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Mother, Sumatra, grooming her 9 year old son, Solibra. Photograph by L.S.

Mother, Sumatra, grooming her 9 year old son, Solibra. Photograph by L.S.

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Humans are unusual among animals for continuing to provision and care for their offspring until adulthood. This "prolonged dependency" is considered key for the evolution of other notable human traits, such as large brains, complex societies, and extended postreproductive lifespans. Prolonged dependency must therefore have evolved under conditions...

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... This reduction in time spent in association with their mothers could have lasting effects on weaned immature orangutans. Among chimpanzees, weaned immatures who experience maternal loss suffer various adverse effects, including hampered physical development, later age at first reproduction, lower overall reproductive success, higher risk of mortality, and/or lower overall survival Crockford et al. 2020;Stanton et al. 2020). Similar effects of maternal loss post weaning, but prior to the onset of sexual maturity, have been observed in red deer (Cervus elaphus: Andres et al. 2013), baboons (Papio cynocephalus: Tung et al. 2016), and hyenas (Crocuta crocuta: Watts et al. 2009). ...
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As climate change continues to fundamentally alter resource landscapes, the ability to flexibly respond to spatio-temporal changes in the distribution of preferred food sources is increasingly important for the overall health and fitness of animals living in seasonal, variable, and/or changing environments. Here, we investigate the effects of an uncharacteristically long period of fruit scarcity, following widespread thick haze caused by peat and forest fires in 2015, on the behaviour and sociality of female Bornean orangutans ( Pongo pygmaeus wurmbii ). We collected data from 2010 to 2018 at Tuanan, Central Kalimantan, Indonesia, and compared the activity, diet, and association patterns of adult females during low-fruit periods before the fires, i.e., regular, seasonal periods of low fruit availability (“pre-fire”), and after the fires, i.e., during the extended period of low fruit availability (“post-fire”). First, we found that, post-fire, female orangutans adopted a more extreme energy-saving activity pattern and diet — resting more, travelling less, and diet-switching to less-preferred foods — compared to pre-fire. Second, we found that the probabilities of association between females and their weaned immature offspring, and between related and unrelated adult females were lower, and the probability of agonism between unrelated females was higher, post-fire than pre-fire. This change in energetic strategy, and the general reduction in gregariousness and social tolerance, demonstrates how forest fires can have lasting consequences for orangutans. Fission–fusion species such as orangutans can mitigate the effects of changes in resource landscapes by altering their (sub)grouping patterns; however, this may have long-term indirect consequences on their fitness.
... Whether early maternal investment influences adult male chimpanzee competitive ability is unknown. At Taï, the presence of mothers was positively associated with post-weaning growth, although this applied to both offspring sexes (Samuni et al. 2020). Likewise, association between mothers and sons before maturity, but after weaning, was positively associated with male reproductive success (Crockford et al. 2020), but causality, if any, was unclear. ...
... At Taï, the presence of mothers was positively associated with post-weaning growth, although this applied to both offspring sexes (Samuni et al. 2020). Likewise, association between mothers and sons before maturity, but after weaning, was positively associated with male reproductive success (Crockford et al. 2020), but causality, if any, was unclear. Any causal influence is likely to act via early growth and nutrition provided by maternal milk, as happens in bighorn sheep (Festa-Bianchet et al. 2000). ...
