ENERGY LIMITS TO BODY SIZE IN A GRAZING REPTILE, THE GALAPAGOS MARINE IGUANA

Abstract

Galapagos marine iguana (Amblyrhynchus cristatus) populations show con- siderable differences in body size. Adult body mass varies by more than 10-fold, and body length (snout-vent length, SVL) varies by -2.2-fold. Predation and interspecific food com- petition are largely absent and can be excluded as potential forces explaining differences in body size among populations. This provides an ideal system in which to determine how proximate environmental factors affect adult body size. We compared the small iguanas from Genovesa Island (mean adult male SVL 250 mm) to the larger Santa Fe iguanas (mean adult male SVL 400 mm). Marine iguanas forage on intertidal algae pastures in scramble competition. Energy availability was lower on Genovesa than on Santa Fe, because of lower marine productivity on Genovesa. The length of grazable algal turf decreased with increas- ing sea surface temperature (SST). SST was -20C lower on Santa Fe than on Genovesa, implying 1.5 mm lower algae pastures on the latter. Genovesa showed a fivefold lower standing algal biomass and a twofold lower productivity of algae pastures than did Santa Fe. The smallest iguanas of each island had 1.5-fold higher bite rates during foraging, and their absolute food intake per day was 35% that of the largest iguanas. However, food intake per bite per gram of body mass was about twice as high for small iguanas as for large iguanas. Large iguanas of both islands showed a marked decline in body mass during the two study years, whereas small iguanas (SVL 200 mm SVL on Genovesa and >310 mm SVL on Santa Fe significantly lost mass; in the more productive year (1992/1993), thresholds were 230 mm and 350 mm SVL, respectively. Thus, food abundance (length and turnover of algal swards) explained differences in adult body length and mass between islands as a result of energetic limitation. On a given island, foraging efficiency (intake/bite) explained the negative energy balance of large compared to small iguanas. This also explained why, on both islands, the largest animals suffered higher mortality rates than did medium-sized ones when food was scarce. The finding that small animals outcompeted larger ones because of their higher foraging efficiency resembles the grazing succession in ungulate herbivores.