Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 414

Lack of Oxygen
Free oxygen is essential for life. The composition of the atmosphere is relatively con-
stant, containing about 21 percent free O 2 both at sea level and in the upper part of the
atmosphere. Nevertheless, there is less oxygen in the air at higher altitudes, because air
is compressible and has greater density and more molecules of oxygen per unit volume
at lower elevations than at greater heights. The availability of oxygen is expressed as
the partial pressure of oxygen (PO 2 ), which is derived by multiplying the total atmo-
spheric pressure by 21 percent. Thus, normal atmospheric pressure at sea level is 760
mm, so the pO 2 is 159 mm. With increasing elevation and decreasing atmospheric pres-
sure, the pO 2 decreases proportionately.
The reduced oxygen concentration at high altitudes does not appear to have a great
effect on the biota. Oxygen deficiency has no noticeable effect on vegetation, insects,
or reptiles and amphibians (Bliss 1962; Mani 1962; Hock 1964a). Little is known about
its effects on birds. The South American condor ( Sarcorhamphus gryphus ) nests in the
high Andes, but migrates daily to and from sea level, where it feeds on dead fish along
the Pacific coast (Lettau 1967). The bar-headed goose ( Anser indicus ) winters in the
lowlands of India, but flies over the summit of Everest on its way to nesting grounds
in the high lakes of Tibet (Swan 1970). Many mammals have overcome the effects of
oxygen deficiency so that they can occupy almost any environment where there is suf-
ficient food. However, experience shows that for humans and their livestock, elevation
does make a difference.
This is shown by the pronounced symptoms experienced when certain low-altitude
mammals (including humans) go to high altitudes. With rapid ascent, acute mountain
sickness or other maladies may develop, possibly resulting in death. If they go up slowly,
acclimatization to the changing conditions takes place, but there are limits beyond
which lowland mammals cannot go. For lowland cattle, this is approximately 3,000 m
(10,000 ft) (Alexander and Jensen 1959).
Survival Strategies
Animals respond to environmental stresses first by behavioral adjustments, followed
by physiological adjustments which, in consequence of natural selection, result in mor-
phological adaptations, followed by possibly new community relations and interac-
tion. Plants, being relatively immobile, respond primarily through morphological and
physiological adaptations, while animals respond primarily through behavior (Kendeigh
1961). Animals often deal with environmental extremes by escaping them through mi-
gration, hibernation, burrowing, or the use of micro-habitats. The nature and timing
of growth and reproduction also comprise an important part of survival strategy in ex-
treme environments. Only a few species expose themselves to the full brunt of the cli-
mate throughout the year; those that do, however, display the broad range of adapta-
tions necessary for existence under such conditions. Escaping environmental extremes
is primarily a function of behavior (although some metabolic adjustments are also in-
volved), but for an animal to withstand extreme conditions, it must, like the plants, rely
on morphological and physiological adaptations.