How was this lizard able to survive?

I started working outside early this morning, and found a fence lizard clinging to a board. The air temperature was close to freezing, and there was frost on exposed surfaces. I touched the lizard, and thought I saw it move ever so slightly, so I picked it up and brought it inside, and put him in a jar. Within a half an hour, he was trying to escape. I am going to wait until the sun is higher in the sky, and put him back outside.

So, how was it able to survive? Even if it didn’t freeze, it’s body temperature must have been close to freezing for several hours.

His body is so small that it’s temperature drops before tissue damage can take place I would imagine.

I have no idea about fence lizards (I’m not a reptile expert), but I know that many lizards can survive cold temperatures, sometimes even a bit below zero. Their metabolism slows pretty dramatically and they may end up lethargic or even unable to move until they warm up, but as long as ice crystals don’t form in their bodily fluids they don’t suffer any permanent damage.

Some frogs can be frozen solid. They have a special type of antifreeze in their blood that not only prevents ice crystals from forming in the first place, but also when ice does form, it forms in the blood first and draws water out of the cells so that the ice crystals don’t damage the cells when they form.

I don’t know if lizards have a similar mechanism or not. Even if they don’t, since their blood isn’t pure water, it can probably drop a bit below the freezing point of water before ice crystals start to form in it.

Assuming lizard blood has a similar concentration to normal saline solution of 0.9% NaCl, the freezing point depression would bring the freezing point 26 F. So the lizard still has a ways to go before ice crystals would form even without special measures.

OK, but even if it didn’t freeze, why did it survive?
Humans die if their core temperature falls below 70°F or so - is the difference due to the lizard being an Ectotherm?

I think that’s largely it. Some lizards hibernate, their body temperature lowers and their metabolism comes close to a stand still. I think all lizards can survive core temperatures well below 70F, they’ll just get slower.

Interesting summary of a study detailing two mechanisms in one species of lizard( note: pdf ) - Freezing Lizards.

Animals are complex beings. Humans for instance die way before anything freezes, because our systems are designed to keep everything at the same temperature, and when the temperature decrease counter measures fail to work everything goes hay-wire. If recovery is managed properly we can revive people who’ve been cooled way beyond the “death point” with modern techniques, so it’s not that we break stuff.

A lizard evolved to survive in places with sub-freezing temperatures will, as other’s have mentioned, have systems that just slow down and keep slowing down. But they don’t stop, so even without the amazing systems mentioned in TamerLane’s cite, they will still be metabolising, which could keep them just a nudge above ambient.

That’s a good part of it. As has been said, in homeotherms like mammals all physiological processes are set up to run efficiently at a single temperature (which is one of the main advantages of being a homeotherm). In poikilotherms (animals with variable temperatures) like lizards physiological processes are more forgiving and can still run even when temperatures are much lower than optimal.

And cold tolerance varies considerably between temperate and tropical reptiles. Temperate zone reptiles have to be able to survive near-freezing temperatures in order to survive there. Tropical reptiles suffer much higher mortality if subjected to such temperatures. For example, introduced Burmese pythons in the Everglades are often killed by cold snaps when native snakes survive perfectly well.