Variation in sexual signals and defensive strategies elicits receiver-dependent shifts in attractiveness

Abstract

Sexual selection often favors the evolution of conspicuous mating displays. Emitting such overt displays carries the risk of interception by eavesdropping enemies, i.e. predators, parasitoids and parasites that exploit communication systems to find and attack their signaling victims. Yet, many signalers respond to variation in perceived eavesdropper risk, protecting themselves through risk-dependent inducible defenses to mitigate potential costs. Given that signalers are embedded in communication networks in which they interact with other signalers, target receivers and multiple eavesdropping enemies, here we investigated how variation in signaling and defensive strategies impacted by an eavesdropping enemy (frog-biting midges; Diptera: Corethrellidae) affects other receivers in a communication network. Ultimately, we aimed to determine whether and to what extent effects that cascade throughout the network shape relative fitness among chorusing males. Using female choice experiments with túngara frogs (Engystomops pustulosus) and predation experiments with eavesdropping, fringe-lipped bats (Trachops cirrhosus), we show that variation in the call elaboration and defensive strategies of competing males shapes their relative fitness. Defensive strategies targeting eavesdropping frog-biting midges indirectly shift a male's relative attractiveness to females and predatory bats, though the mechanisms and impacts are context and receiver specific. These findings showcase how the frequency-dependent effects of micropredation can dynamically shape variation in secondary sexual characteristics and thus influence the mechanisms driving sexual selection.

Keywords: Engystomops pustulosus; Eavesdropper; Female choice; Foraging; Inducible defense; Predator–prey interactions; Vibratory cues.

Fig. 1.

The effects of male túngara frog call and swat rates on the probability of females selecting treatment (1) or control (0). Effects of call rate (A), swat rate (B) and their interaction (C) on female choice, and the effects of the interaction between a male's call complexity (i.e. number of chucks) and swat rate (D). All other parameters in the model were held at mean values to derive these functions. The solid lines and shading indicate median estimates and their associated 90% credible intervals.

Fig. 2.

Probability of fringe-lipped bat selecting treatment (1) or control (0). For this estimate, swat rate and total chucks were held at mean values. The solid lines and shading indicate median estimates and their associated 90% credible intervals.