Raymond B. Huey and Eric R. Pianka
When physiological ecologists evaluate whether body temperatures of lizards and other ectotherms (such as insects, fishes) are “normally distributed,” they are not referring to whether a lizard’s body temperature averages 98.6°F, a so-called “normal” body temperature for a human. Rather they are discussing whether the shape of the distribution of body temperatures (when temperatures are plotted on a histogram) fits a normal distribution or familiar “bell curve.” Normal distributions, which are symmetrical in shape about the mean, are common for many measurements in biology and other fields.
Physiological ecologists have long suspected that distributions of body temperatures of active diurnal lizards should actually be left-skewed and have a relatively long left tail (that is, an excess of “cool” body temperatures). One reason is that lizards emerging from cool nocturnal retreats will have low body temperatures until they start basking and behaviorally thermoregulating. Another is that lizards might avoid extremely high temperatures, which are stressful or even lethal.
Whether these distributions are skewed vs. symmetrical is important for performance, as the distribution’s shape will affect how performance integrates over time. For example, if distributions are strongly left skewed, then lizards are often active at temperatures too cold for optimal performance. Even though skewness is clearly relevant to lizard ecology, it has been quantified only a few times.
During the last half century, we have studied the ecology of desert lizards on three continents (Africa, Australia, and North America) and have measured body temperatures of many thousands of active diurnal lizards. To evaluate whether lizard temperatures are in fact typically left skewed, we compiled and analyzed our data for 45 species (8,827 individuals). Most species do have left-skewed distributions of body temperature, but the skewness is smaller than expected.
The limited skewness evident in our samples suggests that lizards do a very good job of avoiding activity at low temperatures. They might do this by pre-warming inside burrows before emerging in the morning, or by emerging only when environmental temperatures are high enough to speed warming to optimal body temperatures.
Image caption: Thorny devil (Moloch horridus) in the Western Australia Desert. Photo credit: Eric R. Pianka