[Show abstract][Hide abstract] ABSTRACT: Sociality should evolve when the fitness benefits of group living outweigh the costs. Theoretical models predict an optimal group size maximizing individual fitness. However, beyond the number of individuals present in a group, the characteristics of these individuals, like their sex, are likely to affect the fitness payoffs of group living. Using 20 years of individually based data on a social mammal, the Alpine marmot (Marmota marmota), we tested for the occurrence of an optimal group size and composition, and for sex-specific effects of group characteristics on fitness. Based on lifetime data of 52 males and 39 females, our findings support the existence of an optimal group size maximizing male fitness and an optimal group composition maximizing fitness of males and females. Additionally, although group characteristics (i.e., size, composition and instability) affecting male and female fitness differed, fitness depended strongly on the number of same-sex subordinates within the social group in the two sexes. By comparing multiple measures of social group characteristics and of fitness in both sexes, we highlighted the sex-specific determinants of fitness in the two sexes and revealed the crucial role of intrasexual competition in shaping social group composition.
[Show abstract][Hide abstract] ABSTRACT: In social species, the hierarchical status of an individual has important consequences for its fitness. While many studies have focused on individual condition to explain access to dominance, very few have investigated the influence of the social environment, especially during early life. Yet it is known that environmental conditions early in life may influence several traits at adulthood. Here, we examine the influence of early social environment on accession to dominance by investigating the influence of litter size and sex composition on survival and the probability of ascending to dominance later in life using a 20-year dataset from a wild population of Alpine marmots (Marmota marmota). Although litter size had no effect on the fate of individuals, litter sex composition affected male juvenile survival and both male and female probabilities of reaching dominant status when adult. Male juveniles incur lower survival when the number of male juveniles in the litter increases, and individuals of both sexes from male-biased litters are more likely to become dominant than individuals from female-biased litters. However, the absolute number of sisters in the litter, rather than the sex ratio, seems to be an important predictor of the probability of acquiring dominant status: pups having more sisters are less likely to become dominant. Several potential mechanisms to explain these results are discussed.
[Show abstract][Hide abstract] ABSTRACT: Studying the different roles of adaptive genes is still a challenge in evolutionary ecology and requires reliable genotyping of large numbers of individuals. Next-generation sequencing (NGS) techniques enable such large-scale sequencing, but stringent data processing is required. Here, we develop an easy to use methodology to process amplicon-based NGS data and we apply this methodology to reliably genotype four major histocompatibility complex (MHC) loci belonging to MHC class I and II of Alpine marmots (Marmota marmota). Our post-processing methodology allowed us to increase the number of retained reads. The quality of genotype assignment was further assessed using three independent validation procedures. A total of 3069 high-quality MHC genotypes were obtained at four MHC loci for 863 Alpine marmots with a genotype assignment error rate estimated as 0.21%. The proposed methodology could be applied to any genetic system and any organism, except when extensive copy-number variation occurs (that is, genes with a variable number of copies in the genotype of an individual). Our results highlight the potential of amplicon-based NGS techniques combined with adequate post-processing to obtain the large-scale highly reliable genotypes needed to understand the evolution of highly polymorphic functional genes.Heredity advance online publication, 11 March 2015; doi:10.1038/hdy.2015.13.
[Show abstract][Hide abstract] ABSTRACT: Although long neglected, female competition is widespread and may have stronger evolutionary consequences than previously thought. In singular breeders, reproductive success is conditional on social status, and intrasexual competition for the dominant position can be particularly severe in females. Because the ability of females to secure the dominant position may strongly influence their fitness, a better understanding of the determinants of dominance maintenance is needed. Using a 21-year data set on Alpine marmots, Marmota marmota, we investigated the potential drivers of female dominance loss in a singularly breeding mammal. Particularly, we tested whether the dominant females' ability to retain their social position depended on the number and the characteristics (age, relatedness to dominant individuals) of potential competitors in the social unit. To identify the potential underlying mechanism, we further investigated how the number of subordinate females affected females' competitive ability. We found that the risk of losing the dominant position increased with the number of subordinate adult females in the group, but that the age of the subordinate females and their parentage relationships to the dominant individuals were unlikely to be driving dominance loss in this species. Moreover, when the number of potential competitors in the social unit increased, we observed a difference in body mass increase between the dominant and her subordinates leading to a decrease in the body mass difference between dominant females and their subordinate adult females, and ultimately to a higher risk of females losing the dominant position. Overall, our results showed that the number of potential competitors in a social group affects the females' ability to secure their dominant position, and suggested that this effect is mediated through changes in female body mass.
