Aspects of the mating behaviour of male mollies ( Poecilia spp.)

The unisexual fish Poecilia formosa (the Amazon molly) reproduces by gynogenesis, a process in which sperm from the males of the host bisexual species activates development of its eggs. Unisexuals live with one of the host species in nature and compete with bisexual females for the males. It was long thought that male discrimination and mate selection established a balance between the unisexual and bisexual populations. Thus, hierarchies of males were set up in which dominant males mated with their conspecific females and subordinates mated with the Amazon molly. Recent evidence suggested, however, that male fish do not discriminate between their own females and the Amazon molly, and that there always are more males available than sexually receptive females of both species. Our findings indicate that male behaviour may be more complex than suggested by either hypothesis. Mate discrimination and courtship behaviour appear to increase with age, so that large males show almost complete preference for their conspecific females, but smaller males will mate with the Amazon molly. In complex groups, small males often dart in and mate with their own females whilst the large males are engaged in courtship activities and defence of territory.