Widespread male sex bias in mammal fossil and museum collections – PNAS

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PNAS – Proceedings of the National Academy of Sciences of the United States of America
[Accessed 7 Sep2019]

Widespread male sex bias in mammal fossil and museum collections
Graham Gower, Lindsey E. Fenderson, Alexander T. Salis, Kristofer M. Helgen, Ayla L. van Loenen, Holly Heiniger, Emilia Hofman-Kamińska, Rafał Kowalczyk, Kieren J. Mitchell, Bastien Llamas, and Alan Cooper
PNAS first published September 3, 2019. https://doi.org/10.1073/pnas.1903275116
The extent to which the fossil record provides an accurate picture of past life is an important issue that is often difficult to assess. We genetically sexed 277 mammalian subfossils using high-throughput sequencing of ancient DNA, and found a strong male bias (approx. 75%) in Pleistocene bison (n=186) and brown bears (n=91), matching signals previously reported for mammoth. Similarly, a male bias was also found in species of nearly all mammal orders in 4 large museum collections. For mammals, we suggest both male behavior and appearance can lead to increased chances of representation in fossil and museum collections, and this previously unrecognized sex bias could have substantial implications for views of past population and ecological processes.
A recent study of mammoth subfossil remains has demonstrated the potential of using relatively low-coverage high-throughput DNA sequencing to genetically sex specimens, revealing a strong male-biased sex ratio [P. Pečnerová et al., Curr. Biol. 27, 3505–3510.e3 (2017)]. Similar patterns were predicted for steppe bison, based on their analogous female herd-based structure. We genetically sexed subfossil remains of 186 Holarctic bison (Bison spp.), and also 91 brown bears (Ursus arctos), which are not female herd-based, and found that ∼75% of both groups were male, very close to the ratio observed in mammoths (72%). This large deviation from a 1:1 ratio was unexpected, but we found no evidence for sex differences with respect to DNA preservation, sample age, material type, or overall spatial distribution. We further examined ratios of male and female specimens from 4 large museum mammal collections and found a strong male bias, observable in almost all mammalian orders.

We suggest that, in mammals at least, 1) wider male geographic ranges can lead to considerably increased chances of detection in fossil studies, and 2) sexual dimorphic behavior or appearance can facilitate a considerable sex bias in fossil and modern collections, on a previously unacknowledged scale. This finding has major implications for a wide range of studies of fossil and museum material.