Articel Content
Definitions of sex and gender by Walter Salzburger.
Text: Walter Salzburger
In today’s society, it is widely accepted that there are more facets to gender than just male and female. How is an individual’s sex defined? Answers from the field of
evolutionary biology.
In biology, an individual’s sex is defined according to its function in sexual reproduction – in particular, according to the reproductive cells (gametes) produced in the germ line: Females produce a comparatively small number of larger reproductive cells (eggs), whereas males produce numerous small, usually highly mobile reproductive cells (sperm). Sexual reproduction occurs through the fusion between egg and sperm cells.
In the overwhelming majority of animals, including humans, gametes are formed in dedicated reproductive glands, or gonads – ovaries in females, and testes in males. However, in many species, the differences between the two sexes go far beyond the presence of either ovaries or testes, and may, for example, be apparent in attributes such as body shape, coloration or behavior.
In other species, the distinction between the sexes is less clear-cut, and in many cases is not possible at all. For instance, most species of snails, many worms and most plants are hermaphroditic, meaning that a single individual produces both types of gamete. In the case of simultaneous hermaphrodites, an individual is both male and female at the same time, which can even lead to self-fertilization. Other species undergo a sex change: Clownfish, for example, always begin their lives as males, with only those that live long enough and occupy a dominant position in an anemone later becoming females.
Incidentally, in the animal kingdom there is no strong link between an individual’s sex and its role in raising offspring. For example, many cichlid fishes incubate the fertilized eggs in their mouth and continue to offer their progeny refuge there after they hatch. Depending on the species, this practice – known as “mouthbrooding” – might be performed by females only, by both parents, or only by males. The males of many seahorse species, on the other hand, carry the fertilized eggs in a ventral brood pouch for incubation.
Finally, the factors that determine gonad formation (and consequently an individual’s sex) also vary widely in nature: In mammals, it is well known that the sex of an individual is determined by the constellation of the two sex chromosomes X and Y, with the combination XX yielding females and the combination XY yielding males. Birds too have specific sex chromosomes (W and Z), with females being determined by the combination WZ and males by ZZ. In reptiles, the ambient temperature an egg is exposed to often determines whether the individual later becomes a female or a male, while fish offer examples of virtually every sex determining mechanisms known to date – from individual sex-determining genes and chromosomal sections to ambient temperature or social behavior within the group.
The question of why sexual reproduction emerged in the first place has yet to be conclusively resolved. However, it is widely believed that the reshuffling of the genetic material from the two parents associated with sexual reproduction plays a key role. In any case, that so many species rely solely or primarily on sexual reproduction to reproduce, in spite of the efforts involved, leaves no doubt that it is – in evolutionary terms at least – a major success story.
Walter Salzburger is Professor of Zoology and Evolutionary Biology at the University of Basel. His research examines how animals evolve, adapt to their environment, and diversify. He is especially interested in cichlid fishes, which he studies in Lake Tanganyika in Africa.
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