Biology subjects

Lerch, B. A.

Publications and source records attributed to Lerch, B. A..

7 recordsLinked to original sources

bioRxiv2026

Social costs of group life: Larger groups have less affiliative behavior and more social conflict

The costs and benefits of sociality depend in part upon the size of groups. Therefore, individual behavior should vary across groups of different sizes, reflecting group size-dependent costs and benefits. Here, we test this prediction using a long-term dataset on wild female baboons (Papio cynocephalus x P. anubis). We find that individuals who live in larger groups suffer social costs: compared to animals in smaller groups, they experience more agonistic interactions, fewer grooming interactions, and sparser social networks. These social costs arise, in part, from a pattern in which relationships within dyads become more agonistic as group size increases (i.e., a given dyad exhibits fewer grooming interactions relative to agonistic interactions when it is living in a larger group). While individuals in large groups generally maintain the same number of strong bonds as individuals in small groups, they tend to develop weak bonds with a greater number of social partners that results in, on average, weaker bonds overall. Our results demonstrate that, despite potential benefits to large groups in terms of predation risk and intergroup competition, the social lives of individuals in larger groups involve less affiliation and more conflict in this wild primate population.

animal behavior and cognition↗

bioRxiv2026

Activity budgets, social behavior, and fitness outcomes associated with a baboon group fusion

Many primates exhibit female philopatry and live in stable, female-bonded groups, where females are seldom exposed to novel same-sex conspecifics. Permanent group fusions are unique in bringing unfamiliar female conspecifics together but are rarely documented in these populations. We present a case study on a fusion of two groups from a hybrid population of baboons (Papio cynocephalus x P. anubis ) living in the Amboseli basin of Kenya. We compared the behavior of eight adult females after the fusion to the behavior of females in groups that had not fused and compared pre- and post-fusion fitness outcomes. Following the fusion, the group's activity budget and patterns of agonistic interactions were typical for the study population. Females preferred familiar grooming partners for a short period following the fusion; however, after three months, female grooming patterns were comparable to other groups, indicating rapid social integration. Females from the smaller pre-fusion group were lower ranking than females from the larger group. The fusion occurred following increased human-induced mortality in one of the two groups. From our limited sample, we observed no detectable fitness-related costs of fusion in terms of birth rates or offspring survival, while adult female mortality was low following the fusion. These results demonstrate the flexibility of female baboons in navigating exposure to novel same-sex conspecifics despite a species-typic pattern of female philopatry. Our results support previous research indicating that group fusions most likely occur when groups are too small to retain adult males, defend against predators, or compete with other groups.

animal behavior and cognition↗

bioRxiv2025

Demographic, behavioral, and ecological data from a long-term field study of wild baboons in Amboseli, Kenya

Long-term data sets on individually recognized animals and their environments are critical to understanding animal behavior, evolution, and ecology. However, they are resource- and time-intensive and seldom made publicly available. The Amboseli Baboon Research Project (ABRP) is one of the longest-running studies of a wild mammal population in the world and has collected extensive data on the baboon population of the Amboseli ecosystem in Kenya since 1971. Here, we describe four ABRP data sets newly available to the evolutionary biology, behavioral ecology, and primatology communities: (1) the sizes and demographic compositions of 21 social groups from 1971-2023; (2) the activity budgets of adult females and immatures from 1984-2023; (3) behavioral data on diet for adult females and immatures from 1984-2023; and (4) weather data, including precipitation from 1976-2023 and temperature from 1976-2022. Data are aggregated annually and monthly to enable cross-data set analyses. These data offer a rare longitudinal perspective on behavioral and ecological change in a wild mammal population.

animal behavior and cognition↗

bioRxiv2025

Adaptive dynamics of sexual signaling and discrimination under different mating conditions