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Maternal lactational investment can affect female reproductive rates and offspring survival in mammals and can be biased towards infants of one sex. We compared estimates of lactation effort among mothers, assessed as their potential milk contribution to age-specific infant diets (mother-infant differences in fecal stable nitrogen isotopes, δ ¹⁵ N), to the timing of weaning (infant age at last nursing bout) and to maternal inter-birth interval lengths for male and female infant chimpanzees ( Pan troglodytes ) at Ngogo, Uganda. Infant males had greater proportions of milk in their age-specific diets, indicated by higher mother-infant differences in δ ¹⁵ N (Generalized Estimating Equation, GEE: p < 0.01). This may mean that mothers of sons showed greater lactation effort than mothers of daughters. Infant males stopped nursing at older ages than infant females (Kaplan–Meier product limit estimate, Breslow estimator: p < 0.05). Mothers of sons showed longer interbirth intervals than mothers of daughters (GEE: p < 0.01). All three measures indicated maternal lactational investment was higher for sons. Male infants may cost mothers more to ensure infant survival than female infants because males are more vulnerable and/or because maternal genetic returns on investment are greater for sons than daughters, as male philopatry means that chimpanzee mothers can have more influence on the reproductive success of sons. Chimpanzee females may trade off growth-related benefits of high lactational investment in male offspring against reduced reproductive rates. Significance statement Maternal investment via lactation affects the reproductive success of female mammals and their offspring and can be biased towards infants of one sex. We investigated lactational variation among wild chimpanzees in relation to infant sex using three proxies for maternal lactational investment: fecal stable nitrogen isotopes, a physiological biomarker that may provide an estimate of lactation effort; observations of nursing, which we used to establish weaning ages; and the lengths of intervals between births of surviving infants. Chimpanzee mothers biased lactational investment toward sons on all three indicators and showed reduced fecundity due to longer inter-birth intervals for mothers of sons than for mothers of daughters. These results would be expected if greater maternal investment toward sons leads to better condition and higher reproductive success for sons later in life, thus to greater inclusive fitness for mothers.
... In several nonhuman primates, maternal condition affects offspring survival and reproductive success post weaning. For instance, in both chimpanzees and bonobos, the presence of mothers enhances the reproductive success of their adult sons, likely by helping them in status competition with other males for social rank (Crockford et al., 2020;Surbeck et al., 2011). Further, a recent comparative study showed that in five of seven primate species studied, offspring born in the last 4 years before a female's death are more likely to die at a young 6 We have not discussed several other sources of morbidity and mortality, such as congenital problems, accidents and environmental causes of death (e.g., typhoons), and labor, which do not typically qualify as "threats" in psychology. ...
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In psychological research, there are often assumptions about the conditions that children expect to encounter during their development. These assumptions shape prevailing ideas about the experiences that children are capable of adjusting to, and whether their responses are viewed as impairments or adaptations. Specifically, the expected childhood is often depicted as nurturing and safe, and characterized by high levels of caregiver investment. Here, we synthesize evidence from history, anthropology, and primatology to challenge this view. We integrate the findings of systematic reviews, meta-analyses, and cross-cultural investigations on three forms of threat (infanticide, violent conflict, and predation) and three forms of deprivation (social, cognitive, and nutritional) that children have faced throughout human evolution. Our results show that mean levels of threat and deprivation were higher than is typical in industrialized societies, and that our species has experienced much variation in the levels of these adversities across space and time. These conditions likely favored a high degree of phenotypic plasticity, or the ability to tailor development to different conditions. This body of evidence has implications for recognizing developmental adaptations to adversity, for cultural variation in responses to adverse experiences, and for definitions of adversity and deprivation as deviation from the expected human childhood.
... 183grouping decisions. In addition, by independence, males begin exhibiting adult behaviour through mating 184(Crockford et al. 2020;Constable et al. 2001; lower-bound for paternity in Budongo is 9-years), engaging in 185 boundary patrols(Mitani and Watts, 2001), and forming strong social bonds with adult males(Sandel et al. 186 2020). The transition from subadult to adult is fluid, variable, and not reliably predicted by age alone (e.g., 187Reynolds, 2005; Goodall, 1986), by including all independent males we also avoid making potentially arbitrary 188 decisions about the minimum age for males to be considered adultsdata were extracted from the Budongo long-term dataset for the period between192 October 2015 and October 2019. ...
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Individuals of social species face a trade-off between the competitive costs and social benefits of group living. Species show a range of social strategies to deal with this trade-off, for example atomistic fission-fusion dynamics in which temporary social groups of varying size and membership form and re-form; or molecular fission-fusion dynamics which contain stable sets of multilevel nested subgroups. Chimpanzees are considered an archetypical atomistic fission-fusion species, using dynamic changes in day-to-day association to moderate the costs of within-group competition. It has been argued that humans’ highly flexible social organisation allows us to live in extremely large groups. Using four years of association data from two neighbouring communities of East African chimpanzees ( Pan troglodytes schweinfurthii ), we describe new levels of flexibility in chimpanzee social organisation and confirm the presence of subgrouping in a second, large community of chimpanzees. We show that males from the larger Waibira community (N males 24-31) exhibited additional levels of semi-stable subgrouping, while males from the smaller Sonso community (N males 10-13) did not. Subgroup membership showed stability across some years, but flexibility across others. Our data support the hypothesis that chimpanzees can incorporate strategies other than fission-fusion to overcome costs of social living, and that their social organisation may be closer to that of modern humans than previously described. SIGNIFICANCE STATEMENT Social living offers benefits and costs; groups can more easily locate and defend resources, but experience increased individual competition. Many species, or individuals, flexibly adjust their social organization when faced with different competitive pressures. It is argued that humans are unique among primates in combining multigroup social organisation with fission-fusion dynamics flexibly within and across groups, and that doing so allows us to live in extremely large groups. Using four-years of association data we show new levels of flexibility in chimpanzee social organization. Males from a typically-sized community employed atomistic fission-fusion dynamics, but males in an unusually large community incorporated additional semi-stable levels of subgrouping. Our data support the hypothesis that chimpanzee males combine social organization strategies, and that doing so may allow them, like humans, to mitigate individual costs at larger community sizes.
