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

The idea that female mate choice might be adaptive is relatively easy to un derstand in species with resource-based mating systems in which females gai n access to a territory, food, or other forms of parental care from the mal es 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 c an obtain information on male fertility (i.e., direct fertility benefits) v ia cues used during mate choice. Specifically, we examined whether male gup py colour patterns, body size, and mating behaviour signal their functional fertility, that is, their ability to supply a large number of sperm at cop ulation. We found significant correlations between male phenotype parameter s and the number of sperm in male guppies originating from two wild Trinida dian populations. There were, however, significant interpopulation differen ces with respect to which traits were good predictors of sperm load. In the low-predation Paria River population, larger males and males with relative ly more carotenoid colouration had significantly larger sperm loads, but ma ting behaviour (i.e., sigmoids) and melanin colouration were not good predi ctors of sperm load. In the high-predation Tacarigua River population, larg er 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. Over all, our results suggest that there is the potential for direct fertility b enefits through mate choice in the promiscuous, non-resource-based mating s ystem of the guppy.