Are big brains bad news for mammals?

As the earth plods through its latest geological epoch, the Anthropocene, it may be undergoing the so-called sixth extinction. With Homo sapiens at the steering wheel, our planet is losing an incredible amount of its biodiversity, at an unprecedented pace. But not all species are equally at risk. Some species are faring quite well in human-dominated ecosystems, others have found pockets of suitable habitat scattered in between disrupted landscapes, but most face an existential dilemma: adapt, or die trying. Mammals in particular are hard hit: 25% are assigned to some flavor of endangerment classification, while another 15% of mammal species are “data deficient,” meaning that we simply don’t know enough about them to say just how badly they’re doing. The first step towards solving these problems is understanding what makes a species more or less likely to race toward extinction. One researcher has found a useful strategy: measuring brain size.

Stanford University biologist Eric S. Abelson (now at UCLA) reasoned that lots of researchers have looked at variables like body size, home range size, fecundity, litter size, and so on in an effort to predict which species are most vulnerable. The idea is that understanding which variables predict endangerment would allow us to more efficiently direct our limited time, energy, and resources into their conservation. But conservation biologists have all but ignored animal behavior, in part because it’s hard to measure. It turns out that brain size, relative to body size, is a useful way to get at this complex variable.

Take the rabbit. With an encephalization factor of 0.4, its brain is about less than half what you’d expect given the size of its body. By contrast, humans and dolphins clock in with factors of 7.4 and 5.3, followed by chimps, monkeys, elephants, whales, and others. That means our brains are far larger than you’d expect given the size of our bodies.

That ratio—the size of the brain compared to the size of the animal’s body—is known as “relative encephalization,” and it’s been linked to a heap of behavioral variables. Animals with higher encephalization tend to have longer lifespans, complex intelligence, complex social relationships, tool use, increased learning abilities, more varied diets, and more. When Abelson compared encephalization with IUCN endangerment status, he found that the two were correlated. The more highly encephalized a species was, the more likely it was to risk extinction. Humans notwithstanding, bigger brains turn out to be bad news.

But why? Brains are “expensive” tissue, but despite its energy-hogging ways, some species have clearly found it to be evolutionarily advantageous. The problem is that the human-dominated world is largely different from the one in which these large-brained mammals evolved, and it could be that such large brains are no longer adaptive or advantageous. While it’s not yet clear just why this pattern exists, it does provide conservationists with a roadmap to work more efficiently at conservation of mammals. It’s fairly easy to measure relative encephalization, which makes the measurement a really useful one for assessing the extinction risk of the 15% of mammal species classified as data deficient. “The degree to which [relative encephalization], serving as a proxy for behavioral traits, is correlated to conservation status makes it a potentially beneficial tool in understanding future extinction risk and aiding in conservation planning,” Abelson writes. – Jason G. Goldman | 02 March 2016

Source: Abelson, E. S. (2016). Brain size is correlated with endangerment status in mammals. Proc. R. Soc. B, 283(1825). DOI: 10.1098/rspb.2015.2772.

Header image: (from L to R) human (Homo sapiens), rhinoceros (Diceros bicornis) and common dolphin (Delphinus delphis) brains, ©Escaladix/Wikimedia Commons