Birds that live on oceanic islands have larger brains than their relatives on the mainland, new research shows. And the difference holds not only for notably smart species, such as crows, but also for not-very-clever ones, such as pigeons.
Evolution is a complex process, with environmental changes and genetic drift setting species off on any number of paths, and evolutionary biologists are keen to find evidence of the process at work according to predictable trends. Some selective pressures are more common in island environments than on the mainland, resulting in trends such as convergence in body shape in lizards, and the loss of flight in birds.
The tool-use behavior in island birds such as the New Caledonian crow (Corvus moneduloides), the Hawaiian crow (Corvus hawaiiensis) and the Galapagos woodpecker finch (Camarhynchus pallidus) suggests that island life may also lead to the evolution of advanced cognitive abilities, and larger brain size.
In birds, overall brain size is an accurate proxy for cognitive ability, as the larger brain has a larger pallial region, the area responsible for spatial learning.
Researchers had previously searched for evidence of the ‘brain-island’ hypothesis, in studies comparing brain size in mainland species and their island counterparts. Neither of these studies, on crows and ravens, as well as another on primates, found a conclusive trend.
For the latest study, led by Ferran Sayol from the Ecological Applications and Forest Research Centre in Barcelona, Spain, measured the brain size of 110 avian species living on islands, and 1821 continental species. The dataset involved 11,554 specimens from 1931 bird species.
To tease out the confounding factor of whether large brain size is a precursor to, or the result of, successful island colonisation, the researchers applied a Bayesian phylogenetic framework – a detailed examination of evolutionary relationships and niche breadth.
The advantage of such a big dataset is that it includes large-brained species such as corvids and parrots, alongside other island colonisers such as pigeons and rails which have smaller brains and are not quite as well known for their intelligence. The trend stayed true across multiple families: island birds have bigger brains than their mainland counterparts.
“Our findings indicate that island birds tend to have larger brains than their mainland close relatives and that these differences have evolved in situ and independently in several lineages,” says Sayol.
The paper, published in the journal Nature Communications, suggests that island living is more unpredictable, which in turn selects for larger brains despite the substantial energetic and developmental costs involved in running them.
The island environment limits dispersal. When environmental conditions deteriorate, bigger brains with enough behavioural plasticity to switch to novel foods or change foraging techniques are selected. For example, the Galapagos woodpecker finch is more likely to use its tiny grass stick tool to extract insect prey when the island is in drought conditions.