Influences of climatic and social environment on variable maternal allocation among offspring in Alpine marmots

In an environment with limited resources, parents may trade-off the number of offspring produced against offspring mass. To maximize fitness under unpredictable environments, females must not only maximize mean annual reproductive success but also minimize between-year variation in reproductive success. Thus, preferred strategies of maternal allocation might be to maximize the mass of their offspring or to produce a number of offspring of variable body masses. Many social species have evolved in variable and unpredictable environments where only the social environment can be predicted. If mothers seem to alter their total reproductive allocation to offspring depending on their social environment, how the total expenditure is allocated between the different offspring is still unknown. Here, we analysed how climatic and social environments influence strategies of maternal allocation and how these strategies impact pup first-year survival in a wild population of Alpine marmots monitored between 1990 and 2016. We found that females acted as income breeders using resources immediately available for reproduction. Our results showed that the proportion of maternal mass allocated to offspring varied mainly with litter size. However, how maternal allocation is shared between pups depended on climatic and social environments. In general, mothers tended to have litters of greater average mass and small variability in favourable social environments or when resources are abundant and lighter average pup mass but high variability in unfavourable social environments or when resources are scarce. This variable allocation could correspond to dynamic bet-hedging such that mothers influence the variance of pup mass within the litter in response to poor current environmental conditions. Our analysis of first-year survival showed that females should maximize the body mass of their young whatever the conditions will be because pups of higher mass have higher survival, regardless of environmental conditions. When resources are scarce, this strategy might not be achievable for all pups so that mothers produced variable pups. In large litters, this strategy increased first-year survival. Because pup variability affects parental fitness, differential allocation between pups of the same litter could have large consequences on fitness and thus on reproductive strategies of social species.