A comprehensive study by the Natural History Museum in London has found that male specimens of birds and mammals outnumber female specimens in natural history collections.
The research looked at more than two million specimen records held by five large natural history collections worldwide. It found that only about 40% of bird specimens were female, and around 48% of mammal specimens. The same underlying sex bias exists even in specimens collected more recently, the study found.
A more extreme bias was observed in type specimens – the specimens on which a species description is based – where only 25% of bird specimens and 39% of mammal specimens were found to be female.
The imbalance means that studies using natural history collections are likely to be skewed, which could impact scientific understanding in areas such as biogeography, morphology, taxonomy, genetics, parasitology and immunology. Museums Journal spoke to Natalie Cooper, the researcher behind the study, about her findings and what they mean for museums.
What inspired you to begin the research?
Stem disciplines tend to have more male than female researchers, especially in senior positions. We were collating this data for the museum and wondered if we’d see similar patterns in the specimens in our collections too. Luckily many large museums now have all their records online so we were able to investigate this across five large international museums including the Natural History Museum.
Did you have any expectations about what you'd discover before you started?
We suspected we’d probably see more males in certain groups, for example in birds where males are really colourful, or in mammals where the males have large horns or antlers. But we thought this would mainly be an issue for the early years of our collections and that it would improve towards the present as our attitudes and trapping methods improved.
What most surprised you about the findings?
We were really surprised that the male bias still exists now as we assumed there would be more active attempts to balance things. We were also surprised by how extreme the bias was in name bearing types - only 25% of bird and 39% of mammal types are female. This makes sense if the diagnostic characters for that species are on the male (for example plumage or baculum characters) but in those cases we’d hope to see a female paratype, but this wasn’t the case for most species.
What do you believe are the main reasons are for the male bias in collecting?
We think some of this is likely not deliberate. In many mammal species males have larger home ranges so are more likely to encounter a hunter or trap. Likewise, some trapping methods, for example using male territorial calls to attract birds into a mist net, are more likely to attract males. Males may also be more conspicuous where they are the larger or more colourful sex. Some bias, however, appears to be due to deliberate selection for males. In groups where the males are more colourful, or substantially larger, or have antlers or horns for example, we find the strongest skew towards males.
In what ways could the lack of female specimens be skewing our knowledge and research of these animals?
Biological sex affects many aspects of species ecology and evolution. For example, the sexes can differ in their parasite loads, diets, behaviours, size and shape. Thus any research that doesn’t take account of this could be getting a biased picture of the natural world if we only look at males.
What impact could the unequal sex ratio have on the way natural history collections are displayed and interpreted to the public?
There’s excellent work on this by Rebecca Machin, the curator of natural sciences at Leeds Museums and Art Gallery, and Jack Ashby, the manager of the University of Cambridge Museum of Zoology, showing that museums tend to display more males, and display them in more dominant poses compared to females. This is unconsciously biasing everyone’s view of what the natural world looks like, and reflects Victorian gender ideals of the dominant male and nurturing female. In reality female animals can be just as brutal, ruthless and sexual as males. This has led people claim that women are meant to be “meek and mild” because female animals behave this way, which simply isn’t true.
What do you think museums can do to address this issue?
The first step is recognising it’s an issue. In developing natural history exhibition spaces I think it’s becoming much more common to think about a variety biases, not just sex, in both the exhibits and the people mentioned in them. Hopefully we can start changing these things, although change is sadly very slow at large museums due to costs.
For our collections, we aren’t suggesting “burning all the males” as one radio DJ suggested to me a few weeks ago, instead we’d suggest thinking about how collecting is done and if there are ways to reduce bias. For example we could use female calls to attract birds to mist nets as well as male calls. If there are options for collecting a subset of specimens captured then making sure the collection is sex balanced would be ideal. We also recommend assigning opposite sex paratypes when naming new species to ensure the full range of variation is represented.
How will you be taking this research forward? Are there other collecting biases you'd like to examine?
As a group of authors we are really interested both in exhibitions and collections. On the exhibitions side we want to try to understand how extreme these biases are across a wide range of museums, and also to understand how we can tell more diverse stories in our exhibitions. From a collections point of view we’re interested in exploring why so many specimens don’t even have a recorded sex, and specimens that don’t fit neatly into the male or female categories.