Charles Darwin confidently stated that male mammals were mostly bigger than females, and this view has been held for over 150 years.

New research out this week in the journal Nature Communications, however, finds that Darwin's statement isn’t true and there is actually no difference in size between most male and female mammals.

In the study, the researchers compared the body masses of 429 wild animal species. They found that in many species including lemurs, golden moles, horses and zebras, the size of both sexes is the same.

Contrary to popular opinion, males were typically larger than females in only 28 percent of the mammal species studied.

Now there were some species that had significantly larger males in the cohort, including the elephant seal in which the males typically weigh in at three times the size of females.

However, to counter that the study also found that female peninsula tube-nosed bats were typically 40 percent larger than the males.

So why do so many of us believe that males are bigger?

Well part of it is to do with Darwin, but part of it is to do with what we are surrounded by locally and on TV.

Many familiar mammals, including pets like cats and dogs as well as livestock like sheep and cows have males that are larger than females. We also tend to watch wildlife documentaries that focus on charismatic species like primates and carnivores which also have larger males as they have great fight and dominance scenes in them. Typically, the male of the species is heavier when the males are required to fight to acquire a female mate.

Bias in the scientific literature has played a part in the misconception that males are normally bigger with most of the research being carried out by male scientists. One of the first studies that countered their view was in 1977 by female scientist Katherine Ralls, but her research was drowned out by all of the male dominated opposing studies.

The science-backed truth is that in the majority of mammal species, monogamy is the trend and males don’t need to fight for access to females. When this is the case, the females are typically larger which helps them to produce and carry more offspring.

It’s also a great lesson in how even scientific evidence can be shaped by our own pre-existing beliefs and why it's important to increase diversity in the science sector.

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