Geography Reference
In-Depth Information
to thousands of individuals per square meter. The largest single mass assemblage ever
documented was in the western Himalaya at an elevation of 4,200 m (13,850 ft). Dis-
covered on a snowfield, where they covered a solid patch 10 m (33 ft) in diameter, they
were estimated to number about 200,000 individuals per m 2 (11 ft 2 ). This was in May,
and most of the beetles were alive but inactive. It was possible to scoop up handfuls of
the creatures and they would stir only slightly (Mani 1962).
The exact purpose of ladybird-beetle migration to mountain peaks is unclear, since
they apparently do not mate or feed there. Some scientists believe that they go there
to hibernate (Mani 1962), while others do not believe that they spend the winter on the
high peaks intentionally, but think that they simply get caught by cold weather and have
no other choice (Edwards 1957). Whatever the case, it is not uncommon to find large
numbers of dead beetles under rocks and in crevices in the spring, while others are
alive and preparing to move into the lowlands (as demonstrated by the example in the
Himalaya, cited above). Such mass assemblages serve as food for other high-altitude
creatures—mainly birds and predatory insects, but also large mammals. The Himalayan
bear ( Ursus thibetanus ) is known to overturn stones in search of ladybird beetles, and
the grizzly bear ( Ursus arctos ) also feeds on them (Chapman et al. 1955; Mani 1962).
Other causes of migration among mountain animals are outbreaks of bad weather
and poor food availability. Migrations for these reasons are well known for the Arc-
tic—for example, the fluctuating populations of lemmings and voles and their predators,
or the Arctic fox ( Alopex lagopus ) and snowy owl ( Nydea scandiaca ), but such cyclical
fluctuations are not usual in alpine environments (Hoffmann 1974). Nevertheless, the
downward migration of species is frequently observed when especially bad conditions
exist (Verbeek 1970; Ehrlich et al. 1972; Geist 1971). A specialized illustration of this
is provided by the Clark's nutcracker ( Nucifraga columbiana ), a large corvid that lives
in the sub-alpine zone of many northern hemisphere mountains and feeds through the
winter on pine seeds it has cached in the ground. During years of poor seed production,
lack of food may force the nutcracker to leave the high country. In the Sierra Nevada of
California, seven major invasions of these birds into the lowlands have taken place since
1898 (Davis and Williams 1957, 1964), apparently the result of poor seed production.
Hibernation
Another method of effectively escaping the winter cold and lack of food is hibernation.
In this amazing adaptation, an organism becomes inactive and passes the stressful peri-
od in a state of dormancy. Hibernation is essentially a deep and prolonged sleep. The
metabolic rate in warm-blooded mammals may be reduced by up to two-thirds, and the
internal temperature may be lowered to within a few degrees above freezing. Many
cold-blooded mammals and insects survive internal temperatures below 0°C (32°F)
(Hesse et al. 1951; Mani 1962). This results in vast energy savings for the organism.
Hibernation is an extremely efficient survival mechanism for species not mobile enough
to migrate, or unable or disinclined to remain active all winter feeding themselves. This
includes members of almost all divisions of animal life except for the birds, which, ow-
ing to their mobility, find it much more expedient to migrate (Carpenter 1974, 1976).
Among alpine mammals, hibernation is best developed in ground squirrels and mar-
mots. Of these, the ground squirrel ( Citellus spp.) is perhaps the most remarkable: Its
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