The Evolution of the Avian Brain after the K-Pg mass extinction.

Ernst Haeckel – Kunstformen der Natur (1904), plate 99: Trochilidae .

Birds are extraordinarily intelligent. Studies have shown that corvids and some parrots are capable of cognitive achievement comparable to those of great apes. They manufacture and use tools, solve puzzles, and plan for future needs. Futhermore, they share with humans and a few other animal groups a rare capacity for vocal learning. Previous studies have shown that bird brains have more neurons than mammalian brains and have very high neuronal densities.

Birds and mammals have undergone remarkable encephalization, in which brain size has increased without corresponding changes in body size. Brain size has been correlated with major evolutionary innovations like cognition, flight, environmental adaptability and enhanced sensory capabilities. But the early evolutionary history of the hyperinflated brain that distinguishes birds from other living reptiles remains elusive. Using a large dataset comprising more than 2,000 birds and non-avian dinosaurs, a team of scientist lead by Daniel Ksepka reconstructed part of that story. Their results indicate that the avian brain size was profoundly impacted by the K-Pg mass extinction event.

Avian Brain-Body Size Evolution. From Ksepka et al., 2020.

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

The new findings reveal at least seven brain-body scaling events in birds right after the mass extinction event. The initial shift in the expansive neoavian radiation appears to have been driven by larger brains and smaller bodies, since the evolution of large brains provide a buffer against frequent or unexpected environmental changes via enhanced capacity for flexible behavioral responses. Birds only reached their apex in relative brain size during the Neogene when crown corvids and crown parrot radiated. Song-birds (including corvids), and hummingbirds (Trochilidae) are the only major groups of birds known to be capable of vocal learning, an ability controlled by additional brain pathways not found in other birds.

 

References:

Ksepka et al., Tempo and Pattern of Avian Brain Size Evolution, Current Biology (2020), https://doi.org/10.1016/j.cub.2020.03.060

Kabadayi, C., & Osvath, M. (2017). Ravens parallel great apes in flexible planning for tool-use and bartering. Science, 357(6347), 202–204. doi:10.1126/science.aam8138 

Lee, M.S.Y., Cau, A., Naish, D., and Dyke, G.J. (2014). Dinosaur evolution.Sustained miniaturization and anatomical innovation in the dinosaurian an-cestors of birds. Science345, 562–566 DOI: 10.1126/science.1252243

 

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