Evolution favors brainier birds. The most crowded branches of the avian tree of life generally hold the most intelligent birds because larger-brained birds split into new species faster than smaller-brained ones, according to recent work published in Evolution.
“Being smart means you’re more likely to make new species,” explains University of Chicago, Illinois, evolutionary biologist Trevor Price, who was not involved in the study.
Evolutionary biologists have long wondered why some branches on the tree of life are more crowded than others—a feature also known as their degree of diversification, or the net effect of species arising and going extinct. Environmental factors and traits of the species themselves influence this dance between life and death.
Brain size is often thought to be associated with diversification, through rates of speciation and extinction in birds. The idea, says evolutionary ecologist and study coauthor Ferran Sayol, is that bigger brains help birds avoid extinction, invade new ecological niches where they speciate, or lead to more innovative and adaptive behaviors that then lead to speciation. Indeed, evolutionary biology’s most famous bird-watcher, Charles Darwin, thought new behaviors could lead to new species. In On the Origin of Species, he wrote about woodpeckers that do not climb trees, and whose “anomalous habits,” compared with familiar tree-climbing woodpeckers, could lead to the rise of new species. Despite these oft-cited ideas, previous studies hadn’t thoroughly tested and clarified the relationship between brain size and diversification, Sayol says. He led the recent work as “an empirical test to an old idea.”
To tackle the new work, scientists from the Center for Research on Ecology and Forestry Applications in Barcelona, Spain, and the University of Gothenburg, in Sweden, employed two different methods. First, they collected data on many different traits suspected to influence diversification, including brain size, body size, geographic distributions, migratory habits, diet, and habitat, for thousands of bird species from museum specimens and published sources. Then, using a computational model, the team tested how brain size varied with speciation and extinction rates in a bird phylogenetic tree published in 2012. They found a positive correlation between relative brain size and speciation rates, suggesting that brain size promotes diversification through speciation.
However, when it came to higher rates of speciation, this first method could not distinguish between the effects of brain size and the effect of other traits, such as migration and habitat. To tease them apart, the team used a second method, which compared diversification rates throughout the bird phylogeny to brain size data based on measurements of museum skulls of more than 1,900 species.
For every bird species in the phylogeny, the researchers calculated diversification rates by essentially estimating the number of twigs splitting off each evolutionary branch divided by the length of that branch, Sayol explains. Birds on short branches with many twigs have higher diversification rates than species on long branches with only a few splits.
Once they’d calculated these rates, the team tested the independent effects of brain size, habitat, migration, and other traits on the diversification rate. “We found that brain size was affecting diversification rates, but other factors were also important,” says Sayol. Habitat generalists and those on islands had higher diversification rates, as did migratory species, “but brain size still explains some of the variation.”
The fact that both sets of analyses point to larger brain size promoting speciation, “is a much more convincing result than if you just did one or the other,” Price says. Previous work came to similar conclusions, but through less rigorous analyses, he adds. “The idea that brain size promotes speciation has been there, but many people didn’t believe it,” Price says. “They’ve done all they can now. I believe this result.”
The next step is to figure out exactly how big brains enable diversification in wild populations by “getting down to the practicalities of what brain size means for behavior,” Price says. “Having a big brain, perhaps you can explore more niches, or you can persist,” he explains. “Either way, you do behaviors in the field better than someone with a small brain.”