FLOCKING AND FEEDING IN THE FIDDLER-CRAB (UCA-TANGERI) - PREY AVAILABILITY AS RISK-TAKING BEHAVIOR

For a full understanding of prey availability, it is necessary to stud y risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on t he surface of the intertidal flats during low tide, thereby exposing t hemselves to avian predators. A study in an intertidal area along the coast of Mauritania showed that small crabs always stayed in the vicin ity of their burrow, but large crabs wandered in large flocks (also re ferred to as droves) to feed on sea-grass beds downshore. Transplantin g downshore feeding substrate to the burrowing zone of the small crabs proved that they too preferred to feed on it. Since small crabs can b e preyed upon by more species of birds, this suggests that the decisio n not to leave the burrowing zone might be related to the risk of bein g fed upon by birds. We calculated predation risk from measurements on the density and feeding activity of the crabs, as well as the feeding density, the intake rate and the size selection of the avian predator s. Per hour on the surface, crabs in a flock were more at risk than cr abs feeding near their burrow. Thus, though flocking crabs may have be nefited from 'swamping the predator' by emerging in maximum numbers du ring some tides only, this did not reduce their risk of predation belo w that of non-flocking crabs. Furthermore we found that irrespective o f activity, large crabs suffered a higher mortality per tide from avia n predators than small crabs. This suggests that large crabs could not sufficiently reduce their foraging time to compensate for the increas ed risk while foraging in a flock, even though they probably experienc ed better feeding conditions than small crabs staying near their burro w. The greater energy demands of large crabs were reflected in a great er surface area grazed. Thus, with increasing size a fiddler crab has to feed further away from its burrow and so may derive less protection from staying near to it. It seems that growing big does not reduce th e risk of predation for fiddler crabs, as it does in many other specie s with indeterminate growth. As in such species, the most probable adv antage of growing big is increased mating success. Ultimately, therefo re, prey availability must be understood from the life-history decisio ns of the prey species.