Effective crypsis and its maintenance by immobility in Craugastor frogs

Prey that are cryptic when at rest should limit movement to avoid disrupting crypsis. We studied effectiveness of crypsis and degree of reliance on immobility versus escape by Craugastor frogs approached by a predator. We studied detectability of C. fitzingeri resting in a 2 m diameter circle of leaf litter using four human observers. Nearly 40% of frogs were not detected within 60 s, and mean detection time was more than 30 s despite a small search area. Given a much lower density in natural habitats, It is likely that these frogs are rarely detected visually without area-concentrated search, except perhaps by predators sensitive to wavelengths not detected by human beings. When approached by a human, over 90% of individuals belonging to five species of Craugastor remained immobile until the predator reached them. Only C. noblei fled, and this species fled more frequently than the others at higher approach speed. Approach speed did not affect probability of lumping or distance jumped. Distance jumped varied among species, but was unrelated to body length except for C. mimus during fast approaches. Extreme reliance on crypsis reinforced by immobility suggests that crypsis is so effective in these frogs that survivorship may be higher for individuals that do not attempt to escape. The increased probability of being detected and attacked upon moving combined with limited escape capacity presumably raises the probability of being captured for frogs that jump. The importance of crypsis Is emphasized by observations that almost all frogs that jumped reestablished crypsis after a single jump by becoming immobile, and some burrowed into leaf litter upon landing, concealing the head and part of the body.