The Alpine Marmot Project

Welcome to the Alpine Marmot Project

Reproduction

The mating system

Alpine marmots are generally monogamous (they keep the same partner for one or more reproductive events). Within a family group, only the dominant pair regularly mates and raises, young since the dominant pair supresses the reproductive functions of subordinate males and females (Arnold et Dittami 1997; Hackländer et al. 2003). Vehrencamp (1983) thus classifies this species as “despotic”, as ooposed to egalitarian.

Genetic studies show that monogamy is not always the rule for Alpine marmots. In 14 years of studying Alpine marmots in La Grande Sassiere, researchers observed only one case of polygyny (males mating with more than one female). But in the most common form of alternative mating system, females mate with more than one male (polyandry).

Arnold (1990) was the first to clearly demonstrate the existence of extra-pair paternity in the Alpine marmot, estimating that 5% of litters contained at least one individual from an extra-pair mating. However, given that that allozyme markers show a very low genetic variability, this estimate can be considered unreliable (Preleuthner and Pinsker 1993). Subsequently, Arnold et al. (1994) used DNA-fingerprinting to show that 43% of litters of Alpine marmots contained young from extra-pair matings; in those litters, 13% of individuals showed extra-pair paternity. Goossens et al. (1998), using a set of six microsatellite DNA loci, obtained results similar to those of Arnold et al. (1994) in La Grande Sassière population: 37% of litters contained young from extra-pair matings, and almost 20% of individuals in those litters showed extra-pair paternity.

The identity of males who fathered extra-pair young usually remained unknown. Arnold (1990) assumed that subordinate males of the family, especially the subordinate males related to the dominant male, were responsible for extra-pair young. This opportunity to mate might encourage subordinates to remain in the family and thus contribute to social thermoregulation (see the hibernation section). Indeed, subordinate males and females are fertile, as shown by increased androgen levels in the males (Arnold and Dittami 1997) and increased estradiol levels and occasional pregnancies in females (Hacklander et al. 2003).

However, during the mating season, the dominant male increased his antagonistic interactions with subordinate males, while during gestation, the dominant female similarly increased her antagonistic interactions with subordinate females. This rise in antagonistic interactions results in increased glucocorticoid levels in the subordinates of both sexes, which suppresses their reproductive functions (Arnold et Dittami 1997; Hackländer et al. 2003).

Moreover, competition between dominant and subordinate males, and between dominant and subordinate females, becomes much stronger as the genetic relatedness between dominant and subordinate individuals decreases. For example, dominant individuals frequently attack unrelated subordinates of the same sex, whereas interactions between dominant individuals and their sons and daughters generally remain amicable (Arnold et Dittami 1997; Hackländer et al. 2003). These behaviors suggest that suppression of reproduction is mainly directed towards subordinates who are not closely related genetically to the dominant individuals (Arnold et Dittami 1997).

Nevertheless, in La Grande Sassière, microsatellite markers indicate that only 5.7% of litters and 2.2% of young were fathered by a subordinate male of the family group, and this independently of the degree of genetic relatedness between the subordinate male and the dominant male (Goossens et al . 1998).

Reproduction and Rearing the young

Alpine marmots reproduce annually. The two dominant individuals mate within 15 days of coming out of hibernation. The female is receptive for only 24 hours (as observed in a zoo, Müller-Using 1957). It is not uncommon for a dominant female to skip a year between litters: only 66% of dominant females breed any given year (Arnold 1993). Gestation lasts thirty days, with the females giving birth in May. The newborn pups remain in the burrow where they are breast fed for forty days. The weaned pups then emerge from the burrow in between mid-June and mid-July.

Weaned litters contain from one to seven young, with an average of 4.1 young per litter for the population of La Grand Sassière (Allaine et al. 2000). Taken as a whole, more males than females are born in this population (Allaine et al., 2000). However, within a given family group, the male/female birth ratio can vary. In the population of La Grande Sassière, litters born in the absence of subordinate males have a sex ratio skewed in favor of males, whereas litters born in the presence of subordinate males have a balanced sex ratio (Allaine 2004).

breeding

Although only the dominant female provides parental care to its young, the Alpine marmot is considered as a cooperative breeder (Blumstein and Armitage 1999). Individual subordinate males indirectly help raise the young not only through participation in surveillance of the family’s territory, but also through social hibernation (see the hibernation section). Cooperative breeding could explain why the sex ratio at birth is biased towards males: juvenile survival increases in the presence of subordinate males but not in the presence of subordinate females (Arnold 1993; Allaine et al. 2000; Allaine 2004).

Infanticide

Infanticide is far from being an exceptional event. Thus, since 1990, seven instances of infanticide were observed at La Grande Sassière. Each of these infanticides was perpetrated by a male who had recently ousted a dominant male and taken over his territory (Perrin et al. 1994; Coulon et al., 1995; Hackländer and Arnold 1999).

This rate of infanticide is probably an underestimate, given that researchers can identify with certainty only the infanticide of pups who have emerged from the burrow. However, it is extremely rare to see the emergence of a litter when a male takes possession of a territory early in the active period, suggesting that reproduction was somehow blocked or that infanticide occurred inside the natal burrow (Hackländer and Arnold 1999).

Although many hypotheses exist concerning the adaptive value of infanticide, the sexual selection hypothesis seems most likely to apply to the Alpine marmot. This hypothesis suggests that infanticide is a way for the new dominant male to help insure his future reproductive success. Even though the female will not begin a new estrus until the following spring spring in Alpine marmot, the new male would still be limitingover  maternal investment in unrelated young, thus preserving the female’s reproductive potential for the future (Coulon et al., 1995; Hackländer and Arnold 1999).

Bibliography

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