Rodent Societies: An Ecological & Evolutionary Perspective

productions, and M. baibacina, M. caligata, M. camtschat-
ica, M. caudata, M. menzbieri,andM. vancouverensisoften
skip 2 or more years (Blumstein and Arnold 1998; Armi-
tage 2000; Armitage and Blumstein 2002).
Because the length of the active season is short (mean
length is 4.8 months), marmots must initiate reproduction
as early as possible in the spring so time will be sufficient for
all members of the population, but especially for reproduc-
tive females and young, to accumulate fat for hibernation
(Armitage 2000). However, if a female emerges from hiber-
nation too early, she may use considerable resources coping
with a cold, snowy environment. At least six marmot species
reduce early post-hibernation energetic stress by mating be-
fore emergence (Armitage and Blumstein 2002). Birth and
early development of the young (e.g., M. broweri;Rausch
and Bridgens 1989), may occur in the burrow before sur-
face activity begins.
These known biological and climatic factors that dem-
onstrate that marmots live in a harsh environment raises the
question: what are the major adaptations for coping with
environmental harshness?

Hibernation and large body size

Hibernation is a means of conserving energy during a period
of food unavailability. Energy savings for animals hibernat-
ing singly during the hibernation period (compared to con-
stant euthermy) averaged 83.3% for the yellow-bellied mar-
mot, 43.8% for the alpine marmot (Armitage et al. 2003)
and 43.2% for the woodchuck (Armitage et al. 2000). Mar-
mots are the largest true hibernator; large body size in-

creases energetic efficiency (French 1986). Energy (fat)

stores scale directly with mass; energy use scales to mass3/4

at environmental temperatures within thermoneutrality and

mass1/2at colder temperatures. Thus larger marmots accu-

mulate more fat and use it relatively more slowly.

Body mass varies considerably among marmot species;

emergence mass of the largest species, M. bobak,is 1.76

times that of M. flaviventris,the smallest species (Armi-

tage 1999a). Larger species of marmots not only accumu-

late more mass because of larger size; they may accumulate

more mass than expected based on body size. The accumu-

lated fat not only fuels the metabolism of hibernation, it

also sustains activity, including reproduction, following ter-

mination of hibernation and is the source of energy for cop-

ing with unfavorable climatic conditions until vegetation

again becomes available. Large marmot species store more

fat because they use more fat to cope with their harsh envi-

ronment (Armitage 1999a).

Large body size has a second major advantage. A larger

body size has a larger absolute gut capacity relative to meta-

bolic rate. Larger animals can use more fibrous diets by

means of longer retention of digesta in the gut (Hume et al.

1993). The gastrointestinal tract is one of the most inten-

sive energy-use organs of the vertebrate body (Hume et al.

2002) and could exact high costs during hibernation. How-

ever, the system is reduced in size at the time of hibernation

and increases in size and activity following emergence

(Rausch and Rausch 1971; Bassano et al. 1992; Hume et al.

2002). The body-size effects of diet and hibernation form

an integrative system; large body size enables marmots to

exploit a diet consisting of grasses and forbs (Armitage

Evolution of Sociality in Marmots: It Begins with Hibernation 359

Figure 30.1 The Olympic marmot overlooking a snowfield in mid June. Photo by K. Armitage.