A team of researchers from Princeton University (New Jersey, United States) has concluded that, contrary to what was commonly believed, male mammals are not, as a general rule, larger than females in the vast majority of species. species.
In total 429 species have been analyzed. According to the results presented today in Nature Communications, in 45% of the males were larger than their counterparts of the opposite sex. In 16%, females are usually larger than males. In the remaining 39%, males and females have similar sizes.
These results contradict the narrative that, as a general rule, male mammals are larger than females. The researchers consider that this misconception is based on the fact that “rough measures of dimorphism and taxonomically biased sampling” have been used for years.
In this way, sexual dimorphism – the difference in size between sexes of the same species – is questioned, as proposed by Darwin in his theory of evolution published in the book The Origin of Species in 1859. The British naturalist explained that the The development of secondary sexual characteristics, in the context of the reproductive strategy for survival, resulted in greater size in male mammals than in females. Already in the 1970s, Katherine Ralls published an article in The American Naturalist that questioned Darwinian theory.
The biologist concluded at that time that mammals “are not especially dimorphic between sexes” and that species with little or no size differences “were extremely common in the vast majority of species.” Despite this, Ralls’ findings were overshadowed by a narrative that continued to grant greater size to the males of the vast majority of species, without having reliable scientific evidence of this and despite some additional evidence that supported his conclusions in the years later.
This theory that has dominated until today is also explained because the focus of much of the research on sexual size dimorphism has been on even-toed ungulates (cervids, giraffids or bovids, among others), carnivores (especially pinnipeds, such as seals) and primates. All of them present a marked sexual dimorphism in favor of males.
The species with the greatest dimorphism is the northern elephant seal, whose males can be up to 3.2 times larger than females. At the other extreme, the most pronounced bias in larger females occurs in the tube-nosed bat, which can have a wingspan up to 1.4 times larger than males. However, a good part of the mammal species correspond to rodents and bats, which are usually underrepresented in this type of studies.
Research has covered only 5% of mammal species. But in the species not analyzed there probably still exists a higher rate of monomorphism than what they have found, the authors of the work argue. Therefore, there is surely greater evidence that “males are not predominantly larger than females,” they conclude.
“Given growing evidence for a greater prevalence of sexual size monomorphism than is commonly recognized heretofore, the theoretical basis for the evolution of sexual dimorphism in mammals deserves rethinking,” the researchers write in Nature Communications.
It is commonly assumed that all available females will choose the strongest and most dominant male as a mate, or will be coerced by him to copulate. But “many populations have shown great variation in female mate preferences, as well as aggressive competition among them for mates and for resources that are crucial to their fitness. Until now, some of these phenomena have been dismissed as capricious behaviors, rather than considered adaptive and strategic,” the article concludes.