... In addition to ecological crises, infants may experience stressful social events, like weaning-related rejection, maternal death, or birth of a sibling (Tung et al. 2016;Maestripieri 2018), or they are impacted by maternal senescence (Ivimey-Cook and Moorad 2020), all of which tend to reduce the amount of maternal investment they receive. There is empirical evidence from studies of both humans and other vertebrates for rippling effects of early life adversity on offspring development and fitness (Lummaa and Clutton-Brock 2002;Weinstock 2008;Giesing et al. 2011;Berghänel et al. 2017;Crockford et al. 2020;McGhee et al. 2021), but the proximate mechanisms underlying this phenomenon remain poorly known, especially in wild animals (Edes and Crews 2017;Langenhof and Komdeur 2018;Lea et al. 2018;Hawkley and Capitanio 2020). However, one factor that has been identified as important mediator of the relationship between individuals' early experiences and health outcomes in several species and taxa is the exposure to maternal stress hormones, i.e., glucocorticoids (GC; Love et al. 2013), both pre-and postnatally. ...
Article
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Maternal effects mediated by nutrients or specific endocrine states of the mother can affect infant development. Specifically, pre- and postnatal maternal stress associated with elevated glucocorticoid (GC) output is known to influence the phenotype of the offspring, including their physical and behavioral development. These developmental processes, however, remain relatively poorly studied in wild vertebrates, including primates with their relatively slow life histories. Here, we investigated the effects of maternal stress, assessed by fecal glucocorticoid output, on infant development in wild Verreaux’s sifakas ( Propithecus verreauxi ), a group-living Malagasy primate. In a first step, we investigated factors predicting maternal fecal glucocorticoid metabolite (fGCM) concentrations, how they impact infants’ physical and behavioral development during the first 6 months of postnatal life as well as early survival during the first 1.5 years of postnatal life. We collected fecal samples of mothers for hormone assays and behavioral data of 12 infants from two birth cohorts, for which we also assessed growth rates. Maternal fGCM concentrations were higher during the late prenatal but lower during the postnatal period compared to the early/mid prenatal period and were higher during periods of low rainfall. Infants of mothers with higher prenatal fGCM concentrations exhibited faster growth rates and were more explorative in terms of independent foraging and play. Infants of mothers with high pre- and postnatal fGCM concentrations were carried less and spent more time in nipple contact. Time mothers spent carrying infants predicted infant survival: infants that were more carried had lower survival, suggesting that they were likely in poorer condition and had to be cared for longer. Thus, the physical and behavioral development of these young primates were impacted by variation in maternal fGCM concentrations during the first 6 months of their lives, presumably as an adaptive response to living in a highly seasonal, but unpredictable environment. Significance statement The early development of infants can be impacted by variation in maternal condition. These maternal effects can be mediated by maternal stress (glucocorticoid hormones) and are known to have downstream consequences for behavior, physiology, survival, and reproductive success well into adulthood. However, the direction of the effects of maternal physiological GC output on offspring development is highly variable, even within the same species. We contribute comparative data on maternal stress effects on infant development in a Critically Endangered primate from Madagascar. We describe variation in maternal glucocorticoid output as a function of ecological and reproductive factors and show that patterns of infant growth, behavioral development, and early survival are predicted by maternal glucocorticoids. Our study demonstrates how mothers can influence offspring fitness in response to challenging environmental conditions.
