Toxic males: Density‐dependent male mating harassment can explain geographic parthenogenesis

Understanding evolutionary forces that maintain sexual reproduction has been a central question in evolutionary biology, because the well‐known “two‐fold” cost of sex demands an explanation, especially since asexual reproduction (vegetative reproduction and parthenogenesis) often arises by mutation and hybridization. Geographic parthenogenesis, where asexual reproduction by close relatives of sexual lineages tends to be found in “marginal” habitats, has been considered a key pattern for studying the maintenance of sex. Recent studies of sexually reproducing populations have revealed that mating attempts by males can be harmful to females due to sexual conflict. Where that occurs and where there is a mixture of both sexual and asexual females, if male mating harassment is more harmful to asexual females than to sexual females, this may prevent the invasion of rare asexuals. Here, we use a simple ecological model with density‐dependent male mating harassment to show how asexual reproduction can become prevalent in habitats when environmental conditions are poor (i.e., low carrying capacities). The maintenance of sex is possible in a bistable system where abundant males are so harmful that they prevent the invasion of rare asexuals. However, male density is not large enough to suppress asexuals and sex is lost when carrying capacities are low or when sex ratio is highly male‐biased. While previous theoretical studies employed a frequency‐dependent model to study male mating harassment, we demonstrate that density‐dependence may be important to explain the dynamics in low population densities where there is geographical parthenogenesis.