Fitness consequences of delayed dispersal in alpine marmots [Conséquences de la valeur sélective de la dispersion tardive chez les marmottes alpines].

Résumé

Alpine marmot social groups usually consisted of a dominant territorial pair and their progeny which always remained at the natal site beyond sexual maturity. Among females, reproduction was restricted to dominant territorials, whereas subordinate males sometimes copulated. Subordinates had a fair chance of becoming territorial close to the natal site or to inherit the natal territory. On the other hand, long distance dispersal bore a high mortality risk. Benefits from group living arose during hibernation. The presence of adults additional to the territorials reduced winter mortality, particulary when infants were present. Subordinates were less likely to disperse in years when immatures lived in the group. They lost extra weight during winter when they hibernated with infants. Infant winter mortality was further decreased in such groups. Prolonged tolerance of offspring in ground squirrels has been hypothesized to be continued parental investment (Armitage, 1981, 1987). Delayed dispersal in alpine mamots demands further explanation. Subordinates accepted an avoidable cost during winter if closely related to infants, presumably because they warmed them. At least for females, this can be explained only by invoking kin selection. Subordinate males caring for infants may potentially be their fathers. Territorial males may yield part of their fitness to sons to prevent their dispersal and to secure their help in raising young.