In many species males have showy body parts or behaviours that females use to help them decide which male to mate with, such as the peacock’s tail. It is believed that these displays indicate the quality of the males’ genes, but it has proved difficult to find evidence that this is the case. Good genes might have many effects, not only making strong signals but improving the chance of survival. Studies looking for relationships only consider the genes of adult males, but in most species a significant proportion of individuals die before reaching adulthood. Survival to adulthood isn’t only affected by genes but by where the animal lives. Some individuals are luckier than others, such as in the amount of food they can find. It is possible that this variability in environment combined with mortality before adulthood affects any possible relationships between genes and sexual displays.
We made a computational model of an animal that varies in the quality of its genes and the quality of the environment individuals are born in. Both of these determine both the chance that an individual survives to adulthood, and the quality of its sexual display if it does. The model shows that if animals grow-up in variable environments and when many animals do not survive to reach maturity then the male sexual characteristics might not be a reliable indicator of the fitness of male genes. This happens because individuals in poor environments only survive if they have good genes but end up with a poor display, but individuals in good environments always survive, and if the effect of genes is pretty small in good environments then all individuals will have roughly the same quality of display. So, poor displays are more likely to be shown by individuals with good genes than poor genes. In some extreme cases, such as when the environmental quality has a particularly strong effect, the relationship between genes and display can even be negative.
The results show that the showy body parts or behaviours used by females to choose a mate may often not be very good indicators of the quality of males. This is especially likely when the environment strongly affects survival to adulthood, as it does in species with little parental care or very variable habitats. Hence, we have provided an explanation for how there is still lots of variability in genes and sexual displays, because the variable environment makes it impossible to avoid breeding with males who have bad genes. This explanation suggests that females will often select males for their other characteristics, such as the territory they own or their absence of parasites.