PHYSICS

tures mainly by moving into locations of favourable temperature
(e.g., in the shade of a desert rock). Warm-blooded (homoiothermic)
organisms normally keep practically constant body temperatures,
independent of environment. Homoiothermic animals, including
man, are able to control their body temperature not only
by moving into favourable environments but also through internal
regulatory effects on the nervous system modulating heat production
and loss.
A more recent and sensible terminology separates organisms
into endotherms, who have a body temperature that is primarily
dependent on internally generated metabolic heat, and ectotherms,
who have a body temperature that is primarily dependent
on external heat sources (the Sun or a heated substrate). These
terms, therefore, emphasize the heat sources used, rather than
the setting or constancy of achieved body temperature.
There are dual setpoint regulatory systems in mammalian and
reptilian nervous systems, with separate neural thermostats for
controlling heat gain and heat loss. Some neurons ("warm receptors")
increase pulse frequency when the hypothalamic temperature
rises, and are probably linked to neurons that activate heat
dissipation systems such as shade-seeking, sweating or vasodilation.
Other neurons ("cold receptors") decrease in pulse frequency
and may initiate vasoconstriction, basking and other behavioral
effects. Still others only increase in pulse frequency when
the hypothalamic temperature falls below a specific set-point;
these are likely to trigger heat production mechanisms (i.e., shivering).
13.2. HEAT PRODUCTION
Vasoconstriction and vasodilation. Heat is produced in all body
tissues, but particularly in the core organs of a resting animal and
in muscles during activity. Heat must be distributed around the
body, princi pally by the blood or other body fluids, and
dissi pated at the surface according to need. The simplest form of
thermal control is to manage the rate and volume of the flow of
blood to the surface relative to the core. It is possible to
distinguish vasoconstriction - the narrowing of surface blood
vessels in response to cold temperature, keeping heat in the core
96
(fig. 13.1, a), and vasodilation - the expansion of a surface blood
vessels in response to warm temperatures (fig. 13.1, b). For
example, elephants use their large ears for thermoregulation, to
cool themselves in the hot equatorial sun. As the elephant flaps its
ears, blood vessels in the ear are cooled. The cooled blood circulates
throughout the elephant and helps regulate overall body
temperature. Another example, the fennec (Fennecus zerda) , is
the smallest of the wild canid species. The fennec has incredibly
large ears (fig. 13.2), which measure up to 15 ern in length,
making it extremely sensitive to sound. It also assists in thermoregulation,
which is essential in its North Africa desert habitat.
Vasoconstriction
Shunt vessels
(A-V anastamoses)
-+ Blood flow
~ Heart transfer
Low
Surface conductance
t
t t Air
vessels
15°C or less
Skin~
a
Vien
High
conductance
I I I 35°C
Vasodilation
Fig. 13.1. Thermal regulation in animals:
a ­ through vasoconstriction - the narrowing of surface blood vessels
in response to cold temperature and keeping heat in the core;
b - through vasodilation - the expansion of a surface blood vessels
in response to warm temperatures
Huddling and aggregation. Endothermic animals in polar regions,
such as penguins and some seals, cluster together when
the temperature drops substantially (e.g., -50 0 q , typical in an
Antarctic winter and when the wind speeds are over 160 kph.
Penguins form much denser huddles than seals with up to
b
97