolution of queen–worker conflict in the workers' favour. Although the theory has had considerable success, it does not predict some patterns of sex allocation, which are relatively frequent in social insects. Split sex ratios have been found in species where there is no variation in relatedness. In some cases, the queen universally mates a single time (Vargo 1996; Helms 1999; Foitzik & Heinze 2000), but sex ratios are split. In other cases, split sex ratios are associated with variation in queen number, but because the queens are unrelated to one another, split sex ratios are not expected (Fournier et al. 2003; Helms et al. 2005). Additionally, split sex ratios may be absent when there is variation among colonies in relatedness due to mating frequency (e.g. MacKay 1981; van der Have et al. 1988) or the number of queens (Chan et al. 1999). Finally, split sex ratios are sometimes observed in cases where there is variation among colonies in relatedness; however, the variation in relatedness is opposite to the expected direction (Pamilo & Seppä 1994). These counterexamples have been explained either by assuming that workers do not control sex ratios or that they cannot tell the type of colony they are in. Observations of split sex ratios in colonies without variation in relatedness have been interpreted as evidence that the queen controls the sex ratio (Mehdiabadi et al. 2003).
In the Boomsma–Grafen theory, the fitness pay-off to colonies is assumed to be proportional to investment, which makes the conversion of investment in females into pay-off from female investment straightforward. Under this assumption, biomass is used instead of simply the numbers to correct for differences in body size (and may be corrected for the different energetic costs of producing males and females as a function of body size, e.g. Boomsma 1989; Boomsma et al. 1995) as the measure of pay-off. The Boomsma–Grafen theory assumes that investment, and the pay-off from investing in one sex, directly affects the investment, and therefore the pay-off, from the other sex (e.g. Charnov 1982). The assumed relationship is completely proportional—25 per cent greater investment in one sex increases the pay-off from that sex by 25 per cent and causes a corresponding decrease in investment and pay-off in the other sex. If the size of a reproductive alters the pay-off in a nonlinear way (e.g. Wiernasz et al. 2001; Wiernasz & Cole 2003), then investment no longer accurately predicts the pay-off. The goal of this paper is to examine how nonlinear relationships between investment and pay-off can influence the stability of the ESS and the distribution of colony sex ratios in a population.
Recent approaches (Helms et al. 2005) that consider the coevolution of worker and queen sex ratio strategies have shown that split sex ratios can evolve without variation in relatedness within populations. The underlying reason for the evolution of split sex ratios remains a conflict about the optimal sex ratio, but now it is between the strategies of queens and workers. Whether or not split sex ratios evolve depends on the relative magnitude of the costs incurred by workers and queens for biasing the sex ratio, the size of mutational effects and the particular worker strategy.
We adapt a model from plant life-history evolution (Lloyd 1984) that makes explicit the relationship between investment and fitness to explore the evolution of split sex ratios, independent of the effects of genetic relatedness. This model is not based on a specific mechanism, such as conflicts of interest between colonies of high and low relatedness, or between queens and workers within colonies. Instead, we focus on how the stability of the ESS sex ratio leads to either split or mixed sex ratios. Because the model is based on fitness pay-offs through male and female functions, we are not constrained to assume that investment is directly proportional to fitness. We shall explore the effect of control of investment and changes to the social structure on the predictions of the model, and test the same with the data collected from a long-term study of reproduction in the western harvester ant, Pogonomyrmex occidentalis.
2. The dioecy model
Our model is a modification of that proposed in Lloyd (1984). One can express the fitness that a colony acquires through male or female offspring as a function of the proportion of investment in males or females. Although the usual assumption is that the relationship is linear, it is more general to allow male or female fitness to accrue to a colony in a nonlinear manner. The formulation used by Charlesworth & Charlesworth (1981), Charnov (1982), Lloyd (1984) and Campbell (1998, 2000) has been to assume a power function, so that the fitness achieved through male and female functions is
w♂ ∝ ay, w♀ ∝ (1−a)z, where y, z > 0
with a being the proportion of investment in males (figure 1). If both the exponents y, z=1, the relationships are linear and the assumption of most work on sexual investment in social insects is recaptured.
