Is there an optimal number of helpers in Alpine marmot family groups?

Abstract

The consequence of helping behavior on breeders fitness is still controversial. We used multivariate analyses to investigate for the effects of male and female subordinates on breeders' components of fitness in the Alpine marmot, Marmota marmota. We found that male and female subordinates, respectively, increased and decreased juvenile survival during winter. Thus, we give evidence that male subordinates should be considered as helpers, and that helpers provided breeders with immediate reproductive success gains, whereas subordinates females were costly. Helpers had no positive effects on female body condition, on persistence (future survival) of dominants, and on future reproduction (occurrence and size of a litter). Helpers thus did not provide breeders with delayed fitness benefits, and therfore, the load-lightening hypothesis was not supported. On the contrary, helpers had delayed fitness cost for dominant males and, consequently, for dominant females. Immediate benefits counterbalanced by delayed costs suggested an optimal number of helpers in the family group both from male and female perspectives. An optimality model well predicted the observed mean number of helpers in Alpine marmot family groups. Optimal numbers of helpers were slightly different for males and females, suggesting a potential conflict of interest between dominants. We finally discuss the possible mechanisms of helping that may explain the observed pattern in the Alpine marmot.

Helpers influence on territory use and maintenance in Alpine marmot groups

“…However, it should be pointed out that the potential benefits deriving from the presence of helpers in a group might be outweighed by interference competition for food among members of a group (Vahl et al 2007), or by possible costs for the dominant male of having a larger pool of possible competitors in the group. Indeed the probability of dominant male persistence in a group has been shown to drastically drop down when helpers are more than three in Alpine marmots (Allainé & Theuriau 2004). Relationship between a) balanced Male Index and b) Group Index on the relative shared area (i.e.…”

“…Like all the species of the genus Marmota, Alpine marmots hibernate during the winter, and the active season usually starts in April and lasts until October, depending on climate conditions (Zelenka 1965). Alpine marmots are cooperative breeders (Allainé & Theuriau 2004) that form family groups of up to 20 individuals (Arnold & Dittami 1997), generally made of a pair of dominant individuals, subordinate adults and yearlings, and young of the year (Arnold 1990;Allainé 2000). Dominants generally inhibit the reproduction of subordinates (Arnold & Dittami 1997;Hackländer et al 2003).…”

Cooperative breeding and monogamy in mammalian societies

“…We include species as cooperative breeders only if a proportion of females do not breed regularly and show alloparental care (such as contributing to provisioning or carrying young born to other females) [3,4,11]; as communal breeders, if most adult females breed regularly and share care such as allonursing or feeding offspring [1]; and as social breeders, if females live in groups and are neither cooperative or communal breeders. Our definition of cooperative breeding consequently excludes some species that are sometimes regarded as cooperative breeders, including species, like alpine marmots [16], where there is no evidence that females modify their behaviour in order to benefit juveniles born to other females and others, like banded mongooses [17], where breeding females are assisted by males whose contributions to cooperative rearing could represent a form of parental investment or courtship [18,19]. Information on the occurrence of cooperative and communal breeding was collected from relevant reviews, as well as during our systematic search of the available literature on social systems (see below).…”

Life-History Responses to the Altitudinal Gradient

“…Similarly, pups of the alpine marmot Marmota marmota, due to small sizes, have reduced thermal inertia and take advantage of the energy spent by all family members during hibernation ('social thermoregulation'; Arnold 1988). Young survival, in particular, is positively associated with the number of subordinate males, which also participate in the surveillance of the family's territory (Arnold 1993;Allainé and Theuriau 2004).…”