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In many human societies, there are laws put in place to combat any likelihood of incest. However, inbreeding is bad news not just among humans; incest is generally frowned upon everywhere in the animal kingdom. However, without laws, how can animals be sure to avoid ending up with an incestuous family tree? In the case of butterflies, would-be mates avoid breeding with mates who are inbred, because they can smell it.

Animals who breed with their siblings are more likely to produce offspring with congenital problems. In the case of men, for example, men who are the product of inbreeding are more likely to be weaker, which means that they will be less able to protect the nest and provide food for their offspring. That means that it is in females' best interest to avoid inbred males, because offspring will have a decreased chance of survival.

In the case of the butterfly Bicyclus anynana, it is even more critical to avoid mating with inbred males, because there is a 50 percent chance that he is sterile. Therefore, none of her eggs will hatch and she will produce no offspring.

Researchers decided to discover how animals knew how to avoid reproduction with inbred individuals. The first part of the experiment meant that researchers created conditions in which sisters could only mate with their brothers. Following that part of the experiment, researchers tested the offspring for their flight performance and sex pheromones. They found that the offsprings' general condition was much worse, and they produced fewer pheromones than their peers.

After that part of the experiment, researchers released males and females in a cage. Some of the males were inbred. Researchers coated the genitals of the males with a fluorescent dust that was marked with different colors for inbred or outbred males. That dust is transferred to females during mating, which allowed researchers to track who mated with whom. Then researchers coated the antennae of some of the female butterflies with nail polish, which would prevent them from smelling properly.

The researchers found that the females who could not smell properly mated with inbred and outbred males equally. However, the females whose antennae were left untouched were significantly more likely to mate with outbred males than with inbred males.

The study was published in the journal Proceedings of the Royal Society B.