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The interaction between infant feeding and maternal lactational physiology influences female inter-birth intervals and mediates maternal reproductive trade-offs. We investigated variation in feeding development in 72 immature wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda, and made inferences about maternal lactation over the course of infancy. We compared the percentage (%) of time that mothers nursed infants as a function of infant age and assessed how hourly rates and bout durations of nursing and foraging varied in association with differences in offspring age, sex, and maternal parity. Nursing % times, rates and durations were highest for infants ≤ 6 months old but did not change significantly from 6 months to 5 years old. Nursing continued at a decreasing rate for some 5- to 7-year-olds. Infants ≤ 6 months old foraged little. Foraging rates did not change after 1 year old, but foraging durations and the % time devoted to foraging increased with age. Independent foraging probably became a dietary requirement for infants at 1 year old, when their energy needs may have surpassed the available milk energy. Infants spent as much time foraging by the time they were 4 to 5 years old as adults did. No sex effect on infant nursing or foraging was apparent, but infants of primiparous females had higher foraging rates and spent more time foraging than the infants of multiparous females did. Although no data on milk composition were collected, these findings are consistent with a working hypothesis that like other hominoids, chimpanzee mothers maintained a fixed level of lactation effort over several years as infants increasingly supplemented their growing energy, micronutrient and hydration needs via independent foraging. Plateauing lactation may be a more widespread adaptation that allows hominoid infants time to attain the physiology and skills necessary for independent feeding, while also providing them with a steady dietary base on which they could rely consistently through infancy, and enabling mothers to maintain a fixed, predictable level of lactation effort. [ABSTRACT FROM AUTHOR]

Many animals engage in aggression, but chimpanzees stand out in terms of fatal attacks against adults of their own species. Most lethal aggression occurs between groups, where coalitions of male chimpanzees occasionally kill members of neighboring communities that are strangers. However, the first observed cases of lethal violence in chimpanzees, which occurred at Gombe, Tanzania in the 1970s, involved chimpanzees that once knew each other. They followed the only observed case of a permanent community fission in chimpanzees. A second permanent fission recently transpired at Ngogo, Kibale National Park, Uganda. Members of a large western subgroup gradually ceased associating peacefully with the rest of the community and started behaving antagonistically toward them. Affiliation effectively ended by 2017. Here, we describe two subsequent lethal coalitionary attacks by chimpanzees of the new western community on males of the now separate central community, one in 2018 and the second in 2019. The first victim was a young adult male that never had strong social ties with his attackers. The second was a high-ranking male that had often associated with the western subgroup before 2017; he groomed regularly with males there and formed coalitions with several. Other central males present at the start of the second attack fled, and others nearby did not come to the scene. Several western females joined in the second attack; we suggest that female–female competition contributed to the fission. This event highlighted the limits on protection afforded by long-term familiarity and the constraints on costly cooperation among male chimpanzees. [ABSTRACT FROM AUTHOR]

Highly frugivorous primates like chimpanzees ( Pan trogolodytes) must contend with temporal variation in food abundance and quality by tracking fruit crops and relying more on alternative foods, some of them fallbacks, when fruit is scarce. We used behavioral data from 122 months between 1995 and 2009 plus 12 years of phenology records to investigate temporal dietary variation and use of fallback foods by chimpanzees at Ngogo, Kibale National Park, Uganda. Fruit, including figs, comprised most of the diet. Fruit and fig availability varied seasonally, but the exact timing of fruit production and the amount of fruit produced varied extensively from year to year, both overall and within and among species. Feeding time devoted to all major fruit and fig species was positively associated with availability, reinforcing the argument that chimpanzees are ripe fruit specialists. Feeding time devoted to figs-particularly Ficus mucuso (the top food)-varied inversely with the abundance of nonfig fruits and with foraging effort devoted to such fruit. However, figs contributed much of the diet for most of the year and are best seen as staples available most of the time and eaten in proportion to availability. Leaves also contributed much of the diet and served as fallbacks when nonfig fruits were scarce. In contrast to the nearby Kanywara study site in Kibale, pith and stems contributed little of the diet and were not fallbacks. Fruit seasons (periods of at least 2 months when nonfig fruits account for at least 40% of feeding time; Gilby & Wrangham., Behavioral Ecology and Sociobiology 61:1771-1779, 2007) were more common at Ngogo than Kanyawara, consistent with an earlier report that fruit availability varies less at Ngogo [Chapman et al., African Journal of Ecology 35:287-302, 1997]. F. mucuso is absent at Kanyawara; its high density at Ngogo, combined with lower variation in fruit availability, probably helps to explain why chimpanzee population density is much higher at Ngogo. [ABSTRACT FROM AUTHOR]

