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

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

The biomass of foraging marine iguanas per square meter of the intertidal area was higher on Santa Fe (364 and 397 g[multiplied by][m.sup.-2][multiplied by][d.sup.-1], in 1991/1992 and 1992/1993, respectively) than on Genovesa (108 and 120 g[multiplied by][m.sup.-2][multiplied by][d.sup.-1], in 1991/1992 and 1992/1993, respectively). On Santa Fe, marine iguanas foraged for much shorter periods (23.4 [ or -] 13.6 min/d, n = 219 individual foraging days) than on Genovesa (66.0 [ or -] 21.6 min/d, n = 592 individual foraging days; Mann-Whitney U test, P [less than] 0.001). There were no detectable differences in foraging durations between years (Mann-Whitney U test, P [greater than] 0.05); thus, we lumped data on foraging durations within each island. Within an island, iguanas of different body length classes showed no significant differences in foraging durations (ANOVA, P [greater than] 0.05; Table 2). The mean number of bites per second of a foraging bout differed between iguanas of different body lengths. On each island, smaller iguanas had higher bite rates (Table 2, cf. Wikelski and Trillmich 1994). In combination, this resulted in a higher total calculated number of bites per day for small vs. large iguanas (Table 2). The total number of bites per day was roughly five times higher at Genovesa than at Santa Fe, because iguanas at Genovesa foraged about three times longer per day (Table 2). On each island, the second smallest body length class had the highest food intake per bite per gram of body mass, and thus foraged most efficiently (Table 2). Foraging efficiency decreased on each island for larger animals, but also for the smallest body length class.

Daily food intake

The total food intake (dry mass of algae in stomach) per foraging day increased with body length on each island during each year [ILLUSTRATION FOR FIGURES 3A-D OMITTED]. For the following analyses, log-transformed data of food intake and body length were used (instead of the raw data presented in Fig. 3) to make the residual variation less heteroscedastic. We found no significant heterogeneity of slopes of food intake against body length among islands (ANCOVA: body length x island, P = 0.33) and among years (ANCOVA: body length x year, P = 0.24; the factor effect body length x island x year was not significant, and was discarded). Therefore, we assume that there is one common slope of food intake against body length, under different conditions. A factorial model showed that food intake was influenced by body length, and differed within islands and between islands in different years (ANCOVA factorial model: food intake island (P = 0.04) year (P = 0.37) island x year (P [less than] 0.001) body length (P [less than] 0.01) error; [F.sub.4,107] = 10.6, P [less than] 0.001).

On both islands, iguanas had a higher mean food intake during a "normal" year than during an El Nino year [ILLUSTRATION FOR FIGURE 3 OMITTED]. This was not attributable to differences in body length of the animals sampled in the different years (Mann-Whitney U test for Santa Fe: [n.sub.1] = 25, [n.sub.2] = 46, P = 0.3; for Genovesa: [n.sub.1] = 26, [n.sub.2] = 14, P = 0.97) but to algae blade length. To highlight this relationship, we determined the blade length of algae in the foraging areas from the data on food abundance, using the data points from both islands when algae sampling coincided closest in time with the respective stomach flushing of iguanas. We found that the food intake of iguanas of similar body length (200-250 mm SVL) was predicted from blade length of the algae in the respective foraging area ([ILLUSTRATION FOR FIGURE 4 OMITTED]) linear regression: mass of stomach contents (grams) = 0.61 ([ or -] 0.55) 0.80 ( 0.20) x algae blade length; [F.sub.1,22] = 15.8, [R.sup.2] = 0.43, P [less than] 0.001).