Ranging behaviour is often linked to resource availability within a given habitat, and site fidelity to specific ranges is expected to improve foraging efficiency and thus impact fitness. Yet social animals must also navigate affiliative, in addition to competitive, dynamics of a shared environment. The ranging patterns of highly social species should thus reflect the compromise between both foraging and social needs, and range shifts may reveal important information about socioecology. Species with high fission–fusion dynamics present an interesting system to evaluate these factors since individuals often range alone to mitigate feeding competition but seek out social opportunities when advantageous. Here we assess the influence of habitat quality and social bonds on female range shifts after the death (i.e. removal) of other females in a population of eastern chimpanzees. We combine behavioural, ecological and geospatial data from 40 years, 29 death events and 45 female chimpanzees. We found significant differences in the patterning of range overlap following death events depending on the relationship of surviving females to the deceased. Females with strong social ties to a female prior to that female's death significantly decreased range overlap with the dead female's vacated range compared to females with weak associations to the dead female. Furthermore, females that lost maternal kin subsequently lost habitat quality whereas those who lost strong nonkin associates gained habitat quality. Our findings add to the body of work suggesting that female social bonds shape ranging decisions and highlight the complex balance females must strike between their social investments and ability to access high-quality resources.

In humans and other social mammals, more socially connected females often have higher fitness. Yet evidence linking female sociality to offspring survival remains inconsistent, and is limited to a handful of primate species in which females depend on close female kin for social status. Here, we examine the relationship between female social integration and offspring survival in eastern chimpanzees. We find that females that were more socially integrated with other females in the year before giving birth had higher offspring survival to age 1 (the period of highest mortality) and age 5 (the approximate age of weaning). Furthermore, social integration remained a strong predictor of offspring survival among females without close female kin. Our results thus add to a small set of studies linking sociality with offspring survival, here in the dispersing sex. As in humans, more socially connected female chimpanzees have higher offspring survival, despite primarily residing with non-kin.

Maintaining water balance is imperative for organismal survival. Various socioecological conditions such as sex, reproductive state, and dominance rank can impact water intake and hydration status. Eastern Chimpanzees (Pan troglodytes schweinfurthii) present an interesting biological model to investigate the socioecological correlates of hydration status, given adult sex differences in behavior. Males are more gregarious than females, which may afford them less behavioral flexibility to reduce dehydration risk. However, females invest heavily in their offspring, producing a water heavy milk for up to 5 years, and adult females experience disparities in access to water in foods dependent on dominance rank, which may cause variation in hydration status. In the present study, we investigated correlates of adult chimpanzee hydration status at Gombe National Park, Tanzania, using a measure of urine concentration-urine specific gravity (USG)-highly concentrated urine results in a higher USG, indicating dehydration. We found that both males and females had significantly higher USG in the dry season, but males had lower USG than females during the dry season. Within females, reproductive state did not significantly impact hydration status; however, females in middle lactation tended to have higher USG than nonpregnant, nonlactating females. Additionally, high-ranking females had lower USG than mid- and low-ranking females. While our study showed consistency with USG values that have been demonstrated at other Chimpanzee research sites, we found evidence that this population may be chronically dehydrated by human standards, but more research is needed across populations to determine the health and fitness effects of chronic dehydration in Chimpanzees. Wild chimpanzees live in challenging, seasonal environments-and the population at Gombe National Park, Tanzania-show signs of chronic dehydration. Here we explore how hydration status varies based on socioecological variables such as sex, season, and dominance rank.

