IS THERE AN ENDOGENOUS TIDAL FORAGING RHYTHM IN MARINE IGUANAS

As strictly herbivorous reptiles, Galapagos marine iguanas graze on al gae in the intertidal areas during low tide. Daily foraging rhythms we re observed on two islands during 3 years to determine the proximate f actors underlying behavioral synchrony with the tides. Marine iguanas walked to their intertidal foraging grounds from far-off resting areas in anticipation of the time of low tide. Foraging activity was restri cted to daytime, resulting in a complex bitidal rhythm including consp icuous switches from afternoon foraging to foraging during the subsequ ent morning when low tide occurred after dusk. The animals anticipated the daily low tide by a maximum of 4 h. The degree of anticipation de pended on environmental parameters such as wave action and food supply . ''Early foragers'' survived in greater numbers than did animals arri ving later at foraging sites, a result indicating selection pressure o n the timing of anticipation. The timing of foraging trips was better predicted by the daily changes in tabulated low tide than it was by th e daily changes in actual exposure of the intertidal foraging flats, s uggesting an endogenous nature of the foraging rhythms. Endogenous rhy thmicity would also explain why iguanas that had spontaneously fasted for several days nevertheless went foraging at the ''right'' time of d ay. A potential lunar component of the foraging rhythmicity of marine iguanas showed up in their assemblage on intertidal rocks during neap tide nights. This may indicate that iguanas possessed information on t he semi-monthly rhythms in tide heights. Enclosure experiments showed that bitidal foraging rhythms of iguanas may free run in the absence o f direct cues from the intertidal areas and operate independent of the light:dark cycle and social stimuli. Therefore, the existence of a ci rcatidal oscillator in marine iguanas is proposed. The bitidal foragin g pattern may result from an interaction of a circadian system with a circatidal system. Food intake or related stimuli may be used as tidal zeitgebers in synchronizing the foraging rhythms of these reptiles un der natural conditions.