Larger daughters, not sons, rule size-chart in some species of mammals

Despite the typical narrative that male mammals are larger than females, a few mammalian species display a contrasting pattern.

According to a study published inNature Communications,females in some mammals are heavier than males. In an analysis of over 400 mammalian species, 16 per cent of species females were the heavier gender.

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The study showed that males outweighed females in 45 per cent of the mammals, exhibiting a condition known as sexual dimorphism, where both sexes show different morphological characteristics. In nearly 39 per cent of the mammals, both sexes had the same mass.

Kaia Tombak, a postdoctoral researcher at Purdue University and the author of the new study, told the New York Times, "What we found is that there is no norm."

In three categories of mammals, carnivores, primates, and even-toed hoofed animals, usually males are larger than females. These species have often been the focus of research. In 87 per cent of carnivores, 79 per cent of even-toed hoofed animals, and 62 per cent of primates, the male was bigger than the females.

Rodents and bats account for over half of the mammals on Earth, and 48 per cent of rodent species analysed during the species were the same in size. Moreover, males were larger in 44 per cent of rodent species.

However, females were heavier in 46 per cent of the bat species.

According to Dr Tombak, there are several factors females might benefit from being the bigger gender. For instance, in bats, flying requires an enormous amount of energy. Large female bats might be better equipped to carry their developing foetuses and young offspring through the air.

The "big mother" hypothesis by biologist Katherine Ralls from the 1970s suggests that larger females produce better offspring. They are likely to survive and provide better care and nourishment.

If this hypothesis favours the selection of larger females and sexual competition favours the selection of larger males, the result sizes of both genders of a species might be the same. Dr Tombak said, "You have these two forces that are kind of pushing body size up on both sides."

However, these findings are based on only five per cent of all mammalian species. The researchers said that they are far from the topic. However, the study demonstrated how animals vary from each other. Robert Cox, an evolutionary biologist who was not involved in the study said, "It's nice to be reminded that some of the things that we say as generalisations break down when you consider the specific details."

(With inputs from agencies)