Sex biases in natural history collections

Study of two million birds and mammals finds bias towards males—especially among name-bearing type specimens

GBIF-mediated data resources used : 6.6 M species occurrences
Colaptes auratus luteus
Female specimen of a northern yellow-shafted flicker (Colaptes auratus subsp. luteus) collected in Evanston, Illinois, USA. Photo via The Field Museum (CC-BY-NC 4.0)

Often treated as a nuisance variable—or overlooked altogether—the biological sex of an organism is an important factor affecting many aspects of the individual's ecology and behaviour. Among museum collection specimens, biased sex ratios could have serious implications for studies that rely on them.

This study examines GBIF-mediated records of more than 2.4 million bird and mammal specimens from five major international museums. For birds, 20 per cent were female, 31 per cent male and 49 per cent unsexed, while for mammals—likely easier to sex—only 15 per cent of specimens were unsexed, 44 per cent were male and 41 per cent were female.

This bias was not due to unsexed specimens being female. Even species that exhibit female-skewed ratios in the wild, showed male bias among specimens. The authors found no significant improvement in sex balance over the last 130 years.

The most notable finding of the study was among the name-bearing type specimens (holotypes, syntypes, lectotypes, and neotypes), where only 25 per cent of birds and 39 per cent of mammals were females.

Original article

Cooper N, Bond AL, Davis JL, Portela Miguez R, Tomsett L and Helgen KM (2019) Sex biases in bird and mammal natural history collections. Proceedings of the Royal Society B: Biological Sciences. The Royal Society 286(1913): 20192025. Available at: