Sexual selection and ecological-demographical context drive the evolution of same-sex sexual behaviour

The occurrence of same-sex sexual behaviours (SSB) has long attracted the attention of researchers in evolutionary biology since these interactions do not directly lead to reproduction. The frequency of SSB is expected to vary depending on sexual context, but whether it responds to populations' ecological and demographical factors is unknown. Using experimental evolution in the seed beetle Callosobruchus maculatus, we explored the interactive effects of sexual selection intensity (polygamy vs. monogamy) and population spatial structure (undivided populations vs. divided metapopulations) in shaping the frequency of same-sex interactions. Males engaged more often in SSB than females. In undivided populations polygamy led to higher frequencies of male-male interactions, while SSB was more frequent in monogamous compared to polygamous lines when populations were divided. Our findings suggest that males are subject to stronger selection for higher frequency of SSB than females, and that the intensity of sexual selection is responsible for the frequency of SSB in large undivided populations. However, our results indicate that in divided populations softer selection operating in small subpopulations may lead to relaxed selection for mate discrimination, especially under monogamy. Our results point to a complex interaction between sexual selection evolutionary history and softness of selection shaping SSB.