Abstract
The turkey vulture (Cathartes aura) is a widespread, scavenging species in the Western Hemisphere that locates carrion by smell. Scent guided foraging is associated with an expansion of the olfactory bulbs of the brain in vertebrates, but no such neuroanatomical data exists for vultures. We provide the first measurements of turkey vulture brains, including the size of their olfactory bulbs and numbers of mitral cells, which provide the primary output of the olfactory bulbs. Comparative analyses show that the turkey vulture has olfactory bulbs that are 4× larger and contain twice as many mitral cells as those of the sympatric black vulture (Coragyps atratus), despite having brains that are 20% smaller. The turkey vulture has the largest olfactory bulbs in absolute terms and adjusted for brain size among birds, but the number of mitral cells is proportional to the size of their olfactory bulbs. The combination of large olfactory bulbs, high mitral cell counts and a greatly enlarged nasal cavity likely reflects a highly sensitive olfactory system. We suggest that this sensitive sense of smell allowed the turkey vulture to colonize biomes that are suboptimal for scavenging birds and become the most widespread vulture species in the world.
Highlights
The overall structure of the olfactory system in vertebrates is highly conserved[1]
Differences in morphology were readily apparent in the brains of the two species (Fig. 1); the turkey vulture has significantly larger olfactory bulbs than the black vulture (Table 1)
Previous studies suggested that turkey vultures have a very good sense of smell[8,26,32] and that black vultures do not[8,28]
Summary
The overall structure of the olfactory system in vertebrates is highly conserved[1]. This generality even extends to the relatively small olfactory bulbs of birds[2,3], which were originally considered to be microsmatic or anosmic[4]. Turkey vultures have large and complexly folded nasal sinuses, indicating a relatively large olfactory epithelial surface area compared with other vultures and appear to have enlarged olfactory bulbs, based on linear measurements of endocasts[2,8]. These data provide preliminary evidence that turkey vultures have enlarged olfactory bulbs, endocast data do not accurately reflect olfactory bulb size[3,19] and direct neuroanatomical measurements are lacking for any brain regions in vultures. We investigate olfactory bulb size and mitral cell counts in turkey and black vultures and compare them with those of other bird species
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