Beyond infant death: the hidden costs of male immigration in geladas

Male-mediated early maturation unlikely to evolve via adaptive evolution

The Vandenbergh effect, or male-mediated maturation, occurs when females reach sexual maturation upon exposure to a novel male. Male-mediated maturation is found across mammals, including in geladas, Theropithecus gelada, where it may be an adaptive counterstrategy to infanticide that follows the immigration of a new male; maturing after male immigration maximizes a female's chances of weaning her first offspring before the next infanticidal male immigrates (the 'optimal timing hypothesis'). Alternatively, the nonadaptive 'Bruce effect by-product hypothesis' posits that male-mediated maturation in geladas (and possibly other mammals) is triggered by the same physiological changes that, in pregnant females, produce spontaneous abortion (the Bruce effect). We test both hypotheses using theory and observational data. We show that neither male-mediated maturation nor its associated hormonal changes occur in baboons (Papio cynocephalus × P. anubis), a primate without the Bruce effect. An individual-based model suggests that male-mediated maturation should not evolve via adaptive evolution in either geladas or baboons. Finally, we derive the selection coefficient for male-mediated maturation and show it is likely to be very small because male-mediated maturation yields only marginal potential benefits unless the system is extremely fine-tuned. We conclude that male-mediated maturation in geladas is a by-product of the Bruce effect and more broadly that the Vandenbergh effect may be nonadaptive.

Social correlates of androgen levels and dispersal age in juvenile male geladas

Androgens offer a window into the timing of important male life history events such as maturation. However, when males are the dispersing sex, piecing together normative androgen profiles across development is challenging because dispersing males are difficult to track. Here, we examined the conditions that may be associated with male androgen status (via fecal androgen metabolites, fAMs) and age at dispersal in wild male geladas (Theropithecus gelada). Gelada male life histories are highly variable - dispersal may occur before sexual maturation, dispersal itself can be immediate or drawn out, and, due to their multi-leveled society, social conditions affecting dispersal can vary for juveniles living in different reproductive units within the same band. Using longitudinal data from known natal males, we examined how androgen levels and age at dispersal were associated with: (1) access to maternal resources (i.e., maternal rank, birth of a younger sibling, experiencing maternal loss), and (2) access to male peers (i.e., number of similar-aged males in their unit). We found that androgens were significantly lower in males with high-ranking mothers (in males >2.5 years of age; infant androgens were unrelated) and that having more male peers in their social group and larger groups overall predicted an earlier age at dispersal. Moreover, dispersal in geladas was not preceded or followed by a surge in androgen levels. Taken together, results suggest that social environments can cause individual variation in androgens and dispersal age. Whether this variation leads to differences in male fitness in later life remains to be determined.

Fitness consequences of outgroup conflict

In social species across the animal kingdom, conspecific outsiders threaten the valuable resources of groups and their members. This outgroup conflict is recognised as a powerful selection pressure, but we argue that studies explicitly quantifying the fitness consequences need to be broader in scope: more attention should be paid to delayed, cumulative, and third-party fitness consequences, not just those arising immediately to group members involved in physical contests. In the first part of this review, we begin by documenting how single contests can have survival and reproductive consequences either immediately or with a delay. Then, we step beyond contests to describe fitness consequences that can also result from interactions with cues of rival presence and the general landscape of outgroup threat, and beyond single interactions to describe cumulative effects of territorial pressure and elevated outgroup-induced stress. Using examples from a range of taxa, we discuss which individuals are affected negatively and positively, considering both interaction participants and third-party group members of the same or the next generation. In the second part of the review, we provide suggestions about how to move forward. We highlight the importance of considering how different types of outgroup conflict can generate different selection pressures and of investigating variation in fitness consequences within and between species. We finish by discussing the value of theoretical modelling and long-term studies of natural populations, experimental manipulations, and meta-analyses to develop further our understanding of this crucial aspect of sociality.

