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, Yoccoz, Silva, Goossens, & Allainé, 2006;
Hacklander, Mostl, & Arnold, 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). At each capture, a
blood sample (2 ml.kg-1) was taken from the saphenous
vein within 30 minutes after capture.
As senescence should not occur prior to first reproduction (Hamilton,
1966), we restricted the subsequent analysis to the sole dominant
marmots: fully grown and reproductive individuals aged from 3 to 12
years-old (47 born on the study site and 5 immigrants, removing
immigrants did not change qualitatively the results).