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Ontogenetic development in reptiles entails major changes in size-related foraging options. We studied the changes in foraging behavior of marine iguanas. In this species, size increases about twenty- to hundredfold from hatching to full adult size. The foraging strategy of marine iguanas was studied at Miedo on Santa Fe Island in the Galapagos archipelago During low tide, large marine iguanas (>250 mm snout vent length (SVL)) foraged more in the lower intertidal than small ones (<250 mm SVL) which preferred the upper intertidal with higher temperatures and less frequent wave washing. Animals usually returned to the same foraging site day after day and had lower food intake after changing their foraging site. Feeding accounted for 60% of the time spent in the intertidal. Smaller animals fed every day, larger ones only every other day. Smaller individuals shuttled faster between foraging and basking sites than larger ones. Total feeding time per day was, however, the same for both size classes. At neap tides (= high water level at low tide) animals had shorter foraging bouts than at spring tides with much lower water levels at peak low tide. Length of feeding bouts depended most on wave action, time of low tide (during the daylight period), and body mass of an animal. Small animals fed significantly less at higher than at lower wave activities. All animals on Santa Fe spent more time feeding in the intertidal than in the subtidal. Only large males additionally foraged subtidally and the more so the bigger they were. During the reproductive season, territorial males were less likely to go foraging, but when feeding, territorials fed more subtidally than non-territorials, went foraging earlier, and spent less time foraging in the IT than non-territorials. Ability to resist wave drag increased with body size but did not decrease at lower body temperature, whereas running speed did so significantly. Bite frequency during foraging also decreased with decreasing body temperature and smaller, younger animals had higher bite rates than older, bigger ones. White-painted animals rewarmed slower than naturally black ones and partially compensated for this by shortening foraging bouts but increasing their number. The observed age-related changes in foraging behavior can be explained by postulating a rule of the form 'forage while warm and warm up when getting inefficient at grazing'. Of course, animals will also stop feeding should the stomach be filled before the end of the low tide cycle. To explain age- and motivation-related differences in foraging behavior, the only change that needs to be postulated is in the thresholds of body temperature inducing switches from foraging to warming-up and back. These changes are adaptive responses to size-related changes in costs and benefits of foraging in a cool, wave-washed environment.