The sense of smell might indeed be as important to birds as it is to fish or even mammals. This is the main conclusion of a study by Silke Steiger (Max Planck Institute for Ornithology) and her colleagues. The sense of smell in birds was, until quite recently, thought to be poorly developed. Recent behavioural studies have shown that some bird species use their sense of smell to navigate, forage or even to distinguish individuals.
Silke Steiger and her colleagues chose a genetic approach for their study. Their research focused on the olfactory receptor (OR) genes, which are expressed in sensory neurons within the olfactory epithelium, and constitute the molecular basis of the sense of smell. The total number of OR genes in a genome may reflect how many different scents an animal can detect or distinguish. In birds such genetic studies were previously restricted to the chicken, hitherto the only bird for which the full genomic sequence is known.
In addition to the chicken, the researchers compared the OR genes of eight distantly related bird species. They estimated the total number of OR genes in each species' genome using a statistical technique adapted from ecological studies where it is used to estimate species diversity. They found considerable differences in OR gene number between the nine bird species. The brown kiwi from New Zealand, for example, has about six times more OR genes than the blue tit or canary. "When we looked up the relative sizes of the olfactory bulb in the brain, we also noticed similar big differences between species", said Steiger.
"It is likely that the number of OR genes correlates with the number of different smells that can be perceived. As the olfactory bulb is responsible for processing olfactory information, we were not too surprised to see that the number of genes is linked to the size of the olfactory bulb." Wide variation in numbers of OR genes, and sizes of olfactory bulbs, has also been found amongst mammals. The implication of this finding is that different ecological niches may have shaped the OR gene repertoire sizes in birds, as has been suggested for mammals.
The high number of OR genes in the kiwi could be explained by this bird's unusual ecological niche. Unique among birds, the nostrils of the night-active kiwi are at the tip of the bill. When kiwis probe the forest floor in search of food, they are guided by smell rather than sight. Indeed the snuffling, nocturnal kiwis are sometimes considered to be New Zealand's equivalent of a hedgehog!
Besides the total number of OR genes, the researchers estimated which proportion of these genes are functional. This was done because, in mammals, a reduced dependence on the sense of smell is associated with OR genes gradually accumulating mutations and so becoming non-functional. For example, in humans, which have a poor sense of smell compared with most other mammals, only about 40% of all OR genes may be functional. However, in the bird species studied by Steiger et al., the large majority of the OR genes were functional, again indicating that the sense of smell is much more important in birds than previously thought.
From the analysis of the chicken genome three years ago a new class of OR genes was found. Now Silke Steiger and her colleagues have shown that this class of genes seems to be a shared feature of all birds, while such OR genes are not found in other vertebrates such as fish, mammals or reptiles. The specific function of this class of bird-specific OR genes remains unknown.
Citation: Silke S. Steiger, Andrew E. Fidler, Mihai Valcu & Bart Kempenaers
Avian olfactory receptor gene repertoires: evidence for a well-developed sense of smell in birds?
Proceedings of the Royal Society B, published 16.07.2008, doi:10.1098/rspb.2008.0607
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