1.Contests between conspecific males are an important method of establishing mating rights or territories, yet the potential costs of injuries are high. To reduce potential risks, males can use signals to convey information about their underlying strength to competitors, and although individuals could signal deceptively to gain an advantage, most signals seem to be reliable.
2.Theory suggests that signal reliability is maintained because individuals that signal unreliably may be punished (i.e. receiver-imposed costs). Manipulative studies support this idea, showing that low-quality individuals that are given high-quality signals bear substantial costs.
3.Here, we explore the importance of receiver-imposed costs in natural, un-manipulated populations. Specifically, we show how the likelihood of being exposed (i.e. potential receiver density) and the potential severity of punishment (i.e. average receiver size) affect the predominance of unreliable signalling among populations.
4.Male fiddler crabs, Uca vomeris, use their enlarged claws as signals and weapons in combat, and the relationship between claw size (signal) and strength (quality) determines whether or not a male is a reliable signaller. We predicted that the prevalence of unreliable signalling among individual males would increase in populations where the receiver-imposed costs of deception were low. That is, males should produce unreliable (large but weak) claws.
5.We show that individual crabs produce more reliable signals of strength in populations with a high biomass, where there are both higher densities and larger average body sizes of potential receivers. Our study provides evidence that receiver-imposed costs can maintain signal reliability in natural un-manipulated populations and supports contemporary models of aggressive signalling.