Energy limits to body size in a grazing reptile, the Galapagos marine iguana

Ecology, Oct, 1997 by Martin Wikelski, Victor Carrillo, Fritz Trillmich

INTRODUCTION

Why do animals have certain body sizes, and how is the actual body size of an animal determined? In answering these questions, one can gain insight into the action of selective pressures that form the life history of animals (Peters 1983) and also into the proximate constraints that finally set limits to body sizes (e.g., Van Valen 1973, Schmidt-Nielsen 1984, Maurer et al. 1992, Brown et al. 1993). In explaining body sizes of animals, one usually has to deal with a wide array of selectional forces and mechanisms for which the influence on body size is difficult to quantify (Dunham et al. 1978, Gaston and Lawton 1988), such as predation pressure (Owen-Smith 1993) or interspecific competition for food (Illius and Gordon 1987, 1992). By studying two island populations of Galapagos marine iguanas (Amblyrhynchus cristatus), a herbivorous reptile, one can exclude some of these factors from the outset. Predation is absent there for animals larger than hatchling size. Genovesa Island has no terrestrial predators; hawks and owls occur on Santa Fe, but, only the former prey on marine iguanas, and only on hatchlings and egg-laying females. Such predation pressure is low, as only a few females fall prey to the Galapagos Hawk every year (Curio 1965, Laurie and Brown 1990b). There is no interspecific food competition (Trillmich and Trillmich 1986), food abundance is quantifiable by height and degree of cover of the algal pasture (Wikelski and Trillmich 1994), and food patches are not defendable (Trillmich and Trillmich 1984, 1986). Marine iguanas occur on all islands of the Galapagos archipelago. The island populations show pronounced differences in adult body mass, ranging from island maxima of 1 kg to 12 kg body mass (Laurie 1989; W. A. Laurie, personal communication). Similar phenomena have been observed in lizards (see Schoener 1969, Case 1976, Pregill 1986, Case and Schwaner 1993). Strictly herbivorous throughout life, marine iguanas feed on macrophytic marine algae in the intertidal zone or by diving (Darwin 1883, Trillmich and Trillmich 1986, Wikelski et al. 1993, Wikelski and Trillmich 1994, Wikelski and Hau 1995). Large size confers a mating advantage to males (Trillmich 1983, Wikelski et al. 1996) and a fertility increase to females (Laurie 1990, Dellinger 1991). We wanted to explore why body size differs so widely among islands and which factors influence growth and final size of marine iguanas on Santa Fe and Genovesa. Santa Fe lies in the center of the Galapagos archipelago and was chosen because, building on the initial work of Laurie (1989), we had already followed that population for 10 years at the beginning of this study. Genovesa was chosen for comparison because it is home to the population of smallest marine iguanas on the Galapagos. In addition, this island is more tropical than the central islands of the archipelago and, therefore, is more frequently exposed to warm, equatorial waters that move in around the island whenever the intertropical convergence zone moves slightly south (Houvenaghel 1978)

Our analysis focuses on proximate mechanisms that may cause different selection regimes and, hence, differences in adult body sizes between islands. We approach this problem by quantifying foraging efficiencies for different-sized iguanas, using mass-specific food intake as unit of measurement (Wikelski et al. 1993). We then compare this with an independently derived measure of physiological performance of different-sized iguanas (growth and change in body mass). Finally, we scale our findings against published data on energy expenditure in this species (Nagy and Shoemaker 1984), and explain why larger animals (but also hatchlings) of each island suffer higher mortality rates than do smaller ones (Laurie and Brown 1990a, Wikelski 1994, Wikelski and Trillmich 1997).

METHODS

Physical setting and study animals

Marine iguana foraging ecology was studied intensively on the two islands, Genovesa and Santa Fe, at the study sites "Salvaje de Corazon" (89 [degrees] 59 [minutes] W, 0 [degrees] 19 [minutes] N) and "Miedo" (90 [degrees] 02 [minutes] W, 0 [degrees] 50 [minutes] S), respectively, from 1991 to 1994. These islands differ in their position relative to the intertropical convergence zone (ITC). Genovesa is much more affected by warm waters from the Panama area, which seasonally influence the Galapagos (Houvenaghel 1978, Feldman 1986), and is less exposed to cold, upwelled waters from the Humboldt and Cromwell currents (Fahrbach et al. 1991). Upwelled waters are important for the nutrient supply of algae (Houvenaghel 1978).

Both study sites initially possessed high-density iguana populations ([greater than]2000 iguanas/km of coastline). The total population size on the Genovesa study site decreased from [approximately equal to]2500 iguanas in spring 1991 to 250 in spring 1995; on Santa Fe, it fluctuated around 7000 individuals. About one-third of the animals of both populations were marked individually by branding (details of methods in Laurie 1989, 1990). Both sites held extensive intertidal feeding grounds, which consisted of mostly steep cliffs in Santa Fe, but rock flats in Genovesa.