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Fear. (01/2022)

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.

Walter Salzburger
Prof. Dr. Walter Salzburger

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.

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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