Chimpanzees ( Pan troglodytes) are ecologically flexible omnivores with broad diets comprising many plant and animal foods, although they mostly eat fruit (including figs). Like other ecologically flexible nonhuman primates (e.g., baboons, Papio spp.) with broad diets, their diets vary across habitats. Much data on diets come from short studies that may not capture the range of variation, however, and data are scant on variation within habitats and populations. We present data on diet composition and diversity for chimpanzees at Ngogo, in Kibale National Park, Uganda, collected over a 15-year period, with a focus on the plant components of the diet. We compare Ngogo data to those on chimpanzees at the nearby Kibale site of Kanyawara, on other chimpanzee populations, and on some other frugivorous-omnivorous primates. Results support the argument that chimpanzees are ripe fruit specialists: Ngogo chimpanzees ate a broad, mostly fruit-based diet, feeding time devoted to fruit varied positively with fruit availability, and diet diversity varied inversely with fruit availability. Comparison of Ngogo and Kanyawara shows much similarity, but also pronounced within-population dietary variation. Chimpanzees fed much more on leaves, and much less on pith and stems, at Ngogo. Figs accounted for somewhat less feeding time at Ngogo, but those of Ficus mucuso were quantitatively the most important food. This species is essentially absent at Kanayawara; its abundance and high productivity at Ngogo, along with much higher abundance of several other important food species, help explain why chimpanzee community size and population density are over three times higher at Ngogo. High inter-annual variation at Ngogo highlights the value of long-term data for documenting the extent of ecological variation among chimpanzee populations and understanding how such variation might affect population biology and social dynamics. Am. J. Primatol. 73:1-16, 2011. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Several recent studies have documented considerable intraspecific and intrapopulation ecological variation in primates. However, we generally lack an understanding of how such variability may be linked to concomitant demographic variation among groups or populations of the same species, particularly in regard to large-bodied and wide-ranging species with high ecological flexibility, such as chimpanzees ( Pan troglodytes). We compared the feeding ecology of chimpanzees inhabiting 2 sites in Kibale National Park, Uganda that differ 3-fold in chimpanzee density and support notably different plant communities. Chimpanzees at Ngogo, a site with the largest known chimpanzee community and unusually high chimpanzee density, spent a significantly lower percentage of time resting (and pregnant and lactating females spent more time feeding), incorporated higher percentages of ripe fruit in their diet, had lower dietary diversity values, and had shorter and less variable average patch residency times than did their counterparts at the nearby Kanyawara site, which supports a relatively low density of chimpanzees. In addition, feeding party size was significantly and positively related to feeding patch size at Ngogo, but not at Kanyawara. Together these findings aid in explaining the noted disparity in chimpanzee community size and density between Ngogo and Kanyawara by suggesting that the diet of Ngogo chimpanzees is of higher overall quality than that of Kanyawara chimpanzees. They also highlight the potentially profound influence of even small-scale habitat heterogeneity on the ecology of primates. Researchers must take such influences into account when attempting to draw conclusions about species- or population-level characteristics. [ABSTRACT FROM AUTHOR]

