in Evolutionary Biology

What are Sexes?

There is much confusion in our current culture as to what ‘sexes’ are and what they are not. When biologists make a claim about the number of sexes in a species, they are not making a claim about chromosomes, body types, or personal identity; rather, they are making a claim about the number of distinct reproductive strategies in that species.

A reproductive strategy is an evolved system for propagating genes and forming a new individual. In sexually reproducing species, producing a new individual requires the combination of at least two distinct and complementary reproductive strategies. These strategies are fulfilled through the delivery of genetic material in sex cells called gametes, which have half the genetic material of the parent. When two gametes fuse, they form a genetically unique individual with a full set of chromosomes.

Some species reproduce through gametes of the same size (isogamy) and can have many unique reproductive strategies called mating types, which control what gametes can fuse with one another, but their differences do not go far beyond the molecular level.[1] On the other hand, most species in the plant and animal kingdoms reproduce through gametes of differing size and form (anisogamy), where there is an asymmetry in size and behavior between the interacting gametes and often the individual organisms themselves.[2]

When gamete sizes are differentiated (anisogamy), there are typically exactly two sexes, no more and no less.[3] In such systems, the reproductive strategy that produces the smaller gametes is designated as “male” and the reproductive strategy that produces the larger gametes is designated as “female.”[4] It is not the physical size of the gametes themselves that differentiates the male and female reproductive strategies, but rather what those size differences represent.[5]

Universal Biological Definition

In biology, the two sexes are defined this way:

Biologically, the female sex is defined as the adult phenotype that produces the larger gametes in anisogamous systems.

Biologically, the male sex is defined as the adult phenotype that produces the smaller gametes in anisogamous systems.

Lehtonen & Parker (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

The male strategy involves producing many small motile gametes (sperm) that can find the larger gamete quickly but have little to no resources for the future zygote, while the female strategy involves producing few large sessile gametes (eggs) that do not move fast but provide almost all the resources for the future zygote and provide a large target for the smaller gametes. Because the sexes are ultimately defined as reproductive strategies involving the production of two different gamete types, understanding the origins of the two sexes means understanding why gametes come in two different sizes.

I wrote a short essay on this here.

The divergence of gametes into two extreme sizes, forms, and behaviors is almost mathematically inevitable, because it maximizes the efficiency of the sexual system.[6] When such divergence occurs, two reproductive strategies are generated, which forms the two sexes—male and female—along with the distinct reproductive anatomy to support the two strategies.[7] Because anisogamy is almost universal across sexually reproducing multicellular plants and animals, it is perfectly accurate to say that there are only two sexes in most complex species.[8] This is what sexes ultimately are: discrete reproductive strategies involving the production of two different gametes.

The near ubiquitous existence of two sexes does not limit the diversity of organismal body types, however, because the male and female strategies can be composed in a variety of ways.[9] Sometimes they are composed in the same individual organism (an individual is both male *and* female), and many other times they are composed in different individuals (an individual is *either* male or female). Despite the diversity of body types, what consistently defines the systems as male or female is the type of reproductive strategy implemented in the individual organism, ultimately differentiated by gamete type.[10] Thus, the term ‘sexes’ describes the implementation of discrete reproductive strategies across space and time, not simply the appearance of body types.

Non-universal Definitions

When biologists and others claim that there are only two sexes in most complex species, theorists and activists interested in redefining sex will counter by defining ‘sexes’ through non-universal definitions—descriptions that do not apply to all anisogamous species or ones that narrowly or incorrectly apply to humans.

Some of these definitions might be useful in specific contexts, whether in social or medical situations, but they are not biological definitions for ‘sexes’, as they differ across time and space. Furthermore, because of their contradictory, inconsistent, or narrow scope, such definitions cannot be used to understand and contribute to the extensive body of knowledge for the evolution and development of male and female in humans and across different species.

These non-universal definitions of sexes can be broken into three categories: chromosomal, phenotypic, and sociocultural.

I am going to explain why all three fail to describe what sexes ultimately are and how such definitions create absurd conclusions.

Chromosomal Definitions

A common erroneous assumption is that sexes are defined by chromosomes. Activists will often say that there are more than two sexes, and when asked what these additional sexes are, they will list atypical chromosome combinations of XXY, XO, XXX, XYY, and so on. They assume that XX and XY means female and male, respectively, while the atypical combinations do not. But this is a category error: conflating sex determination mechanisms with sexes.

Sex determination mechanisms are the regulatory systems—genetic and environmental—that determine the development path the organism will take for their reproductive strategy, and thus, their sex.[11] Confusing chromosome combinations for sexes would be like confusing the ingredients of a cake for the cake itself. Chromosomes are the ingredients; sexes are the result.

