Twice a day, coinciding with tidal motion, climbing crabs Sesarma leptosoma leave the canopy of the mangroves they inhabit and migrate down the trunks to the swamp below. But nature being what it is, they run the risk of being eaten on their journey by another crab, Epixanthus dentatus.

Stefano Cannicci et al. decided to investigate what visual cues alert the climbing crabs to the predator (Animal Behaviour 63, 77–83; 2002). They wrapped two 70-cm lengths of mangrove trunk with PVC sheeting, and placed raffia on the top to create a test pathway. The authors then recorded their subjects' response to the dummies shown here.

The first is a preserved specimen of E. dentatus in ambush posture. The second is a wooden rectangle, painted naturalistically, but with real claws attached. The third is a wooden trapezium, the same size and coloration as E. dentatus, but otherwise not like the real thing. Cannicci et al. kept all the dummies in mud for a week to give them an authentic swampy tang. In tests, S. leptosoma recognized the first two dummies as dangerous from about 30 cm away, and took avoiding behaviour, but they were unperturbed by the third. So it seems that claws in particular provide the warning. With up to six predators per tree, the climbing crabs probably don't get a second chance to mistake identity.