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Same-sex sexual behaviour and selection for indiscriminate mating


The widespread presence of same-sex sexual behaviour (SSB) has long been thought to pose an evolutionary conundrum, as participants in SSB suffer the cost of failing to reproduce after expending the time and energy to find a mate. The potential for SSB to occur as part of an optimal strategy has received less attention, although indiscriminate sexual behaviour may be the ancestral mode of sexual reproduction. Here, we build a simple model of sexual reproduction and create a theoretical framework for the evolution of indiscriminate sexual behaviour. We provide strong support for the hypothesis that SSB can be maintained by selection for indiscriminate sexual behaviour, by showing that indiscriminate mating is the optimal strategy under a wide range of conditions. Further, our model suggests that the conditions that most strongly favour indiscriminate mating were probably present at the origin of sexual behaviour. These findings have implications not only for the evolutionary origins of SSB, but also for the evolution of discriminate sexual behaviour across the animal kingdom.

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Fig. 1: Optimal discrimination strategies and resulting SSB.

Data availability

This manuscript does not use data.

Code availability

Code to replicate results can be found on Dryad at


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We thank M. Moore and M. Lambert for comments on early versions of the manuscript and A. Deconinck for suggesting the name ‘targeted’ sex. M.R.S. was supported by the National Science Foundation with award no. DEB-1939290.

Author information




B.A.L. conceived of the project and the optimization models. B.A.L. and M.R.S. designed the population genetic models. B.A.L. led the writing on the manuscript with input from M.R.S.

Corresponding author

Correspondence to Brian A. Lerch.

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Supplementary information

Supplementary Information

Supplementary Fig. 1, Methods and Appendices 1–4.

Reporting Summary

Supplementary Video 1

Video showing the fitness gradient as a function of attempted discrimination a changing with each parameter. The default setting for each parameter is c = 0.1, s = 1, σ = 0.5, d = 0.8, f = 1, p = 0 and r = 0. Each parameter is allowed to vary between 0 and 1, except for c, which is allowed to vary only between 0 and 0.2, and d, which is allowed to vary between 0 and 0.9. Evolutionary optima occur wherever the line crosses the x axis (and has a negative slope). If the line is always positive, the optimal strategy is to always discriminate. If the line is always negative, the optimal strategy is to never attempt sexual discrimination.

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Lerch, B.A., Servedio, M.R. Same-sex sexual behaviour and selection for indiscriminate mating. Nat Ecol Evol 5, 135–141 (2021).

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