Individuality, Social Behavior, and Reproductive Success in Yellow-Bellied Marmots

Current theory suggests that population dynamics are the consequence of the repro? ductive strategies of individuals. Individual differences should be expressed in reproductive output, dispersal, social behavior, and recruitment. Mirror-image stimulation (MIS; i.e., exposure ofthe animal to a large mirror) was used as an independent measure of individuality, which could be distributed continuously or which could be grouped into two or more types. Three axes derived from a factor analysis of behavioral data obtained during MIS accounted for 85% ofthe variance among individual marmots. The rank order of 19 adult females on each of the three MIS axes was not correlated with the rank order of lifetime reproductive success measured as number of young weaned, number of yearlings produced, or number of young or yearlings produced per year of residency. This result suggests that individual differences are not continuous. Each female was assigned to one of three groups according to the MIS axis on which she had her highest factor score. Rankings for the number of female yearlings, number of recruits, and number of 2-yr-old resident daughters varied significantly among the MIS groups. Mean values of these measures were highest for females in the "sociability" group. Although none of eight measures of lifetime social behavior for 18 females was significantly related to the three MIS groups, several measures of lifetime amicable behavior were correlated with the production and recruitment of female yearlings. Behavior in the field is affected not only by individual behavioral phenotypes, but also by kinship and patterns of space use. Marmots may have a strategy of phenotypic plasticity. By producing young of varied phenotypes, a female increases the probability that over the long term some of her descendants will survive in varied and unpredictable social and ecological environments.