Marine iguanas face a unique challenge in that it is most active during diving, while other lizards only submerge themselves for hiding purposes. Marine iguanas must fight the strong currents while foraging, and in doing so face many energetic and thermoregulatory challenges.

Marine iguanas initially dive with their lungs inflated, having to overcome buoyancy in order to reach algae on the rocky sea bottom. However, once they reach deeper waters they achieve neutral buoyancy that enables them to swim more freely. In shallow waters, buoyancy is not much of an issue as the iguanas use their curved claws to hang on to the algae (Bartholomew et al. 1976).

So, how do marine iguanas survive and forage so efficiently in the cold waters?

• First, marine iguanas have a particularly high lactate burden, such that they can endure long periods of diving while still able to forage effectively. After 2 minutes of burst swimming (sprinting) and 5 min of cruise swimming, the mean increase in lactate levels is 60 mg%, much lower than the 100 mg% found in other animals put under similar conditions (Bartholomew et al. 1976).
• Second, marine iguanas combine periods of burst and cruise swimming in order to minimize lactate levels and maximize performance (Bartholomew et al. 1976). Bartholomew et al. predict that cruise swimming is sustained aerobically while burst swimming is sustained anaerobically.
• Finally, marine iguanas drastically lower their heart rate during dives. This, along with vasoconstriction, keeps blood flow restricted to the core so that cooling due to the cold waters is reduced (Bartholomew and Lasiewski, 1965).

Figure 1. Heating and cooling rates in water and air. Change in temperature is difference between body temp and ambient temp. During heating, ambient temperature is 40 C, during cooling, ambient temperature is 20 C (Adapted from Bartholomew and Lasiewski 1965).

A marine iguana dives to the bottom to feed of algae

Photo used with permission from Rob Stewart, National Geographic