Distinguishing between unreliability and dishonesty: a comparative study of aggressive communication in crayfish

A major challenge in the study of animal communication is distinguishing
whether signals convey honest or dishonest information. Biologists infer
the honesty of a signal from its correlation with the information being
signaled (e.g., fecundity or fighting ability) — the better the
correlation, the more honest the signal. However, this view of signaling
potentially conflates unreliable indicators with dishonest signals. Just
because a trait conveys unreliable information does not mean that the
structure serves as a dishonest signal; developmental noise, genetic
drift, and environmental constraints can reduce the covariation between a
putative signal and an organism’s quality. Moreover, a trait must
influence the behavior of other organisms to qualify as a signal. We
studied how a putative signal, claw size, affects physical performance and
social dominance in three species of stream-dwelling crayfish, which fight
routinely over resources. For comparison, we measured the relationship
between claw size and claw strength in three species of burrowing
crayfish, which do not fight. In all species, crayfish with larger claws
were not necessarily stronger, indicating that claw size poorly indicates
claw strength. However, claw size is unlikely to function as a dishonest
signal of fighting ability in these species for two reasons. First, claw
size unreliably indicates claw strength in both the stream-dwelling
species and the burrowing species. Second, relative claw size poorly
predicted whether stream-dwelling crayfish would escalate aggression,
questioning whether claw size serves as a signal. Instead, stream-dwelling
crayfish escalated aggression based on their relative body size. Our
results highlight the importance of observing behavior when determining
the honesty of a signal. As such, future studies must distinguish between
phenotypes that are unreliable indicators (or reliable indicators) from
phenotypes that have been selected for dishonest (or honest)
signaling.