Sophie Lardy

Paternity and Dominance Loss in Male Breeders: The Cost of Helpers in a Cooperatively Breeding Mammal

Paternity insurance and dominance tenure length are two important components of male reproductive success, particularly in species where reproduction is highly skewed towards a few individuals. Identifying the factors affecting these two components is crucial to better understand the pattern of variation in reproductive success among males. In social species, the social context (i.e. group size and composition) is likely to influence the ability of males to secure dominance and to monopolize reproduction. Most studies have analyzed the factors affecting paternity insurance and dominance tenure separately. We use a long term data set on Alpine marmots to investigate the effect of the number of subordinate males on both paternity insurance and tenure of dominant males. We show that individuals which are unable to monopolize reproduction in their family groups in the presence of many subordinate males are likely to lose dominance the following year. We also report that dominant males lose body mass in the year they lose both paternity and dominance. Our results suggest that controlling many subordinate males is energetically costly for dominant males, and those unable to support this cost lose the control over both reproduction and dominance. A large number of subordinate males in social groups is therefore costly for dominant males in terms of fitness.

Do pre- and post-copulatory sexually selected traits covary in large herbivores?

BackgroundIn most species, males compete to gain both matings (via pre-copulatory competition) and fertilizations (via post-copulatory competition) to maximize their reproductive success. However, the quantity of resources devoted to sexual traits is finite, and so males are predicted to balance their investment between pre- and post-copulatory expenditure depending on the expected pay-offs that should vary according to mating tactics. In Artiodactyla species, males can invest in weapons such as horns or antlers to increase their mating gains or in testes mass/sperm dimensions to increase their fertilization efficiency. Moreover, it has been suggested that in these species, males with territory defence mating tactic might preferentially increase their investment in post-copulatory traits to increase their fertilization efficiency whereas males with female defence mating tactic might increase their investment in pre-copulatory sexually selected traits to prevent other males from copulating with females. In this study, we thus test the prediction that male’s weapon length (pre-copulatory trait) covaries negatively with relative testes size and/or sperm dimensions (post-copulatory traits) across Artiodactyla using a phylogenetically controlled framework.ResultsSurprisingly no association between weapon length and testes mass is found but a negative association between weapon length and sperm length is evidenced. In addition, neither pre- nor post-copulatory traits were found to be affected by male mating tactics.ConclusionsWe propose several hypotheses that could explain why male ungulates may not balance their reproductive investment between pre- and post-copulatory traits.

Sex-specific determinants of fitness in a social mammal

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.

Litter sex composition influences dominance status of Alpine marmots (Marmota marmota)

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.

Stress levels of dominants reflect underlying conflicts with subordinates in a cooperatively breeding species

Maintaining dominance status had long been considered to be less stressful than subordination. However, no consistency in stress levels of dominant and subordinate individuals has been demonstrated. Tactics used to achieve and maintain dominance could be determinant. In cooperatively breeding species, conflicts between dominants and subordinates are expected since dominant individuals tend to monopolize reproduction while subordinates seldom reproduce. Reproductive skew models predict that subordinates’ reproductive opportunities are either allotted or subject to competition with dominants. In the former case, no policing of subordinates by dominants is expected. In the latter, dominant should exert a control over the subordinates possibly leading to higher stress levels in dominants than in subordinates, which could be further elevated as the number of potential competitors in the group increases. In the present study, we aimed to test these hypotheses by assessing individual’s stress level using the neutrophils to lymphocytes ratio (N:L) in a wild cooperatively breeding rodent, the Alpine marmot (Marmota marmota). We found that dominants exhibit higher N:L ratio than subordinates and that dominants’ N:L ratio increases with the number of unrelated same-sex subordinates in the group. We conclude that controlling unrelated subordinates is stressful for dominants, as expected under tug-of-war models. These stress patterns reveal conflicting relationships between dominants and subordinates over the reproduction and social status acquisition. This study highlights the influence of the nature, strength, and direction of conflicts on stress levels.Significance StatementIn cooperatively breeding species, reproductive skew models predict that subordinates’ reproductive opportunities are either allotted or subject to competition with dominants and, thus, can modulate the relative stress level between dominants and subordinates. In the first case, no policing of subordinates by dominants is expected, while in the second, dominant should exert a control over the subordinates which should lead to higher stress level in dominants than in subordinates, and particularly when subordinates are unrelated to the dominants. In Alpine marmots, we found that dominants exhibit higher stress level than subordinates and dominants’ stress level increased with the number of unrelated same-sex subordinates. These patterns are in agreement with the predictions of the tug-of war models of reproductive skew and indicate that controlling subordinates is costly for dominants.