Objectives: Many nonhuman primate diets are dominated by plant foods, yet plant tissues are often poor sources of sodium-a necessary mineral for metabolism and health. Among primates, chimpanzees (Pan troglodytes), which are ripe fruit specialists, consume diverse animal, and plant resources. Insects have been proposed as a source of dietary sodium for chimpanzees, yet published data on sodium values for specific foods are limited. We assayed plants and insects commonly eaten by chimpanzees to assess their relative value as sodium sources. Materials and Methods: We used atomic absorption spectroscopy to determine sodium content of key plant foods and insects consumed by chimpanzees of Gombe National Park, Tanzania. Dietary contributions of plant and insect foods were calculated using feeding observational data. Results: On a dry matter basis, mean sodium value of plant foods (n = 83 samples; mean = 86 ppm, SD = 92 ppm) was significantly lower than insects (n = 12; mean = 1549 ppm, SD = 807 ppm) (Wilcoxon rank sum test: W = 975, p < 0.001). All plant values were below the suggested sodium requirement (2000 ppm) for captive primates. While values of assayed insects were variable, sodium content of two commonly consumed insect prey for Gombe chimpanzees (Macrotermes soldiers and Dorylus ants) were four to five times greater than the highest plant values and likely meet requirements. Discussion: We conclude that plant foods available to Gombe chimpanzees are generally poor sources of sodium while insects are important, perhaps critical, sources of sodium for this population.

Maintaining water balance is essential for organismal health, and lactating females must balance individual needs with milk production and offspring hydration. Primate milk is dilute and presumed to be the primary source for infant hydration for a considerable time period. Few studies have investigated the hydration burden that lactation may place on female primates. In this study, we investigated sources of variation in female and offspring drinking frequency among wild chimpanzees (Pan troglodytes). We hypothesized females would experience seasonal and lactation hydration burdens and adjust their drinking behavior to accommodate these, but this hydration burden would vary between females of different dominance ranks. We also predicted that parity would relate to maternal drinking frequency since primiparous females are still investing in their own growth. Finally, we predicted that offspring would drink more in the dry season and as they aged and lost milk as a water source, but that offspring of high-ranking females would be buffered from these effects. Using 41 years of long-term data on the behavior of mothers and offspring of Gombe National Park, we found that mothers drank more in the dry season, but there was no significant difference between mothers of different ranks during this period. Low-ranking females drank significantly more than mid- and high-ranking females during late lactation. Offspring also drank more in the dry season and as they aged, but there was no evidence of buffering for those with high-ranking mothers. While chimpanzees in our study population drank infrequently, they do demonstrate noticeable shifts in drinking behavior that suggests seasonal and reproductive hydration burdens.

A large body of literature demonstrates the adaptive benefits of social relationships between kin, including fitness and survival. Given that most social mammals are characterized by male-biased dispersal, the majority of research on kin selection and associated advantages focuses on social relationships between female kin. Meanwhile, research on social relationships between adult male and female kin has primarily focused on inbreeding avoidance or the benefit to adult sons, with less attention on potential advantages these social relationships may provide females. The general pattern of male dominance over females in most mammal species suggests that females may benefit from protective associations with adult male kin. Using 43 years of behavioural data on the wild chimpanzees, Pan troglodytes schweinfurthii, of Gombe National Park, Tanzania, we examined association patterns between females and their adult maternal male kin. We specifically focused on how these associations may represent a trade-off between inbreeding avoidance and protection for females, particularly against infanticide. In accordance with inbreeding avoidance, we predicted that females' association with adult kin would decrease when they were maximally tumescent, signalling sexual receptivity. To determine whether female-male kin associations provide protection to females, we examined female associations with adult male kin during their first year postpartum when infants are most vulnerable to infanticide. We predicted that during this first year postpartum, females would have a higher association with male kin than with unrelated males. We found that females associated more with adult sons and brothers than with unrelated males when they did not have a sexual swelling. Female association increased with all males across tumescence but females associated less with their brothers than they did with their sons and unrelated males when they were maximally tumescent, providing equivocal support for the inbreeding avoidance hypothesis. Furthermore, females associated more with both sons and brothers than with unrelated males in the first 6 months of the postpartum period. Higher association with brothers, relative to unrelated males, persisted throughout the first year postpartum. Together, these results speak to the cost-benefit trade-off in female and adult male kin associations, highlighting the potential protective advantage for females, especially during the postpartum period. (c) 2022 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Existing data on bonobo and chimpanzee dental eruption timing are derived predominantly from captive individuals or deceased wild individuals. However, recent advances in noninvasive photographic monitoring of living, wild apes have greatly expanded our knowledge of chimpanzee dental eruption in relatively healthy individuals under naturalistic conditions. We employ similar methods to expand on this knowledge by reporting deciduous dental eruption ages in living, wild bonobos and chimpanzees from LuiKotale, Democratic Republic of the Congo and Gombe National Park, Tanzania, respectively. Deciduous dental eruption ages in our sample generally fall within the range of variation previously documented for captive chimpanzees. We also found substantial variation in deciduous canine eruption timing, particularly among bonobos. One bonobo had a deciduous canine present by 227 days old while another did not have a deciduous canine present at 477 days old. As more data accumulate from these populations, future studies should consider sources of variation in deciduous canine eruption timing and relationships with other aspects of life history.

