In many species, sexually selected traits serve as indicators of individual quality to potential mates. Much of this understanding comes from studies on sexually dimorphic species, focusing on traits perceived by humans as exaggerated. However, while some traits may appear exaggerated or dimorphic to humans but not to the species itself, others may be exaggerated in the species’ perception but not to humans. The lack of studies of non-exaggerated sexual traits (at least to human perception) may bias our comprehension of sexual selection. To reduce such bias, here we studied an apparently monomorphic fish Poecilia vivipara, to examine the potential role in mate selection of the dark spot present on the flank of both males and females. Since the dark spot appears in sexually mature individuals and is costly to produce, we tested whether it (1) indicates nutritional quality and (2) plays a role in mate choice. We analyzed the spot’s reflectance spectrum, correlated its area with morphophysiological measures of individual quality, and conducted a mate choice experiment. Results show that males have spots, on average, 72% larger than those of females, and the spot indicates individual quality in both sexes. While the correlation between spot size and body length is steeper for males, the correlation between spot size and body fat does not differ between males and females. However, the spot does not influence mate choice. Given the trait’s production cost, we suggest it may play a role in species recognition or post-copulatory processes.
Massote et al. (Wed,) studied this question.