The total fitness through males or females as a function of the investment sex ratio. The maximum fitness for each function is normalized to a value of 1 for ease of presentation. In (a) exponents describing the fitness function from relationship 1 are...
To solve for the ESS sex ratio and find the conditions for split or mixed sex ratios, we first note that the fitness of a colony that has an alternative sex ratio can be written as
where a1 is the sex ratio in the population and a2 is the sex ratio of a ‘mutant’ colony. The first term is the fitness through females and the second is the fitness through males. Female fitness is given by the pay-off function (2.1). Male fitness is achieved by mating and sperm transfer, and is a function of the proportion of females with which males can mate. This function gives fitness from the perspective of the queen. We will derive the relationship for the worker point of view below. By solving , we find the ESS: . When the functional relationship between investing in males or females is the same (y=z), the ESS sex ratio is 1/2. To investigate the stability of the equilibrium, we obtain the second derivative evaluated at the ESS
When , the ESS sex ratio is stable; sex ratios will tend to an intermediate value. Alternatively, when , the ESS is unstable and colonies will become specialists, producing only males- or only females-split sex ratios. The ESS is stable, i.e. mixed sex ratios are expected, when 2zy<z+y (figure 2). Our formulation is identical to that of Lloyd (1984) and Campbell (2000) for diploid plants. The sex ratio is determined by the functional relationship between investment and fitness pay-off. Split sex ratios are expected, even from the point of view of the queen, when there is a disproportional pay-off to specializing on males or females (y, z>1), and they are not expected when there is a less than proportional pay-off to specializing on a single sex (y, z<1). As the exponents increase significantly above 1, there is a disproportionate pay-off to increasing investment in one sex compared with the fitness penalty for underproducing the other sex—the ESS becomes unstable. The stability of mixed sex ratios refers to the population sex ratio rather than that of individual colonies. In the region where mixed sex ratios are expected, there may be little selection on a colony to have a particular sex ratio, resulting in a large variation in the distribution of colony sex ratio, a pattern that characterizes studies of sex allocation in cosexual plants (e.g. Fox 1993; Masaka & Takada 2006).
Parameter space for the evolution of split sex ratios under the dioecy model. The solid curves are for queen control and worker control of investment in a colony with a single queen who mates a single time. The dashed curve gives the conditions for the...
From the point of view of a worker in a colony where the queen has mated a single time,
The difference between the queen and worker fitness measures is that workers value the production of queens more than that of males. Solving for the equilibrium sex ratio: , which when y=z=1 leads to , in other words a 3 : 1 female-biased sex ratio. Solving for the conditions of instability, , so that split sex ratios will occur with worker control yields 2zy>(3z+y)/2 (figure 2). There is a substantial portion of parameter space in which both the workers and the queen agree either that sex ratios will be split (above both curves) or that sex ratios should be mixed (area below both curves). The areas between the curves correspond to conditions where either worker control or queen control of sex ratio can produce split sex ratios. This is not a conflict between sex ratio strategies in the usual sense, rather that mixed sex ratios are stable under queen control and unstable under worker control. However, in certain portions of the parameter space, split sex ratios are predicted under queen control even when they are not predicted under worker control.
Because variation in relatedness drives split sex ratio evolution in the Boomsma–Grafen model, we next consider the effect of variation in relatedness in the dioecy model. Relatedness can vary among colonies because of differences in the mating frequency of the queen or differences in the queen number, and thus changes conditions from the worker's perspective.
When the queen mates with n males who contribute equally to the reproductive output of a colony, fitness becomes
Females, while still more valuable to workers than males, are relatively less valuable when the queen mates multiple times because the workers are not equally related to all females. Because the males are produced by unfertilized eggs, the number of matings by the queen is irrelevant. The population equilibrium sex ratio becomes
and the condition for the development of split sex ratios becomes y(z+k)>k+y, where k=z(1+(2/n)). As the number of matings increases (n→∞), the relationship becomes identical to that of the queen or of a diploid, and the worker curve is warped into that of the queen (figure 2). When the queen is highly polyandrous, the region of parameter space in which the predictions based on worker control and queen control differ decreases.