Chimpanzees make and use a wide variety of tools in the wild. The size and composition of their toolkits vary considerably among populations and at least to some extent within them. Chimpanzees at several well documented sites mostly use tools in extractive foraging, and extractive tool use can substantially increase their foraging efficiency. They also use tools for hygiene and for several other purposes, including attracting the attention of conspecifics, as in leaf-clipping. Some of the interpopulation variation in toolkits results from ecological variation, but differences in the efficiency of social transmission, perhaps related to differences in social tolerance, presumably also contribute. I describe tool use by chimpanzees in an unusually large community at Ngogo, in Kibale National Park, Uganda. Researchers have described some tool use for the community previously, but this is the most extensive report and is based on observations over 11 yr. The Ngogo chimpanzees have a small toolkit and use tools rarely except in leaf-clipping displays and to clean body surfaces; notably, males often use leaf napkins to wipe their penes after copulation. Extractive tool use is rare and is limited mostly to leaf-sponging and, less often, honey-fishing. Social tolerance is not low at Ngogo, but use of tools for extractive foraging, in ways documented at other field sites, may have little potential to increase foraging efficiency. Future research will undoubtedly show more tool use by females, which were underrepresented in my observations, but will probably not document much increase in the toolkit or in the use of extractive tools. [ABSTRACT FROM AUTHOR]

Chimpanzees (Pan troglodytes) have hostile intergroup relations throughout most or all of their geographic range. Hostilities include aggressive encounters between members of neighboring communities during foraging and during patrols in which members of one community search for neighbors near territory boundaries. Attacks on neighbors involve coalitions of adult males, and are sometimes fatal. Targets include members of all age/sex classes, but the risk of lethal intergroup coalitionary aggression is highest for adult males and infants, and lowest for sexually swollen females. The best-supported adaptive explanation for such behavior is that fission-fusion sociality allows opportunities for low-cost attacks that, when successful, enhance the food supply for members of the attackers' community, improve survivorship, and increase female fertility. We add to the database on intergroup coalitionary aggression in chimpanzees by describing three fatal attacks on adult males, plus a fourth attack on an adult male and an attack on a juvenile that were almost certainly fatal. Observers saw four of these attacks and inferred the fifth from forensic and behavioral evidence. The attackers were males in two habituated, unprovisioned communities (Ngogo and Kanyawara) in Kibale National Park, Uganda. We also summarize data on other intercommunity attacks at Ngogo. Our observations are consistent with the “imbalance of power” hypothesis [Manson & Wrangham, Current Anthropology 32:369–390, 1991] and support the argument that lethal coalitionary intergroup aggression by male chimpanzees is part of an evolved behavioral strategy. Am. J. Primatol. 68:161-180, 2006. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Territorial boundary patrols are a distinctive and unique aspect of wild chimpanzee, Pan troglodytes, behaviour. Although patrolling has been frequently observed in nature and several proposed functional explanations for it exist, scant information is available regarding the proximate factors that affect this activity. We found that there is considerable temporal variation in the frequency of patrolling by chimpanzees at Ngogo, Kibale National Park, Uganda. We examined several ecological and social factors that potentially influence this variation. Results of logistic regression analyses revealed that male party size was the single best predictor of the tendency to patrol. Lethal intergroup aggression occurs in chimpanzees, and patrols are likely to be dangerous and costly. Our findings are consistent with the hypothesis that chimpanzees at Ngogo reduce these costs by patrolling together in large parties. [Copyright &y& Elsevier]

