2.3 Hibernators as eutherms
Hibernating endotherms are not the easiest animals to study. Thus, until the late 1960s many biologists believed that mammalian hibernation was a process in which thermoregulation was simply ‘switched off’, following the receipt of a set of ‘cues’. These cues included a declining T a, a shortening daylength, the extent of body fat and a lack of food etc. With this model, the hibernator essentially becomes an ectotherm whose T b follows the T a quite closely and who is at great risk if the T a in the hibernaculum (hibernatory winter retreat) falls below freezing. Indeed, prior to the 1960s, many workers assumed that a poor thermoregulatory ability was a prerequisite for hibernation.
What evidence casts doubt on these views?
You may be aware that many animals overwinter in hibernacula that are not very deep or well defended against cold. If they were indeed so dependent on T a, there would be a very high mortality rate. In addition, the ability to hibernate occurs in species very closely related to non-hibernators; it is unlikely that thermoregulatory ability would vary so widely between closely related genera of rodents such as the non-hibernating Djungarian or Siberian hamsters (Phodopus sp.) and the hibernating Turkish hamster (Mesocricetus brandti) and black-bellied or European hamsters (Cricetus cricetus).
Note that there is some confusion over the common names of the species of Phodopus. Until about 1980 Phodopus sungorus was considered to have two subspecies – Phodopus sungorus sungorus and Phodopus sungorus campbelli. Phodopus sungorus sungorus was known as the Djungarian hamster. There was no commonly used name for Phodopus sungorus campbelli. Then studies showed that these two subspecies were in fact different species and they were renamed as Phodopus sungorus and Phodopus campbelli. For some reason, Phodopus sungorus was then referred to as the Siberian hamster and Phodopus campbelli as the Djungarian hamster. Scientists who study these animals may well use either the older or newer common name. The moral here – always use the correct scientific name for a species as well as what might be its common name. Common names are common only to the culture in which they are used. In this text we refer to Phodopus sungorus as the Siberian hamster.
It has been recognized for many years that hibernators can arouse at intervals throughout the hibernating season without an apparent rise in T a and that arousal could occur when animals were handled or disturbed in the cold. Both of these observations imply a remarkably high degree of control by the animal (Table 3).
|T a/° C||T b rectal/° C||Relative rate of heat production per day/arbitrary units|
In what way does the data in Table 3 suggest that T b is controlled in the hibernating marmot?
The T b in each experiment remained relatively constant, varying only between 4.1° C and 4.7° C, but the heat production varies, being very much higher when T a was held at the lowest temperature.
Nevertheless, it was not until the effects of manipulating hypothalamic temperatures (see Section 6) of otherwise cold hibernating rodents were investigated, that researchers began to consider that hibernating mammals might be exercising control similar to non-hibernating (normothermic) ones – a view that is now generally held.