The effects of queen number (polygyny) are very similar to the effects of multiple mating. If the queens are not related, workers of one queen do not value the offspring of the other queens, regardless of sex. Workers still overvalue females by a 3 : 1 ratio, and no change in sex ratio is expected. Although colonies vary in relatedness, from the viewpoint of the workers, relatedness to the reproductives that matter has not changed. Consequently, split sex ratios are not expected under the Boomsma–Grafen hypothesis. In many cases, however, multiple queens are related. The relatedness of workers to the reproductive offspring of a queen that is not their mother is r/4, where r is the relatedness of their mother to the other queen. Because the only pathway of relatedness from workers to the offspring of another queen is through the queen, both her male and female offspring are equally valuable. When a colony has multiple equivalent gynes, the workers value the production of females more than they value the production of males by a factor,
where N is the number of gynes and r is the relatedness among them. When there is a single queen, the workers value females three times more than they value males, and as the number of gynes becomes large, F approaches 1. When the relatedness among gynes is zero, the relative value of females is unchanged. The equilibrium sex ratio, , and the conditions for the production of split sex ratios are changed, with split sex ratios expected when yz(1+F)>y+zF (figure 2).
Over most of the parameter space, the conditions for the production of split sex ratios by either party are the same, and are independent of mating frequency, queen number or queen versus worker control. The dioecy model does not require variation in relatedness, although details about the control of sex ratio and intracolony relatedness influence the boundaries between parameter regions where split sex ratios are expected. It is consistent with split sex ratios arising in colonies whose single queens mate a single time or with multiple queens that are unrelated, as well as the absence of split sex ratios even when they are predicted by the Boomsma–Grafen model.
3. The harvester ant study system
As part of our study on the life history of P. occidentalis, we have measured the reproductive output of approximately 200 colonies beginning in 1994 (for a description of the study site, and sampling methods see Wiernasz & Cole 1995). We obtained the total reproductive output for colonies by watering them, which stimulates the flight of all reproductives (for details of the methods for inducing mating flights and capturing reproductives, see Cole & Wiernasz 2000). Each colony has been genotyped using highly variable microsatellites (Wiernasz et al. 2004). Colonies vary substantially in relatedness, with the number of patrilines ranging from 3 to 10 (mean=6.2).
We obtained sexual investment ratios for colonies in 1994, 1995, 1996, 1997, 1998, 2001 and 2003 by collecting all reproductives produced. In 1999, 2000, 2005 and 2006, early rainfall produced mating flights before we could measure the reproductive output. In 2002, although we measured the reproductive output, only one colony reproduced, due to severe drought. When calculating the sex ratios, we used only the data from colonies that produced at least 20 reproductives. This minimizes the over-representation of apparent split sex ratios due to binomial sampling. The mean number of reproductives in our study colonies was 244. We estimated the investment in each sex by individually weighing the reproductives. During 1994–1998, we used a randomly chosen sample of 30 males and 30 females (or all, if fewer than 30) from each colony; in 2001 and 2003, we weighed all the reproductives produced by a colony. Colonies vary in the sizes of both males and females that they produce, hence we used colony- and year-specific sizes for males and females. To measure male fitness, we estimated the number of sperm transferred as a function of body mass. Both the total sperm count of a male and the number of sperm transferred to females during mating are functions of body mass, and larger males transfer disproportionately more of their sperm than small males (Wiernasz et al. 2001). We used the published relationship to calculate the total number of sperm transferred by all of the males that were produced by each colony in each year. For 2001 and 2003, we summed the expected number of sperm transferred by all males. For the period 1994–1998, we calculated the mean male fitness based on the sample of 30 and estimated the total fitness for the entire male output. Fitness through female function is based on the size-specific probability of survival of queens (Wiernasz & Cole 2003). We used the published relationship between body mass and survival probability to estimate the probability of survival of each queen collected from the colonies. We summed the probability of survival over all females giving the expected number of queens that survive colony founding. For both the sexes, although individual fitness increases as a function of body size, the pay-off to colonies for making reproductives of increasing size is a decelerating function (Wiernasz et al. 2001; Wiernasz & Cole 2003). Although these measures are correlates of total fitness, they represent the best estimates that can be obtained for this species. They are also equivalent to the fitness estimates used for plant sex ratio studies (pollen transfer and seed set/survival, e.g. Campbell 2000). For each year, we plotted the fitness of a colony through male and female functions against the proportional investment in males. We fitted a power function to the data in order to obtain the exponents y (for females) and z (for males) that are important for determining the stability of the sexual investment ratios.
