Recent study traces the evolution of brain size along the avian tree of life.
Despite the growing number of excellent popular science writers, many misconceptions about evolution are still surviving among the general public. One especially tenacious one is “The March of Progress“, mostly depicted a series of walking primates from an ape-like creature over primitive hominids to modern Homo sapiens. This iconic picture misrepresents evolution as a linear process from primitive creatures to complex organisms. This thinking is often extended to the last universal common ancestor and summarized in the phrase “From monad to man.” In each case, modern humans are considered the ultimate goal of the evolutionary process. Or the pinnacle of evolution. Nothing could be further from the truth. Evolution is not a linear sequence, but an ever-branching tree. Humans are just one of the many twigs on this tree of life. We might be more advanced in terms of brain power compared to a bacterium. But some bacteria are clearly superior to us when it comes to processing methane or living in boiling lakes. Which organism can be considered the pinnacle of evolution is thus a non-sensical question.
Given that most evolutionary biologists are aware of these misconceptions, I was surprised to read the following in a recent Current Biology paper: “two groups—parrots and corvids—independently acquired relative brain sizes, neuronal densities, and sophisticated cognitive potential near the pinnacle of the vertebrate world. [my emphasis]” This sentence seems to suggest that there is an avian March of Progress towards larger brains with parrots and corvids at the finish line. This reasoning is obviously incorrect. Brain size is just one of numerous traits under selection in different species. In one environment a large brain and a small body might be beneficial, while in another environment a small brain and large body have the highest chances of survival and reproduction. Hence, the small-brained, large-bodied species could be considered the pinnacle of evolution in its habitat.
Now that I have clarified that misconception and ventilated some frustration, we can finally delve into the Current Biology study on avian brain size evolution. Apart from the hick-up at the end of the paper, this is a beautiful piece of work. The researchers amassed an impressive dataset of brain endocasts of 284 extant bird species, 22 extinct bird species, and 12 non-avian theropod dinosaurs, complemented with data for more than 1,900 extant species from another study.
Next, they investigated the relationship between brain volume and body mass for different bird groups. These relationships can be captured in simple mathematical formula that you probably remember from high school: y = ax + b. The coefficients a and b correspond to the slope of the regression line and the intercept (i.e. the point where the regression line hits the y-axis), respectively. By comparing these coefficient between related bird groups, the researchers were able to reconstruct the evolution of brain size across the avian phylogeny. An increase in the intercept indicates that one bird group increased in brain volume and body mass, but that the relative brain size remained the same. However, a steeper regression line (i.e. an increase in slope) points to an increase in relative brain size in one bird group.
Detailed analyses of these regressions revealed several evolutionary shifts in relative brain size. Interestingly, most significant changes occurred after the mass extinction at the end the Cretaceous when the non-avian dinosaurs disappeared. This catastrophic event might have set the stage for an adaptive radiation in brain size, a scenario that fits the “cognitive buffer hypothesis”. This hypothesis suggests that large brains provide a buffer against frequent or unexpected environmental changes via enhanced capacity for flexible behavioral responses.
After this adaptive radiation, several bird groups showed independent changes in relative brain size. Birds of prey in the orders Accipitriformes, Strigiformes, Falconiformes, and Cariamiformes, for instance, experienced a small increase in relative brain size. A carnivorous lifestyle probably led to the evolution of a larger body, but was not accompanied by a proportional increase in brain size. In the woodpeckers (order Piciformes), we see the opposite pattern where a decrease in body size was not followed by a decrease in brain volume, resulting in a significant increase in relative brain size. This pattern is also apparent in hummingbirds and swifts (order Apodiformes)m and in sandpipers and buttonquails (order Charadriiformes).
Different Evolutionary Paths
And that brings us to the “pinnacle” of avian brain size: the parrots and the corvids. The method outlined above allowed the researchers to pinpoint the exact mechanisms behind the high relative brain sizes of these birds. It turns out that they each took a different path to the top: parrots primarily reduced their body size, whereas corvids increased body and brain size simultaneously. A nice example of convergent evolution.
It is no surprise that parrots and corvids have large relative brain sizes. These species are known for their clever tricks, such as mimicking sounds and using tools. But brain size is just one aspect of intelligent behavior. Other studies have found that the structure of the brain and the connections between the neurons are also important. For example, parrots have an additional vocal learning pathway that is absent in songbirds. And both corvids and parrots have the highest cerebral neuronal densities in birds. There is more to avian life than a large brain.
Ksepka, D. T. et al. (2020). Tempo and Pattern of Avian Brain Size Evolution. Current Biology.
Featured image: New Caledonian Crow using a tool © National Geographic