Ecological, evolutionary and social constraints on reproductive effort: are hoary marmots really biennial breeders?

Cumulative reproductive costs on current reproduction in a wild polytocous mammal

The cumulative cost of reproduction hypothesis predicts that reproductive costs accumulate over an individual's reproductive life span. While short-term costs have been extensively explored, the prevalence of cumulative long-term costs and the circumstances under which such costs occur alongside or instead of short-term costs, are far from clear. Indeed, few studies have simultaneously tested for both short-term and cumulative long-term reproductive costs in natural populations. Even in mammals, comparatively little is known about cumulative effects of previous reproduction, especially in species with high variation in offspring numbers, where costs could vary among successful reproductive events. Here, we quantify effects of previous short-term and cumulative long-term reproduction on current reproduction probability and litter size in wild female yellow-bellied marmots (Marmota flaviventer) and test how these effects vary with age and between two contrasting environments. We provide evidence for cumulative long-term effects: females that had both reproduced frequently and weaned large litters on average in previous years had decreased current reproduction probability. We found no evidence for short-term reproductive costs between reproductive bouts. However, females weaned larger litters when they had weaned larger litters on average in previous years and had lower current reproduction probability when their previous reproductive success was low. Together these results suggest that, alongside persistent among-individual variation, long-term reproductive history affects current reproductive success.

Socially mediated effects of climate change decrease survival of hibernating Alpine marmots

In the context of global change, an increasing challenge is to understand the interaction between weather variables and life histories. Species-specific life histories should condition the way climate influences population dynamics, particularly those that are associated with environmental constraints, such as lifestyles like hibernation and sociality. However, the influence of lifestyle in the response of organisms to climate change remains poorly understood. Based on a 23-year longitudinal study on Alpine marmots, we investigated how their lifestyle, characterized by a long hibernation and a high degree of sociality, interacts with the ongoing climate change to shape temporal variation in age-specific survival. As generally reported in other hibernating species, we expected survival of Alpine marmots to be affected by the continuous lengthening of the growing season of plants more than by changes in winter conditions. We found, however, that Alpine marmots displayed lower juvenile survival over time. Colder winters associated with a thinner snow layer lowered juvenile survival, which in turn was associated with a decrease in the relative number of helpers in groups the following years, and therefore lowered the chances of over-winter survival of juveniles born in the most recent years. Our results provide evidence that constraints on life-history traits associated with hibernation and sociality caused juvenile survival to decrease over time, which might prevent Alpine marmots coping successfully with climate change.

Seal mothers expend more on offspring under favourable conditions and less when resources are limited

In mammals, maternal expenditure on offspring is a complex mix of several factors including the species' mating system, offspring sex and the condition and age of the mother. While theory suggests that in polygynous species mothers should wean larger male offspring than females when resources and maternal conditions allow, the evidence for this remains equivocal. Southern elephant seals are highly dimorphic, polygynous capital breeders existing in an environment with highly variable resources and should therefore provide clear evidence to support the theoretical expectations of differential maternal expenditure in male and female pups. We quantified maternal size (mass and length) and pup size at birth and weaning for 342 elephant seal mothers at Macquarie Island. The study was conducted over 11 years of contrasting sea-ice and Southern Annular Mode values, both indices of maternal prey resources. Overall, large females weaned male pups that weighed 17 kg (15·5%) more than female pups. Maternal condition varied by as much as 59 kg among years, and was positively related to Southern Annular Mode, and negatively to maximum sea-ice extent. Smaller mothers weaned relatively larger male pups under favourable conditions, this effect was less apparent for larger mothers. We developed a simple model linking environmental variation to maternal masses post-partum, followed by maternal masses post-partum to weaning masses and then weaning masses to pup survival and demonstrated that environmental conditions affected predicted survival so that the pups of small mothers had an estimated 7% increase in first year survival in 'good' vs. 'bad' years compared to 1% for female pups of large mothers. Co-occurrence of environmental quality and conservative reproductive tactics suggests that mothers retain substantial plasticity in maternal care, enhancing their lifetime reproductive success by adjusting reproductive expenditure relative to both prevailing environmental conditions and their own capabilities.