We tested whether sex ratio was consistent within a colony using a two-way ANOVA with year and colony as classification variables. We sought further evidence of consistency by testing whether a colony's sex ratio was correlated across years. Because colonies do not reproduce in each year, we cannot use a repeated-measures ANOVA. To avoid multiple comparisons, we calculated the correlation between sex ratios for two pairs of years (1994–1995 and 2001–2003). These two years have the greatest time separation that mitigates the potential non-independence of the results.
For each year, we tested for a correlation between colony sexual investment ratio and the within-colony relatedness. The correlation was not significant in any year or for the overall dataset (p>0.2 for each year).
For the entire dataset, the colony investment ratios were biased towards females. There was no evidence of split sex ratios. A few colonies produce highly male- or female-biased broods, but overall the distribution is not U-shaped (figure 3; mean a=0.383 (0.34–0.42=95% confidence limits)). If we use the Boomsma (1989) correction for investment ratios based on probable energetic differences in the production of males and females, the investment ratio is slightly different (mean a=0.432, 95% confidence limits=0.39–0.47), but still significantly different from the 0.5 expected with queen control of investment. The predicted sexual investment ratio under worker control of investment based on the effective number of patrilines is 0.427, which is not different from the observed sex ratio.
Sex ratio (proportion of investment in males) distribution for all colonies combined across years. Sex ratio distributions within a year have the same overall pattern.
Although the population sex ratio varied over time, a given colony's sex ratio was stable. Both year (F6,225=5.55, p<0.001) and colony (F151,255=2.82, p<0.001) effects were highly significant. Both of the across-year comparisons of sex ratios were significant (1994–1995: r=0.48, p=0.008; 2001–2003: r=0.30, p=0.028).
For each year except 1994, the year with the least data (33 colonies), the combination of exponents falls outside of the split sex ratio region of parameter space (figure 4). The overall exponent (large shaded circle), from the summed fitness over eight years, predicts that sex ratios should not be split. Using the Boomsma (1989) correction (smaller shaded circle), we calculate slightly different exponents, but the conclusion is unaltered.
The relationship of exponents for the male and female fitness functions. The points are labelled by year. The shaded points are the exponents using the total investment and fitness for a colony (smaller grey circle with the correction of Boomsma 1989...
Under the dioecy model, the expectation is that colonies in this population, even though they vary in relatedness, will not have split sex ratios. While the exponent for females is on average near unity (mean=1.03 (95% confidence limits=0.62–1.45)), the exponent relating male fitness to sex ratio is significantly less than 1 (mean=0.49 (95% confidence limits=0.36–0.62)). Although female fitness is proportional to investment, male fitness decelerates substantially with increasing investment.
Sex ratio is a colony characteristic in P. occidentalis; sexual investment patterns were stable over more than a decade. While sex allocation was consistent, it did not reflect intracolonial relatedness. If sex ratios were uncorrelated with relatedness, and also inconsistent across years, we could argue that they resulted from factors that we did not measure (e.g. a function of local environmental conditions that were expressed in particular colonies in particular years). The population sex ratio did differ among years, which may have been a function of reproduction by different subsets of colonies, and differences in environmental conditions among years. However, because the sex ratio for a particular colony remained consistent, it should be considered an aspect of the colony's phenotype.
