Tuesday, May 19, 2015


You probably saw on your news website of choice last week the discovery of the first “warm-blooded” fish- the opah.

First off, let’s get rid of that term, as we did in big gurl hongry. It’s really not very descriptive, nor is “cold-blooded.” The terms we’re looking for are endothermic and exothermic.

Every animal has a temperature at which it can most optimally do its thang. Ours is 98.6-ish. If it gets too far above that, things start going wrong. If it drops too far below that, things start going wrong.

I can be slow on the
uptake sometimes.
Same thing goes for animals across the phyla- endothermic and exothermic alike. Jellyfish dislike getting too cold just as much as Jack Nicholson.

The real difference in “warm-blooded” and “cold-blooded” animals is the source from which their heat comes. Endothermic animals generate their own heat from inside, while exothermic animals obtain heat from outside their bodies, from the environment. Exo and endo; it’s almost like they planned that.

When I was first learning about all this thermoregulation business, and I heard the sentence “endothermic animals generate their own heat from inside,” an image of tiny campfires throughout my body flashed in my head. But obviously this isn’t right. Really, the heat is a collective result of our cellular metabolisms. Every single living cell in your body is constantly ticking: taking in nutrients, transforming them into proteins and building… well, you. The by-product of all this chemical work is excess energy in the form of heat. So in a way, each cell is acting as a microscopic chemically-powered campfire, putting off tiny amounts of heat that when added up together collectively, keep your tissues at about 98.6.

One advantage of being endothermic is adaptability. Since we make our own heat, we don’t have to depend on one specific environment to maintain it for us. Wanna take a trip to Antarctica? Bundle up and bring some food and you’re good to go. Bring an exotherm to see Santa and no matter how many Northfaces you put him in or flies you feed him, he’s going to freeze to death.

The opah- very pretty!
Another advantage of being endothermic is energy levels. Yes, we have to eat more (some reptiles can go years without eating! I can go about 1.5 hours before I get cranky), but we can not only perform rapid motions, but sustain thems. A frog may be able to quickly hop away, but only for short bursts. When you look at predator species in the endorthermic clades Mammalia and Aves, you’ll see animals that can hunt intensely and for long periods of time before their tissues tire, since their energy reserve is internal. We make darn good hunters because of our thermoregulation systems.

Combine these two advantages, and you’ve got a strong case for while the opah has evolved endothermy. Here you’ve got this predator with morphology fit for cold ocean depths, but with a metabolism that allows it to function in colder waters while simultaneously having the energy capacity of warm water dwellers. And in those colder waters, where it is almost certainly the only endotherm in its food chain, its high energy levels give it the ability to quickly chase down those slow-moving exotherm prey species and eat their asses. It’s kicking butt down there.

No doubt about it- it has evolved away from exothermy. This is the first known fish to be an endotherm. Ever. It’s a big deal.

Let’s welcome our ray-finned, bony friend to the club, and wish him many metabolism-sustaining meals to come. Opah!