Think of a
Sexual dimorphism is the term used when males and females of the same species look visibly quite different. It can be as simple as
Back in the 1970s, mammologist Katherine Ralls found that there were many species in which there was
Previous studies have used arbitrary cut-offs for size measurements, or been held back by data availability in judging whether mammalian orders have true SSD, write the authors. Furthermore these studies have rarely included rodents or bats in the datasets, instead focusing on primates and large carnivore species including seals. Fortunately, in this new study the team was able to use large datasets across a wide range of mammalian taxa, and sample each order and family according to how many species are in each.
The final dataset for the researchers included data on body mass for 429 different mammalian species. Their results show that 38.7 percent of mammalian species have males and females of the same size (sexually monomorphic), while 45.1 percent of species have males that are bigger than the females, and 16.2 percent have females larger than the males.
Examples of the most extreme size difference occur in the peninsular tube-nosed bat (Murina peninsularis) where the females are 1.4 time heavier than the males, and in the
However, as Ralls thought, most of the dimorphisms did not show in such an extreme way. The team found that half the species in the rodents (the order with the largest number of species in this research) were the same size across males and females, while in the bat order
Overall, the researchers found that their results did not fit the persistent narrative that most mammal species have larger males than females. In fact, species being the same size across males and females occurred almost as often as larger males. Where sexual dimorphism does occur, the males do tend to be larger, but not necessarily to an extreme degree.
They suggest that the idea that males are larger than females has persisted for so long because a lot of early zoology work was based on
The study is published in