Lecture4

Lecture4 - 1 Biol 585 R. D. Howard Fall 11 Life History I:...

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1 Biol 585 Fall 11 R. D. Howard Life History I: Allometry When discussing temperature adaptations of endothermic homeotherms (birds and mammals), I made reference to an underlying generalization between body size and temperature regulation that pertains to all organisms, endotherms or ectotherms, homeotherms or poikilotherms. The generalization is that there is a non-linear relationship between surface area and volume. In particular, surface area becomes smaller relative to volume as linear dimensions increase. For temperature regulation, this means: a. The rate at which heat is lost or gained is proportional to surface area. b. However, for the same amount of heat transfer, a larger volume will have its temperature changed less than a smaller volume. The mathematical relationships relating surface area and volume to body size are Surface area (length) 2 AND Volume (length) 3 THUS, OVERALL THE LARGER THE ORGANISM, THE SMALLER THE EFFECT WHEN ENVIRONMENTAL AND BODY TEMPERATURES DIFFER. Larger organisms have lower surface to volume ratios than smaller organisms do, and thus are less affected by external temperatures. That is, to maintain a constant body temperature, larger animals need to expend less energy relative to smaller animals (hence the problems I alluded to before concerning shrews and hummingbirds). It is the simple questions in biology that are the most difficult to answer; this is certainly true for body size. One of the simplest questions that you could ask of an organism is: why are individuals of this species the size they are? Why aren’t all individuals of this species much larger or smaller? If you thought long and hard about this, you probably could come up with some guesses. It may surprise you to know that we cannot fully answer this question for any organism. Notice that when I posed the question, I hedged a bit and asked why aren’t individuals usually of a particular size rather than all of a particular size. I did this because, if we collected a bunch of individuals of a certain species and measured their body length, we would find some variation. For most species if we plotted the distribution of body sizes, we would find a normal distribution: Number of Individuals Body Length
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2 Of course, this doesn’t answer the question, it just points out that variation exists. However, as we know phenotypic variation is one of the key requirements of natural selection. So, we can rephrase our simple question to: why hasn’t natural selection favored larger (or smaller) size, on average, given this variation? Perhaps, there is no genetic basis for differences in body size. Just for grins, lets assume there is (which is not an unrealistic assumption for most species). We can ask our question again. Our response might then be there is no fitness advantage to being larger or smaller? Now, we should begin to discover why the simple question we posed is an
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Lecture4 - 1 Biol 585 R. D. Howard Fall 11 Life History I:...

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