# Can you boil a frog alive, by turning up the heat so slowly it never notices?

This image is often used as a metaphor for public complacency in the face of environmental damage, erosion of civil liberties, or any other danger that might come on by gradual and barely perceptible stages. But I have no idea if it works IRL. On frogs, I mean.

No, it’s completely false. See Snopes: Boiled Beef

In Herpetology class we had a lab where we determined the critical thermal maxima for various salamander species. Once the water became hotter than their comfort range they began to swim around actively in an attempt to get out. I have no doubt a frog would do the same thing.

As a logic exercise, it makes no sense. Are creatures only able to sense a high-level rate of change of an environmental temperature but not the instantaneous value? It seems more likely that when it got hot, they’d say, “Jeez, it’s hot in here!” and take appropriate measures no matter how fast or slow it happened.

Anyone know the origin of the metaphor/image/story?

Actually, to some degree this is true, even in humans. If the rate of change is slow enough, your body has a hard time detecting the change. Humans (and most animals, as far as I am aware, but I’m no expert) are good at detecting fast changes in temperature, but not so good at determining absolute temperature. An easy way to prove this is stick one hand in cold water and one hand in hot water until both hands get used to the temperature they are in. Then run both hands under a faucet at a temperature in between those two extremes. To one hand, the faucet water will feel warm, and to the other hand the water will feel cold.

This effect does not work to silly extremes like frog boiling, though. While it’s easy to trick your body into being off by 5 deg C or maybe a bit more when guessing absolute temperature, it’s harder to trick it to be more than 10 deg C off. Additionally, once you get over a certain temperature, pain receptors start firing and alert you to the fact that you are getting too hot. Humans have two pain receptors related to heat. One fires off at about 45 deg C. The other doesn’t kick in until about 50 deg C (I think - somewhere around there). The first one also fires off in the presence of certain spices, which is why “hot” peppers seem hot to us. Many animals, like birds, are missing the first type of heat pain receptor, which is why birds can eat hot peppers and not be bothered by them.

I wasn’t sure which types of pain receptor frogs had, so I did a bit of googling. I found this:

This is an article entitled “Cellular Mechanisms of Nociception in the Frog” published in the journal of neurophysiology. As you can imagine, this isn’t a nice friendly article written in small words for us dummies to understand. But, as far as neurophysiological articles go, it is actually easy to understand. I designed equipment for a neurobiologist many years ago, and I can tell you that I’ve certainly read worse.

This quote is particiarly relevant:

What this is saying is that frogs don’t respond to hot peppers, but they do respond to heat at about 50 deg C. This means that Kermit is going to jump out of the pot long before it boils, and in fact will jump out of the pot before he even suffers any serious damage from the heat.

Incidentally, the “critical thermal maximum” mentioned in the Snopes article is defined as the temperature at which an animal’s locomotor responses become so disorganized that it can no longer escape from conditions that will eventually lead to its death. In other words, it is measured in the laboratory by noting the temperature at which the animal stops moving (or can no longer move in a coordinated fashion), so the technique would not work if animals did not normally try to escape. (I should mention that when an experimental animal reaches this point, it is immediately removed and placed in cool water until it recovers. Animals rarely actually die in the course of such experiments.) The thermal maximum for frogs is often below 30 C (86 F), thus far below boiling.

Some past threads on this though I don’t think there’s anything there that isn’t covered in this one. I’ve added this to my Unofficial GQ FAQ.