The Snell-Rood lab finds brain size tracks human-induced changes to the environment in some species.
Does adapting to environments shaped by humans make animals smarter? A paper from the lab of Emilie Snell-Rood published in the journal Proceedings of the Royal Society B, suggests that animals are showing complex cognitive responses to both urban and rural areas with cranial capacity tracking human-induced change in some cases. In other words, for some species the answer is yes and for others it is no.
"Which species are going to be able to deal with 100 percent conversion of prairie to agriculture?" says Snell-Rood. "Which species are going to be able to deal with forests being supplanted by cities?" Ultimately, says the researcher, being able to understand which species are going to be able to cope with that kind of human-induced change will provide vital context when prioritizing conservation objectives.
Previous research across bird species suggests a link between brain size and ability to adapt to urban environments, but doesn’t address the question of whether cities select for increases in cognition within species. That’s where Snell-Rood and undergraduate student Naomi Wick come in.
"We wanted to find out if dealing with a new environment is just a matter of species sorting (in which a particular species is "pre-adapted" for success in a particular environment) or actually changes going on within species in addition to that species-level variation," says Snell-Rood.
The researchers turned to the Bell Museum of Natural History collections to find out. Drawing on specimens dating back to the early 20th century, Snell-Rood and Wick examined the cranial capacity of 10 species, including varieties of shrews, voles, bats, and squirrels, along with a mouse and gopher, from locations in and around the Twin Cities metro. They found that the urban populations of two of the species did, in fact, possess significantly greater cranial capacity. They also predicted that if more smarter were favored in urban environments cranial capacity should also increase over time. "We didn’t see cranial capacity increases over time in the urban specimens," she says. "And, actually, for two species we see a decline over time, while the bats and shrews show an increase in cranial capacity in rural populations."
While there are a number of factors that could account for this such as the time frame in which the specimens were collected, Snell-Rood points out that cities aren’t entirely unpredictable. "Things are laid out in a regular fashion. There are certain resources that might be exploitable. There’s a reduction in predators and the new predators are follow predictable patterns."
Learning is costly, says Snell-Rood. "Neural tissue is incredibly expensive metabolically. There are trade-offs in investing in brains and investing in reproduction, which may be why we see a reduction in cranial capacity over time in two of the species – larger brains may be favored just during the initial colonization of the city." Moreover, humans are changing rural environments in ways that could be just as challenging as cities, resulting in the changes in cranial capacity over time seen in bats and shrews.
While the researchers can’t say for certain whether the changes are evolutionary or developmental, evidence suggests the former. "We tended to not see changes in body size which suggests it’s not just nutritional, but rather an evolutionary response," says Snell-Rood. And while the study raises as many questions as it answers, she considers the findings cause for some optimism. "It reminds us of the fact that populations adapt, and that at least some species are tracking human-induced environmental change."
– Stephanie Xenos