... Firstly, our finding that bonobo but not chimpanzee females with adult sons have the highest average numbers of females in their parties (Supplementary Fig. 2A) indicates that bonobo female grouping decisions are either influenced by potential stronger gains in indirect fitness benefits through supporting their sons or reflect a stronger tendency of adult sons to associate with their mothers, which attracts other females 27 . While maternal presence during adolescence has been shown to affect male reproductive parameters in chimpanzees 85 , in bonobos the presence of a mother in a party increases the likelihood of her son to mate 27 and her presence in the community increases his likelihood to reproduce more than in chimpanzees 86 . Similar effects of mother presence on the reproductive success of their sons have been described in other species including orcas 87 . ...
Article
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Here we show that sexual signaling affects patterns of female spatial association differently in chimpanzees and bonobos, indicating its relevance in shaping the respective social systems. Generally, spatial association between females often mirrors patterns and strength of social relationships and cooperation within groups. While testing for proposed differences in female-female associations underlying female coalition formation in the species of the genus Pan, we find only limited evidence for a higher female-female gregariousness in bonobos. While bonobo females exhibited a slightly higher average number of females in their parties, there is neither a species difference in the time females spent alone, nor in the number of female party members in the absence of sexually attractive females. We find that the more frequent presence of maximally tumescent females in bonobos is associated with a significantly stronger increase in the number of female party members, independent of variation in a behavioural proxy for food abundance. This indicates the need to look beyond ecology when explaining species differences in female sociality as it refutes the idea that the higher gregariousness among bonobo females is driven by ecological factors alone and highlights that the temporal distribution of female sexual receptivity is an important factor to consider when studying mammalian sociality. Surbeck and colleagues investigate the proximate drivers of female gregariousness in bonobos and chimpanzees across different observed communities. Their findings indicate that varied levels of sexual signalling in these two species result in different social behaviours regarding female grouping and potentially cooperation.
... This score was then inverted so that maternal loss at an earlier age is associated with a higher value. We include maternal loss after the period of nutritional independence because death of mother continues to have substantial effects on offspring survival and fitness even following weaning (Foster et al. 2012;Nakamura et al. 2014;Crockford et al. 2020;Samuni et al. 2020;Stanton et al. 2020). We use 4 years of age as a cutoff because we are interested in early life experiences and 4 years marks the earliest age at menarche in this population before translocation (Strum and Western 1982). ...
... This score was then inverted so that maternal loss at an earlier age is associated with a higher value. We include maternal loss after the period of nutritional independence because death of mother continues to have substantial effects on offspring survival and fitness even following weaning (Foster et al. 2012;Nakamura et al. 2014;Crockford et al. 2020;Samuni et al. 2020;Stanton et al. 2020). We use 4 years of age as a cutoff because we are interested in early life experiences and 4 years marks the earliest age at menarche in this population before translocation (Strum and Western 1982). ...
Article
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Adverse experiences during early life exert important effects on development, health, reproduction, and social bonds, with consequences often persisting across generations. A mother’s early life experiences can impact her offspring’s development through a number of pathways, such as maternal care, physiological signaling through glucocorticoids, or even intergenerational effects like epigenetic inheritance. Early life adversity in female yellow baboons (Papio cynocephalus) predicts elevated glucocorticoids, reduced sociality, shortened lifespan, and higher offspring mortality. If baboon mothers with more early life adversity, experience poorer condition and struggle to provide for their offspring, this could contribute to the persisting transgenerational effects of adversity. Here, we examined the effects of mothers’ early life adversity on their maternal effort, physiology, and offspring survivability in a population of olive baboons, Papio anubis. Mothers who experienced more adversity in their own early development exerted greater maternal effort (i.e., spent more time nursing and carrying) and had higher levels of glucocorticoid metabolites than mothers with less early life adversity. Offspring of mothers with more early life adversity had reduced survivability compared to offspring of mothers with less early life adversity. There was no evidence that high maternal social rank buffered the effects of early life adversity. Our data suggest early life experiences can have lasting consequences on maternal effort and physiology, which may function as proximate mechanisms for intergenerational effects of maternal experience. Significance statement Animals exposed to early life adversity experience both immediate and lasting consequences. If early life adversity exerts developmental constraints that affect a mother’s ability to provide for her offspring, this could explain the transgenerational effects of early life adversity. In our study of wild olive baboons, we examined how a mother’s own early life adversity predicts her maternal effort (i.e., nursing and carrying time), maternal fecal glucocorticoid levels, and offspring outcomes. We found that female baboons who experienced more early life adversity had higher glucocorticoid levels during pregnancy and lactation, exerted more maternal effort, and produced offspring with higher mortality risk than females with less early life adversity. Our results suggest that female baboons with more early life adversity experience developmental constraints and struggle to invest in offspring, which likely contributes to persisting effects of early life adversity across generations.
