This is repost from September of '06. I never had the chance to finish the series because of school, so I will be finishing in three parts this week.
Why have the dinosaurs been relegated to little kid stuff? Why can't I find a purple triceratops t-shirt in XL?
Were the dinosaurs warm-blooded or cold?
I want to talk about dinosaurs and body temp for a couple of posts, but I think the best place to start is with a little discussion of why this issue is important and the evolutionary implications of endothermal/ectothermal states.
Ectothermy is the state of commonly referred to as "cold-bloodedness" (an inaccurate term; it's more about precise regulation) which is observed in most reptiles (though not all), most fish (not all) and basically every animal that is not a bird or a mammal (though not all). Ectotherms rely on heat from outside sources to maintain their body temperature. The sun is the main source of this heat energy (infrared), which can be transferred directly through absorbing the sun's rays, or indirectly through convection - movement of heat between objects (including organisms) and the air.
When a lizard suns itself, its blood vessels just below the skin open up, allowing more blood to flow next to the skin and the source of heat (the sun's radiation). The lizard's blood warms and transmits heat to the rest of the body.
Endotherms, on the other hands, are animals that regulate their own heat, like us. We don't need external sources because heat production is built in to our metabolic machinery. In fact, most endotherms spend more time trying to dissipate heat than acquire it. As you might expect, there is a formal range of classifications (lower lethal, lower critical, thermoneutral, upper critical, etc.) that biologists can use to determine the tolerances of certain endotherms.
The most important thing to remember is the necessity of temperature regulation in animals, the "why." Metabolism is all about burning calories, and the processes that burn those calories are driven by chemical reactions. Chemical reactions within the body are finicky; they depend on enzymes to catalyze which only work within a relatively narrow band of temperatures.
Endotherms have an advantage in this respect. A constant high body temperature increases the activity of the central nervous system, and subsequently neurotransmitter and enzymatic activity. Ectotherms do not have this advantage; on cool days/nights, they lose the ability to be as active. Keep in mind that this does not mean that endotherms are better, just different.
So what does this have to do with dinosaurs?
The main problem in assessing the body temperature of dinosaurs is that we have no direct evidence. There are no extant dinos, so scientists have to look to their descendents, birds and reptiles. But, as we covered earlier, temperature regulation is far from uniform in these animals.
There is one other problem: temp regulation is a special problem in the case of such huge animals.
Next time we'll look into the evolution of endothermy and how it might have arisen from the dinosaurs.