Cold-blooded in cold water

I’ve recently watched several BBC clips about the marine iguanas of Galapagos, and all of them make a big deal about the danger the cold water poses to the cold-blooded iguanas. This clip goes into the most detail:

Down on the shoreline live the most extraordinary of the island’s many strange inhabitants: marine iguanas, the only saltwater lizards in the world.

They eat algae — seaweed — growing on rocks between the tides, so they have to wait for the water to go down before they can feed. They live only on shores exposed to cold currents. The arrival of the cold water is a double-edged sword. Its nutrients stimulate the growth of the algae they eat, but because the lizards are cold-blooded, cold water slows them down and could even kill them. The best algae grow lose to the low tide mark, so it’s a race to eat all they can before the rocks are covered again and their bodies are chilled to danger point. Strong claws and a good grip are essential if you’re not to be swept away.

For most iguanas, life is ruled by the tides, but the big males have another option. Below the low tide mark, the growths of algae are more luxurious because the rocks are always covered. The males use the heat of the tropical sun to exploit them. They expose the greatest possible surface to its warming rays. Because their bodies are larger, they can store more heat and don’t chill down so quickly. When they’ve warmed to an optimum of 25 [or 35? unclear pronunciation] degrees, they take to the water. Down here they can take advantage of a food supply that’s out of reach for the smaller iguanas. They can hold their breath for 20 minutes or more, but they have to feed fast. Every minute they spend here, the heat is draining from their body. If their temperature falls too low, they’ll be unable to move, and they’ll die. It’s time to go.

As the voiceover  goes on about how dangerous the cold seawater is to the iguanas (on account of they’re cold-blooded, you see), the camera shows plenty of equally cold-blooded fish swimming around without any problem. The obvious question this raises is never addressed. The program talks as if the iguanas’ way of life were almost unheard of — “Get this!” it seems to say, “a marine animal that’s cold-blooded!” — when in fact ectothermy is the norm for marine life, even in the coldest parts of the sea. (What do penguins eat? Fish, squid, and crustaceans.) Warm-bloodedness is a terrestrial trait which has never evolved in aquatic animals. The only endotherms in the sea are whales, seals, penguins, and other air-breathing animals whose warm blood is a legacy of their terrestrial ancestors. (That’s actually pretty strange, when you think about it, given that the sea — especially the deep sea — tends to be much colder than the land. My guess at an explanation would be that the ambient temperature doesn’t change as much in the sea as it does on land, so marine animals can just adapt to whatever the ambient temperature happens to be. On land, there are seasons and weather to cope with.)

So why is cold-bloodedness such a big problem for iguanas but not for fish? At first I thought it might have something to do with the antifreeze proteins some fish (but not reptiles) have — but antifreeze proteins are only for dealing with water so cold that it would otherwise literally freeze a fish’s blood, and, according to this map, the sea around the Galapagos, while certainly a bit chilly by equatorial standards, isn’t anywhere near cold enough for freezing to be an issue. According to the range map here, cold-blooded sea turtles seem to do just fine without antifreeze both in the Galapagos area and in waters a good 10 degrees cooler.

So far I haven’t figured this out — this post has been in my drafts folder for quite some time now waiting for me to find the answer — but I’m going to go ahead and post it in hopes that some knowledgeable person will happen upon it and leave an enlightening comment. I’ll keep reading and thinking and post again if I find anything that sheds any light. In the meantime, here are some more entertaining marine iguana clips.

With an annoying sea lion:

With lots of spitting:

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