Infectious disease outbreaks pose a significant threat to the conservation of chimpanzees (Pan troglodytes) and all threatened nonhuman primates. Characterizing and mitigating these threats to support the sustainability and welfare of wild populations is of the highest priority. In an attempt to understand and mitigate the risk of disease for the chimpanzees of Gombe National Park, Tanzania, we initiated a long-term health-monitoring program in 2004. While the initial focus was to expand the ongoing behavioral research on chimpanzees to include standardized data on clinical signs of health, it soon became evident that the scope of the project would ideally include diagnostic surveillance of pathogens for all primates (including people) and domestic animals, both within and surrounding the National Park. Integration of these data, along with in-depth post-mortem examinations, have allowed us to establish baseline health indicators to inform outbreak response. Here, we describe the development and expansion of the Gombe Ecosystem Health project, review major findings from the research and summarize the challenges and lessons learned over the past 16 years. We also highlight future directions and present the opportunities and challenges that remain when implementing studies of ecosystem health in a complex, multispecies environment.

Although rare among group-living primates, infanticide by females has been reported in several chimpanzee, Pan troglodytes, populations. We examined 13 infanticidal attacks over 47 years at Gombe National Park, Tanzania to evaluate three adaptive hypotheses. (1) Exploitation of the infant as a food resource -by eating a vulnerable neonate, attackers gain calories that may be important during periods of food scarcity or energetic stress. (2) Resource competition -Gombe females concentrate their foraging in overlapping core areas and dominance rank influences foraging success. By killing the infant of a female with high core area overlap, the perpetrator removes a current and future competitor, improving her access to food. (3) Low cost -female chimpanzees mature and reproduce slowly, and longevity increases reproductive success. Physical aggression causes risk of severe injury or death, so females will only mount attacks when risks to the perpetrator are low. In support of hypothesis 1, females usually consumed the carcass. However, attacks were not more likely in times of resource or energy scarcity. In support of hypothesis 2, females attacked others with whom they shared core areas, but attacks did not cause shifts in ranging patterns. In support of hypothesis 3, one or more attackers always outranked the victim, the attacks often involved coalitions and victims usually lacked kin support. Attacks were more likely to be successful when attackers were not hindered by clinging infants and victims could not retreat. Our results provide further evidence for female competition and the adaptive value of female-led infanticide in this species. (c) 2021 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Fission-fusion dynamics have evolved in a broad range of animal taxa and are thought to allow individuals to mitigate feeding competition. While this is the principal benefit of fission-fusion, few studies have evaluated its costs. We compared gregariousness, foraging budgets, and social budgets between lactating bonobos and chimpanzees from wild populations to evaluate potential costs. Both species exhibit fission-fusion dynamics, but chimpanzees, particularly in East African populations, appear to experience higher feeding competition than bonobos. We expected lactating chimpanzees to be less gregarious than lactating bonobos; reduced gregariousness should allow lactating chimpanzees to mitigate the costs of higher feeding competition without requiring more foraging effort. However, we expected the reduced gregariousness of lactating chimpanzees to limit their time available for affiliative interactions. Using long-term data from LuiKotale bonobos and Gombe chimpanzees, we found that lactating chimpanzees were indeed less gregarious than lactating bonobos, while feeding and travel time did not differ between species. Contrary to our predictions, lactating females did not differ in social interaction time, and lactating chimpanzees spent proportionately more time interacting with individuals other than their immature offspring. Our results indicate that lactating chimpanzees can maintain social budgets comparable to lactating bonobos despite reduced gregariousness and without incurring additional foraging costs. We discuss potential explanations for why lactating bonobos are more gregarious.

