Sex-specific behavior is still likely to bias collections of modern mammals, but the team thinks that factor is eclipsed by the quirks of the ways humans collect dead animals. Lots of research specimens are donated to museums by hunters, who tend to target males because they’re larger and have flashy features like horns and manes, and because it’s less morally icky than killing a mother who might be caring for young. Even animals trapped expressly for research can end up with a lopsided sex ratio, often for conservation reasons: Lanier says, for example, that the state of Oklahoma recently asked her and her colleagues to collect as few female bats as possible for a project, and preferentially take the males.
For curators, the upshot of the new paper isn’t so much the why of the sex bias, but the what now? Gower, Lanier, and Roberts all point out that the relative scarcity of female research specimens could lead scientists to draw skewed conclusions about a species. Studying a male-heavy sample as if it’s representative of a whole species could erase sex differences in diet, size, behavior, and individual animals’ responses to changing habitats. “I think some of this bias actually speaks to a larger issue that we’ve also seen in medical science, which is that we tend to select one sex” as the default model of how a body works, Lanier says. “And I think that those biases really leave us with an incomplete understanding of how the world works.”
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Lanier even speculates that the sex bias in Pleistocene samples could affect one of our most fundamental ideas about the era: that after it ended, giant mammals gave way to regular-size ones. In mammals, males tend to be larger than females, and that dimorphism becomes more extreme the larger a species gets. If three-quarters of the bones paleontologists collect from Pleistocene bison are male, they’ll end up estimating an average size that would have been larger than the actual average during the Ice Age—when, presumably, 50 percent of bison were female—and unintentionally inflating the size difference between 12,000-year-old bison and modern ones.
To capture a more accurate picture of nature, Roberts says, museum curators have to keep growing their collections. “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,” she says, “it’s important that we continue to collect.”
More collecting runs up against an obvious concern in a world where species both beloved and crucial to their ecosystems are fading out of existence: How many animals, exactly, is too many to remove from their natural populations? According to Lanier, museum collecting of small mammals usually doesn’t hurt their populations, and larger ones are almost always collected in partnership with hunters, or as salvaged roadkill. Still, as of last year, there are nearly 6,500 recognized mammal species on Earth, and countless more extinct ones. Out in the world, there are okapi stretching to reach the tenderest leaves, harvest mice skittering along flower stems, water buffalo roaming through grassy swamps. Museums can make their stores as balanced as possible, but they can only ever contain a static slice of the spectacle outside.