Living on an Island Enlarges the Brain

A comparative study indicates that species of birds living on islands developed brains larger than their relatives on the continent. The authors believe that this adaptation is due to the higher levels of uncertainty in these environments.

source: Jon Sullivan

The Petroica Macrocephala is a small bird that lives in New Zealand and whose most striking feature is reflected both in its scientific name and in its anatomy: its head is noticeably larger than its relatives on the continent and one wonders if they are also smarter than their cousins. This type of questions are those raised by the Spanish researcher Ferran Sayol four years ago when he began to develop his doctoral thesis on the evolution of brain size in birds. His research began with one of the best-known examples of birds that evolved on islands and are especially skilled, the crows of New Caledonia. Their great brain and their ability to develop tools are usually cited as a possible proof that the birds that live on the islands have to sharpen their wits, but is it an isolated exception or is there a hidden evolutionary pattern?

To prove it, Sayol and his team have made a comparative and systematic study of the brain size of birds living on islands and those living on continents and have discovered that, indeed, the former have brains larger than their relatives. continental In a paper published Tuesday in the journal Nature Communications, Sayol details the analysis of the brain sizes of 11544 specimens from 1931 species of birds (110 islands and 1821 continental) and suggests that the fact of living on an island, with more environments unpredictable, may have been a selective factor for the development of larger brains in proportion to the body. "We wanted to see if in the islands the species had a bigger brain than in the continent because we had some predictions as to why it should be like that," he explains to Next. "And we found that it was like that, but that in itself does not mean that bigger brains have evolved on the island: it could also be that those with larger brains can colonize the islands more easily."

To verify this, the authors carried out several phylogenetic reconstructions of the species that live on islands to find out if the species already had the largest brain before colonizing the new environment or if the change happened later. "What we saw is that it seems that a large brain does not help you colonize an island, but once there is an environment that favors a large brain," says Sayol. Until now, the authors indicate, there was no global analysis comparing all species of birds that live on islands with their continent siblings, it had only been done with crows and in primates and in none of the previous studies were differences found. that they attribute to that the sample was not as global as his. Now, for the first time not only is this trend found, but they can discard the fact that it was a trait that birds had before reaching these environments in the middle of the ocean.

Birds are a particularly suitable group to study this subject because there is a lot of information about where they are in each island and at the same time good information about the size of their brains ", explains the researcher. On the possibility of the pattern repeating itself in other animal groups, such as mammals, scientists do not rule it out, but believe that the fact that birds can be dispersed so easily between islands may have favored greater specialization and differentiation . "In hominids we have the case of Homo floresiensis, but it is a single case and it is difficult to compare on a large scale, as we have done with birds," says Sayol.

The result is also contradictory with the well-known tendency that takes place in the islands to reduce the size of the species, as it happens with the man of the island of Flores, nicknamed "the hobbit" because of its size. "This also happens in birds, although in reality two things happen: the species that are rather large seem to become smaller and those that are small in the continent become larger on the island, as if they tend to reach a size medium. And it might seem that the brain becomes large because the body becomes small, but we find that it is not the case, that in all of average the brain gets bigger in proportion ".

With these results, ensuring that living on an island makes you "smarter" would be "extrapolating to the gross," according to Sayol, but he has no doubt that living in this environment favors those who have bigger brains. "Basically because the islands can change the environment from one year to another, and there are more uncertainties," he concludes, "and this has already been found in other works is a factor that can select the big brain to solve problems and inclement those who have to face. "

Reference: Predictable evolution towards larger brains in birds colonizing oceanic islands (Nature Communications) DOI 10.1038/s41467-018-05280-8 https://www.nature.com/articles/s41467-018-05280-8