In an attempt to describe hormone–behavior interactions in a sample of wild male chimpanzees, we quantified testosterone in 67 fecal samples obtained from 22 adult male chimpanzees at Ngogo, Kibale National Park, Uganda. A mixed-model methodology that controlled for age-class identified a significant positive association between testosterone levels and dominance rank. The results are consistent with those reported from a separate, smaller chimpanzee community in the same population in a study that analyzed testosterone levels in urine [Muller & Wrangham, 2004]. As in that earlier study, our results held during a period of social stability, which is not consistent with predictions of the “challenge hypothesis.” We concur with Muller and Wrangham [2004] that the challenge hypothesis requires modification to explain the chimpanzee data, because fission-fusion sociality in chimpanzees makes challenges unpredictable. We also discuss the utility of fecal samples and a mixed-model statistical method for behavioral endocrinology studies. Am. J. Primatol. 64:71–82, 2004. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Chimpanzees (Pan troglodytes ) prey on a variety of vertebrates, mostly on red colobus (Procolobus spp. ) where the two species are sympatric. Variation across population occurs in hunting frequency and success, in whether hunting is cooperative, i.e., payoffs to individual hunters increase with group size, and in the extent to which hunters coordinate their actions in space and time, and in the impact of hunting on red colobus populations. Also, hunting frequency varies over time within populations, for reasons that are unclear. We present new data on hunting by chimpanzees at Ngogo, Kibale National Park, Uganda, and combine them with earlier data (Mitani and Watts, 1999, Am. J. Phys. Anthropol. 109: 439–454) to examine hunting frequency and success, seasonality, and cooperation. The Ngogo community is the largest and has the most males of any known community. Chimpanzees there mostly hunt red colobus and are much more successful and make many more kills per hunt than at other sites; they kill 6–12% of the red colobus population annually. The number of kills and the offtake of meat per hunt increase with the number of hunters, but per capita meat intake is independent of hunting party size; this suggests that cheating occurs in large parties. Some behavioral cooperation occurs. Hunting success and estimated meat intake vary greatly among males, partly due to dominance rank effects. The high overall success rate leads to relatively high average per capita meat intake despite the large number of consumers. The frequency of hunts and of hunting patrols varies positively with the availability of ripe fruit; this is the first quantitative demonstration of a relationship between hunting frequency and the availability of other food, and implies that the chimpanzees hunt most when they can easily meet energy needs from other sources. We provide the first quantitative support for the argument that variation in canopy structure influences decisions to hunt red colobus because hunts are easier where the canopy is broken. [ABSTRACT FROM AUTHOR]

Chimpanzees are among the few mammals that engage in lethal coalitionary aggression between groups. Most attacks on neighbors occur when parties made up mostly of adult males patrol boundaries of their community's range. Patrols have time, energy, and opportunity costs, and entail some risks despite the tendency of males to attack only when they greatly outnumber their targets. These factors may lead to a collective action problem. Potential benefits include protection of community members, particularly infants; range expansion and increases in the amount and quality of food available; and incorporation of more females into the community. Males may not share these equally; for example, those able to obtain large shares of matings may stand to gain most by participating in patrols and to lose most by refraining. Despite the attention that boundary patrolling has attracted, few relevant quantitative data are available. Here, we present detailed data on boundary patrolling and intergroup aggression in a chimpanzee community at Ngogo, Kibale National Park, Uganda, that is unusually large and has more males than any other known community. Males there patrolled much more often, and patrol parties were much larger on average, than at two other sites for which comparative data exist. Our findings support the argument that male participation varies along with variation in potential gains, in willingness to take risks, and in skill at handling these risks. Both the overall frequency with which individual males patrolled and their willingness to join patrols as others set off on them were positively associated with variation in mating success, in participation in hunts of red colobus monkeys, and in hunting success. Males patrolled relatively often with others with whom they associated often in general, with whom they often groomed, and with whom they formed coalitions in withincommunity agonism. This indicates that they were most willing to take risks associated with patrolling when with others they trusted to take the same risks. [ABSTRACT FROM AUTHOR]

