The Alpine Marmot Project

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Dispersal

Natal dispersal can be defined as permanent removal of an organism from its natal site to a site where it will settle and reproduce (Howard 1960). In the alpine marmot, reproduction takes place through the acquisition of dominant status, the primary goal of natal dispersal is therefore to acquire this status.  Dominant status can be acquired by filling the vacancy created by the death of a dominant individual, or by the eviction of a dominant marmot from his or her territory.  After eviction only rarely does a dominant marmot manage to become dominant in another territory.  99 percent of the time the evicted dominant will die. (Arnold 1990; Stephens et al. 2002).

Studies on the population of Alpine marmots in Berchtesgaden, Germany (Arnold 1990a, Frey-Roos 1998, Stephens et al. 2002; Grimm et al. 2003) and the site of  la Grande Sassière. (Magnolon 1999) all show similar dispersion patterns.

Both sexes are significantly involved in natal dispersal: 91.5% males and 80.0% of females engage in a dispersion phase (Arnold 1993, Perrin 1993, Frey-Roos 1998; Magnolon 1999). Subordinate individuals delay dispersal beyond sexual maturity (two years) and often three, four and even five years (Arnold 1990, 1993, Perrin 1993, Frey-Roos 1998; Magnolon 1999): 21.3% respectively ( 35/164 individuals), 72.2% (91/126 individuals), 48.4% (15/31 individuals), and 91.7% (11/12 individuals) of individuals of two, three, four and five scatter (Grimm et al. 2003).

While 8.5% of males and 20.0% of females remain philopatric (that is they contribute to the raising of their younger siblings) and later access dominant status by inheriting their natal territory, overall another 64.5% males and 55.9% females access dominant status:  either by inhabiting a neighboring territory (within 500 m from their natal territory) 25.6% and 21.1% of males and females, or in a distant territory for 38.% of males and females 34.8% (Frey-Roos 1998, Stephens et al. 2002).

Although the majority of dispersants acquire dominant status in a territory close to their natal territory, natal dispersal can lead individual marmots far away from their place of birth.  Radio-tracking indicates dispersal distances above 10 km (Arnold 1993, Stephens et al. 2002). The ultimate range of potential dispersants remains unknown thus far, but probably exceeds ten kilometers.

Despite a similarity of dispersal patterns in both sexes, the differences are worth noting. Males tend to disperse in greater proportion than females (p = 0.10, Arnold 1993).  The proportion of philopatric individuals is lower in males than in females (8.5% against 20.0%).  And males disperse earlier than females (59.5% against 31.5% males females disperse at two years, p <0.05, Magnolon 1999).

Gender differences have also been analyzed in relationship to the causes of departure. Males leave their family group under the influence of endogenous factors, particularly increased levels of circulating testosterone.  The odds of their dispersal double with the arrival of a new dominant male, whereas exogenous factors, in particular the nature of interactions with the dominant female, cause the departure of females, and females are four times more likely to disperse if their dominant male is related (Magnolon 1999).

Dispersing individuals face an uncertain fate, and this makes dispersal a risky strategy biologically speaking (Frey-Roos 1998, Stephens et al. 2002). Indeed, a dispersing individual sees his chance of survival drop from an average of 0.98 for a subordinate male to 0.79 for a dispersing male and from 0.97 for a female subordinate to 0.81 for a dispersing female (Frey-Roos 1998, Stephens et al. 2002 ). This survival probability decreases with the distance from the natal territory, and is nearly zero for a dispersing individual which fails to settle on a territory during the period of activity and then hibernates alone. (Arnold 1993; Stephens et al. 2002).

Bibliography

Arnold W (1990) The evolution of marmot sociality: I. Why disperse late? Behavioral Ecology and Sociobiology 27, 229-237.

Arnold W (1993) Social evolution in marmots and the adaptive value of joint hibernation. Verhandlungen der Deutschen Zoologischen Gesellschaft 86, 79-93.

Frey-Roos F (1998) Geschlechtsspezifisches Abwanderungsmuster beim Alpenmurmeltier (Marmota marmota). Thèse de doctorat, Marburg. Philipps University, Germany.

Magnolon S (1999) Dispersion natale chez la Marmotte Alpine (Marmota marmota). Modalités et effets de quelques facteurs proximaux. Thèse de doctorat, Université de Tours, Tours, pp 160.

Perrin C (1993) Organisation socio-spatiale et distribution des activités chez la Marmotte Alpine (Marmota marmota Linné 1758). In. Université Denis Diderot, Paris.

Stephens PA, Frey-Roos F, Arnold W, Sutherland WJ (2002) Model complexity and population predictions: The Alpine Marmot as a case study. Journal of Animal Ecology 71, 343-361.