Frogs With a Southern Drawl: Wide Recognition Space Facilitates Heterospecific Aggression in Territorial Cricket Frogs

ABSTRACT Species recognition enables both females and males to make adaptive behavioral decisions in sympatric zones. Research on species recognition usually concentrates on how female mate choice prevents interspecies matings. Male–male competition also plays an important role in reproduction but is relatively understudied in the context of speciation. The present study investigates the aggressive thresholds for species recognition during male–male competition in northern and southern cricket frogs: two sympatric “sibling” species of frogs that are distinguishable based on advertisement calls. We conducted a field playback experiment that presented synthetic calls to male northern cricket frogs ( Acris crepitans ) that varied in temporal properties, spanning the range from conspecific to heterospecific call properties. Males responded aggressively to calls ranging from average conspecific calls to average heterospecific calls, notably excluding extreme heterospecific calls. We also modeled how optimal thresholds for species recognition depend on local proportions of conspecific and heterospecific individuals, and predicted as optimal the thresholds we measured under conditions of very few heterospecifics. Overall, our results show that recognition space for territorial aggression in male A. crepitans is larger than signal space, and their thresholds for recognition would enable heterospecific aggression. Such thresholds could be adaptive under social environments dominated by conspecifics, or in noisy choruses in which males compete with both conspecifics and heterospecifics to attract females by minimizing acoustic interference from rivals. This research provides insight into the interplay between speciation and male–male competition in two closely related species and demonstrates how signal recognition can be decoupled from signal variation.