ALGAL FOOD PREFERENCES AND SEASONAL FORAGING STRATEGY OF THE MARINE IGUANA, AMBLYRHYNCHUS CRISTATUS, ON SANTA CRUZ, GALÁPAGOS

The abundance of intertidal algae, algal food preferences, and foraging behavior of the marine iguana, Amblyrhynchus cristatus Bell, were studied at two sites on Santa Cruz Island, Galapagos, over a tropical cool and hot season. At both sites iguanas had a consistent, seasonally changing hierarchy of preferred algal species, selected according to their availability. During high low tides, Ulva lobata (Kutzing) Harvey, usually avoided, was eaten more often because preferred red algae were submerged and unavailable. At other times, one or other of the 4–5 red algal species, seasonally abundant at the sites, were preferred. Feeding preferences changed from the cool to hot season, as algal biomass and cover declined. Foraging behavior also changed between seasons. Only in the cool season did iguanas delay their arrival when low tide was early in the morning, but they anticipated late afternoon low tides. Foraging efficiency increased with temperature and increasing algal biomass. At the site of high algal abundance, bite rates increased and feeding duration decreased, with increasing ambient temperature. At the site where algae were scarce in the hot season, both bite rates and foraging duration increased with increasing temperature. The proportion of time that iguanas on the feeding ground were engaged in feeding also varied seasonally, and increased during high swell and high low tides, which compensated for less grazing time. The total number of feeding bites per day declined with temperature at the site of high algal abundance, but increased with temperature at the site of algal scarcity. Thus, feeding behavior optimized the intake of preferred species at the highest rate of re-warming within constraints imposed by tide and swell. Large males maximized food intake and feeding efficiency by submerged feeding in rock pools. We present a variable “sawtooth” model to explain differences in foraging duration according to seasonally varying algal abundance and temperature. The Galapagos Islands are a dynamic environment. Periodic El Nino events in crease sea temperatures 2–5 o C, and reduce algal abundance in the inter- and shallow subtidal. The marine iguana, Amblyrhynchus cristatus Bell, unique among reptiles in feeding almost exclusively on marine macroalgae (Darwin, 1845), is variously stressed by El Nino events, when only algae of poor food quality persists (Laurie, 1989), and by expanding anemone barrens that smother grazing habitats (Okey et al., 2003). Hence, it is important to understand the algal diet of iguanas, and the variability in algal abundance. Previous studies have rarely considered the botanical aspects of iguana foraging. Here we briefly summarize earlier studies on iguana diet, provide information on algal abundance and seasonality at two feeding sites on Santa Cruz, Galapagos, and describe the food, feeding preferences, and foraging behavior of the iguana over 8 mo spanning a cool and a hot season. At these sites iguanas have low densities of < 1000 per km of shore, skewed toward older age classes, because of high predation on hatchlings by feral dogs and cats (Laurie, 1983). We undertook this study of the seasonally changing availability of algae and the iguanas’ responses in order to reveal algal preferences and other factors influencing diet. We collected quantitative data