Being social isn't for dummies. Animals that gather into packs, herds or troops never mind into cities and countries need to be smart. How else to negotiate the complex rules and hierarchies of their cultures? It's not for nothing that sharks, among the dimmest of the large carnivores, are loners, or that humans far and away the smartest are so enthusiastically collectivist.
What this ought to mean is that social animals have bigger brains than solitary ones, and the research has indeed suggested as much. A landmark 2007 paper called "Social Brain Hypothesis," published in the journal Evolution, showed that increased sociality was linked to steadily bigger brains in at least three orders of mammals: primates like us, carnivores like lions and ungulates like zebras and bison. (Watch TIME's video "Chimps & People: Dangerous Bedfellows.")
That widely accepted truth might be coming undone, however, thanks to a new study published in the Proceedings of the National Academy of Sciences. According to the authors, evolutionary biologists John Finarelli of the University of Michigan and John Flynn of the American Museum of Natural History, there's a much murkier link than we thought between big brains and big societies. As it turns out, it was our favorite nonhuman critters dogs that threw off previous data.
Finarelli and Flynn did not study as broad a collection of animals as the authors of the earlier paper. They studied just one order, the carnivores, but they did so in depth. Sampling both living terrestrial carnivores and the fossils of extinct ones, they analyzed overall brain volume relative to body mass in fully 289 species. They also factored in what is known (or, in the case of fossils, theorized) about each species' social behavior. What they got was a surprising mix of findings. (See pictures of 10 species near extinction.)
In general, carnivore brains followed one of several developmental arcs, some growing larger over time, some fluctuating up and down, some remaining relatively steady, some actually growing smaller. Most of the larger members of the feliform suborder which includes large cats as well as hyenas and mongooses pretty much stuck with the brain size they had from the start. The extinct bear-dog a family of animals that died out 9 million years ago and were, as their name suggests, related to both bears and dogs actually became more pea-brained over time. Common dogs, like humans, have enjoyed a comparatively recent expansion of cranial capacity.
What doesn't seem to track, however, is a consistent connection between these measures and the complexity of the animals' communities. "The universality of the social-brain hypothesis does not apply," says Finarelli.
The researchers cite no shortage of examples. Meerkats, whose societies are rich enough to have sustained a wildly popular television series Meerkat Manor don't weigh in with a whole lot of gray matter relative to their body size. The same holds true for hyenas and mongooses albeit without the TV following. Bears, small cats and weasels, on the other hand, pack a lot of brain into their heads yet prefer to go it alone. (See pictures of animals in love.)
The problem with the old study, the researchers believe, was its overemphasis on dogs and its use of living species alone. Canines have co-evolved with humans, growing more social as we selected for those traits. That essentially skewed the results, and the absence of fossil ancestors from the data meant there was no information about whether the brains of the earliest dogs grew or shrank or did both over time. Finarelli and Flynn acknowledge that the modern canine brain has grown along with its sociability, but they do not know which is the cause and which is the effect or if the two things are linked in any meaningful way at all.
That's not to say that the shape of the brain tells you nothing about the characteristics of social species, particularly when the species in question are primates. Studies from the 1990s that have stood up over time showed that among social apes like gorillas and chimps, brain and behavior evolve in ways peculiar to an individual's sex. Males have more bulk in the region of the brain connected with aggression and competition and less in the region that tempers those tendencies which better equips them for the socially competitive world into which they're born. Females have more heft in the neocortex, a higher-order region that wires them for complex tasks like nurturing and reading social cues. Again, it's not clear whether brain size drove traits or vice versa, but they do appear linked.
But none of this suggests that within a species Homo sapiens, say brain size tells you a lick about intellect. Across the centuries, eugenicists and practitioners of other junk sciences argued that cranial volume could reveal important things about the intelligence or other traits of one race compared with another. That was rubbish. The new carnivore studies, by contrast, offer a tantalizing window into the things that help an entire species evolve the way it does or, more important, the things that don't.