Allogrooming contributes to the development and maintenance of social relationships, including those that involve alliances, in many primate species. Variation in relatedness, dominance rank, and other factors can produce variation in the value of others as grooming partners. Several models have been developed to account for variation in the distribution of grooming in relation to dominance ranks. These start from the premise that individuals are attracted to high-ranking partners, but time limits, direct competition, and prior grooming engagement between high-ranking individuals can constrain access to them. Sambrook et al. (1995) formalized some of these models and showed the importance of taking group size variation into account when assessing them. Chimpanzees form multimale communities in which males are the philopatric sex. Males commonly associate and groom with each other; they also form dominance hierarchies and form alliances that influence dominance ranks and mating success. Both male rank and the rank distance between partners are significantly correlated with the distribution of grooming between males in an extremely large chimpanzee community at Ngogo, Kibale National Park, Uganda, that has more males than any other known community. High-ranking males had more grooming partners than mid- or low-ranking males. Grooming predominantly went up the dominance hierarchy, but was also concentrated among males that were close in rank. Rank and rank distance apparently both affected grooming independently of reciprocity in grooming and independently of the frequency with which males associated in temporary parties. However, the data do not clearly indicate how constraints on access to partners might have operated. Published data from a smaller chimpanzee community at Mahale show no rank or rank distance effect on male grooming. These results and earlier, conflicting findings on the association between dominance rank and grooming in male chimpanzees indicate that variation in group size, i.e., the number of males per community, probably influences the strength of any such effects, as happens for grooming between females in several cercopithecine species. Data on coalitions at Ngogo support the argument that high-ranking males are valuable social partners, and similarity in strategies of alliance formation may influence the distribution of grooming. [ABSTRACT FROM AUTHOR]

Allogrooming serves many social functions in primates. Grooming can help individuals to service social relationships generally, sometimes reciprocally, and may be particularly important in the development and maintenance of alliances. However, time constraints limit the number of partners with whom one individual can groom enough to maintain cooperative relationships. As a result, the size of its grooming network may reach an asymptote as the size of its group increases, and it may distribute its grooming less equally among potential partners. Chimpanzees live in multimale, fission-fusion communities; males are philopatric, and commonly associate and groom with each other. Males form within-community alliances that influence dominance rank and access to mates, and allies groom with each other regularly; males also cooperate in aggression between communities. The chimpanzee community at Ngogo, in Kibale National Park, Uganda, is unusually large and has more males than any other known community. Field data show that adult Ngogo males groomed far more with other adult males than with females or with adolescent males, in contrast to a previous report (Ghiglieri, 1984). Adolescent males groomed adults much more than the reverse; males groomed and were groomed by females about equally. Individual males groomed mostly with a small number of other males. On average, males at Ngogo had only slightly more male grooming partners overall and had the same number of important partners as those of males in a much smaller community in the Mahale National Park, Tanzania, and they distributed their grooming less equitably. These results fit those expected if limits on available grooming time cause males to have a loyalty problem as the number of potential grooming and alliance partners increases. Despite differences in the extent and equitability of their grooming networks, males at both Ngogo and Mahale showed reciprocity in grooming. Grooming reciprocity has been demonstrated for captive chimpanzee males, but the Ngogo findings are the first demonstrations of reciprocity in wild communities. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the demographic effects on the hunting behavior of a community of wild chimpanzees at Ngogo in the Kibale National Park, Uganda. Methods; Results; Discussion and comparisons with other studies.

Humans are argued to be unique in their ability and motivation to share attention with others about external entities—sharing attention for sharing’s sake. Indeed, in humans, using referential gestures declaratively to direct the attention of others toward external objects and events emerges in the first year of life. In contrast, wild great apes seldom use referential gestures, and when they do, it seems to be exclusively for imperative purposes. This apparent species difference has fueled the argument that the motivation and ability to share attention with others is a human-specific trait with important downstream consequences for the evolution of our complex cognition [M. Tomasello, Becoming Human (2019)]. Here, we report evidence of a wild ape showing a conspecific an item of interest. We provide video evidence of an adult female chimpanzee, Fiona, showing a leaf to her mother, Sutherland, in the context of leaf grooming in Kibale Forest, Uganda. We use a dataset of 84 similar leaf-grooming events to explore alternative explanations for the behavior, including food sharing and initiating dyadic grooming or playing. Our observations suggest that in highly specific social conditions, wild chimpanzees, like humans, may use referential showing gestures to direct others’ attention to objects simply for the sake of sharing. The difference between humans and our closest living relatives in this regard may be quantitative rather than qualitative, with ramifications for our understanding of the evolution of human social cognition. [ABSTRACT FROM AUTHOR]

