Ripple effects in a communication network: Anti-eavesdropper defence elicits elaborated sexual signals in rival males

Emitting conspicuous signals into the environment to attract mates comes
with the increased risk of interception by eavesdropping enemies. As a
defence, a commonly described strategy is for signallers to group together
in leks, diluting each individual’s risk. Lekking systems are often highly
social settings in which competing males dynamically alter their
signalling behaviour to attract mates. Thus, signalling at the lek
requires navigating fluctuations in risk, competition, and reproductive
opportunities. Here, we investigate how behavioural defence strategies
directed at an eavesdropping enemy have cascading effects across the
communication network. We investigated these behaviours in the túngara
frog (Engystomops pustulosus), examining how a calling male’s swatting
defence directed at frog-biting midges indirectly affects the calling
behaviour of his rival. We found that the rival responds to swat-induced
water ripples by increasing his call rate and complexity. Then, performing
phonotaxis experiments, we found that eavesdropping fringe-lipped bats
(Trachops cirrhosus) do not exhibit a preference for a swatting male
compared to his rival, but females strongly prefer the rival male.
Defences to minimize attacks from eavesdroppers thus shift the mate
competition landscape in favour of rival males. By modulating the
attractiveness of signalling prey to female receivers, we posit that
eavesdropping micropredators likely have an unappreciated impact on the
ecology and evolution of sexual communication systems.