The evolution of risky behaviour in the presence of a sexually transmitted disease

Sexually transmitted diseases (STDs) are widespread in nature, often sterilizing their hosts or causing other pathogenic effects. Despite this, there is a widespread occurrence of behaviours that are likely to increase the risk to an individual of contracting an STD. Here, we examine the evolution of behaviours such as promiscuity or mate choice that increase the risk of contracting an STD, but also provide a fitness benefit. As might be expected, the balance between risk and fitness benefit defines the optimal strategy, but this relationship is not straightforward. In particular, we often predict the coexistence of highly risky and highly risk-averse individuals. Surprisingly, very safe strategists that only suffer a small cost will tend to coexist with highly risky strategists rather than outcompete them as might have been expected. Rather than selecting for monogamy or for reduced mate choice, therefore, the presence of an STD may often lead to variability in either promiscuity or mate choice.