Laboratory 7 — Zoology 250: Thermoregulation

Introduction

Imagine you are a lizard in the desert. The desert gets cold at night and your body temperature drops with it. As a result, each new day dawns and your brain starts out pretty cold. This, of course, matters since temperature governs how fast enzymes catalyze reactions and the speed of other biochemical processes. If you are a desert lizard, your brain does not get a fast start most days.

Temperature critically affects performance in both terrestrial and aquatic animals. The temperature animals operate at is affected by a number of factors that we have talked about in lecture. These include an animal’s overall regulatory strategy (ectotherm or endotherm), size, shape, habitat, climatic conditions, behaviors which influence body temperature (do you bask or live underground?), evaporative cooling, and whether you are insulated or not. In general, ectotherms tend to be small, exchange heat rapidly with the environment, have little or no insulation, and have low metabolic rates. This is not always true though; some insects have very high metabolic rates when active. Endotherms, in contrast, tend to be larger, exchange heat more slowly with the environment, are well insulated, and have much higher metabolic rates than ectotherms (at least ~7X higher at rest typically). You might think about ectothermy as life on a "tight budget" and endothermy as an active, expensive lifestyle.

Body size plays a big role in thermoregulation. Hummingbirds let their body temperatures drop significantly overnight and small mammals in high latitudes let their temperature drop for months. Neither can afford the cost of maintaining a high body temperature when they are not taking in a lot of energy by feeding and the temperature of the environment drops. In contrast, bears do not let their body temperature drop significantly while "hibernating" (bears are not considered true hibernators for this reason) and a large animal like a whale cools very slowly even when no longer generating heat. For example, the core temperature of a dead baleen whale on a beach drops only about one degree per hour even in quite cold weather. Part of this slow temperature drop relates to blubber, a very effective insulator. What really slows cooling though is the animal’s enormous size. This matters because a whale has relatively little surface area given its huge volume. Stated simply, heat is generated by the volume (essentially the number of respiring cells) and lost through the body surface. If an animal is large enough or in a hot habitat, its problem can actually be heating up too much. This may be the reason African elephants have such large, well-vascularized ears and bull male killer whales have large, well-vascularized dorsal fins.

Goal and Procedure

This laboratory will be an investigative one in which you test the factors affecting animal heating and cooling. We want you and your group to pick a variable to investigate, state testable hypotheses related to the effect of this variable, and use the thermistor temperature probes of the biopac monitoring apparatus to test these hypotheses. Your tests will be in examining how fast your experimental animals heat and cool. The "animals" will be made of tin foil and heated by a lamp.
 
 

Part A: To start and become comfortable with these measurements, do the following:
 
1. make a cube approximately 10 cm on a side
2. insert the thermistor probe
3. place this cube 30 cm from your heating lamp
4. begin recording with the Biopac apparatus
5. turn on the heating lamp
6. measure the temperature over several minutes and, ideally, until it levels off (i.e., stops increasing).
7. You will get data here with which to create a heating curve. Now, turn the lamp off and keep recording. You will generate a cooling curve this way.

Part B: Individual experiments

I mentioned several factors above that affect how quickly an animal heats or cools and most of these can be experimentally investigated using this apparatus. For example, "What is the effect of overall body size?" Placing a temperature probe in experimental "animals" of similar shape, but different sizes, and examining heating and cooling curves will allow you to approach this question. In contrast, you might construct animals of similar sizes, but different shapes, to investigate the effect of shape while holding size constant. As an example, may dinosaurs appear to have had crests. This might have been for signaling to conspecifics, but would have also served well as heat collectors. Other ideas: what is the effect of evaporative cooling (e.g., sweating)? Insulation? Wind? Color? We will have materials on hand to test the influence of all these variables. If you have other ideas that you think are testable and feasible, great! Talk to us and we can try to make them work.

When making your "animals," remember that this doesn’t have to be Jurassic Park and you don’t need t compete with the Dreamworks team! What do I mean? Your differing animals can differ quite simply. For example, what is the effect of having animals which are shaped like a cube as above as compared with being more flattened (and therefore having greater surface area)?

A second thing to remember. There is always error in measurements and procedures. This is why we want sample sizes of more than one. You should try to repeat your experimental measurements at least a few times if possible (i.e., let the animal cool back down and run the heating curve again).

This lab will move more quickly and smoothly if you have thought about what you want to do in advance. The actual measurements may go quite quickly once you are set up, but the critical part is always figuring out what you are asking and therefore what to set up.

You will be using the Biopac Student Lab Pro software for this exercise. We will have detailed instructions for this available at your bench and, if necessary, the way data is taken can be extensively modified in this package to allow you to test anything (almost) you would like. Also, to help in describing your experimental treatments, we will try to have a digital camera on hand to record and print pictures of your creations. We’d rather look at a picture than read a thousand words!