April 14, 2011
Feathers, air sacs, nesting behavior—the earliest birds owed a lot to their dinosaurian ancestors. The first birds also inherited a strong sense of smell.
Modern birds have not been thought of as excellent scent-detectors, save for some super-smellers such as turkey vultures, which detect the scent of rotting carcasses. We typically think of avians as more visual creatures, and in some birds, the part of the brain that processes information from smells is relatively small.
But birds actually have a diverse array of scent-detecting capabilities, and a poor sense of smell may be a more recent characteristic of some lineages. After all, birds have been around for over 120 million years. We wouldn’t expect that birds have always been the same from the time they originated.
We obviously can’t directly test the ability of fossil organisms to detect scents, but, as shown in a study published this week by Darla Zelenitsky and colleagues, the shape of prehistoric brains may hold some crucial clues about the senses of extinct animals. The key was the olfactory bulb. This is a part of the brain—highlighted by the yellow flash in the video above—that is specialized for perceiving scents.
To estimate how important an animal’s sense of smell was, the scientists looked at the size of the olfactory bulb. This follows from a well-established principle in brain anatomy called proper mass—the more important the function of a brain part is to an animal, the larger that brain region will be. In other words, if an animal had a relatively large olfactory bulb it likely relied heavily on scent, whereas a tiny olfactory bulb would indicate the unimportance of scent to that animal. By comparing modern bird brains with virtual brain casts of extinct birds and non-avian dinosaurs, Zelenitsky and co-authors tracked how the sense of smell developed in dinosaurs and the earliest birds.
The brain anatomy of 157 living and fossil species was examined in the study. What the scientists found did not match the conception that birds lost their smelling skills early. Quite the opposite.
Multiple lines of evidence have confirmed that birds evolved from maniraptoran dinosaurs—a subgroup of coelurosaurs containing dinosaurs such as Deinonychus, Struthiomimus, Oviraptor and others—and the brain studies showed that sense of smell improved during the evolution of this group. The dinosaur Bambiraptor, for example, had a sense of smell comparable to that of turkey vultures and other birds that rely on scents to track down food.
This strong sense of smell was passed on to the earliest birds. Rather than decreasing, the relative olfactory bulb size remained stable during the evolutionary transition between non-avian dinosaurs and the first birds. Unexpectedly, olfactory bulb size then increased as archaic bird lineages proliferated, and the earliest members of the modern bird group—the neornithes—were even better-skilled at picking up scents than their predecessors. In fact, Zelenitsky and colleagues suggest, the improved sense of smell in the neornithes might have made them better foragers than earlier types of bird, and this may have some bearing on why they survived the end-Cretaceous mass extinction 65 million years ago while more archaic bird lineages perished.
The results of the new study reverses one of the long-standing misconceptions about birds and their evolution. Some modern bird lineages lost their powerful scent detecting abilities over time, but, early on, birds were as adept at picking up smells as their dinosaur ancestors. Paired with future studies focused on the parts of the brain associated with vision, studies like this will help us better understand how birds and dinosaurs navigated through their prehistoric worlds.
Zelenitsky, D., Therrien, F., Ridgely, R., McGee, A., & Witmer, L. (2011). Evolution of olfaction in non-avian theropod dinosaurs and birds Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.0238
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