... In mammals, mothers are essential for the early development of their infants since they provide postnatal care (Maestripieri and Mateo, 2009). Maternal loss in mammals reduces growth , survival (Watts et al., 2009;Andres et al., 2013;Tung et al., 2016;Stanton et al., 2020), and long-term reproductive success (Andres et al., 2013;Strauss et al., 2020;Crockford et al., 2020;Zipple et al., 2021, reviewed in Clutton-Brock, 2016. The biological embedding model (BEM; Power and Hertzman, 1997;Miller et al., 2011;Berens et al., 2017) posits that adversity experienced early in life, including exposure to severe stressors, can have deleterious consequences on an individual's physiology and health across their lifespan. ...
... Specifically, our dataset allowed us to assess both the short-and long-term effects of maternal loss on the HPA axis activity in wild chimpanzees. We thereby investigated one of the potential physiological mechanisms explaining the fitness costs associated with maternal loss reported in wild chimpanzees such as reduced growth, survival, and reproductive success (Nakamura et al., 2014;Samuni et al., 2020;Stanton et al., 2020;Crockford et al., 2020). Furthermore, studying physiological effects using diurnal cortisol slopes is an underused paradigm in wild animal subjects despite its prevalence in the human health literature. ...
... While the effect of maternal loss on wild animal survival and reproduction has been recently established (Foster et al., 2012;Andres et al., 2013;Tung et al., 2016;Walker et al., 2018;Surbeck et al., 2019;Crockford et al., 2020;Zipple et al., 2021), the mechanisms underlying these fitness costs remain understudied. Our study provides one of the rare empirical tests of the BEM (see also Rosenbaum et al., 2020) and ACM in wild long-lived mammals by assessing the short-and long-term physiological impacts of early maternal loss. ...
Article
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The biological embedding model (BEM) suggests that fitness costs of maternal loss arise when early-life experience embeds long-term alterations to hypothalamic-pituitary-adrenal (HPA) axis activity. Alternatively, the adaptive calibration model (ACM) regards physiological changes during ontogeny as short-term adaptations. Both models have been tested in humans but rarely in wild, long-lived animals. We assessed whether, as in humans, maternal loss had short- and long-term impacts on orphan wild chimpanzee urinary cortisol levels and diurnal urinary cortisol slopes, both indicative of HPA axis functioning. Immature chimpanzees recently orphaned and/or orphaned early in life had diurnal cortisol slopes reflecting heightened activation of the HPA axis. However, these effects appeared short-term, with no consistent differences between orphan and non-orphan cortisol profiles in mature males, suggesting stronger support for the ACM than the BEM in wild chimpanzees. Compensatory mechanisms, such as adoption, may buffer against certain physiological effects of maternal loss in this species.
... This is not the case for male offspring. Mother-son cohabitation during reproductive ages of sons has the potential to improve the reproductive success of sons, for instance in bonobos 54 and in this population of chimpanzees 55 . Here, we showed that females with sons approaching reproductive age in the community were more likely to participate in intergroup encounters in comparison to females with no such sons in the community. ...
Article
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Humans maintain extensive social ties of varying preferences, providing a range of opportunities for beneficial cooperative exchange that may promote collective action and our unique capacity for large-scale cooperation. Similarly, non-human animals maintain differentiated social relationships that promote dyadic cooperative exchange, but their link to cooperative collective action is little known. Here, we investigate the influence of social relationship properties on male and female chimpanzee participations in a costly form of group action, intergroup encounters. We find that intergroup encounter participation increases with a greater number of other participants as well as when participants are maternal kin or social bond partners, and that these effects are independent from one another and from the likelihood to associate with certain partners. Together, strong social relationships between kin and non-kin facilitate group-level cooperation in one of our closest living relatives, suggesting that social bonds may be integral to the evolution of cooperation in our own species. Strong social bonds are known to affect pairwise cooperation in primates such chimpanzees. Here, Samuni et al. show that strong social bonds also influence participation in group-level cooperation (collective action in intergroup encounters) using a long-term dataset of wild chimpanzees.
