Local competition sparks concerns for fairness in the ultimatum game

Humans reject uneven divisions of resources, even at personal cost. This is observed in countless experiments using the ultimatum game, where a proposer offers to divide a resource with a responder who either accepts the division or rejects it (whereupon both earn zero). Researchers debate why humans evolved a psychology that is so averse to inequity within partnerships. We suggest that the scale of competition is crucial: under local competition with few competitors, individuals reject low offers, because they cannot afford to be disadvantaged relative to competitors. If one competes against the broader population (i.e. global competition), then it pays to accept low offers to increase one''s absolute pay-off. We support this intuition with an illustrative game-theoretical model. We also conducted ultimatum games where participants received prizes based on pay-offs relative to immediate partners (local competition) versus a larger group (global competition). Participants demanded higher offers under local competition, suggesting that local competition increases people''s demands for fairness and aversion to inequality.     

Species and sex differences in the screams of chimpanzees and bonobos

We examined screams of chimpanzees and bonobos to investigate interspecific and intraspecific variability in call structure. Measurement of 11 acoustic features of screams revealed differences between and within species. One-way analyses of variance and discriminant function analyses show that the calls of chimpanzees and bonobos differ primarily in spectral characteristics. Spectral features also account for acoustic differences between the sexes. These acoustic variations may be attributable to differences in body size and social dispersion between the two species and sexes. The effectiveness with which an acoustic feature could be used to discriminate the two species and female bonobos from male bonobos is negatively associated with its relative variability. These data are consistent with the hypothesis that optimal signals for group identification vary little within groups but differ widely between groups.     

Volumetric and lateralized differences in selected brain regions of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

The two species of Pan, bonobos and common chimpanzees, have been reported to have different social organization, cognitive and linguistic abilities and motor skill, despite their close biological relationship. Here, we examined whether bonobos and chimpanzee differ in selected brain regions that may map to these different social and cognitive abilities. Eight chimpanzees and eight bonobos matched on age, sex and rearing experiences were magnetic resonance images scanned and volumetric measures were obtained for the whole brain, cerebellum, striatum, motor‐hand area, hippocampus, inferior frontal gyrus and planum temporale. Chimpanzees had significantly larger cerebellum and borderline significantly larger hippocampus and putamen, after adjusting for brain size, compared with bonobos. Bonobos showed greater leftward asymmetries in the striatum and motor‐hand area compared with chimpanzees. No significant differences in either the volume or lateralization for the so‐called language homologs were found between species. The results suggest that the two species of Pan are quite similar neurologically, though some volumetric and lateralized differences may reflect inherent differences in social organization, cognition and motor skills. Am. J. Primatol. 71:988–997, 2009. © 2009 Wiley‐Liss, Inc.     

Comparative locomotor behavior of chimpanzees and bonobos: The influence of morphology on locomotion

Results from a 10 month study of adult male and female bonobos (Pan paniscus) in the Lomako Forest, Zaire, and those from a 7 month study of adult male and female chimpanzees in the Tai Forest, Ivory Coast (Pan troglodytes verus), were compared in order to determine whether there are species differences in locomotor behavior and substrate use and, if so, whether these differences support predictions made on the basis of interspecific morphological differences. Results indicate that bonobos are more arboreal than chimpanzees and that male bonobos are more suspensory than their chimpanzee counterpart. This would be predicted on the basis of male bonobo's longer and more narrow scapula. This particular finding is contrary to the prediction that the bonobo is a “scaled reduced version of a chimpanzee” with little or no positional behavior difference as had been suggested. This study provides the behavioral data necessary to untangle contradictory interpretations of the morphological differences between chimpanzees and bonobos, and raises a previously discussed (Fleagle: Size and Scaling in Primate Biology, pp. 1–19, 1985) but frequently overlooked point–that isometry in allometric studies does not necessarily equate with behavioral equivalence. Several researchers have demonstrated that bonobos and chimpanzees follow the same scaling trends for many features, and are in some sense functionally equivalent, since they manage to feed and reproduce. However, as reflected in their morphologies, they do so through different types and frequencies of locomotor behaviors. © 1993 Wiley-Liss, Inc.     

