An international team of
evolutionary biologists and paleontologists have reconstructed the evolution of
the avian brain using a massive dataset of brain volumes from dinosaurs,
extinct birds like Archaeopteryx and the Great Auk, and modern
birds.
The study, published online in the journal Current Biology, reveals that
prior to the mass extinction at the end of the Cretaceous Period, birds and
non-avian dinosaurs had similar relative brain sizes. After the extinction, the
brain-body scaling relationship shifted dramatically as some types of birds
underwent an explosive radiation to re-occupy ecological space vacated by
extinct groups.
“One of the big
surprises was that selection for small body size turns out to be a major factor
in the evolution of large-brained birds,” says Dr. Daniel Ksepka, Curator of
Science at the Bruce Museum and lead author of the study. “Many successful bird
families evolved proportionally large brains by shrinking down to smaller body
sizes while their brain sizes stayed close to those of their larger-bodied
ancestors.”
In order to
understand how bird brains changed, a team of 37 scientists used CT scan data
to create endocasts (models of the brain based on the shape of the skull
cavity) of hundreds of birds and dinosaurs, which they combined with a large
existing database of brain measurements from modern birds. They then analyzed
brain-body allometry: the way brain size scales with body size.
“There is no
clear line between the brains of advanced dinosaurs and primitive birds,” notes
co-author Dr. Amy Balanoff of Johns Hopkins University. “Birds like emus and
pigeons have the same brains sizes you would expect for a theropod dinosaur of
the same body size, and in fact some species like moa have
smaller-than-expected brains.”
The two groups
of birds with truly exceptional brain sizes evolved relatively recently:
parrots and corvids (crows, ravens, and kin). These birds show tremendous
cognitive capacity, including the ability to use tools and language, and to
remember human faces. The new study finds that parrots and crows exhibited very
high rates of brain evolution that may have helped them achieve such high
proportional brain sizes.
“Several groups
of birds show above average rates of brain and body size evolution,” remarks
co-author Dr. N. Adam Smith of the Campbell Geology Museum at Clemson
University. “But crows are really off the charts — they outpaced all other
birds. Our results suggest that calling someone ‘bird-brained’ is actually
quite a compliment!”
“Crows are the
hominins of the bird kingdom,” says co-author Dr. Jeroen Smaers of Stony Brook
University. “Like our own ancestors, they evolved proportionally massive brains
by increasing both their body size and brain size at the same time, with the
brain size increase happening even more rapidly.”
Source: https://brucemuseum.org/site/news_detail/bruce-museum-science-news-how-birds-evolved-big-brains
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