Last week, I talked about how cold blooded animals could grow huge in hot environments. Today, I think it would be interesting to talk a bit more about what it means to have, “cold blood.”

What does it mean to be cold blooded?
It has nothing to do with Rick James. Having cold blood basically means that the animal depends on ambient temperature to power its metabolism. Outside heat does everything from getting enzymes running to warming up muscles. While some animals do use geothermal vents for this, heat from the sun is the most common source.

What kind of animals are cold blooded?
The simple answer is most fish, and all present day reptiles and amphibians. Birds and mammals are, by design, endotherms. (There is exactly one mammal that doesn’t apply to this rule-read more about God’s mistake; the naked mole rat.)

Any exceptions?
You bet. Tuna actually produce their own body heat through working their muscles. This allows them to be fast, powerful swimmers even in colder waters. This phenomenon is called poikilothermy. Other cold blooded animals manage to keep a more or less constant body temperature simply by merit of being huge. Because of a greater mass and proportionally less surface area, animals that exhibit gigantothermy stay warm in colder areas. Everyone’s favorite gigantotherm is the Great White Shark, but bigger sea turtles, and perhaps dinosaurs had this trait too. It was unlikely that last week’s Titanoboa was a gigantotherm, because a slender, long shape has a comparatively high surface area.

Despite being cold blooded, Great Whites are capable of sustained, acrobatic activity through their size alone. Courtesy of Charles Luyt

What are the pluses of being cold blooded?
To sum it up in a sentence; being cold blooded means you don’t need to spend a lot of energy to get energy. To keep your body temperature up at a constant 98.6 degrees, which allows our enzymes, muscles, and brains in tip top shape 24/7 takes a lot of energy. We eat tons more food per year compared to a similar sized ectotherm. Even giant reptiles like Nile Crocodiles and Anacondas only need one to two big meals per year. Since less energy is being spent finding another meal, more energy can be spent on reproduction.

This guy weighs about 1200 pounds and eats about one or two 500 pound wildebeest per year. For comparison, if you weigh about 180 pounds, you eat nearly a ton of food per year. Picture courtesy of Marco Schmidt.

What are the dangers of being an ectotherm?
For one thing, cold blooded creature don’t have nearly as much stamina as warm blooded ones. This is because a fast metabolism quickly regenerates ATP and gets rid of harmful acids that build up during sustained activity. The second danger, which I went over last week, is that warming up each day can be dangerous; time spent absorbing the sun’s rays is not time spent avoiding predators.