Sex pheromones let female butterflies avoid inbred males

Research which has appeared in the journal Proceedings of the Royal Society B by University of Cambridge zoologists has shown that female butterflies of the species Bicyclus anynana use the quality of his sex pheromones to decide whether or not a male butterfly is inbred and hence a good choice for a mate.

First, the scientists produced inbred butterflies by ensuring that sisters could only mate with their brothers. Tests showed that the general condition (as measured by flight performance) of the inbred males was significantly worse than that of outbred males, and that they were able to produce less sex pheromones. Female butterflies were found to be twice as likely to mate with inbred males, but how could they tell that they had a better chance of reproductive success with the outbred males?

The researchers subsequently attempted to determine whether the attractiveness of the inbred males could be restored. They painted the antennae of female butterflies with nail polish to block their ability to detect sex pheromones. The technique, which involved marking the genitals of inbred and outbred males with different coloured fluorescent dust, showed that the preference for outbred males was no longer observed when the females were deprived their pheromone detectors.

Lead author Erik van Bergen is currently a PhD student based in Professor Paul Brakefield’s lab at the University Museum of Zoology, but he conducted most of the research at Leiden University. He satisfied ScienceOmega.com’s curiosity by answering some questions about the findings…

How much do we know about strategies for avoiding inbreeding? Do they depend on the social structures a species does/doesn’t have in place?
It is important to separate two different processes here, namely, the act of ‘inbreeding’ and being ‘inbred’ because these are slightly different issues. Animals will often try to minimise inbreeding – the act of mating with a relative – because their inbred offspring are more likely to have genetic disorders which make inbred individuals weaker in general.

If she can recognise him, a female will try to avoid mating with a relative to make sure her offspring will have a lower chance of genetic disorders. This strategy is called inbreeding avoidance and is not the theme of my paper. A female is also expected to try to avoid mating with an inbred male (again, if she can recognise him) to make sure her offspring will have a high chance of survival. This is the theme of my paper – picking the perfect father. The perfect father, in the case of our butterfly, is not the sterile inbred male. Remember, inbred males are normally weaker making them less able to defend the nest or provide food for the youngsters, for instance.

To answer the question, ‘How much do we know about strategies for avoiding mating with inbred individuals?’: we don’t actually know very much about different strategies. We do know that inbred males quite often have a low rate of mating success compared to normal outbred males. We also know that the traits males use to attract females – such as songs, colours and pheromones – are often strongly affected by inbreeding. We showed that female butterflies use these cues to avoid mating with an inbred male.

Is it more important for Bicyclus anynana females to avoid inbred males than it is for other female butterflies?
Well, we know that inbred males of B. anynana are likely to be sterile so mating with them will not fertilise the female’s eggs. For B. anynana females there is a direct cost of mating with an inbred male, so she should definitely avoid them if she can. I am not sure if inbred male sterility has ever been examined in other butterflies, but inbreeding has been found to affect fertility and sterility in many other species. In general, if mating or pairing up with a weak inbred male has direct costs or fewer benefits it is better to pick an outbred male.

Is there any evidence to suggest or reason to believe that males might avoid inbred females?
To be honest, we never tested this because males are normally less choosy. They are able to produce many reproductive cells each day and therefore it is less costly for males to make a mistake; they will simply try to fertilise another female or females the next day. Females invest a lot in reproduction; they put many nutrients in their eggs and would prefer them to be fertilised by a ‘good’ male. Mistakes are extremely costly in these circumstances, so they are normally the more choosy gender. We do know, however, that even inbred females prefer outbred males.

Were you surprised to find that inbred males produce less sex pheromones?
Not entirely, because we already knew that sex pheromones are extremely important for mate choice decisions. We also knew that traits used by males to attract the opposite sex are often affected by inbreeding. What did surprise me is that we were able to restore the mating success of inbred males completely by blocking female perception. So apparently these sex pheromones are the only cues used by the females to detect inbred males. Their general condition, which was also worse, played no discernible role in reducing the mating success of inbred males. 

What are the effects of isolation on butterfly populations? Would females prefer not to mate at all than mate with an inbred male?
It is not that females of this species do not mate with inbred males at all. In our experiments outbred males were roughly two times more successful in mating than severely inbred males. If we released 18 females in a cage with an equal number of inbred and outbred males, for example, the next morning we would find that about 12 females had mated with a normal outbred male and about six had mated with a severely inbred male.

Do you have further research in the pipeline on B. anynana or other species?
I have actually changed the scope of my research slightly. We are now focussing on the evolution of all Bicyclus species, of which there are approximately 90, and investigating the processes underlying speciation events in this group.