Natural history collections across five museums show bias towards male specimens

The study found a slight bias towards males in birds (40% females) and mammals (48% females), but this varied among species.

A new paper published today has found that the collections of birds and mammals from five international natural history museums have a slight bias towards male specimens. The study led by Dr Natalie Cooper from the Natural History Museum in London investigated sex ratios of over two million bird and mammal specimen records from five museums including the Natural History Museum, and the Smithsonian.

The study found a slight bias towards males in birds (40% females) and mammals (48% females), but this varied among species. The specimens used in the study were obtained between 1751 and 2018, mostly through hunting or trapping, and sexed based on internal or external genitalia or secondary sexual characters, for example plumage colouration or antlers.

The study found male bias was strongest in name-bearing types; only 27% of bird and 39% of mammal types were female. These results imply that previous studies may be impacted by undetected male bias, and going forward vigilance is required when using specimen data, collecting new specimens, and designating types. Researchers found the proportion of female specimens had not significantly changed in 130 years, but had decreased in species with showy male traits like colourful plumage and horns.

There are many reasons why natural history collections may have a male bias. In particular a major suspected source of male bias in collections for some species is deliberate selection for large, “impressive” male specimens, especially where males are larger or more colourful than females, or possess ornaments or weaponry such as antlers. Another reason could be the active avoidance of females with young due to legislation, ethical or conservation considerations.

Dr Natalie Cooper said: “Given the age of most major natural history collections, some male bias may be related to the changes in attitudes towards sex through time, therefore, we expect male bias to decrease towards the present due to changes in collection methods and motivations over the last century, but interestingly we see no improvement through time, even recent collections are male biased. Male bias may be accidental, for example due to trapping biases (i.e. trapping method, season of collecting, conspicuous male behaviours or traits), difficulties identifying females to species-level, or in some cases simply because there were more males in a population”.

Museum specimens are used extensively in studies of taxonomy, systematics, biogeography, genomics, comparative anatomy and more. The sex of a specimen is an important factor that influences many aspects of an individual’s ecology and life-history but it is often treated as a nuisance variable, overlooked entirely, or data collection focuses on just one sex. The researchers suggest that if there is a bias in the sex composition of collections, this could have large implications for studies that assume their samples are representative of the whole population or species. Dr Alex Bond from the Natural History Museum said: “It’s important to understand the biases in museum collections so that we can make sure that their limitations are acknowledged. But at the same time, it shows how useful it can be to look across millions of specimens by using digital records”.

Dr Cooper continues: “No large-scale study of sex ratios in bird and mammal museum collections exists, therefore investigating this is of vital importance as the number of studies using museum specimens continues to rise. By doing studies like this we can start to understand our biases, and start trying to fix them.”

The paper was published in Proceedings of the Royal Society B on Wednesday 23rd October 2019.

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Notes for editors

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