FLIGHTLESS equals bird-brained, right? Not any more. Earthbound birds have brains just as large as their flying cousins, suggesting that they may not be as dim-witted as once thought.
Many modern birds have lost the ability to fly, including penguins, kiwis, emus and ostriches. And many species that are now extinct, such as the great auk, dodo and Madagascan elephant bird, were also unable to take to the sky. But previous studies had concluded that flightless birds had relatively small brains compared with flyers.
Now a team from the National Museum of Natural History in Washington DC and Monash University in Melbourne, Australia, has used statistical analysis to compare brain size and body mass in 417 species of living and fossil birds (Journal of Zoology, vol 263, p 317).
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The researchers found that the brain sizes of flightless birds, relative to body mass, are not significantly different to those of birds that fly. Exceptions to this rule were the extinct fish-eating great auk (Pinguinus impennis), and the kakapo, a rare New Zealand parrot (Strigops habroptilus) 鈥� both of which had relatively small brains. The team suggests that the kakapo has developed a large gastrointestinal tract for digesting tough plant material, at the expense of brain size. The great auk may have grown larger to aid thermoregulation, without a proportional increase in brain volume. Alternatively, says team member Andrew Iwaniuk, 鈥渂y increasing its body mass to an extreme level, the great auk could probably dive deeper, but as a result could not fly鈥�.
The team proposes that birds that evolved flightlessness did so slowly enough for brain size to keep pace with body mass. The study also refutes the theory that the evolution of flight accompanied a relative increase in brain size. 鈥淐learly this is not true based upon our comparisons,鈥� says Iwaniuk. 鈥淭he flightless species with relatively smaller brains actually possess much heavier bodies as a result of adaptations to a new feeding style.鈥�
It is possible that different parts of the brain may have changed size as flightlessness evolved, while overall relative brain size was not affected. 鈥淐hanges could be occurring in regions within the brain or spinal cord. We just haven鈥檛 measured them yet,鈥� says Iwaniuk.
鈥淚gnoring penguins, flightless birds fall into two main divisions: those that lost their flight tens of millions of years ago, mainly ratites, and those that became flightless less than a million years ago, such as rails and many island species,鈥� says ornithologist Walter Boles of the Australian Museum in Sydney. 鈥淚t is interesting to see that this relationship holds for both groups.鈥