The majority of museum specimens—including both ancient and modern mammals—are male, a landmark survey published in the Proceedings of the National Academy of Sciences suggests.

Per the Atlantic’s Rachel Gutmann, researchers led by the University of Copenhagen’s Graham Gower analyzed hundreds of bison and brown bear fossils collected in the field or borrowed from museums across Europe and North America. Ultimately, the team identified 74 percent of the bison and 64 percent of the bears as male.

In addition to assessing prehistoric species, Gower and his colleagues studied modern specimens housed at New York’s American Museum of Natural History, the Smithsonian’s National Museum of Natural History, London’s Natural History Museum and the Royal Ontario Museum. Of 19 mammalian orders represented, 17 were predominantly male; Chiroptera, an order constituting bats, and Pilosa, an order including anteaters and sloths, were the only groups in which females outnumbered males.

As Hakim Bishara reports for Hyperallergic, the scientists attribute this surprising sex bias to an array of factors, including male mammals’ typically larger size, herd distribution, sex-specific geographic ranges, individual animal behavior and human collecting preferences.

To study bison specifically, the team drew on findings outlined in a 2017 Current Biology study. This paper, centered on an analysis of 95 sets of mammoth remains, found that 69 percent of featured specimens were male—a trend explained not by unequal sex ratios at birth, but by male mammoth behavior.

Mammoths, much like bison, either traveled solo, in herds headed by a single male or in smaller groups made solely of males. (Those unable to establish their own herd often resorted to roaming with other would-be herd leaders, according to Phys.org’s Bob Yirka.) Separated from matriarchal herds, male mammoths and bison often engaged in risky activities with high mortality rates.

“They were more likely to do silly things, like die in tar pits,” Gower tells Atlas Obscura’s Sabrina Imbler. Tar pits and similarly mucky death sites—from bogs to crevices and lakes—then inadvertently preserved the animals’ remains for thousands of years.

Male specimens’ dominance among brown bear fossils, meanwhile, may stem from the fact that these solitary creatures traversed larger swaths of land than their female counterparts. As Gutmann writes for the Atlantic, “If you’re a paleontologist excavating a slice of 12,000-year-old rock, … you’d be more likely to come across a wandering male than a homebody female.”

Human collection habits also contribute to museums’ skewed sex ratios. Per Gutmann, the hunters who donate animal specimens largely target males, as they are larger, boast showy features such as horns and manes, and—unlike mammal mothers—are not responsible for ensuring offspring’s welfare.

Uneven representation among museum specimens could produce skewed research results. Hayley Lanier, an assistant curator of mammalogy at the University of Oklahoma’s Sam Noble Museum who was not involved in the study, tells the Atlantic that “some of this bias speaks to a larger issue that we’ve also seen in medical science, which is that we tend to select one sex” as the primary model of how living things function, thereby ignoring sex differences in such areas as diet, size and behavior.

Lanier says, “I think that those biases really leave us with an incomplete understanding of how the world works.”

To address the imbalance described in the study, the authors suggest museums continue diversifying their collections, adding specimens of different sexes, ages and geographic origin.

Speaking with the Atlantic, Trina Roberts, a researcher at the Natural History Museum of Los Angeles County who was also not involved in the new research, concludes, “If what museums are trying to do is create a better and more complete archive of biodiversity on Earth, and we know biases exist like the one this paper is pointing out, it’s important that we continue to collect.”