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).