Major claws make male fiddler crabs more conspicuous to visual predators: a test using human observers

One of the possible costs of the male fiddler crabs enlarged claw can be co nspicuousness to predators. This hypothesis was tested using human observer s as a model of visual predators. In the European fiddler crab, Uca tangeri Eydoux, the males' major claw is white contrasting with the orange-brownis h colour of the carapace and of the feeding claw, and the mudflat backgroun d. The following morphotypes were created from close-up photographs taken i n nature using an image processing software: male, male without claw, femal e, female with enlarged claw, male with enlarged claw of the same colour of the feeding claw, male with 75% sized claw, male with 50% sized claw. Thes e morphotypes were then presented in a randomised order to students, using a psychology test software, which allows the measurement of response time i n msec. The subjects were allowed to look at the images for an unlimited am ount of time, until they detected the individual or until they decided to p ass on to another image. Backgrounds (i.e. mudflat picture) without individ uals were also presented as a control. Male crabs were detected significant ly sooner than females. When we compared males with the claw removed with f emales with an enlarged claw added, the pattern is reversed and the latter are detected significantly faster. Thus, the enlarged claw seems to be the key feature that makes the individuals more conspicuous. Size and colour se em to be the main aspects of the claw's conspicuousness. The data of these experiments support the initial prediction of males being more conspicuous than females because of their enlarged claw. The possible costs and benefit s of this trait, related to predation, are discussed.