Dynamic Sexual Dichromatism Promotes Rapid Mate Recognition in an Explosive Breeding Toad

Many species reproduce by participating in explosive breeding events, where hundreds of individuals converge at specific locations for a short time period to mate. During these spectacular aggregations, many males must actively search and compete for a limited number of females. One idea is that sexual selection in explosive breeding species favors the evolution of communication strategies facilitating easy detection of males and females. As such, the emergence of dynamic sexual dichromatism, a reversible change of body coloration of one sex, might act as visual signal promoting rapid mate recognition in dense breeding aggregations. We tested this idea in Asian Common Toads (Duttaphrynus melanostictus), an explosive breeder in which males quickly turn from brown to bright yellow for up to two days during mating. Thus, we first explored whether color differences of brown and yellow are sufficient to be perceived and distinguished by toads using a color vision model. In a set of behavioral experiments, we then tested the interactions of male toads with a yellow (resembling a breeding male) and a brown (resembling a female) 3D model toad to determine the functional significance of yellow male coloration during mating. Finally, we compared male body color, morphological parameters, and body condition of mated and non-mated males to investigate if these traits are influenced by securing a mate or female mate choice. Our results show that dynamic yellow coloration displayed by males can be easily discriminated against brown female conspecifics from a toad’s point of view. During model experiments, males had twice as much physical contact with brown models and clasped them 40 times more often than yellow models. Coloration and several morphological traits did not differ between mated and non-mated males; hence, securing a mating partner was not influenced by variations in color or morphology. Our study supports the idea that dynamic sexual dichromatism repeatedly evolved as a visual signal mediating fast mate recognition in tropical and temperate anuran species solving a common problem in time-limited, dense breeding aggregations, by minimizing same-sex harassment and mismatch.