The house finch Carpodacus mexicanus, a native of North America, may have been snared by an evolutionary trap. Karen Bouwman and Dana Hawley draw this conclusion from their video evidence of feeding behaviour in experiments with caged finches (K. M. Bouwman and D. M. Hawley Biol. Lett. doi:10.1098/rsbl.2010.0020; 2010). The trap is the preference of healthy males to feed alongside finches of the same sex that are infected with the bacterium Mycoplasma gallisepticum, so making them more vulnerable to debilitating infection by this contagious agent.
But why does this preference exist, when the expectation would be that healthy individuals would avoid their diseased fellow finches, well, like the plague? The authors' observations show that the short-term gain for healthy males, such as the fine example pictured here, is that they have less of a battle over a meal, given that infected and so weakened males are less aggressive. Bouwman and Hawley don't rule out the involvement of chemical cues in this behaviour. But they lean towards the explanation that the visible recognition of such signs of debilitation as lethargy, and the conjunctivitis produced by M. gallisepticum infection, is the prompt.
A further curious aspect of the results is that it was males, not females, that preferred to associate with diseased individuals of the same sex at meal times. The video evidence showed that whereas healthy males were much more likely to win encounters with infected males, females showed no such edge in confrontations with infected females. There is no obvious reason why this should be so, beyond the possibility that other factors of intraspecific dominance came into play.
The bigger picture, however, concerns the evolutionary trap. House-finch infection with M. gallisepticum emerged only recently in evolutionary terms, some nine finch generations ago. Although the bacterium has since produced seasonal epidemics in finch populations, Bouwman and Hawley's view is that the birds have not yet caught on to the full significance of the signs of disease. Rather, the males, at least, continue to interpret these 'sickness behaviours' not as warnings to keep clear, but as indications that they have simply picked a loser in contests for food. Such a misapprehension, say the authors, can only contribute to the persistence of disease epidemics among these birds.
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Lincoln, T. An ill wind for finches. Nature 463, 888 (2010). https://doi.org/10.1038/463888a