It was a long-standing paradigm in ecology that reptiles were consummate thermoconformers, essentially at the whim and mercy of environmental conditions. In 1944 this idea was challenged by seminal work by Cowles and Bogert who definitively demonstrated behavioral thermoregulation in lizards. This important paper sparked a series of new studies on the evolutionary ecology of thermoregulation. Researchers became interested in how lizards utilized different behavioral strategies under varying thermal regimes. They sought to explain and quantify the costs associated with thermoregulation in different environments, and understand how species richness on islands correlates with thermoregulatory strategy. The study by Cowles and Bogert was arguably one of the primary forces behind the “noose ’em and goose ’em” period of reptile biology.
However, initial forays into important questions about behavioral thermoregulation were hampered by the fact that no one knew exactly how the thermal profile of a thermoconforming lizard should look like. This is troublesome, because without information on thermoconformers, there was no null expectation against which to compare field-measured body temperatures. Basically, if we don’t know how a thermoconformer would experience the environment, how do we know the degree to which animals are thermoregulating?
The first solution to this problem was to use air temperature by time of day as a proxy for the thermal profile of a thermoconformer. Deviations from a strict 1:1 relationship between body and air temperature were often interpreted as evidence for thermoregulation. The major problem with this idea is that a lizard’s core body temperature is actually the result of several factors, of which air temperature is only one. Lizard body temperature results from a combination of micrometeorolgical parameters that include air temperature, solar radiation, substrate temperature, and wind velocity. This integrated representation of a lizard’s steady-state temperature is termed the operative temperature. It is the thermal environment, as perceived by the organism, in the absence of metabolic heating and evaporative cooling.
Today we have an array of devices at hand that can accurately estimate lizard operative temperatures. These range from the very literal, such as lizard-shaped models made out of electroformed copper and painted to match their reflectance, to the more abstract, such as TidBit devices, which are temperature sensors only a few centimeters in diameter.
But before all the fancy devices and complicated models revolutionized the field, the first prototype was actually a beer can filled with water. In a paper modestly titled “Reptilian Thermoregulation: Evaluation of Field Methods”, James Heath (1964) proposed the first device to record reptile operative temperature. Heath filled thirteen beer cans with tap water and placed eleven in the sun and the remaining two cans in the shade. He then qualitatively compared the can temperatures to lizard body temperatures through a literature search, and found that they were often quite similar. He also observed that can temperature was consistently higher than air temperature, as is often the case with lizard body temperature.
This paper is unique for many reasons. First, to my knowledge, it is the only paper on behavioral thermoregulation in reptiles where not a single body temperature was measured. Second, while the method is flawed, the idea that assessing behavioral thermoregulation requires an appropriate null model was insightful and pervasive. Today operative temperatures are broadly used in a variety of capacities, from comparing the degree of thermoregulation among species to deriving niche models and quantifying the extent to which lizards will be able to buffer the effects of climate warming through behavioral mechanisms. It is a powerful tool for addressing a wide variety of questions in reptilian evolutionary ecology. For that, we are indebted to the water filled beer can.
Interestingly, he never says what he did with the beer before filling the cans with water. I will leave it to the audience of this blog to offer their guesses. In a similar vein, why did he use exactly thirteen cans of beer? In my experience, a case and a spare is the best formula for a successful day in the field. By the looks of it, Heath was on to more than just operative temperature and null models.