Defining sexes as chromosomes is non-universal and inconsistent for two reasons: 1) it fails to understand the development of humans with atypical chromosome combinations, and 2) it does not apply to species with different sex determination mechanisms than X-Y or without sex chromosomes at all. First, humans with atypical chromosome combinations are not additional sexes, because such humans still develop the male or female reproductive strategy. Those with XY [SRY], XXY [SRY], and XYY [SRY] all develop the small gamete reproductive strategy (they are all male), and those with XX, XO, and XXX all develop the large gamete reproductive strategy (they are all female).[12]

We can accurately observe the sex of these individuals by observing the reproductive system they develop and the role that the reproductive system plays in one of the two gamete-based reproductive strategies. For instance, a person who develops with a reproductive system of testes and a penis can be defined as male because such structures play a direct role in the small gamete reproductive strategy. Thus, we can see how the biological definition of sexes as reproductive strategies (and not chromosomes) allows us to understand the sex of someone even with developmental anomalies like atypical chromosome combinations.

Second, if sexes are chromosomes, as the activists claim, then how do we determine sex in species that do not use the same sex chromosomes as humans? Or species who do not have sex chromosomes at all? For instance, what makes a bird with ZZ chromosomes male and one with ZW chromosomes female?  What makes a crocodile who incubated at 34 degrees Celsius male and one who developed at 30 degrees Celsius female? Were these just arbitrary designations from scientists pushing their own narrow worldview onto other animals? Or was there an overarching logic for their designation as male or female? This is where the second problem arises when using chromosomes as the sole defining feature of sexes: it does not allow us to define males and females in other species.

The correct answer of course is obvious for those who understand the biological definition of sexes. The bird with ZZ chromosomes is male because he develops the small gamete reproductive strategy (a phenotype that produces sperm), and the one with ZW chromosomes is female because she develops the large gamete reproductive strategy (a phenotype that produces eggs). The Z-W system in birds, like the X-Y system in humans, determines the development path the fetus will go down, and thus, their sex. In humans, the male is typically the heterogametic sex (XY) whereas the male in birds is typically the homogametic sex (ZZ).

What about the crocodiles who develop through temperature sex determination? Sex determination in crocodiles exists on a spectrum of temperatures. Within a certain range, males develop, and within another range, females develop. But the end result is still a male or female. The crocodile who incubated at 34 degrees C is male because he develops the small gamete reproductive strategy, and the one who incubated at 30 degrees C is female because she develops the large gamete reproductive strategy.[13] Starting to see the pattern?

We can only designate certain chromosomes (or certain temperature values in incubating crocodile eggs) as associated with male or female because we already know what male and female are in the first place. Thus, chromosomes are not sexes; rather, chromosomes are only ‘sexed’ because of the role they play in the implementation of the two reproductive strategies (sexes).

Phenotypic Definitions

Another common error is the assumption that sexes are defined solely by the composition of an organism’s body (phenotype), and that biologists only designated plants and animals as male or female because of their own preconceived notions of what a male and female are “supposed” to look like. Activists will claim that there are more than two sexes because of the sheer diversity of sex characteristics across humans and other species. Thus, sex must exist on a spectrum.

For example, activists will point out that sex development in humans can produce a wide range of body types, with varying sizes of genitalia and sex characteristics. Furthermore, when discussing other species, activists will point out that females do not always gestate the young. Male seahorses do so. Or that female hyenas have genitalia that looks like a penis and is often larger than the penis of the male hyenas. Or that clownfish can change their sex from male to female, and that slugs have both male and female parts. Therefore, the activists conclude, what separates male from female must simply be an arbitrary decision made by the preconceptions and prejudices of biologists.

But all these cases do not just fail to disprove the biological definition of sexes; they actually reinforce the scientific stability of male and female. Variation of sex development in humans is not proof of additional sexes, because those with varying compositions of genitalia shape and sex characteristics develop phenotypes that play a role in one of the two reproductive strategies like everyone else. They do not form examples of additional reproductive strategies beyond male and female but rather show the diversity of body types within the two strategies. Variation of anatomy and physiology, but not variation of sex. To be examples of additional reproductive strategies beyond male and female, such individuals would have to develop a third strategy with a third gamete type.

When it comes to other anisogamous species, unique body types also represent the diversity within the two sexes, not something outside of them. The female hyena is female not because of the prejudices of biologists, but because she develops the large gamete reproductive strategy (a phenotype that produces eggs). Her ‘penis-like’ structure is actually a giant clitoris through which she urinates, copulates, and gives birth.[14] The male seahorse is defined as male because, despite his ability to gestate the young, he develops the small gamete reproductive strategy (a phenotype that produces sperm).[15] The sex of a clownfish can be determined, despite their ability to change between sexes, by the reproductive strategy they have currently implemented (whether they have a phenotype that produces sperm or eggs).[16] [17] And the slug is an example of how the male and female reproductive strategies can be composed in the same individual (they have a phenotype that produces *both* sperm and eggs).

