THE ADAPTIVE VALUE OF POLYMORPHISM 
IN AN ALPINE ANT, 
FORMICA NEORUFIBARBIS GELID A WHEELER* 
By Ruth A. Bernstein 
Department of Environmental, Population and Organismic Biology 
University of Colorado, Boulder, Colorado 80302 
Introduction 
Organisms that inhabit alpine tundra must be able to survive 
and reproduce under extreme environmental conditions. Most 
of the year the weather is cold, windy, and the ground covered 
with snow. As the snow melts in the spring, the ground becomes 
saturated with water and there is considerable run-off. Summers 
are short and characterized by frequent rains, high solar radiation, 
and extreme temporal and spatial variations in humidity. Animals 
that remain on the tundra year round can be expected to have ac- 
quired habits and associated structures which enable them to over- 
come the difficulties of these high altitude conditions. 
Of the tundra ant species, Formica neorufibarbis gelida is the 
most abundant and often the only ant species inhabiting the alpine 
tundra of the Colorado Rockies. According to Gregg (1963), it 
is the most cold tolerant of all the ant species found in Colorado. 
Additional adaptations include the ability to nest under rocks 
(which provide protection from temperature and humidity ex- 
tremes) and the relatively rapid production of a single brood (from 
eggs to adult workers and sexual forms) within the short summer 
season (Taussig, 1962). 
During preliminary observations on the ecology of this sub- 
species, I noticed color differences in the thorax of foragers — 
some were brownish-black and others red. Both types were found 
within every colony observed. The foragers with red thoraxes also 
appeared to be larger in body size than those with darker thoraxes. 
The possibility that either color or size polymorphism might be 
associated with adaptation to tundra environments stimulated the 
work presented in this paper. 
* Manuscript received by the editor September 7, 1976. 
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