Mammals: most males and females are similar in size – 03/12/2024 – Science

Large males and more modest-sized females seem, at first glance, to be the rule among mammals like us, but the most complete survey carried out on the differences between the sexes in the group to which humans belong indicates that the real situation is much more complicated.

In fact, species in which males and females are similar in size or in which they are usually larger than them predominate. These two situations correspond to 55% of mammals. In the rest of the species in the group, the males are, in fact, stockier.

The analysis has just been published in the specialized journal Nature Communications, being led by Kaia Tombak, a researcher linked to the City University of New York and Princeton University. The study signed by Tombak and his colleagues Severine Hex and Daniel Rubenstein revisits research from the 1970s that already indicated a more equal situation between the sexes with regard to size, but which ended up being relatively ignored by the majority of scientists.

The logic behind the idea that males would normally be the larger sex among mammals has been developed since the pioneering texts of the father of evolutionary biology, Charles Darwin (1809-1882). Taking into account the enormous investment required of females’ bodies during pregnancy and breastfeeding, and the fact that they produce a limited number of eggs in each menstrual cycle, it made sense to imagine that they would be the “choosing” sex, carefully selecting a few partners or even just one.

Males, on the other hand, due to their ability to, in theory, fertilize a much larger number of females in a relatively short time, would have greater incentives to compete with each other and try to monopolize different partners. The larger size would be an important element to be successful in this competition and also to attract females.

However, the new study argues, the model described above for interactions between the sexes appears to have been formulated from a limited number of subgroups of mammals, such as artiodactyls (cattle, deer, pigs, etc.), carnivores and primates, in the which this pattern is much more common.

Concluding based on these subgroups that males are normally brutes could therefore be circular reasoning. “The vast majority of mammal species belong to the groups of rodents and bats, which are poorly represented in studies on the topic”, write the researchers.

To try to avoid these biases, the team created a database with information on 429 species of mammals. The intention is that the sample would represent at least 5% of the species from each of the group’s orders (orders are relatively broad groupings, such as the primates, rodents and carnivores already mentioned), with the exception of the order Eulipotyphla (that of European hedgehogs and moles ).

They also consider mapping at least 5% of the species in each of 66 mammal families (in total, there are 78 families in the group). Families are smaller groupings, such as those of felids (cats) and hominids (orangutans, African great apes and humans).

To increase statistical reliability, they only included species in the list in which there were at least nine individuals of each sex in the measurements. Based on this data, they estimate that 45.1% of mammal species have males with a greater body mass than females, on average. On the other hand, 38.7% of species are “sexually monomorphic in body mass”, with males and females of similar size. Finally, in 16.2% of species, females would be heavier.

If body length is taken into account, the situation is even more similar between the sexes. By this measure, 49.9% of species have a similar size between the sexes, while 28% have larger males and 22.1% have larger females, on average.

When there are differences, the size of males is more exaggerated than that of females. On average, when they are larger, the difference compared to females is 30%. Females, when they are more robust, have an average mass of 13% more than males.

The team found that, in fact, each subgroup of mammals appears to have its own pattern. Primates and artiodactyls in fact tend to have large males, while among rodents, even size is the norm in half of the species. In the case of bats, almost half of the species have females larger than males.

According to the researchers, the data helps to rethink the simplistic model of the reproductive strategies of males and females among mammals. They point out, for example, the existence of the “Big Mother” model – according to which larger females would be able to offer more resources (such as milk) and protection to the offspring, depending on the species.

Furthermore, in many animals, competition for fertilization between males occurs through high sperm production, which would, to a certain extent, eliminate direct confrontation and the need to be larger. On the other hand, competition also occurs between females, depending on other variables.