Foraging strategies of the marine iguana,Amblyrhynchus cristatus

Summary Two foraging strategies were found in marine iguanas (Amblyrhynchus cristatus); (1) subtidal feeding: the animals swam out to sea and dived for algae in the subtidal zone; (2) intertidal feeding: the animals foraged around low tide in the intertidal zone on more or less exposed algae. Most marine iguanas were very consistent in their foraging strategy and so could be classified as subtidal feeders (SFs) or intertidal feeders (IFs). Feeding strategy was weight-related (Fig. 1), not sexspecific. Animals 1,200 g were IFs, animals >1,800 g SFs. Some iguanas in between followed a mixed foraging strategy. SFs foraged independently of the tides, IFs always around low tide (Figs. 2, 3). Feeding time patterns of IFs and SFs are described (Table 1). Sea motion seemed to have little effect on the foraging pattern of SFs, but strongly influenced that of IFs (Fig. 2). The smaller a marine iguana, the faster it cooled when immersed in water (Fig. 4). The difference between water temperature and core temperature of animals returning from foraging was significantly greater in IFs than SFs (Fig. 5). SFs swimming in very cold water regulated their body temperature to prevent excessive cooling. Possible costs and benefits of the two foraging strategies are discussed. Only part of a marine iguana population lives really amphibiously and only ca. 5% of a 24 h day is spent close to or in the water. All social activities, including mating, take place on land. These life history characteristics preclude those adaptations to an amphibious way of life that would at the same time reduce the iguanas' ability to be maximally active at their typical terrestrial body temperature of 35° C.