For Frogs, Bigger Brains Mean Worse Camouflage - The Scientist

Bigger brains can be an advantage for prey animals, as it can help them outsmart their predators. But big brains may be too costly to maintain when predation risk is high, forcing animals to use other strategies to survive, suggests a study on frog camouflage published Wednesday (August 17) in Science Advances.  

Prior research on various species including guppies has shown that animals with larger brains can better avoid predators, and the researchers behind the new work had previously shown that large-brained frogs live longer. But they also knew frogs deploy another successful anti-predation strategy: camouflage. Frogs can vary widely in how they look, from stunningly vibrant to nearly indistinguishable from their surroundings. The study authors were curious about the relationship between brain size and camouflage, and how predation pressure influenced both.  

To find out, they hiked into in the Hengduan Mountains, a biodiverse range located in Southwestern China. At their study sites, they used a spectrometer to measure the color and brightness of 102 frog species and their immediate surroundings, such as a tree trunks, leaves, or grass. They compared these two measurements to get an estimate of crypsis, or how well-camouflaged the frogs are in their habitat. The researchers also captured frogs from each of these species and measured their brain sizes in the lab.

At first, the authors found an inverse correlation between brain size and camouflage: The bigger the brain, the more the frog stood out. Frogs with bigger brains, they posit, can afford to be showier and potentially attract more mates.

Rhacophorus omeimontis, or the Omei treefrog, has many predators in the Hengduan Mountains.

Chuan Chen

But that finding didn't tell the whole story. The researchers hypothesized that increasing pressure from predation would lead species to have larger brains, and then in turn, more conspicuous coloration, study coauthor Ying Jiang, a graduate student at China West Normal University, writes in an email to The Scientist. 

When the researchers specifically examined sites with high densities of snakes (the main predators of frogs in the Hengduan Mountain region), they found an inverse relationship between brain size and predation—the opposite of their prediction. 

That doesn't mean that brain size has absolutely nothing to do with predator defense, the researchers say. They still observed a strong inverse correlation between crypsis and brain size, it was just that overall, the frogs in these predator-dense areas were better disguised and smaller. "Increased predator pressure caused the species to evolve more cryptic" patterns, explains Jiang, "and thus to evolve smaller brain sizes." 

The authors conclude that as predation increases, frogs essentially switch strategies from investing in brainpower to becoming harder to distinguish from their surroundings because they can't afford to spend as much energy on growth. 

With a lot of predators around, the odds increase that the frogs will die young, no matter what strategy they use. That could mean they need to live fast, so those that mature younger and invest in reproducing instead of growing larger have an edge. It's also possible, the authors write, that snakes prefer to eat bigger frogs, so investing in body and brain size—which are usually correlated—might put heartier frogs at a disadvantage. Either way, without the brains to outsmart their predators, they too easily become snacks—so more camouflaged individuals end up being the top frogs despite their small size.

In contrast, when fewer predators are around, frogs can afford to be a bit brainier. This allows them to think fast when they encounter a predator and still stand out a bit more from their surroundings to attract mates. 

The authors further explored the relationships between brains and crypsis using a phylogenetic path analysis: a mathematical model that determines how different ecological factors drive evolution given a group with shared ancestry. The model, which considered brain size, body size, hind limb size, and degree of crypsis, calculated the direct or indirect influence of these variables on one other. And it suggested that predation pressure has a direct influence on brain size, which in turn influences crypsis.

"I really liked how this paper put it all together," says Judith Mank, an evolutionary biologist at the University of British Columbia who didn't work on the study. She says she appreciates—and finds it "really surprising" that brain size directly influenced crypsis. "It means that we need to be thinking more about brain size in an ecological context," she adds.