The only reason we can identify physical characteristics as belonging to a male or female is due to the role they play in one of two reproductive strategies. And these two reproductive strategies of male and female are defined the same across all these species and with all their diverse characteristics. Thus, phenotypes are not sexes; rather, phenotypes are only ‘sexed’ because of the role they play in the implementation of the two reproductive strategies (sexes).

Sociocultural Definitions

The last category of non-universal definitions for sexes is applied to human sociocultural contexts, where the sexes are not defined as discrete reproductive strategies involving biological mechanisms, but as forms of personal expression and identity that change across time and culture. Activists will point to a variety of cultures with a wide diversity of roles, expressions, and attitudes surrounding male and female, and conclude that the categories of male and female are social constructs.

These additional categories “beyond” male and female include the Hijras in India, the Fa’afafines in Samoa, the Metis in Nepal, the Bugis in Indonesia, and the Toms in Thailand.[18] The irony, of course, is that these people are all males and females (like everyone else) who just happen to have unique ways of expressing and seeing themselves. Some are relegated to third categories not due to their own personal liberty, but due to society’s ingrained prejudices and reinforcement of regressive sex-based stereotypes and sex roles.

Using atypical expressions of behavior, interests, and identity to redefine the biological definition of the sexes is a category error, because it conflates the diversity of ways males and females can express themselves (biopsychosocial) with the categories of male and female (biological). Thus, personal identity and expression does not define sexes; rather, personal identity and expression represents the expected evolutionary variation within the two reproductive strategies—male and female.

Why does this matter?

Why does it matter to ultimately define the sexes according to reproductive strategy? Because defining the sexes solely according to chromosomes, body type, or sociocultural contexts limits the ability of biologists and others studying male and female to explore and understand the evolutionary and developmental biology of the sexes in humans and across species. Such narrow definitions fail to provide an overarching framework that unites the extensive body of research on the evolution of sex, anisogamy, and the two sexes across the vast diversity of life.

Even if we decide to redefine the words ‘male’ and ‘female’ according to our own idiosyncratic and anthropocentric views, we are still going to need words to describe the two unique reproductive strategies that unite all anisogamous species. There will always be a need to describe the reproductive strategy that produces many small motile gametes (sperm) and the reproductive strategy that produces few large sessile gametes (eggs).

This is what the sexes are, and they will remain this way regardless of our own attempts at redefining them.

[1] Lehtonen, J., Kokko, H., Parker, GA. (2016). What do isogamous organisms teach us about sex and the two sexes? PTBS, 371(1706), 1-12.

[2] Perrin, N. (2011). What uses are mating types–The ‘Developmental Switch’ Model. Evolution, 66-4, 947-956.

[3] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

[4] Ibid.

[5] Griffiths, P. (2020). A Process Theory of Biological Sex. Society for Applied Philosophy, YouTube.

[6] Bulmer, MG., Parker, GA. (2002). The evolution of anisogamy–a game-theoretic approach. Proceedings Biological Sciences, 269(1507), 2381-2388; da Silva, J. (2018). The evolution of sexes–a specific test of the disruptive selection theory. Ecology and Evolution, 8, 207-219.

[7] Lehtonen, J., Parker, G. (2014); Parker, GA. (2014). The sexual cascade and the rise of pre-ejaculatory sexual selection, sex roles, and sexual conflict. CSH Persp Bio.

[8] Lehtonen, J., Parker, G. (2019). Evolution of the two sexes under internal fertilization and alternative evolutionary pathways. The American Naturalist, 193(5), 702-716.

[9] Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.

[10] Griffiths, P. (2020). The existence of biological sex is no constraint on human diversity. Aeon.

[11] Bachtrog, D., et al. (2014). Sex Determination–Why So Many Ways of Doing It. PLoS Biol, 12(7).

[12] Kimball, J. (2020). Sex chromosomes.

[13] Woodward, D., & Murray, J. (1993). On the effect of temperature-dependent sex determination on sex ratio and survivorship in crocodilians. Proceedings: Biological Sciences, 252(1334), 149-155.

[14] Cunha, G., et al. (2005). The ontogeny of the urogenital system of the spotted hyena. Biology of Reproduction, 73, 554-564.

[15] Stolting, KN, Wilson, AB. (2007). Male pregnancy in seahorses and pipefish: beyond the mammalian model. Bioessays, 29(9).

[16] Thresher, R. (1984). Reproduction in reef fishes. T.F.H. Publications, Inc., New Jersey; Fautin & Allen. (1992). Anemone fishes and their host sea anemones, a guide for aquarists and divers. Western Australian Museum, Perth, p. 160.

[17] Khoo, LM., et al. (2018). Growth pattern, diet, and reproductive biology of the clownfish Amphiprion ocellaris in waters of Pulau Tioman, Malaysia. The Egyptian Journal of Aquatic Research, 44(3), 233-239.

[18] Openly. (2021). The gender binaries of male and female aren’t as universal as you think. Twitter.


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