Think of a lion; think of a gorilla; think of an otter. In your head, are the males bigger than the females? Well, a new study is set to challenge over 100 years of bias in this area of research. By looking at over 400 mammal species, the team found some surprising results – in most cases, the males of the species are not bigger than the females.

Sexual dimorphism is the term used when males and females of the same species look visibly quite different. It can be as simple as different colors or fancy feathers for display, but in some species, it can also come with horns and an increased size of the male, especially in species where males compete with other males for access to females. 

Back in the 1970s, mammologist Katherine Ralls found that there were many species in which there was little sexual size dimorphism (SSD), especially within the larger groups of mammals. However, her research was overlooked and overpowered by the idea that the males of most mammalian orders are bigger than the females.

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 northern elephant seal (Mirounga angustirostris) where the male had a mass 3.2 times that of the females. 

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 Chiroptera the girls tended to be larger than the boys. 

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 male competition for mates. The team also suggest that their results could change again as more data is collected, and suggest more work be carried out in female biology across a wide range of mammalian species. 

The study is published in Nature Communications.