A REAPPRAISAL OF THE AQUATIC SPECIALIZATIONS OF THE GALAPAGOS MARINE IGUANA (AMBLYRHYNCHUS CRISTATUS)

It is tautological to say that an organism is adapted to its environment. It is even tautological to say that an organism is physiologically adapted to its environment. However, just as in the case of many morphological characters, it is unwarranted to conclude that all aspects of the physiology of an organism have evolved in reference to a specific milieu. It is equally gratuitous to assume that an organism will inevitably show physiological specializations in its adaptation to a particular set of conditions. All that can be concluded is that the functional capacities of an organism are sufficient to have allowed persistence within its environment. On one hand, the history of an evolutionary line may place serious constraints upon the types of further physiological changes that are readily feasible. Some changes might require excessive restructuring of the genome or might involve maladaptive changes in related functions. On the other hand, a taxon which is successful in occupying a variety of environments may be less impressive in individual physiological capacities than one with a far more limited distribution. A case in point concerns the functional capacities of the Galapagos marine iguana (Amblyriynchus cristatus), representing the large and diverse family Iguanidae that is easily the major saurian group in the Western Hemisphere. Amblyrhynchus, however, is unique not only among the iguanids but among all lizards in feeding exclusively on intertidal and subtidal marine products, predominantly soft-bodied macrophytic algae. It is of interest to ask how far the marine iguana has departed from general iguanid physiological patterns in following its unique mode of life. The amount of work required to harvest its algal food is of particular interest. These lizards must swim through the surf zone, dive beneath the surface (lungs remain inflated during diving so that the animals must initially overcome their own buoyancy), hold their breath during submergence, surface, and then swim back to shore through the surf (Bartholomew et al., 1976). The exertion associated with this underwater grazing must surpass the foraging of any other large iguanid, all of which are primarily herbivorous (Pough, 1973). The cool water temperatures around the Galapagos Archipelago (15-25 C) potentially hinder development of the intensity of activity required during feeding. All large iguanids so far investigated, including Amblyrhynchus, bask in the sun and reach body temperatures in the vicinity of 35 C. Activity in terrestrial iguanids characteristically occurs at these higher body temperatures, and the Van't Hoff effect would predict greatly depressed rates of functional processes at these lower temperatures. Since physiological investigations have revealed that iguanids possess very little stamina, the locomotor performance required during activity is of particular interest. Other iguanids that have been investigated can sustain vigorous activity for only a minute or two and this activity is accomplished almost exclusively through anaerobic metabolism involving the production of lactic acid, rather than through greatly augmented aerobic metabolism (Moberly, 1968a; Bennett, 1972; Bennett