Females are no smaller than males in bats and many other mammals
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It has been said that male mammals are larger than females, this is known as sexual dimorphism, but it turns out that it is not the norm. An analysis of the body weights of over 400 species of mammals reveals that only about 44% of them are large males.
Kaia Tombak of Hunter College in New York states that there was “great taxonomic bias” in the types of mammals examined for body-size dimorphism. They tended to measure primates, ungulates and carnivores to the exclusion of many other types of mammals, she says.
Dongbak and her colleagues analyzed datasets of male and female adult weights for hundreds of species across mammalian clades, from apes and cats to rodents and bats. The researchers also looked at body length in a subset of the data, as body weight can vary with season and animal health. Additional data were drawn from primary sources to ensure the inclusion of underestimated taxa.
They found that 39% of mammals are actually monomorphic. That is, males and females were similar in size. Further monomorphism was found when body size was used instead of body weight. 48% of mammals have males and females the same size, and 22% of mammals have females that are larger than males. Of the mammals they analyzed for both weight and length data, only 30% had larger males.
To more rigorously estimate the commonality of dimorphism across all mammalian species, Tombak and her team included species that make up 5% of all mammalian families, including more than 10 species. Given the results of this extensive study, “I can’t say that larger males are a hallmark of mammals,” she says.
This isn’t the first time that the theory that male mammals are larger has been studied. In 1977, Catherine Ralls published a much-cited paper challenging this idea. Nonetheless, it persists and informs the theory of sexual selection that male mammals need larger bodies to compete with each other to mate with females.
This idea was popularized by Charles Darwin’s 1871 book. human descent, This bias leads to knowledge gaps all the way to the basic science of mammalian body size.
Jason Kamiller of the University of Massachusetts Amherst agrees with the authors that quantifying sexual dimorphism as a binary variable is unhelpful. Categorizing mammals just by big males or big females doesn’t help, he says, because it doesn’t reveal much about mammals. But including rodents and bats in studies could skew the results, he says. Although they make up the majority of mammalian species, they typically do not compete for mating between males, instead competing through other means such as sperm selection. “I am interested in [the] If only non-bat/rodent mammals were examined, the results would be different. “
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