|"I wouldn't be caught dead in that fur coat you're wearing". Photo by Naypong at freedigitalphotos.net.|
- Conduction is when heat moves from a hotter area to a colder area across a still surface. If you stand barefoot on a cold sidewalk, the heat in your feet is going to transfer to the cooler surface of the sidewalk by conduction and you will get cooler (which is nice in the hot summer, but uncomfortable when the weather starts to get chilly). Conduction can happen when the body is in contact with a solid (like a sidewalk), a liquid (like a bath), or a gas (like the air around you).
- Convection is essentially conduction with movement, and this movement makes the transfer of heat even faster. If you are standing inside and it is 70ºF in the building, you will likely be fairly comfortable. But if you are outside on a windy 70º day, even though the environment is the same temperature, you will get colder faster.
- We are all familiar with the warming effects of the sun's radiation, but in reality, all objects give off electromagnetic radiation. Radiation within the visible spectrum we perceive as colored light, but most radiation is outside our visible range.
- Evaporation happens when water (like sweat or moist breath) converts from a liquid state to a gaseous state, taking heat away from the body. Animals are always in contact with something (like surfaces, air, or water), so conduction is always occurring.
|Imagine this circle is an animal's body, |
Tb is the animal's body temperature and
Te is the environmental temperature.
The bigger (Tb-Te), the faster the
animal will lose heat and cool down.
|This works the other way around, too. |
The bigger (Te-Tb), the faster the
animal will heat up.
But reptiles (as well as amphibians and fish) are ectotherms. They get almost all of their heat from their environments. They maintain their body temperatures behaviorally, by choosing what environment to hang out in and what position to put their body in. If they are cold, they go bask in the sun to absorb radiation heat or lay on a warmed rock to absorb conducted heat. If they are hot, they lay on a cool rock in the shade to lose heat by conduction or soak in a cool stream to lose heat by convection. To maintain a relatively constant body temperature, they are constantly moving between warm and cool areas to adjust their body temperature one direction or another.
Many ectotherms rely on their ability to adjust their body temperatures quickly, and this ability depends on creating large driving forces of heat exchange. If an ectothermic reptile were to have an insulation layer, like fur, it would reduce its ability to adjust its body temperature by conduction and convection. It would lose its heat slowly and not be able to replace it fast enough. In the end, it would become too cold. It may seem paradoxical, but a lizard in a fur coat would likely die of cold-related physical issues (if not embarrassment).
Interestingly enough, just because lizards don't have fur doesn't mean they couldn't have hair. In fact, some of them do have hair, but not how you may think. Hair, fur, feathers, and scales are all made up in large part by keratin proteins. Many gecko species are well known for their wide, sticky toes that help them climb smooth, vertical surfaces (like walls). Their secret? Ultra-thin keratin hairs growing out of the geckos' feet provide a chemical adhesive force to keep the animal secured to the wall surface. So reptiles may not have a need for fur, but some of them have an innovative use for hair.
Want to know more about hairy geckos?
Autumn K, Liang YA, Hsieh ST, Zesch W, Chan WP, Kenny TW, Fearing R, & Full RJ (2000). Adhesive force of a single gecko foot-hair. Nature, 405 (6787), 681-5 PMID: 10864324