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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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... 53 Maternal brothers form particularly strong social ties although most males also form close ties to unrelated males, 54 and males also maintain lasting ties with their mothers. 55 Bonobos do things very differently than chimpanzees. Male bonobos' closest companions are their mothers, 56 and males rarely form coalitions or participate in other forms of cooperative activities. ...
This paper reviews the evolutionary processes that shape the evolution of sociality in mammalian species in an effort to understand the importance of sociality in the lives of modern humans. A body of theory and empirical evidence compiled by behavioral ecologists helps us to understand why (some) other animals live in groups, why group‐living animals form differentiated social bonds, how animals benefit from their social connections, and why some individuals are more social than others in their groups. Together, the answers to these questions help us to understand why humans are such social creatures, and why our social connections play such an important role in our lives.
... P. t. verus has been suggested to be bonobo-like in having reduced tendencies for male aggression compared to P. t. schweinfurthii (Yamakoshi 2004;Pruetz et al. 2017;Wrangham 2021). Specifically P. t. verus has been reported to have lower rates of lethal male aggression than P. t. schweinfurthii (Wilson et al. 2014), more gregarious females (Lehmann and Boesch 2008), a higher frequency of female-female alliances confronting males (Boesch and Boesch-Achermann 2000) and stronger evidence of mothers helping to promote their sons' fitness (Surbeck et al. 2019;Crockford et al. 2020). Reduced male aggressiveness in P. t. verus thus echoes the reduced reactive aggression found in bonobos compared to chimpanzees or in domesticated animals compared to their wild ancestors, but whether such patterns in P. t. verus are due to local ecology, culture or genes is unknown. ...
Objectives
Self‐domestication theory and preliminary data suggest that western chimpanzees ( Pan troglodytes verus ) could have smaller brains than eastern chimpanzees ( P. t. schweinfurthii ), but no large‐scale studies of chimpanzee endocranial volume (ECV) have tested this. This study compares ECV of wild adult P. t. verus and P. t. schweinfurthii , along with femoral head diameter (FHD; an index of body size), bizygomatic breadth (BZB) and palate length (PAL).
Materials and Methods
Adult crania of P. t. schweinfurthii (60 females, 90 males, from Uganda and Democratic Republic of Congo) and P. t. verus (43 females, 37 males, from Liberia and Ivory Coast) were sampled. ECV was measured using 3 mm diameter glass beads, and FHD, PAL, and BZB with digital calipers. Quantities of interest were estimated using Bayesian inference.
Results
No meaningful differences were found between subspecies on average in ECV, FHD, or the relationship between ECV and FHD. Within countries and subspecies, ECV varied widely among individuals, partly because males had higher ECV on average than females. When sex was controlled for, ECV was unrelated to FHD. Within subspecies there was no evidence of meaningful differences in average ECV among countries. PAL was the only measure that differed between subspecies on average, being shorter in P. t. verus females.
Discussion
Current data show that within sexes, mean ECV is similar between P. t. verus and P. t. schweinfurthii . This suggests that average brain size in chimpanzees has remained unchanged for ~0.7 million years, in contrast to orangutans ( Pongo ) and humans.
... This may reflect the paramount importance of maternal support for chimpanzee males, a philopatric species, who continue to benefit from their mother's ecological knowledge (e.g. on resource distribution) into adulthood, while females become more independent upon preparing for dispersal [2,3]. Moreover, male chimpanzees orphaned at an immature age after weaning subsequently reproduce later and less successfully than non-orphaned males [4]. ...
Early maternal loss can have lasting detrimental effects on primate social development. While many rehabilitation settings provide enriching environments to buffer against such effects in orphans, previous research indicates that young bonobo (Pan paniscus) orphans exhibit striking deficiencies in socio-emotional competence compared to their mother-reared peers. However, such studies are generally cross-sectional, without accounting for changes across the lifespan. We conducted longitudinal observations in bonobos living in an accredited African ape sanctuary to examine how rearing background, sex and age predict social tendencies including affiliation, consolation and aggression risk. Affiliative tendencies increased in females and decreased in males with age but were overall lower in orphans compared to mother-reared bonobos. Consolation tendencies decreased with age in mother-reared bonobos, while orphans showed consistently lower consolation (akin to levels of older mother-reared individuals). Young and male bonobos were more likely to receive aggression, while mother-reared and older females were more likely aggressors. Our study highlights the potential that ape sanctuaries like this can have by demonstrating that orphans exhibit decreased affiliative tendencies yet show social functioning ranging within patterns of their mother-reared peers. We discuss these results in the context of bonobos’ natural social ecology and ongoing rehabilitation efforts in this species.
... However, in some carnivores such as the African wild dog (Lycaon pictus) and the spotted hyena (Crocuta crocuta) postweaning food provisioning takes place Watts et al., 2009). Similarly, chimpanzees (Pan troglodytes) and certain bat species provide their oYspring with food after weaning (Crockford et al., 2020;Geipel et al., 2013). In many species, the oYspring stay in close proximity to their mother for protection and learning after lactation, but are not provided with food . ...
