Studied species
Alpine marmots are territorial, socially monogamous and cooperatively
breeding ground-dwelling squirrels (Allainé, 2000). They live in
families of two to 16 individuals composed of a dominant pair
monopolizing reproduction (Arnold & Dittami, 1997; Cohas et al., 2006;
Hacklander et al., 2003), sexually mature (≥ 2 years) subordinates of
both sexes, yearlings and pups of the year (Allainé, 2000). At sexual
maturity, subordinates may keep their status, attempt to reach dominance
in their natal groups or disperse to gain dominance in another territory
(Lardy et al., 2012). Once an individual reaches dominance, it cannot
reverse to subordinate status. Dominance is established for several
years and lasts until the dominant individual is evicted or dies (Lardy
et al., 2011). During the 23 years of study, only three males and one
female lost their dominant status but established dominance in another
territory (Lardy et al., 2011).
Field
methods
As part of a long-term study
at La Grande Sassière Nature Reserve (2,340 m a.s.l., French Alps,
45º29’N, 65º90’E, (see Cohas et al., 2008 for details), we captured
marmots annually, from mid-April to mid-July using live traps placed
close to the main burrows to assign trapped individuals to their family.
Individuals were marked with a transponder and a numbered ear-tag,
combined with a coloured plastic ear-tag for dominant individuals. At
each capture, individuals were tranquilized by an intramuscular
injection of Zolétil 100 (0.1 ml.kg-1), sexed, aged,
weighed and their social status was determined (large scrotum for
dominant males and prominent teats for dominant females, characteristics
of each sex all year round independently of reproduction). Social status
was further confirmed by observations of scent-marking behaviour and
territorial defence that are characteristics of dominants. Exact age was
determined for the individuals born on the study site. For dominant
immigrants (5 individuals), we assigned the age of three when they first
reproduce, as marmots disperse at two years old and never reproduce
before three years old. To determine individual fates, capture histories
were combined with intensive observations (each family being observed on
average 1 hour per day for a minimum of 30 hours per year, for details
see Cohas et al., 2008). In our population the maximum lifespan observed
over the 30 years of the study is 16 years, and the generation time is
about 5 years (Devillard, unpublished data). At each capture, a blood
sample (2 ml.kg-1 up to 5 ml per individual
representing less than 5% of the total volume blood) was taken from the
saphenous vein within 30 minutes after capture.
To ensure that all the individuals included in this study were in a
comparable social status, and all fully grown and reproductive
individuals, we restricted the subsequent analysis to the sole dominant
marmots, (47 born on the study site and 5 immigrants, removing
immigrants did not change qualitatively the results, nor their
interpretations).