Variation between species, populations, groups and individuals in the fitness consequences of out-group conflict

Out-group conflict is rife in the natural world, occurring from primates to ants. Traditionally, research on this aspect of sociality has focused on the interactions between groups and their conspecific rivals, investigating contest function and characteristics, which group members participate and what determines who wins. In recent years, however, there has been increasing interest in the consequences of out-group conflict. In this review, we first set the scene by outlining the fitness consequences that can arise immediately to contest participants, as well as a broader range of delayed, cumulative and third-party effects of out-group conflict on survival and reproductive success. For the majority of the review, we then focus on variation in these fitness consequences of out-group conflict, describing known examples both between species and between populations, groups and individuals of the same species. Throughout, we suggest possible reasons for the variation, provide examples from a diverse array of taxa, and suggest what is needed to advance this burgeoning area of social evolution. This article is part of the theme issue 'Intergroup conflict across taxa'.

Social drivers of maturation age in female geladas

Female reproductive maturation is a critical life-history milestone, initiating an individual's reproductive career. Studies in social mammals have often focused on how variables related to nutrition influence maturation age in females. However, parallel investigations have identified conspicuous male-mediated effects in which female maturation is sensitive to the presence and relatedness of males. Here, we evaluated whether the more "classic" socioecological variables (i.e., maternal rank, group size) predict maturation age in wild geladas-a primate species with known male-mediated effects on maturation and a grassy diet that is not expected to generate intense female competition. Females delayed maturation in the presence of their fathers and quickly matured when unrelated, dominant males arrived. Controlling for these male effects, however, higher-ranking daughters matured at earlier ages than lower-ranking daughters, suggesting an effect of within-group contest competition. However, contrary to predictions related to within-group scramble competition, females matured earliest in larger groups. We attribute this result to either: 1) a shift to "faster" development in response to the high infant mortality risk posed by larger groups; or 2) accelerated maturation triggered by brief, unobserved male visits. While earlier ages at maturation were indeed associated with earlier ages at first birth, these benefits were occasionally offset by male takeovers, which can delay successful reproduction via spontaneous abortion. In sum, rank-related effects on reproduction can still occur even when socioecological theory would predict otherwise, and males (and the risks they pose) may prompt female maturation even outside of successful takeovers.

Effects of climate variability on the demography of wild geladas

Nonhuman primates are an essential part of tropical biodiversity and play key roles in many ecosystem functions, processes, and services. However, the impact of climate variability on nonhuman primates, whether anthropogenic or otherwise, remains poorly understood. In this study, we utilized age-structured matrix population models to assess the population viability and demographic variability of a population of geladas (Theropithecus gelada) in the Simien Mountains, Ethiopia with the aim of revealing any underlying climatic influences. Using data from 2008 to 2019 we calculated annual, time-averaged, and stochastic population growth rates (λ) and investigated relationships between vital rate variability and monthly cumulative rainfall and mean temperature. Our results showed that under the prevailing environmental conditions, the population will increase (λ s = 1.021). Significant effects from rainfall and/or temperature variability were widely detected across vital rates; only the first year of infant survival and the individual years of juvenile survival were definitively unaffected. Generally, the higher temperature in the hot-dry season led to lower survival and higher fecundity, while higher rainfall in the hot-dry season led to increased survival and fecundity. Overall, these results provide evidence of greater effects of climate variability across a wider range of vital rates than those found in previous primate demography studies. This highlights that although primates have often shown substantial resilience to the direct effects of climate change, their vulnerability may vary with habitat type and across populations.

The Goldilocks effect: female geladas in mid-sized groups have higher fitness

The cost-benefit ratio of group living is thought to vary with group size: individuals in 'optimally sized' groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here, we present evidence for optimal group size that maximizes female fitness in a population of geladas (Theropithecus gelada). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized groups exhibited the highest reproductive performance. This group size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized groups are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species.