No preview · Article · Dec 2013 · Animal Behaviour
[Show abstract][Hide abstract] ABSTRACT: The way that plants and animals respond to climate change varies widely among species, but the biological features underlying their actual response remains largely unknown. Here, from a 20-year monitoring study, we document a continuous decrease in litter size of the Alpine marmot (Marmota marmota) since 1990. To cope with harsh winters, Alpine marmots hibernate in burrows and their reproductive output should depend more on spring conditions compared to animals that are active year-round. However, we show that litter size decreased over time because of the general thinning of winter snow cover that has been repeatedly reported to occur in the Alps over the same period, despite a positive effect of an earlier snowmelt in spring. Our results contrast markedly with a recent study on North American yellow-bellied marmots, suggesting that between-species differences in life histories can lead to opposite responses to climate change, even between closely related species. Our case study therefore demonstrates the idiosyncratic nature of the response to climate change and emphasizes, even for related species with similar ecological niches, that it may be hazardous to extrapolate life history responses to climate change from one species to another.
[Show abstract][Hide abstract] ABSTRACT: Sex-specific senescence has been commonly reported in highly dimorphic and polygynous species. However, whether between-sex differences in senescence occur in monogamous and monomorphic species is poorly known. In this study, we used an extensive dataset of 20 years of mass measurements on free-ranging male and female Alpine marmots (Marmota marmota), a medium-sized, long-lived, social and hibernating mammal, to assess sex-specific patterns of senescence in body mass. We tested for the occurrence of both a decrease in body mass scaled to absolute age (called chronological senescence) and a decrease in body mass scaled to individual age at death (called terminal decline). Whereas males showed evidence of both chronological senescence and terminal decline in body mass, females did not show any detectable senescence in body mass. This unexpected between-sex difference of senescence in a species subject to weak sexual selection might be shaped either by costs of an asymmetric intra-sex competition for mates or by costs of social thermoregulation.
[Show abstract][Hide abstract] ABSTRACT: Movement is fundamental to individual and population dynamics, as it allows individuals to meet their basic requirements. Although movement patterns reflect interactions between internal and external factors, only few studies have examined the effects of these factors on movement simultaneously, and they generally focused on particular biological contexts (e.g. dispersal, foraging). However, the relative importance of these factors in driving individual routine movements might reflect a species' potential flexibility to cope with landscape changes and therefore buffer their potential impact on fitness. We used data from GPS collars on Scandinavian brown bears to investigate the relative role of these factors, as well as an additional factor (period of the year) on routine movements at two spatial scales (hourly and daily relocations). As expected, internal factors played a major role in driving movement, compared to external factors at both scales, but its relative importance was greater at a finer scale. In particular, the interaction between reproductive status and period of the year was one of the most influential variables, females being constrained by the movement capacity of their cubs in the first periods of the year. The effect of human disturbance on movement was also greater for females with cubs than for lone females. This study showed how reciprocal modulation of internal and external factors is shaping space use of brown bears. We stress that these factors should be studied simultaneously to avoid the risk of obtaining context-dependent inferences. Moreover, the study of their relative contribution is also highly relevant in the context of multiple-use landscapes, as human activities generally affect the landscape more than they affect the internal states of an individual. Species or individuals with important internal constraints should be less responsive to changes in their environment as they have less freedom from internal constraints and should thus be more sensitive to human alteration of the landscape, as shown for females with cubs in this study.
Full-text · Article · Oct 2012 · Journal of Animal Ecology