Male phenotype and sperm number in the guppy (Poecilia reticulata)

The idea that female mate choice might be adaptive is relatively easy to understand in species with resource-based mating systems in which females gain access to a territory, food, or other forms of parental care from the males with whom they mate. In contrast, the evolution of female mate choice in species exhibiting resource-free mating systems remains controversial. One such species in which males contribute nothing but sperm during mating is the guppy (Poecilia reticulata). Here, we examined whether female guppies can obtain information on male fertility (i.e., direct fertility benefits) via cues used during mate choice. Specifically, we examined whether male guppy colour patterns, body size, and mating behaviour signal their functional fertility, that is, their ability to supply a large number of sperm at copulation. We found significant correlations between male phenotype parameters and the number of sperm in male guppies originating from two wild Trinidadian populations. There were, however, significant interpopulation differences with respect to which traits were good predictors of sperm load. In the low-predation Paria River population, larger males and males with relatively more carotenoid colouration had significantly larger sperm loads, but mating behaviour (i.e., sigmoids) and melanin colouration were not good predictors of sperm load. In the high-predation Tacarigua River population, larger males, males that displayed more, and males with less yellow colouration had significantly more sperm, but other colour pattern components (area of orange and black colouration) were not good predictors of sperm load. Overall, our results suggest that there is the potential for direct fertility benefits through mate choice in the promiscuous, non-resource-based mating system of the guppy.