A new study finds that bird species living on oceanic islands that rise from the ocean floor due to seismic or volcanic activity have larger brains than their closest mainland relatives. The study suggests the birds’ larger brains are the result of evolutionary processes that took place on islands rather than before colonization took place
If you’re an evolutionary biologist, there’s nothing on Earth that’s quite as special as an island. This is because islands are natural laboratories for studying evolution. Yet, at the same time, evolution on islands is, well … peculiar — predictably peculiar. For example, species that colonize islands show some very predictable evolutionary trends: vertebrates tend towards “medium body sizes” (this is the so-called “island rule”; ref), lizards converge towards equivalent eco-morphs (eco-morphs are species that occupy the same niche, have similar morphology and behavior, but are not necessarily close relatives; ref), and birds undergo a reduction or complete loss of flight capacity (ref).
But the propensity for tool-use behaviors, such as those demonstrated by the New Caledonian crow, Corvus moneduloides, the Hawaiian crow, Corvus hawaiiensis, and the Galápagos woodpecker finch, Camarhynchus pallidus, may be another peculiarity of island life for birds. Although we’re not yet certain, this behavioral idiosyncrasy appears to stem from island birds evolving advanced cognitive abilities, a trait that may be accompanied by an enlarged brain (ref). What is it about islands that seem to support the evolution of larger brains in birds? Basically, three main features of island life could set the stage for this: niche expansions, such as eating novel foods, environmental variability combined with constraints on dispersal, and a slower pace of life.
First, the paucity of species on islands gives new colonizers the opportunity to expand into novel niches without competition with other species or pressure from their enemies — although they do end up facing increased competition from members of their own species. But it is this intra-specific competition, along with limited opportunities to disperse, that may drive enhanced cognition and the evolution of larger brains (ref). Indeed, there is evidence to support the idea that niche expansion, along with the tendency for behavioral innovation (ref), are tied to evolution of larger brain size.
Second, the relative isolation of islands prevents individuals from fleeing to other places when environmental conditions deteriorate, which forces individuals to develop more elaborate behavioral responses. Despite taking longer to develop and being physiologically costly to maintain, a larger brain may buffer individuals against environmental changes by supporting new or more complex behaviors (ref). For example, in woodpecker finches, tool use replaces the more usual gleaning technique in years when droughts drive insects from foliage to crevices (ref). Thus, environmental variability combined with constraints on dispersal could be a powerful selective force for an enlarged brain in island bird species.
Last but not least, island life is slower than mainland life (ref), and this could facilitate the evolution of a larger brain in island species — and this larger brain takes longer to develop. Since learning and behavioral flexibility ultimately provide big payoffs to island species when local conditions become more challenging, such an evolutionary innovation could quickly spread through the population.
In spite of these credible arguments, the difference in brain size between island and mainland species has only ever been investigated twice. Neither of those studies uncovered any associations, but both suffered from methodological problems, with the most important being that it was unclear whether large brains evolved before or after island colonization. This is especially problematic because we know that larger brains promote exploration and colonization of novel areas (ref), so this creates a “chicken and egg” scenario: which came first, a larger brain or island colonization?
To address the problems faced by the previous two studies, evolutionary ecologist Ferran Sayol, a PhD candidate at the Centre for Research on Ecology and Forestry Applications (CREAF) in Catalonia, Spain, and an international team of collaborators, tested the ‘brain–island’ hypothesis by asking two questions: Is a larger relative brain size a consequence of island living? Which came first, a larger brain or island living?
Is a larger relative brain size a result of island living?
To answer these questions, Mr. Sayol and his collaborators assembled brain measurements for 11,554 museum specimens (110 species living on islands that rise from the ocean floor due to seismic or volcanic activity, and 1,821 continental species) and phylogenetically analyzed them. These birds represented more than 1900 species that belonged to 91% of all living bird families. Mr. Sayol and his collaborators classified those species as either inhabiting oceanic islands (110) or not (1821) and statistically analyzed whether island species have larger brains than mainland species. They found that endemic oceanic island birds do have bigger brains than mainland-dwelling birds (Figure 1), even after controlling for migratory behavior.
Which came first, a larger brain or island living?
Although it is true that alien species introduced by humans were more likely to become established in their novel homes, including islands, if they had a larger brain to begin with (i.e.; ref), there are two more pieces to this ‘brain–island’ hypothesis puzzle that suggested that the enlarged brains of island bird species primarily evolved after island colonization rather than before. First, island-dwelling bird species were not more likely to be from brainier avian families, since common colonizers of oceanic islands include both small-brained lineages, such as pigeons and rails, and large-brained lineages, such as crows and parrots. Second, when Mr. Sayol and his collaborators reconstructed bird family relationships and mapped island species onto that phylogeny, they saw no difference in relative brain size between the ancestors of oceanic island species and the ancestors of their continental-living cousins (Figure 2a).
Yet, at the same time, the descendants of island species had relatively larger brains than their mainland ancestors, suggesting that evolution towards relatively larger brains occurred after island colonization (Figure 2b & c)
Alternative explanations of the observations
Mr. Sayol and his collaborators examined which of three main mechanisms at play on islands might explain why oceanic islands bring about increases in relative brain size in birds: niche expansions, differences in life history, and environmental variation with limited dispersal opportunities (Figure 3a).
Mr. Sayol and his collaborators fitted a model that allowed the three mechanisms to compete to explain their data. (This is path analysis.) They found that birds living on oceanic islands tend to have larger brains than closely related mainland species because island living makes the environment more unpredictable. Since there are limited possibilities to disperse when conditions deteriorate on oceanic islands, individuals are forced to rely on more elaborate behavioral responses to survive. A larger brain supports behavioral flexibility, so birds with larger brains can learn new coping mechanisms rather than evolving specific responses to unpredictable environmental conditions.
“[O]ur findings fit well with the ‘island syndrome’ theory, which predicts that island dwellers have consistent phenotypic differences when compared to mainland populations,” the authors write in their paper.
Unfortunately for the authors of this study, there are plenty of very intelligent birds that do not live on small oceanic islands. Cockatoos, for example (i.e.; more here). Ravens are another example (i.e.; more here). These examples serve to underscore the idea that we still have much to learn about the nature of intelligence and how it evolves.
Ferran Sayol, Philip A. Downing, Andrew N. Iwaniuk, Joan Maspons, and Daniel Sol (2018). Predictable evolution towards larger brains in birds colonizing oceanic islands, Nature Communications, 9:2820 | doi:10.1038/s41467–018–05280–8
Originally published at Forbes on 31 July 2018.