As humans we are very hard to please when choosing a mate. All sorts of complex characteristics can influence attractiveness: wit, shared interests, kindness, athleticism, creativity, intelligence – and definitely smell.
Women in particular are thought to be especially good at smelling out men who carry different genes from their own, specifically in those genes coding for defences against disease (see this review, for instance). The resulting babies would then have diverse genetic weaponry against the ever-evolving masses of parasites, bacteria and viruses.
But knowing who is the best match (i.e. information about identity) is only half the story. If your ideal genetic match turns out to be hideously ugly, this information is not particularly helpful. It is less clear whether smell can help establish quality, or how attractive that partner is, say, on a scale of 1 to 10. Can we smell a good athlete, or poet, or a good parent? Nobody knows.
According to this new paper in Ecology Letters, butterflies can. The researchers showed that male butterflies of the genus Bicyclus have a peak age when they are strongest and most vigorous. From their smell alone, females can tell not only who the males are (to avoid mating with relatives, for instance) but also how old they are to the nearest 10 days, an amazing degree of resolution. They use this information to choose males that are in their prime.
Smell is hugely important for communication. I find it amazing that it has taken this long to demonstrate a way that smell is used for information about quality, rather than identity (although admittedly I’m not 100% on this specific literature). Studies that claim to show that females make choices between males on the basis of a signal of “male quality” are notoriously difficult. I thought this was a great, thorough paper that nailed almost all sides of a complex story.