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In mammals, males are usually stronger and more competitive than females. Males also show greater inequality in reproductive success – the number of offspring they produce – than females. In spotted hyenas, the opposite is true: reproductive inequality is greater in females than in males. The reason? Female spotted hyenas pass on their social rank and the associated privileges such as priority access to food to their offspring, whereas males do not. These are the findings of a new study on spotted hyenas spanning eight generations across eight clans in the Ngorongoro Crater in Tanzania, carried out by the Ngorongoro Hyena Project of the Leibniz Institute for Zoo and Wildlife Research in Berlin (Leibniz-IZW) and the Institut des Sciences de l’Évolution de Montpellier (ISEM). The study is published in the journal “Science Advances” and may explain why females are the more competitive sex in some species.
In the animal kingdom, not all individuals produce the same number of offspring — and among mammals, this inequality is often greater among males than females. Among sexually reproducing species, males produce numerous, small and motile gametes (sperm), while females produce a small number of large and nutrient-rich gametes (eggs). In mammals, females also bear the physiological cost of pregnancy and lactation. Because females invest far more in each reproductive event than males do, they are more selective about mating. Males benefit more from mating with as many females as possible and therefore compete intensely for access to mates. This leads to conflicts between the sexes and gives rise to sex roles: males want to inseminate as many females as possible and as often as possible, and fight amongst themselves for access to the females, while females are selective and invest heavily in rearing their offspring. These predictions, formalised as the Darwin-Bateman paradigm, have been confirmed across many mammalian species.