In contrast to the predictions of Boomsma–Grafen model, the sex ratios of harvester ant colonies were not split. The absence of split sex ratios could reflect the inability of workers to influence the sex ratio or to determine the genetic diversity within colonies. Two lines of evidence suggest that this is not true. First, the population sex ratio is female biased. It is near the value expected from worker control and different from that expected under queen control—workers appear to control sex ratio in this species. Second, colonies that differ in genetic diversity are very different in their properties, including patterns of colony growth (Cole & Wiernasz 1999; Wiernasz et al. 2004) and activity (Wiernasz et al. 2008). Workers should have a number of cues that enable them to determine the genetic diversity of their colony.
The dioecy model correctly predicted that harvester ant colonies should not specialize on one sex of reproductives. While female fitness is an approximately linear function of investment, the pay-off through males is decidedly nonlinear. Increasing investment in males yields a smaller than proportional pay-off to fitness. Similar differences in the form of the relationship between investment and pay-off have been shown in plants (Campbell 1998, 2000).
In a recent simulation study, Helms et al. (2005) have explored the effects of queen–worker conflict on sex ratio evolution in social insect colonies with no variation in genetic relatedness (one singly mated queen). Their model found that the effect of sex ratio biasing in both castes typically was an accelerating function. Split sex ratios evolved when biasing in one caste opposed the biasing in the other. The degree to which opposing strategies evolved was a function of the relative cost of biasing for each caste and the rate of evolution in each caste. These results are concordant with the general conclusion of the dioecy model—that accelerating returns for specializing in one sex (e.g. figure 1a) lead to an unstable ESS sex ratio and split sex ratios. Our approach made no assumptions about the specific mechanism(s) that could produce accelerating functions, in fact we did not permit conflict (either workers or queens had control over the sex ratio) to emphasize what factors are responsible for split sex ratio generation.
The magnitude of the differences in the exponents for males and females suggests that the processes of investing in males and females differ. The first possibility is that colonies invest different resources in each sex. If resources are removed from females, they are not automatically available for male investment. In harvester ants, supplements of non-storable food (insects) increased male, but not female, size. Supplementation with storable food (seeds) increased the number of both males and females. Because resource addition increases the output of both the sexes, it is unlikely that male and female reproductions depend on entirely different pools of resources. However, the effect of insect supplements suggests that provisioning of male brood is regulated differently from that of female brood, perhaps reflecting decreased efficiencies in male production. Colonies that invest disproportionately in males usually produce only small males, which lower colony fitness pay-offs. The cause of this is unknown—production of numerous small males may reflect extreme queen–worker conflict or local variation in resources (a high proportion of insects relative to seeds).
While the dioecy model can make specific predictions about the occurrence of split sex ratios, it requires considerable data—fitness must be measured through male and female functions. Applying this method to other species will require knowledge about the relationship between body size and fitness in both the sexes, in order to convert investment in terms of either numbers or biomass into fitness estimates. In P. occidentalis, fitness of individual reproductives increases with body size (Wiernasz et al. 2001; Wiernasz & Cole 2003), justifying the use of biomass of males or females as a fitness measure for the colony. In other species, numbers of males or females may more accurately reflect fitness, even if biomass gives a better indication of investment.
The dioecy model provides a comprehensive framework for sex allocation that is based on the pay-offs to the colony via production of males and females, and independent of the genetic variation among colonies. Colonies produce one sex exclusively when the ESS sex ratio is unstable due to nonlinear pay-offs for investment; however, the model does not predict which colonies should specialize on males versus females. Variation in intracolonial relatedness, in local resource availability, or in colony age could influence a colony's propensity to favour one sex over the other. The mechanism of the dioecy model does not preclude a correlation between mating frequency and the sex ratio. When the conditions for the Boomsma–Grafen model hold (relatedness varies, workers control investment and workers can determine relative relatedness), then workers in colonies with higher relatedness will achieve higher fitness if they bias reproduction towards females as predicted by Nonacs (1986). In such populations, kin selection will still lead to an association between sex ratio and relatedness.
We thank Ricardo Azevedo and especially Dave Queller and two anonymous reviewers for their helpful suggestions on the manuscript, and Jonathon Combs, Bao To, Brad Mallberg, Chrissy Perroni, Nick Zarrabi, Mika Turner and Mercedes Bosley for their help with weighing ants. This work was supported by NSF grants IBN-9507470 and IBN-0344896.
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