Summary: Chimpanzees make lethal coalitionary attacks on members of other groups . This behavior generates considerable attention because it resembles lethal intergroup raiding in humans . Similarities are nevertheless difficult to evaluate because the function of lethal intergroup aggression by chimpanzees remains unclear. One prominent hypothesis suggests that chimpanzees attack neighbors to expand their territories and to gain access to more food . Two cases apparently support this hypothesis, but neither furnishes definitive evidence. Chimpanzees in the Kasekela community at Gombe National Park took over the territory of the neighboring Kahama community after a series of lethal attacks . Understanding these events is complicated because the Kahama community had recently formed by fissioning from the Kasekela group and members of both communities had been provisioned with food. In a second example from the Mahale Mountains, the M group chimpanzees acquired part of the territory of the adjacent K group after all of the adult males in the latter disappeared . Although fatal attacks were suspected from observations of intergroup aggression, they were not witnessed, and as a consequence, this case also fails to furnish conclusive evidence. Here we present data collected over 10 years from an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda. During this time, we observed the Ngogo chimpanzees kill or fatally wound 18 individuals from other groups; we inferred three additional cases of lethal intergroup aggression based on circumstantial evidence (see ). Most victims were caught in the same region and likely belonged to the same neighboring group. A causal link between lethal intergroup aggression and territorial expansion can be made now that the Ngogo chimpanzees use the area once occupied by some of their victims. [Copyright &y& Elsevier]

Wild chimpanzees form temporary parties that vary in size and composition. Previous studies have revealed considerable intraspecific variation in party compositions. We examined patterns of association among age, sex, and reproductive classes of chimpanzees at Ngogo in the Kibale National Park, Uganda. We employed a class-based association index and a randomization procedure to control for confounding factors and to test for differences between classes. Results indicate that males associated with other males significantly more than expected if all classes behaved equivalently, while females generally associated with individuals of the same sex less than expected. To interpret these patterns we used two additional indices that separate associations into two components: general gregariousness and preference for particular classes of associates. Males and estrous females were more gregarious than other classes, while anestrous females were less so. After controlling for general gregariousness, adult males as a class showed no specific preference for associating with each other. Anestrous females preferred each other as party members, and estrous females avoided each other. These results are consistent with previous findings that adult males are more gregarious than females. They diverge from the standard picture of chimpanzee society, however, by suggesting a mutual affinity among anestrous females, but not among adult males as a class. [ABSTRACT FROM AUTHOR]

