Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 438

less species are 25 times more abundant above the snowline than in the surrounding
subalpine forests. Even those species with wings rarely fly; they are essentially ground-
dwelling organisms, some with the ability to fly but seldom availing themselves of the
opportunity, others with only partial or abbreviated wings incapable of keeping them
aloft. Thus, at the highest levels it is a common sight to see butterflies, moths, and flies
crawling around on the ground. They flatten themselves against the surface into the
wind, flying only when the air is relatively calm.
The reasons for winglessness and abbreviated wings are not immediately obvious.
Darlington (1943) believed it to be a response to the insular character of mountains
and the limitation of habitats, so that the survival value of flight would be reduced. An
analogy to this would be the flightless birds, such as kiwi and dodo, found on oceanic
islands. The flightlessness of insects on high mountains, however, must also be due to
environmental extremes, especially that of low temperature. Insects require a certain
critical temperature—usually about 10°C—to be able to function and, since the highest
temperatures occur at ground level, there is an advantage in remaining close to the
ground. Insects with the ability to fly must preheat their flying muscles by vibrating or
by pumping them before taking off (Heinrich 1974). Once aloft, muscular activity gen-
erates enough heat to maintain flight under the lower temperature conditions. Another
problem for flying insects is the low atmospheric pressure and thin air of high altitudes.
The major factor, however, is the wind, which is too strong for effective and sustained
flight by insects. For this reason, Darwin thought that flying insects tended to be blown
away from mountaintops, leaving the wingless stock to occupy the area (see Hesse et
al. 1951). Whatever the explanation, winglessness and flightlessness are common traits
of high-altitude insects.
FIGURE 8.17 Heat regulation and temperature sensitivity of cold-climate and tropical mammals. All
warm-blooded animals have similar internal temperatures, but owing to differing amounts of in-
sulation and sensitivity to cold, some must increase their metabolism more rapidly than others
when the external temperature decreases. This is seen in the steepness of the gradient for the
various animals. The basal metabolic rate is the temperature where heat loss begins to exceed
heat production in a relaxed, resting animal. Note that humans are a tropical animal and increase
their metabolism immediately upon a decrease in external temperature, whereas the arctic fox is