Field Energetics and Food Consumption of the Galápagos Marine Iguana, Amblyrhynchus cristatus

Abstract

Metabolic rates (doubly labeled water), daily time budgets, and feeding rates were determined for free-ranging marine iguanas on Isla Fernandina, Galápagos. Field metabolic rates were related to body mass according to the equation kJ/day = 0.079 for all sizes of lizards, and kJ/day = 0.64 for adults only. Hatchlings had surprisingly low field metabolic rates. The daily energy expenditure of a representative 1-kg adult (70.3 kJ/day) was 1.7 times resting metabolism. Metabolic cost of basking (1.9 times resting at 35 C) accounted for 37% of daily energy expenditure; foraging cost was only 8% of the daily cost of living. Foraging efficiency (metabolizable energy gained while foraging/energy spent while foraging) was 12.8, much higher than insectivorous lizards studied to date, but similar to two other herbivorous lizards that forage on land. The plant-eating lizards spend only about 1 h/day foraging, so their foraging expenses are much lower than those of insectivorous lizards which forage 3.5-10 h/day. Adult Amblyrhynchus consumed only algae (primarily Ulva lobata), and about 79% of the energy in their diet was available for metabolism. A 1-kg adult consumed about 8.6 g dry mass (or 37.4 g fresh mass) of food per day. The population of marine iguanas on Punta Espinosa (ca. 1,885 animals) would consume about 27,000-29,000 kg of fresh algae each year.