OBJECTIVESThe prolonged juvenile period exhibited by primates is an evolutionary conundrum. Here we examine wild chimpanzee feeding development in the context of two hypotheses regarding prolonged development in primates: the needing-to-learn hypothesis and the expensive brain hypothesis.MATERIAL AND METHODSWe studied wild chimpanzee (Pan troglodytes schweinfurthii) offspring at Gombe National Park, Tanzania. We analyzed 41 years of observational behavioral data collected between 1975 and 2016 from 81 offspring. We characterized feeding development in the first 10 years of life via four different measures: (1) proportion of observation time spent feeding; (2) diet composition; (3) diet breadth; and (4) diet maturity as measured by similarity to maternal diet. We used mixed effects models to examine changes with age and by sex, while controlling for season.RESULTSFeeding time, diet breadth, and diet maturity exhibited the most substantial increases with age in the first 6 years, with no significant change thereafter. Males and females showed different patterns of change in diet breadth by age, but did not differ by age 10. Diet composition did not change significantly with age and did not differ by sex.DISCUSSIONWe found that chimpanzee offspring attained adult-like feeding behaviors between 4 and 6 years of age, concomitant with the completion of weaning. Thus, our data do not support the needing-to-learn feeding skills hypothesis of a prolonged juvenile period, but additional data are needed to evaluate how and when adolescent chimpanzees are able to make foraging decisions independent of their mothers. Existing data on growth provides support for the expensive brain hypothesis, however, these hypotheses are not necessarily mutually exclusive. As more studies across taxa accumulate sufficient datasets on a range of developmental metrics, we will be able to achieve a more robust understanding of prolonged development in primates.

Highly differentiated and affiliative social relationships are observed in a variety of mammals, including primates, cetaceans, and social carnivores. Although there has been a transformation in our understanding of the form and function of social bonds in the past two decades, the role of early life social experiences in the development of bonds remains less clear. Few studies have examined whether social relationships during infancy and juvenility (aside from those between mothers and offspring) persist into adulthood. In this study, we used longitudinal data on wild chimpanzees (Pan troglodytes schweinfurthii) at Gombe National Park, Tanzania, to investigate the effects of party-level association during infancy and juvenility (hereafter immaturity) on affiliative relationships among adult males (the philopatric sex). In total, we examined behavioral data from focal follows between 1970 and 2015 (N = 23 adult males). We used maternal associations as proxies for the associations of immature males, and we measured adult male social relationships using party-level associations and grooming activity. We found that immature males that associated with individual adult males at higher rates had stronger relationships with those same adult males later in life. By contrast, rates of association between pairs of immature males did not predict the strength of their dyadic relationships as adults. Overall, these findings emphasize the importance of early socialization in male chimpanzee social development. These results also reinforce studies in mountain gorillas (Gorilla beringei) and bottlenose dolphins (Tursiops sp.), suggesting that the persistence of social relationships that do not involve the mother may be more likely to evolve in long-lived species where young adult males face challenges entering an adult hierarchy composed of stronger and/or more socially experienced competitors.

The study of non-human primate thanatology has expanded dramatically in recent years as scientists seek to understand the evolutionary roots of human death concepts and practices. However, observations of how conspecifics respond to dead individuals are rare and highly variable. Mothers of several species of primate have been reported to carry and continue to interact with dead infants. Such interactions have been proposed to be related to maternal condition, attachment, environmental conditions or reflect a lack of awareness that the infant has died. Here, we tested these hypotheses using a dataset of cases of infant corpse carrying by chimpanzees in Gombe National Park, Tanzania (n= 33), the largest dataset of such cases in chimpanzees. We found that mothers carried infant corpses at high rates, despite behavioural evidence that they recognize that death has occurred. Median duration of carriage was 1.83 days (interquartile range = 1.03-3.59). Using an information theoretic approach, we found no support for any of the leading hypotheses for duration of continued carriage. We interpret these data in the context of recent discussions regarding what non-human primates understand about death.