Although numerous ecological and social factors influence range use in vertebrates, the general assumption is that ranging patterns typically accord with principles of optimal foraging theory. However, given temporal variability in resource abundance, animals can more easily meet nutritional needs at some times than at others. For species in which sociality is particularly important for fitness, such as chimpanzees ( Pan troglodytes) and other group-living primates, the influences of social factors can be particularly strong, and likely interact closely with ecological factors. We investigated home range use by a community of chimpanzees at Ngogo, Kibale National Park, Uganda, to determine whether range use corresponded to energy-based optimality principles. Chimpanzees were particularly attracted to areas of the home range where individuals of Ficus mucuso (a large but low-density resource) were found, but only if those areas also offered other preferred or important resource classes. The aggregation of large foraging parties at F. mucuso crowns (frequently seen year-round) facilitates a number of socially beneficial activities for both males and females. Because chimpanzees apparently seek out F. mucuso in areas where other high-quality feeding opportunities exist, these social benefits likely do not come at the expense of fitness benefits accrued from feeding on high-quality resources. Am. J. Primatol. 78:432-440, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Survival in primates is facilitated by commensal gut microbes that ferment otherwise indigestible plant matter, resist colonization by pathogens, and train the developing immune system. 1,2 However, humans are unique among primates in that we consume highly digestible foods, wean early, mature slowly, and exhibit high lifelong investments in maintenance. 3–6 These adaptations suggest that lifetime trajectories of human-microbial relationships could differ from those of our closest living relatives. Here, we profile the gut microbiota of 166 wild chimpanzees aged 8 months to 67 years in the Kibale National Park, Uganda and compare the patterns of gut microbial maturation to those previously observed in humans. We found that chimpanzee gut microbial alpha-diversity, composition, density, interindividual variation, and within-individual change over time varied significantly with age. Notably, gut microbial signatures in infants <2 years old were distinct across all five metrics. Infant chimpanzee guts were enriched in some of the same taxa prevalent in infant humans (e.g., Bifidobacterium, Streptococcus , and Bacteroides), and chimpanzee gut microbial communities, like those of humans, exhibited higher interindividual variation in infancy versus later in life. However, in direct contrast to human infants, chimpanzee infants harbored surprisingly high-diversity rather than low-diversity gut bacterial communities compared with older conspecifics. These data indicate differential trajectories of gut microbiota development in humans and chimpanzees that are consistent with interspecific differences in lactation, diet, and immune function. Probing the phenotypic consequences of differential early-life gut microbial diversity in chimpanzees and other primates will illuminate the life history impacts of the hominid-microbiome partnership. • We profiled changes in the wild chimpanzee gut microbiota over the life course • 600+ samples collected over 3 years from 166 individuals aged 8 months to 67 years • Unlike humans, gut microbial diversity in chimpanzees is highest in infancy (<2 years old) • Divergent patterns may reflect interspecific differences in lactation, diet, and immunity Reese et al. profile the gut microbiota of wild chimpanzees over the life course. High gut microbial diversity among infants <2 years old contrasts sharply with the low diversity observed in human infants. This divergence could relate to interspecific differences in lactation, diet, and immunity and may contribute to lifelong differences in physiology. [ABSTRACT FROM AUTHOR]

Premasticated food transfer, when an individual partially breaks down food through chewing and feeds it to another individual, usually mouth-to-mouth, is described widely across human cultures. This behavior plays an important role in modern humans' strategy of complementary feeding, which involves supplementing maternal milk in infant diets with processed, easily digestible versions of adult foods. The extent to which other primates engage in premasticated food transfer with infants is unclear, as premasticated food transfers have been only occasionally reported in other ape species. We investigated premasticated food transfers in 62 mother-infant pairs of wild chimpanzees at Ngogo, Uganda, as well as unresisted food taking, when mothers passively allow infants to seize food. We evaluated the presence or absence and rates of premasticated food transfer and unresisted food taking relative to maternal parity and infant age and sex and assessed the food species and part used. We found that chimpanzee mothers regularly shared premasticated food with their infants aged between 6 months and 4 years, but they were more likely to share, and more frequently shared, with younger infants. The frequency with which females shared premasticated food may relate to maternal experience, as multiparous females shared premasticated food more often than did first-time mothers, which we did not find with unresisted food taking. Both easy-to-chew, commonly eaten foods and tougher, rarely eaten foods were shared. Premasticated food transfer and unresisted food taking may be infant-rearing strategies to facilitate the transition from a diet of exclusive maternal milk to solid food during early infancy. Premasticated food transfer in particular may provide energetic, immune, or growth benefits to infants through reduced chewing effort and maternal saliva. Given our findings in chimpanzees and earlier reports in other ape species, we suggest that the foundation of complementary feeding, a uniquely hominin strategy, might have been present in a common ancestor shared with the other great apes in the form of premasticated, mouth-to-mouth food transfer by mothers with their offspring. [ABSTRACT FROM AUTHOR]