... Several empirical studies have recently suggested that these post-weaning maternal resources could improve offspring's growth (e.g. chimpanzees, Pan troglodytes [31]), future reproductive success, and longevity (chimpanzees 32,33]; red deer [34]). Monopolizing such maternal resources-if they can be monopolized-could therefore be advantageous for offspring, meaning that sibling competition could last long beyond lactation or nutritional dependency. ...
In monotocous mammals, most individuals experience the birth of a younger sibling. This period may induce losses in maternal care and can be physiologically, energetically and emotionally challenging for the older sibling, yet has rarely been studied in wild primates. We used behavioural data collected from a natural population of mandrills to investigate changes in maternal care and mother–juvenile relationship throughout the transition to siblinghood (TTS), by comparing juveniles who recently experienced the birth of a younger sibling, to juveniles who did not. We found that the TTS was associated with an abrupt cessation of the weaning process for the juvenile, and to a decrease in maternal affiliation. Juveniles’ reactions were sex-specific, as males associated less with their mother, while females tended to groom their mother more often after the birth of their sibling. Despite the substantial loss of maternal care, juveniles did not show an increase in conflict or anxiety-related behaviours. This study contributes to explain why short interbirth intervals often pose a risk to juveniles’ survival in monotocous primates. Our results contrast existing studies and further highlight the importance of examining the TTS in species and populations with various life histories and ecologies.
... Chimpanzees also share with humans a prolonged developmental period and maternal dependency [49][50][51][52][53]. Within the hominin lineage, it has been hypothesized that prolonged juvenile dependency, which is related to parental provisioning, facilitated prolonged brain development (e.g., [54]), which in turn enabled protracted learning capacities needed for complex foraging and tool use [17]. ...
... Chimpanzees also share with humans a prolonged developmental period and maternal dependency [49][50][51][52][53]. Within the hominin lineage, it has been hypothesized that prolonged juvenile dependency, which is related to parental provisioning, facilitated prolonged brain development (e.g., [54]), which in turn enabled protracted learning capacities needed for complex foraging and tool use [17]. Chimpanzees may not continue extensive parental provisioning after weaning at age 5 years old, however, individuals that experience maternal loss after weaning and before adulthood compared to those that do not, lose out on fitness [50,52,53]. This suggests that mothers continue to offer benefits to offspring, although what these benefits are, is not well understood. ...
Tool use is considered a driving force behind the evolution of brain expansion and prolonged juvenile dependency in the hominin lineage. However, it remains rare across animals, possibly due to inherent constraints related to manual dexterity and cognitive abilities. In our study, we investigated the ontogeny of tool use in chimpanzees (Pan troglodytes), a species known for its extensive and flexible tool use behavior. We observed 70 wild chimpanzees across all ages and analyzed 1,460 stick use events filmed in the Taï National Park, Côte d’Ivoire during the chimpanzee attempts to retrieve high-nutrient, but difficult-to-access, foods. We found that chimpanzees increasingly utilized hand grips employing more than 1 independent digit as they matured. Such hand grips emerged at the age of 2, became predominant and fully functional at the age of 6, and ubiquitous at the age of 15, enhancing task accuracy. Adults adjusted their hand grip based on the specific task at hand, favoring power grips for pounding actions and intermediate grips that combine power and precision, for others. Highly protracted development of suitable actions to acquire hidden (i.e., larvae) compared to non-hidden (i.e., nut kernel) food was evident, with adult skill levels achieved only after 15 years, suggesting a pronounced cognitive learning component to task success. The prolonged time required for cognitive assimilation compared to neuromotor control points to selection pressure favoring the retention of learning capacities into adulthood.
... Viviparous mothers have a significant capacity to influence their offspring's traits (Crockford et al. 2020;Malalaharivony et al. 2021). Because of that, maternal effects can have important repercussions on the population dynamics as they contribute to phenotypic variation (Ginzburg & Taneyhill 1994;Rossiter 1994;Mousseau & Fox 1998;Moore et al. 2019). ...
The reproductive mode of viviparity has independently evolved in various animal taxa. It refers to the condition in which the embryos or young develop inside the female’s body during gestation, providing advantages such as protection, nutrition, and improved survival chances. However, parasites and diseases can be an evolutionary force that limit the host’s resources, leading to physiological, morphological, and behavioral changes that impose additional costs on both the pregnant female and her offspring. This review integrates the primary literature published between 1980 and 2021 on the parasitism of viviparous hosts. We describe aspects such as reproductive investment in females, offspring sex ratios, lactation investment in mammals, alterations in birth intervals, current reproductive investment, variations between environments, immune system activity in response to immunological challenges, and other factors that can influence the interaction between viviparous females and parasites. Maintaining pregnancy incurs costs in managing the mother’s resources and regulating the immune system’s responses to the offspring, while simultaneously maintaining an adequate defense against parasites and pathogens. Parasites can significantly influence this reproductive mode: parasitized females adjust their investment in survival and reproduction based on their life history, environmental factors, and the diversity of encountered parasites.