... compared to their peers (e.g., Japanese macaques, Macaca fuscata: Hasegawa and Hiraiwa 1980; long-tailed macaques, Macaca fascicularis: van Noordwijk and van Schaik 1999; Asian elephants, Elephas maximus: Lahdenperä et al. 2016; olive baboons, Papio anubis : Johnson 1987; yellow baboons, Papio cynocephalus: Tung et al. 2016; chacma baboons, Papio hamadryas ursinus: Engh et al. 2009; red deer, Cervus elaphus: Andres et al. 2013;humans, Homo sapiens: Cerel et al. 2006; bonobos, Pan paniscus: Clay and de Waal 2013; chimpanzees, Pan troglodytes: Nakamura et al. 2014;Walker et al. 2018;Crockford et al. 2020;Samuni et al. 2020;Stanton et al. 2020). In some species, mothers retain bonds with their offspring and continue to contribute to their welfare in adulthood (e.g., baboons: Silk et al. 2006aSilk et al. , b, 2009bonobos: Surbeck et al. 2011; orca whales, Orcinus orca: Brent et al. 2015). ...
... It is possible that mothers help their sons form bonds with other females and compete with other adolescent males for mating opportunities. Indeed, maternal presence is associated with sons (and daughters) reproducing at earlier ages (Pusey et al. 1997;Crockford et al. 2020). Some evidence suggests mothers provide such benefits in male-philopatric, relatively egalitarian muriquis (Brachyteles arachnoides), where males do not always maintain strong bonds with their mothers as adults. ...
... Although adolescent and young adult male chimpanzees do not rely on mothers in the same manner that bonobo sons do, they may still receive emotional and agonistic support from mothers which could reduce stress and improve the emotional health of sons. This mechanism could explain why chimpanzee sons who lose their mothers before or during adolescence experience decrements in growth, survival, dominance status, and reproductive success later in life (Nakamura et al. 2014;Crockford et al. 2020;Samuni et al. 2020;Stanton et al. 2020). And, maternal presence may continue to provide these types of health benefits for adult sons in certain circumstances. ...
Article
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Mothers provide indispensable care for infants in many mammalian species. In some long-lived species, the maternal-offspring bond persists after infancy with mothers continuing to provide resources and social support to their adult progeny. Maternal presence is associated with fitness benefits through adolescence for male chimpanzees despite the fact that mature males dominate females and form their strongest bonds with other males. How mothers support grown sons is unknown, because few studies have examined developmental shifts in mother-son relationships during adolescence and adulthood. We investigated social interactions between 29 adolescent (9–15 years) and young adult male (16–20 years) chimpanzees and their mothers at Ngogo in Kibale National Park, Uganda, over 3 years. All males under 12 years old had their mother as their top grooming and proximity partner, as did one-third of the young adult males. As males grew older, the amount of time they associated with, maintained proximity to, groomed with, and kept track of their mothers while traveling decreased. When males were together in the same party as their mothers, however, young adult males affiliated with their mothers as frequently as did adolescent males, with sons initiating the majority of these interactions. In contrast to adult sons, however, adolescent sons became distressed when separated from mothers and relied on their mothers for agonistic support and reassurance after conflicts. These findings indicate that the chimpanzee maternal-offspring bond continues but changes through adolescence and adulthood, with mothers remaining occasional social companions for most adult sons and frequent companions for some. Significance statement Mammalian mothers protect and provision their infants, and in some species, mothers provide social support for their adult offspring. The importance of mothers in the lives of adult sons is clear in humans and in one of our closest relatives, bonobos. Here we show mothers are also important social partners for sons throughout adolescence and into young adulthood in chimpanzees. Despite prevailing knowledge that adult males are key alliance partners for male chimpanzees, we demonstrate that male chimpanzees maintain social bonds with their mothers into adulthood. We also suggest that social bonds reflect not only the total time spent together, but what animals do during those times, as many young adults rarely groomed their mothers, but at times when they were in the same subgroup with her, they did so frequently. Future research will assess the physiological, psychological, and reproductive benefits of such mother-son bonds.