Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 428

White Mountains, California, have two breeding seasons, those at high elevations have
only one (Dunmire 1960). Similar behavior occurs among the reptiles and amphibians
(Saint Girons and Duguy 1970; Goldberg 1974). In addition, the few reptiles that extend
into the alpine zone (only three in the Alps) characteristically carry their young inside
them and give birth to live young rather than laying eggs as lowland forms do. The reas-
on for this is that there is simply not enough heat for cold-blooded animals to bring eggs
to complete development and hatching in high mountains (Hesse et al. 1951). By car-
rying the young inside her and by keeping in the sun, the mother can put the heat she
absorbs to maximum use. The ability to bear living young, therefore, is fundamental to
the entrance of reptiles into cold environments.
The reduction in the number of breeding attempts is offset somewhat by a tendency
for litter size to increase with altitude and latitude (Lack 1948, 1954; Lord 1960; Spen-
cer and Steinhoff 1968). The strategy behind this has been interpreted in different ways,
but the most generally accepted explanation is that the shorter seasons at high altitudes
limit the number of times an animal can reproduce in its lifetime compared to lowland
environments, so it is advantageous to invest in a few large litters. This is despite of the
facts that doing so reduces the life expectancy of the parents and that it is not as effi-
cient as the production of several small litters (it is harder to care for a larger number
of offspring). The production of a few large litters can be viewed as an all-or-nothing ap-
proach adopted because nothing is gained by being conservative (except in poor years,
when they may not reproduce at all) (Spencer and Steinhoff 1968).
The timing of reproduction in relation to environmental severity is demonstrated
by a comparative study of the woodchuck ( Marmota monax ) in southern Pennsylvania,
the yellow-bellied marmot ( M. flaviventris) ) in Yellowstone National Park, Wyoming, and
the Olympic marmot ( M. olympus ) in Olympic National Park, Washington (Barash 1974,
1989). The lowland woodchuck, with the longest growing season, reproduces annually;
the yellow-bellied marmot, with an intermediate growing season, also bears young an-
nually but occasionally skips a year, while the alpine Olympic marmot has the shortest
growing season and reproduces only in alternate years (Fig. 8.11). This is interpreted
as an adaptation to the limited capacity of more extreme environments to support life.
The timing of reproduction is also tied to social behavior which, in some marmots,
increases with environmental severity. Thus, the woodchuck is solitary, aggressive, and
nonsocial, while the Olympic marmot lives in tightly knit colonies and is highly so-
cial. The yellow-bellied marmot in Wyoming and Montana is intermediate in sociabil-
ity. This is a critical factor in determining when the young marmots disperse and form
new colonies. If the young are forced to leave too early, their chances of survival de-
crease. Accordingly, woodchucks disperse the year they are born and become sexually
mature as yearlings; yellow-bellied marmots remain with the parents for the first year
and disperse the next, becoming sexually mature as two-year-olds. The Olympic mar-
mot, however, remains with the parents for two full years and becomes sexually mature
only in the third year. These characteristics are apparently the result of the time re-
quired in each environment for the animals to develop sufficient size and maturity to
be able to disperse and reproduce successfully. For example, woodchucks achieve 80
percent of adult weight as yearlings, yellow-bellied marmots 60 percent, and Olympic
marmots only 30 percent (Barash 1974). At the same time, increased sociability with
increased environmental severity (which allows the young to remain with their parents