... Chimpanzees live in a society where adult males dominate all females and males compete directly with other males for status in an all-male hierarchy that yields reproductive benefits (Bygott, 1974;Wroblewski et al., 2009); offspring begin to earn status independently from their mothers in mid-adolescence (~12 years), around which time maternal intervention reduces in frequency as daughters emigrate and sons begin to travel in subgroups apart from mothers (Enigk et al., 2020;Markham et al., 2015;Pusey, 1983;Reddy & Sandel, 2020;Reddy et al., 2022). Maternal presence throughout adolescence is associated with increased fitness benefits but this ceases by the start of adulthood (~16 years old; Crockford et al., 2020;Goodall, 1986;Samuni, Tkaczynski, et al., 2020;Stanton et al., 2020). In contrast, adult male bonobos live in societies where females often dominate males, remain almost always together in the same subgroups as their mothers and male reproductive success and social status are tightly linked to maternal presence throughout life (Furuichi, 1997(Furuichi, , 2011Kano, 1992;Surbeck et al., 2011Surbeck et al., , 2019Toda et al., 2021). ...
... As that work proceeds, the importance and explanatory reach of life history evolution is expanded by Sarah Hrdy's insights about novel survival challenges faced by infants ancestral to us (Hrdy, 2005(Hrdy, , 2009(Hrdy, , 2014(Hrdy, , 2016Hawkes, 2014) Hawkes and Finlay (2018), Finlay (2019), and Hawkes (2020a). Associations between weaned offspring and their mothers would surely have been important in ancestral populations as they are in chimpanzees now (e.g., Goodall, 1986;Nakamura et al., 2014;Crockford et al., 2020;Stanton et al., 2020), even though infants begin feeding themselves so early (e.g., Bădescu et al., 2017;Matsumoto, 2017;Bray et al., 2018). Ancestral commitment to habitats where not only infants but young juveniles could not feed themselves adequately must have escalated that importance. ...
Greater longevity, slower maturation and shorter birth intervals are life history features that distinguish humans from the other living members of our hominid family, the great apes. Theory and evidence synthesized here suggest the evolution of those features can explain both our bigger brains and our cooperative sociality. I rely on Sarah Hrdy’s hypothesis that survival challenges for ancestral infants propelled the evolution of distinctly human socioemotional appetites and Barbara Finlay and colleagues’ findings that mammalian brain size is determined by developmental duration. Similar responsiveness to varying developmental contexts in chimpanzee and human one-year-olds suggests similar infant responsiveness in our nearest common ancestor. Those ancestral infants likely began to acquire solid food while still nursing and fed themselves at weaning as chimpanzees and other great apes do now. When human ancestors colonized habitats lacking foods that infants could handle, dependents’ survival became contingent on subsidies. Competition to engage subsidizers selected for capacities and tendencies to enlist and maintain social connections during the early wiring of expanding infant brains with lifelong consequences that Hrdy labeled “emotionally modern” social cognition.
... Mothers can assist the reproductive success of their sons [159], as reported in orcas (Orcinus orca) [160] and northern muriquis (Brachyteles hypoxanthus) [161]. Although in chimpanzees, sons with mothers present have a higher rank and reproductive success [96,[162][163][164][165][166], the impact of mothers appears stronger and more direct in bonobos [89,167,168]. Bonobo mothers may support their sons by interrupting copulations between females and other males [168], providing agonistic support to their sons during conflict with other males [52,139]. ...
Reproductive inequality, or reproductive skew, drives natural selection, but has been difficult to assess, particularly for males in species with promiscuous mating and slow life histories, such as bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Although bonobos are often portrayed as more egalitarian than chimpanzees, genetic studies have found high male reproductive skew in bonobos. Here, we discuss mechanisms likely to affect male reproductive skew in Pan, then re-examine skew patterns using paternity data from published work and new data from the Kokolopori Bonobo Reserve, Democratic Republic of Congo and Gombe National Park, Tanzania. Using the multinomial index (M), we found considerable overlap in skew between the species, but the highest skew occurred among bonobos. Additionally, for two of three bonobo communities, but no chimpanzee communities, the highest ranking male had greater siring success than predicted by priority-of-access. Thus, an expanded dataset covering a broader demographic range confirms that bonobos have high male reproductive skew. Detailed comparison of data from Pan highlights that reproductive skew models should consider male–male dynamics including the effect of between-group competition on incentives for reproductive concessions, but also female grouping patterns and factors related to male–female dynamics including the expression of female choice.
This article is part of the theme issue ‘Evolutionary ecology of inequality’.
