ENERGY LIMITS TO BODY-SIZE IN A GRAZING REPTILE, THE GALAPAGOS MARINEIGUANA

Galapagos marine iguana (Amblyrhynchus cristatus) populations show con siderable differences in body size. Adult body mass varies by more tha n 10-fold, and body length (snout-vent length, SVL) varies by approxim ate to 2.2-fold. Predation and interspecific food competition are larg ely absent and can be excluded as potential forces explaining differen ces in body size among populations. This provides an ideal system in w hich to determine how proximate environmental factors affect adult bod y 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 scrambl e competition. Energy availability was lower on Genovesa than on Santa Fe, because of lower marine productivity on Genovesa. The length of g razable algal turf decreased with increasing sea surface temperature ( SST). SST was approximate to 2 degrees C lower on Santa Fe than on Gen ovesa, implying approximate to 1.5 mm lower algae pastures on the latt er. Genovesa showed a fivefold lower standing algal biomass and a twof old lower productivity of algae pastures than did Santa Fe. The smalle st iguanas of each island had approximate to 1.5-fold higher bite rate s during foraging, and their absolute food intake per day was 35% that of the largest iguanas. However, food intake per bite per gram of bod y 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 du ring the two study years, whereas small iguanas (SVL < 170 mm on Genov esa and < 320 mm on Santa Fe) increased in mass, Growth rates of SVL r eflected these findings. A comparison of measured metabolizable energy with published field metabolic rates (FMR) suggested that iguanas abo ve a threshold size were in negative energy balance, because energy in take scaled to body mass with a lower exponent (0.3) than FMR (0.97). Threshold body size was lower on Genovesa than on Santa Fe and differe d between years: In the lean El Nino year (1991/1992), iguanas > 200 m m 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 bet ween islands as a result of energetic limitation. On a given island, f oraging 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 r esembles the grazing succession in ungulate herbivores.