When animals breed with a relative, their offspring are more likely to have genetic disorders. The disorders cause inbred males to be weaker, and less able to defend himself and others. Natural selection tells us that in order for the female to ensure the highest chance of survival for her offspring, she is expected to avoid mating with a weak male, provided the fact that she is able to distinguish between a male who is inbred or not.
Previous studies have shown that the mating success of a male butterfly is lower if they are inbred. But this leaves the question of how the females can distinguish which males to avoid. New research has revealed that inbred male butterflies produce much less sex pheromones than "normal" ones, making them less attractive to females.
One of the researchers, Erik van Bergen, explained that traits used by males to attract females are often strongly affected by inbreeding. "For example, inbred male zebra finches produce a lower number of different individual songs and inbred male guppies have less conspicuous colour patterns. Additionally, in one cricket species, the inbred males are known to produce less acoustic signals while trying to attract females."
For the butterfly pictured above, Bicyclus anynana, it is crucial that the female avoids breeding with an inbred male, due to the fact that approximately 50 percent of inbred males are sterile. Should the female mate with a sterile male, none of her eggs will hatch and she will not produce offspring.
Butterflies were put in the same area in order to ensure inbreeding. The scientists marked the genitals of the males with fluorescent dust of different colors to indicate inbred and outbred males. During mating, the dust is transferred to the female, which can be detected using UV light. The antennae of several females were then painted over with nail polish in order to prevent them from detecting the amount of sex pheromones produced by the males.
The researchers found that the females with covered antennae, had no preference for males and therefore mated with inbred and normal males equally. The females with uncovered antennae, however, mated significantly more often with normal males.
Van Bergen added, "We know that inbreeding contributes to the decline and eventual extinction of small and isolated populations, so it is valuable to have more knowledge about the processes involved in general."