Male mammals have long been said to be larger than females – a phenomenon known as sexual dimorphism – but it turns out that may not be the norm. An analysis of the body mass of over 400 mammal species revealed that only about 44 per cent had larger males.
“There has been huge taxonomic bias”, in what types of mammals have been examined for body size dimorphism, says at Hunter College in New York. There has been a bias toward measuring primates, hoofed animals and carnivores to the exclusion of numerous other mammal types, she says.
Tombak and her colleagues analysed data sets of adult body mass for both sexes of hundreds of species across mammal clades – everything from apes and cats to rodents and bats. Weight can vary by season and the health of an animal, so the researchers also looked at body length in a subset of the data. Additional data was pulled from primary sources to ensure inclusion of underrepresented taxa.
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They found 39 per cent of mammals were actually monomorphic, meaning males and females were a similar size. Using body size instead of weight, they found even more monomorphism: 48 per cent of mammals have males and females of similar size, and 22 per cent of mammals have females that are larger than males. Only 30 per cent of the mammals they analysed with data on both body mass and length had larger males.
To get a more rigorous estimate of the commonality of dimorphism across all mammal species, Tombak and her team included species that made up 5 per cent of every mammalian family that contains more than 10 species. Given the results of this wider survey, “you can’t really say that larger males is a mammalian trait”, she says.
This isn’t the first time that the accepted narrative that male mammals are larger has been researched. In 1977, Katherine Ralls published that challenged this idea. Still, it has persisted, and informs sexual-selection theory – that male mammals need larger bodies to compete with each other in order to breed with females.
This idea was popularised by Charles Darwin’s 1871 book The Descent of Man, and coupled with an overemphasis on male reproductive strategies over female strategies. This bias leads to knowledge gaps all the way down to the basic science of mammalian body size.
at the University of Massachusetts Amherst agrees with the authors that “quantifying sexual dimorphism as a binary variable is not helpful.” It isn’t useful to just categorise mammals as having larger males or larger females because it doesn’t reveal that much about them, he says. But he says the inclusion of rodents and bats in the study may skew the results. These make up a large percentage of mammal species, but they don’t typically exhibit male-male competition to mate, instead competing through other means like sperm selection. “I would be interested to see if [the] results differ if they only examine the non-bat/rodent mammals.”
bioRxiv
