Deception with honest signals: signal residuals and signal function in snapping shrimp

Animals in competitive interactions often assess the competitive ability of opponents using signals. Signals used in competitive interactions are generally predicted to be honest, but open to low levels of deceit. Such "incomplete honesty" in signals can be studied by using signal residuals, the residuals from the regression of a measure of signal structure on competitive ability. Specifically, individuals with positive signal residuals produce signals that exaggerate their competitive ability; deceptive use of these signals may occur if signalers for whom the signal exaggerates their apparent competitive ability use the signal more frequently. I used this framework to examine the use of the open chela display by big-clawed snapping shrimp (Alpheus heterochaelis). Competitive interactions between snapping shrimp are resolved primarily on the basis of body size, and the open chela display is used by males to assess body size. I found that the production of the open chela display by males responding to superior competitors depends on chela residuals, such that individuals for whom the display exaggerates their apparent size produce the display more often. This effect can be seen both in response to isolated chelae and in staged competitive interactions. Interactions involving shrimp with larger chela residuals are long and highly escalated, suggesting that chela residuals affect assessment of competitive ability. Thus, the increased use of the open chela display by males for which the display exaggerates apparent body size is an example of deceptive use of an otherwise honest signal.