Fish: Cold-blooded? Warm-blooded? Other?

The four year old popped up the question over dinner tonight. I honestly hadn’t ever thought about it.

My initial thought was cold-blooded but then I’m thinking…those fish I clean come out of the river pretty damn cold. If they were ectothermic they’d never get anywhere. So then I’m thinking warm-blooded. But I lose my nerve and decide that maybe the entire spectrum just doesn’t apply to fish.

Then Lady Chance says ‘Go ask the dopers and shut up.’

How 'bout it?

Fish are cold-blooded. Hence the term “cold fish” for person who displays few emotions.

According to popular terminology, most fish are "cold-blooded. " More technically, they are ectotherms, receiving their body heat from the outside environment. Their internal temperature is usually within a few degrees of the external temperature. They can be active at low temperatures because their metabolism is adapted to function well at these temperatures.

However, a few fast-swimming fish such as tuna generate a lot of metabolic heat while swimming and function as ectotherms to some degree.

For more info, see the Staff Report by me and Doug:

What makes some animals cold-blooded and others warm-blooded?

There’s an interesting article here about a few fish who have the ability to raise their body temperatures. However, even these fish are still considered to be ectotherms.

Oops. I meant to say “function as endotherms to some degree.”

Maybe I’m oversimplifying , but I have forgotten a lot of biology from college days…

Are only mammals considered “warm-blooded”? In my brain, this is the defining line…

No. Amongst the living aninmals mammals and bviords are both warm blooded. There were various other extinct groups that were also almost certinaly warm blooded.

Read the article. “Warm-blooded” is an imprecise term. However, birds would definitely also be considered warm-blooded.

OK, so fish are ectothermic (for the most part) but survive conditions that would quickly cause an land-based ectotherm to go dormant due to a different metabolism designed to function at lower temperatures?

Preeee-cisely! Think of the difference between the Shuttle Transport Vehicle and a Formula 1 racecar. Each is built to do quite different things. Fish metabolisms are built to function at temperatures from 34 to 80 (F) or so.

FWIW, there is a spectrum between ectothermy and endothermy (and someone with expertise can break out the alternate poikilothermy and homeothermy distinction and clarify the differences in the four terms, if they so choose). But there isn’t a huge supply of creatures spanning the gap. It’s quite possible that the extinct archosaurs thought to be “warm-blooded” were actually much like the living non-avian archosaurs, the crocodilians, which have limited control over body temperature, and a functional four-chambered heart (which seems to be an element in land-dwelling endothermy). While a croc or gator will go torpid in cold weather, it will remain active at moderate-to-cool temperatures that would induce torpidity in a snake, turtle, or lizard.

Yes. The problem for ectotherms is that if they are to function at low temperatures, they must have a suite of metabolic enzymes and other components designed to work well together at those temperatures. When the external temperature goes up, the enzymes get out of sync (the activity of each one will change in a somewhat different fashion as the temperature rises) and cause metabolic problems. A low-temperature fish will become stunned and even die if placed in water that is too warm for it. Some fish can live at a range of temperatures, but it takes them some time to acclimate - they can’t do it over a short period.

In contrast, endothermic homeotherms (“warm-blooded” animals) keep their body temperature within a narrow range. Therefore they can make do with a single set of enzymes closely adapted to work well together at that single temperature.

Some terrestrial ectotherms are well adapted to function at low temperatures. For example, the Wood Frog Rana sylvatica, which ranges into Alaska, becomes active as soon as the snow melts and may begin breeding when there is still ice on the ponds. The Tuatara of New Zealand is also active at relatively low temperatures for a reptile.

The penalty for this is that low-temperature ectotherms cannot stand very high temperatures. Instead, many temperate-zone reptiles are adapted to be maximally active during the warmest parts of the summer, and metabolically “write off” the cooler parts of the year.

If I remember my comparative vertebrate anatomy correctly, isn’t a 4-chambered heart a pre-requesite to be considered warm blooded? I know the crocodilians are on the cusp, but they’re still ectotherms, correct?

As the Staff Report indicates, “warm-blooded/cold-blooded” are rather imprecise terms, and no longer used by biologists (at least formally), so it’s not clear what would be the criterion to “consider” something as being warm-blooded.

According the traditional use, the only real prerequisite to being considered “warm-blooded” would be to be endothermic (with the caveat that some animals that are somewhat or sometimes endothermic, such as tuna, mackeral sharks, and honeybees, are not traditionally considered to be warm-blooded.) The animals that are traditionally considered to be warm-blooded are also more-or-less homeothermic most of the time (although some mammals such as monotremes and edentates aren’t able to control their temperatures as well as other mammals, and some homeotherms may vary in body temperature from their normal condition during hiberation or torpor.)

Warm-blooded animals “traditionally” include only mammals and birds, all of which have a four-chambered heart. This kind of heart is needed for full oxygenation of the blood (preventing mixing of oxygenated blood coming from the lungs with that coming from the body). As such it is connected with the high metabolic rate associated with endothermy. However, as I mentioned tuna and some sharks are to some degree endothermic, and these do not have four-chambered hearts. (Offhand I am not aware if they do have some modifications to their heart anatomy to help separate oxygenated and deoxygenated blood).

Crocodilians are ectotherms, but have a functionally four-chambered heart. The difference from mammals and birds is that they have an internal valve in the heart that allows blood to bypass the lungs during diving (when the lungs are not being used). This is actually a more sophisticated system than that found in mammals in birds, rather than one “on the cusp” of achieving a fully separate pulmonary and systemic circulation. Crocodilians retain a connection between the systems because it is adaptive with regard to their amphibious life style.

This article contends that the crocodilian heart is the most advanced heart we know of.

…And the warm blood flows through the large four-chambered heart
Maintaining the high metabolism rate they have…