Evolutionary biologists have long attempted to explain why individuals of a species differ in appearance and why the choice of a mate is influenced by behaviour and appearance features that cannot reasonably be thought to have any usefulness. Therefore, they have begun to look more and more at the genetics behind what are called secondary sexual characters, such as the tail of a peacock, the stripes of the female gulf pipefish, and the white spot on the forehead of the collared flycatcher. In many species both males and females prefer to mate with those who have the largest or most colourful of these ornaments or who have the most complex song, for instance.
One theory says that the ornaments are clearest on individuals that are in good health and that both the size and the condition of the ornament are heritable. This leads to the question of why evolution did not select the same appearance and good health for all individuals. Is there something in the environment that is constantly changing and can govern the genetics of appearance and health, leading, instead, to diversity?
"More and more evidence indicates that the most changeable part of the environment consists of parasites, bacteria and viruses. All of these, especially viruses, evolve more rapidly than the hosts whose resources they live off of. The host will therefore always be in an important evolutionary race against its diseases," explains Måns Andersson, who directs the research team.
In earlier studies, Andersson and Professor Lars Gustafsson have shown that male Gotlandian collared flycatchers with few dangerous blood parasites have large r forehead spots. The new study shows that males that are vaccinated against Newcastle virus produce more antibodies if they have large forehead spots.
"Thus it seems that the female uses the forehead spot as a health indicator. When she chooses males with a large forehead spots, she takes not only the healthiest males but also the ones with the best immune defence against future virus infections."
The study also shows that the male collared flycatcher can change the size of the forehead spot during the mating season and that males that unfurl their forehead spots most are those that produce the most antibodies.
The findings not only enhance our understanding of why animals behave and look the way they do but also help explain why animals that choose their own mates produce healthier offspring than animals whose mates are selected by humans.