Although many birds are known to have acute vision and hearing, it has been widely thought that they have a very poor sense of smell. Part of the reason for this view is that it is relatively difficult to devise experiments to test the sense of smell in these animals.
Molecular genetic data appeared to have provided support for this assumption when the genome of the domestic chicken and of its wild progenitor, Gallus gallus, was published in 2004. Although around 550 genes were detected that could code for olfactory receptor (OR) proteins, only 15% of these genes (∼80, compared to ∼600 in humans) were thought to be functional. Furthermore, most of these genes seemed to be very closely related, suggesting the olfactory system would have a restricted functional range.
To investigate this assumption that animals in a whole phylogenetic class do not use their sense of smell much, a group from Germany and New Zealand led by Silke Steiger tried to amplify OR genes in DNA extracted from eight species of birds ranging from the prosaic (the blue tit and the mallard) to the exotic(the kakapo, the black coucal or the snow petrel). As a control, they also included the latest version of the G. gallus genome.
To their surprise, all the species – including G. gallus– seemed to have far higher levels of functional OR genes than was expected. The total number of estimated functional OR genes varied from around 70 for the galah (an Australian cockatoo) to around 600 for G. gallus, with most of the species appearing to have around 300 functional OR genes.
Because the degenerate PCR technique the group used will not distinguish functional genes from mutated genes, they checked the accuracy of their procedure by looking in the G. gallus genome to see whether the OR genes they had identified as functional were in fact complete. They found they were over-estimating the number of functional OR genes by about 11%. When they removed these false positives, their data fell into line with unpublished estimates of the proportion of functional ORs from the latest version of the G. gallus genome, which suggest that the chicken's nose may contain up to 500 different kinds of receptor.
These findings challenge the notion that birds do not use their noses much by revealing they possess a substantially larger OR repertoire than was previously imagined.
As a test of their findings, the authors compared the OR repertoires with the relative size of the birds' olfactory bulb and with the ecology of the species. In general, the larger the relative size of the olfactory bulb, the larger the OR gene repertoire, while nocturnal birds like the kakapo and the kiwi, which are more likely to use olfaction, had larger OR repertoires.
This study represents a challenge on a number of levels: it provides a reminder that the results of genomic studies need to be verified by studies of gene expression and it shows the importance of comparative inter-specific studies to test whether findings can be generalised across species. But above all it throws down the gauntlet to experimental biologists: attempt to measure the olfactory capacities of this important class of animals and to test the hypotheses that have emerged from this molecular study. In particular, some of the few experimental studies on avian olfaction suggest that birds with the smallest OR repertoires may not have the poorest olfactory responsive range or acuity. Explaining this apparent enigma will require extensive experimentation.
