About 65 million years ago, most of the dinosaurs and many other animals and plants were wiped off Earth, probably due to an asteroid hitting our planet. Researchers have long debated how and why some species survived the so-called Cretaceous-Tertiary mass extinction, marked in ancient rocks by a transition called the K-T boundary. A new study suggests that one group of survivors, the birds, may have sniffed their way across by evolving an enhanced sense of smell.
Scientists had long thought that birds have a poor sense of smell. But several recent studies show that birds use smell to help them forage for food, communicate with other birds, and even orient themselves in flight. And a 2009 study of dinosaur olfaction, led by paleontologist Darla Zelenitsky of the University of Calgary in Canada, found that dinosaur lineages thought to have given rise to today's birds some 150 million years ago had a keener sense of smell than dinosaurs that went extinct without leaving feathered progeny behind.
To further explore how the sense of smell might have influenced bird evolution, Zelenitsky and her colleagues looked at the olfactory abilities of 157 species of dinosaurs, extinct birds, and living birds. As in the earlier work, the team used a parameter called the olfactory ratio as a proxy for how keen a bird's sense of smell is. In the vertebrates (animals with backbones), smell is processed in the olfactory bulb, which in birds and reptiles is found in the very front of the brain. In birds, the olfactory ratio is the relative size of the bulb compared with that of the brain's cerebral hemispheres and is usually expressed as a percentage. Numerous studies in birds have shown the olfactory ratio, which ranges from less than 10% to more than 30% in a few species, to be a reliable indicator of the sense of smell.
The team tabulated olfactory ratios for 20 species of dinosaurs, seven species of extinct birds, and 130 species of living birds, using data from the published literature as well as performing new computed tomography scans of dinosaur and bird fossil skulls and the skulls of modern birds. These olfactory ratios were then plotted onto evolutionary trees that other researchers had earlier derived for bird evolution.
Researchers have assumed that the relative size of the olfactory bulb shrank during the dinosaur-to-bird transition to make room for brain areas devoted to the sense of sight and balance needed for flight. But Zelenitsky and her colleagues found that the olfactory ratios of the earliest birds hovered well above 20%—at least as high as those of their dinosaur ancestors when adjusted for body size differences between the two groups of animals.
And as birds continued to evolve, their olfactory ratios at first soared to 30% or more, the team reports online today in the Proceedings of the Royal Society B. Only later, perhaps up to 10 million years after the K-T extinction event, did olfactory ratios begin to decline to well below 20% in most birds (although a small number of today's foraging birds, such as kiwis and sea birds that rely heavily on their sense of smell, retain ratios of above 30%).
The team concludes that the sense of smell was initially enhanced during the early evolution of birds, possibly because it increased their ability to forage for food and to navigate while in flight. (Homing pigeons, for example, use their sense of smell to find wind-borne odors associated with their destinations.) Thus a heightened olfactory sense, the authors write, "may have provided these birds with a competitive advantage" that helped get them through the traumas of the K-T boundary relatively unscathed. Only later, when birds began to evolve other cognitive skills—such as advanced foraging and navigating abilities, and even tool use in some species—did the sense of smell become less important, the researchers suggest.
"This is a very neat paper," says Stephen Brusatte, a vertebrate paleontologist at the American Museum of Natural History in New York City. The finding that "the earliest birds and their closest dinosaur relatives had very similar olfactory capabilities provides yet more striking evidence that there was little biological difference" between them, he says.
Luis Chiappe, a paleontologist and early bird expert at the Natural History Museum of Los Angeles County in California, says that the paper represents a "great start," but he adds that the team's hypothesis that the sense of smell got birds through the K-T mass extinction is "clearly a stretch" because there is no solid evidence to support it.