52 Motivation
footpads, and inner thighs dissipate heat efficiently because
they have limited insulation (Stirling, 1988). As we shall
see, these physiological adaptations contribute to an effective
behavioral thermoregulation scheme useful in both hot and
cold environments.
As mentioned earlier, polar bears have several poorly in-
sulated body areas, or hot spots. These hot spots are useful for
behavioral thermoregulation because bears can adopt distinct
postures depending on whether they need to expel or con-
serve heat. In warm environments, bears dissipate heat by
exposing these hot spots, and in colder environments they
conceal these areas (Stirling, 1988; Figure 2.6). Notice that
the form of the bear’s response is sensitive to environmental
temperature. This thermoregulatory scheme is fairly common
among endotherms.
The Rat
Rats are small mammals that live commensally with humans.
These animals populate temperate zones and also live in-
side burrows and buildings in cold climates. Rats are endo-
therms that exhibit a variety of thermoregulatory behaviors
(Hainsworth & Stricker, 1970). The rat’s body temperature
typically varies between 37°C and 38°C at neutral environ-
mental temperatures (approximately 28°C). When environ-
mental temperatures rise above this level, rats display a
constellation of responses that promote metabolic efficiency
and survival. For example, when environmental temperatures
range between 36°C and 41°C, rats exhibit a sustained hyper-
thermia with a magnitude that exceeds the environmental
temperature. This phenomenon is an adaptive and regulated
response. Rats benefit from this increase in body temperature
because it permits them to lose metabolic heat to the environ-
ment via conduction (Hainsworth & Stricker, 1970). Above
41°C rats are unable to sustain hyperthermia relative to the
environment.
Rats also exhibit two behavioral responses to heat stress
within the range that provokes hyperthermia (36°C to 41°C).
At moderate levels of heat stress, rats frequently lay with a
relaxed body posture often called prone extension. Much like
the polar bear, the rat uses this behavior to dissipate heat by
exposing body regions that conduct heat efficiently. In this
case the rat’s tail acts as a thermal radiator because it is both
vascularized and lacking in insulation. Thus, excess body
heat is readily dissipated through the tail (Rand, Burton, &
Ing, 1965). Along with prone extension, rats display saliva
spreading in response to moderate heat stress. This behavior
exploits evaporative cooling as a means to regulate body tem-
perature (Hainsworth, 1967), and it is characterized by the ac-
tive distribution of saliva from the mouth with the forelimbs.
The spreading initially focuses on the head, neck, and paw
regions and later targets the ventral regions with emphasis on
the scrotum and tail. Saliva spreading is prevalent in animals
that lack sweat glands, such as rats, opossums, and desert
rodents. Other terrestrial animals, such as humans, exploit
evaporative cooling by sweating.
Above approximately 41°C, rats can no longer regulate
heat exchange with controlled hyperthermia. Also, the ex-
pression of a relaxed body posture gives way to a pronounced
increase in activity that is probably a manifestation of escape
behavior (Hainsworth, 1967). At higher temperatures, rats
also exhibit saliva spreading. The adaptive advantage of this
behavior is demonstrated by the observation that desalivated
rats die within 1 hr to 2 hr of high heat stress, although nor-
mal rats survive for at least 5 hr of exposure (Hainsworth,
1967).
When a pregnant rat encounters inescapable heat stress, it
responds with the array of thermoregulatory responses that
are typical in her species. For example, the rat will engage in
both body extension and saliva spreading when heat stressed
(Wilson & Stricker, 1979). However, these animals face am-
plified thermal demands because their body mass increases
relative to the size of the available thermal windows that
expel body heat via conduction. Consequently, to regulate
body temperature these mothers compensate by lowering
their threshold for saliva spreading, and pregnant mothers
display saliva spreading at 30°C (Wilson & Stricker, 1979).
Similarly, the animal’s threshold for salivary secretion from
the submaxillary gland decreases, thereby providing an in-
creased saliva reservoir (Wilson & Stricker, 1979). These
measures contribute to successful thermoregulation for both
the mother and her offspring.
Rat mothers bear sizable litters that remain together until
weaning. These pups are particularly susceptible to hypother-
mia because they produce little metabolic heat that is quickly
lost to the environment. Moreover, pups are born with no
fur and little insulation, and they do not exhibit thermogene-
sis via shivering behavior (Hull, 1973). Given these obsta-
cles, rat pups may seem reliant on parental care for thermalFigure 2.6 A polar bear lies on ice to expose
its hot spots and cool off.