How iguanas eat, swim and stay warm

In the morning, a group of marine iguanas lies flat on the sand, as if recovering from an all night bender. After baking for a few hours, they slowly raise their craggy heads and start making their way to the shore, gathering in colonies near the water’s edge. Periodically, one slips into the sea and swims off with a serpentine swish of its tail. They return later to sunbathe on the rocks in the heat of the day.

The clock that times the daily routine is their body temperature, which rises and falls with the environment around them. After a cool night, their goal is to raise their temperature from a low of about 25 to a comfortable 37 degrees Celsius. By laying prostrate on the sand or atop a dark lava boulder – known as their “flat basking posture” – they maximize both the warmth streaming down from above and the conduction of heat from below (White 1973). Once they reach their optimum temperature, they need to start cooling down.

To do that, they orient themselves parallel to the sun, minimizing their direct exposure. Look at a group of iguanas in the late morning or early afternoon and you’ll see most of them are pointing in the same direction. Switching to their “elevated basking posture”, they raise up on their front legs to allow the breeze from the trade winds to pass beneath them. While the temperature of the rocks continues to climb past 50 degrees, they are able to maintain themselves at a comfortable 38 or so.

The reason they take a dip in the water is not to cool down but to eat. Marine iguanas feed on algae clinging to rocks in as much as several meters of water. They are able to graze under water for 20 minutes or more at a time. Younger and smaller iguanas tend to feed in the intertidal zone, where the rocks are temporarily exposed and they don’t have to strain against oncoming waves. Larger iguanas often feed in deeper water, where the algae is richer. Their sharp claws enable them to cling to the rocks while feeding. Temperature plays a role here, too. Larger iguanas have less surface area relative to their mass, so they cool down more slowly. The body temperature of larger iguanas takes about an hour to match the water temperature, compared to just 10 minutes for the smaller ones (Bartholomew 1966). Larger iguanas feed longer and less often – once every few days. Smaller iquanas feed for shorter, more frequent stretches, retreating to shore to warm up in between (Wilelski & Trillmich (1994).

By the time marine iguanas return from lunch, their body temperature is approaching that of the water. Not coincidentally, they tend to return from feeding in the hottest part of the day, resuming their flat basking posture on the rocks to quickly warm back up.

Literature

Bartholomew, G.A. (1966). A field study of temperature relations in the Galapagos marine iguana. Copeia 2: 241-250.

White, F.N. (1973). Temperature and the Galapagos marine iguana – insights into reptilian thermoregulation. Comp. Biochem. Physiol. 45A: 503-513.

Wikelski, M. & Trillmich, F. (1994). Foraging strategies of the Galapagos marine iguana (Amblyrhynchus cristatus): Adapting behavioral rules to ontogenetic size change. Behavior 128(3): 255-279.