Evolution of avian brain sizes: The uncovered connection with body size

Sooraj Shah ’24

Figure 1: Parrots are among the most intelligent of birds as a result of enhanced neuronal density.

It is a common belief that the cumulative size of an individual’s fists taken together results in an approximate size of that individual’s brain. By this interpretation, individuals with larger hand sizes should thus have bigger brains. On the contrary, towards the end of the Cretaceous era, the relative brain size of both small avians (birds) and massive non-avians (dinosaurs) were the same. A study conducted by Dr. Jeroen Smaers, a professor within Stony Brook University’s Department of Anthropology, focused on the evolution of avian brains following the Cretaceous era. 

The team analyzed CT rendered virtual endocasts (models) of Cretacious avians and non-avian theropods (two legged dinosaurs) and compared these with modern birds to create endocasts representing the brain..  These models portrayed 284 existing bird species, 22 extinct bird species, and 12 theropods in combination with 1,900 more existing bird species, and helped to observe the evolving brain to body size scale. By using the Ornstein-Uhlenbeck method, the researchers were able to observe where traits such as brain and body size among the species encountered an evolutionary shift in the phylogeny. A statistical method (pANCOVA) was utilized in order to observe where exactly in the phylogeny of these species that these brain to size shifts occurred. 

The study found that in neoavians (modern birds), the shift to larger brain sizes was a result of a faster reduction in body size coupled with a slower reduction in brain volume. Conversely, telluravian landbird analysis showed a faster increase in body size coupled with a slower increase in brain size. This observation can be linked to the diet of these birds, with telluravians having larger prey than neoavians and a change to carnivorous diet. This drew correlations among diet, body size, and brain size. Additionally, birds such as parrots and crows have larger brain sizes than other birds, indicating that their neuronal density within the brain volume seems to be much higher, which may explain their impressive vocal capabilities. This discovery is extremely important in the context of understanding animal behavior, and the evolution of neuronal density may be the focus of future research. 

Works Cited 

[1] D. Ksepka, et.al, Tempo and Pattern of Avian Brain Size Evolution. Current Biology 30, , 2026-2036 (2020). Doi:  https://doi.org/10.1016/j.cub.2020.03.060.

[2] Image retrieved from: https://images.pexels.com/photos/2317904/pexels-photo-2317904.jpeg?auto=compress&cs=tinysrgb&h=650&w=940

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