Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 437

FIGURE 8.16 Schematic representation of a vascular heat exchange system. The warm blood from
the heart moves toward the extremities and is cooled by transfer of heat to the returning venous
blood, which in turn is warmed. This results in the saving of considerable energy, since the tem-
perature of both core and extremities is maintained at an appropriate level with little effort. (Ad-
apted from Scholander 1955: 19.)
COLD-BLOODED ANIMALS
SMALLER SIZE Among cold-blooded animals and insects at high altitudes, the tendency is
for body size to decrease. The tendency toward smaller body size with altitude can be
seen in almost all cold-blooded vertebrates and invertebrates (Fig. 8.18; Schmidt 1938;
Darlington 1943; Park 1949; Martof and Humphries 1959; Mani 1962, 1968). There is
also a tendency for orders composed of smaller-sized individuals to dominate. Thus,
among the insects, the Diptera (flies, mosquitoes) and Collembola (springtails) become
most important at the highest altitudes (Mani 1968). This must be due in part to the lack
of food, since food is largely restricted to airborne nutrients. The main reason for the
difference in size between warm- and cold-blooded animals, however, is that the warm-
blooded animals maintain a high internal temperature that is essentially independent
of environmental conditions, so that large body size allows efficient heat conservation.
Cold-blooded animals, on the other hand, are totally dependent on environmental tem-
perature, so a smaller size works to their advantage. There is less mass to heat, so re-
sponse is more rapid, and the smaller size means a relatively greater surface area to
capture external heat (Pearson 1954; Pearson and Bradford 1976).
Of course, temperature is only one component of the environmental complex. Other
factors that encourage small size in cold-blooded animals include the short time avail-
able for maturation, the lack of nutrients, and the advantage of small size in escaping
into microhabitats. These factors are important to warm-blooded animals as well, but
not to the same degree, since their level of metabolism is considerably higher.
WINGLESSNESS AND FLIGHTLESSNESS A corollary of smaller size in insects is a tendency to-
ward winglessness or reduced wing size. This is a dominant characteristic of high-alti-
tude insects, observed in many mountain areas (Darlington 1943; Salt 1954; Mani 1962,
1968; Hackman 1964). In the Himalayas, 50 percent of the insects above timberline
have no wings, and at the snowline the proportion is 60 percent (Mani 1962). Wing-