A wide variety of animals engage in same-sex sexual behavior (SSB). Some instances of SSB appear to arise from individuals mating indiscriminately without regard to the sex of their partners. Theory suggests that indiscriminate mating strategies can be selected if sexes are not completely distinguishable--that is, without strong signals of sexual identity. We model the coevolution of sexual signaling and discrimination using the adaptive dynamics approach to long-term trait evolution. Our model relaxes two major assumptions from past phenomenological theory. First, we consider a mechanistic mating process with tunable encounter rates and mating costs. Second, we vary reproductive investment roles to allow for better correspondence with empirical examples of SSB. We confirm that coevolution of sexual signaling and discrimination can lead to two distinct equilibria: one with no sexual signals and indiscriminate mating; the other with perfect signaling and exclusively different-sex sexual behavior. Selection for indiscriminate mating is strongest at very low or very high encounter rates. Mating costs lead to selection for reduced overall mating rate, but this can lead to either more or less SSB, depending on how this reduction is achieved. Overall, our model highlights the importance of ecologically relevant parameters for the evolution of SSB.

evolutionary biology↗

bioRxiv2024

Re-evaluating the relationship between female social bonds and infant survival in wild baboons

Over the past few decades studies have provided strong evidence that the robust links between the social environment, health, and survival found in humans also extend to non-human social animals. A number of these studies emphasize the early life origins of these effects. For example, in several social mammals, more socially engaged mothers have infants with higher rates of survival compared to less socially engaged mothers, suggesting that positive maternal social relationships causally improve offspring survival. Here we show that the relationship between infant survival and maternal sociality is confounded by previously underappreciated variation in female social behavior linked to changes in reproductive state and the presence of a live infant. Using data from a population of wild baboons living in the Amboseli basin of Kenya - a population where high levels of maternal sociality have previously been linked to improved infant survival - we find that infant- and reproductive state-dependent changes in female social behavior drive a statistically significant relationship between maternal sociality and infant survival. After accounting for these state-dependent changes in social behavior, maternal sociality is no longer positively associated with infant survival in this population. Our results emphasize the importance of considering multiple explanatory pathways--including third-variable effects--when studying the social determinants of health in natural populations.

animal behavior and cognition↗

bioRxiv2022

A general theory for the dynamics of social populations: Within-group density dependence and between-group processes.

Despite the importance of population structures throughout ecology, relatively little theoretical attention has been paid to understanding the implications of social groups for population dynamics. The dynamics of socially structured populations differ substantially from those of unstructured or metapopulation-structured populations, because social groups themselves may split, fuse, and compete. These "between-group processes" have been suggested to be important drivers of the dynamics of socially-structured populations, but no general theoretical framework exists that can handle various density-dependent between-group processes within a single model. Here, we develop a general framework for the dynamics of socially-structured populations that considers births, deaths, migration, group extinction, fissions, fusions, and between-group competition within a single model. Both logistic growth and an Allee effect are considered for within-group density dependence. We show that the effect of various between-group processes is mediated by their influence on the stable distribution of group sizes, with the ultimate impact on the population determined by the interaction between the altered group size distribution and within-group density dependence. The group level is important to the dynamics of the entire population, since it drives extinction risk, impacts population growth rates, and leads to the emergence of population-level density dependence (even if birth and death rates depend only on group size and not population size). We conclude with a series of case studies that illustrate different ways that age, sex, and class structure impact the dynamics of social populations. In sum, our results make clear the importance of within-group density dependence, between-group dynamics, and the interactions between them for the population dynamics of social species and provide a general, flexible framework for modeling social populations.

ecology↗

bioRxiv2020

Same-sex sexual behavior and selection for indiscriminate mating

The widespread presence of same-sex sexual behavior (SSB) has long been thought to pose an evolutionary conundrum1-3, as participants in SSB suffer the cost of failing to reproduce after expending the time and energy to find a mate. The potential for SSB to occur as part of an optimal strategy has received almost no attention, although indiscriminate sexual behavior may be the ancestral mode of sexual reproduction4. Here, we build a simple model of sexual reproduction and create a theoretical framework for the evolution of indiscriminate sexual behavior. We provide strong support for the hypothesis that SSB is likely maintained by selection for indiscriminate sexual behavior, by showing that indiscriminate mating is the optimal strategy under a wide range of conditions. Further, our model suggests that the conditions that most strongly favor indiscriminate mating were likely present at the origin of sexual behavior. These findings have implications not only for the evolutionary origins of SSB, but also for the evolution of discriminate sexual behavior across the animal kingdom.

evolutionary biology↗