Coordination strategies of chimpanzees and human children in a Stag Hunt game

Much of human cooperation takes place in mutualistic contexts in which the main challenge for individuals is how to coordinate decisions. In the current studies, we compared the abilities of chimpanzees and young children to coordinate with a partner in two versions of a Stag Hunt game. When risks were low (the hare was of low value) and information was cheap (the partner''s behaviour was readily observable), partners of both species were able to successfully coordinate on the higher value stag more than 90% of the time. By contrast, when the risks were raised and observing the partner was more difficult, the chimpanzees became less successful, whereas the children compensated, and so remained highly successful, by communicating more often and more specifically. This pattern of results is consistent with the hypothesis that humans evolved unique skills of coordination and communication in the context of especially risky coordination problems.     

Comparative analyses of the Pan lineage reveal selection on gene pathways associated with diet and sociality in bonobos

Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) diverged into distinct species approximately 1.7 million years ago when the ancestors of modern-day bonobo populations were separated by the Congo River. This geographic boundary separates the two species today and the associated ecological factors, including resource distribution and feeding competition, have likely shaped the divergent social behavior of both species. The most striking behavioral differences pertain to between group interactions in which chimpanzees behave aggressively towards unfamiliar conspecifics, while bonobos display remarkable tolerance. Several hypotheses attempt to explain how different patterns of social behavior have come to exist in the two species, some with specific genetic predictions, likening the evolution of bonobos to a process of domestication. Here, we utilize 73 ape genomes and apply linkage haplotype homozygosity and structure informed allele frequency differentiation methods to identify positively selected regions in bonobos since their split from a common pan ancestor to better understand the environment and processes that resulted in the behavioral differences observed today. We find novel evidence of selection in genetic regions that aid in starch digestion (AMY2) along with support for two genetic predictions related to self-domestication processes hypothesized to have occurred in the bonobo. We also find evidence for selection on neuroendocrine pathways associated with social behavior including the oxytocin, serotonin, and gonadotropin releasing hormone pathways.     

Statistical fluctuations in population bargaining in the ultimatum game: Static and evolutionary aspects

We explore the emergent behavior in heterogeneous populations where players negotiate via an ultimatum game: two players are offered a gift, one of them (the proposer) suggests how to divide the offer while the other player (the responder) can either accept or reject the deal. Rejection is detrimental to both players as it results in no earnings. In this context, our contribution is twofold: (i) we consider a population where the distribution of used strategies is constant over time and properties of the random payoff received by the players (average and higher moments) are reported from simple exact methods and corroborated by computer simulations; (ii) the evolution of a population is analyzed via Monte Carlo simulations where agents may change independently the proposing and accepting parameters of their strategy depending on received payoffs. Our results show that evolution leads to a stationary state in which wealth (accumulated payoff) is fairly distributed. As time evolves, an increase in average payoff and a simultaneous variance decrease is observed when we use a dynamics based on a probabilistic version of the saying: “One should not comply with small earnings, but one's greed must be limited.”     

Behavioral laterality in captive bonobos (Pan paniscus): Replication and extension

We report observational data on behavioral laterality in 10 captive bonobos (Pan paniscus)at the San Diego Zoo. The unimanual measures include carrying, leading limb in locomotion, self-touching, face-touching, reaching, and gestures. We also recorded bimanual feeding in these subjects. A significant population level left-hand bias exists for carrying. Right-hand biases occur for leading limb in locomotion and gestures. During bimanual feeding, the bonobos hold food items with the left hand while feeding with the right hand. Overall, bonobos exhibit behavioral asymmetries that are similar to previous findings in other pongid ape species. The asymmetries in gestures and bimanual feeding represent novel findings with theoretical implications for the origins of tool use and language.     

Feeding ecology of bonobos living in forest‐savannah mosaics: Diet seasonal variation and importance of fallback foods

Primates along with many other animal taxa are forced to cope with large shifts in basic ecological conditions because of rapid anthropogenically induced changes of their habitats. One of the coping strategies for primates is to adjust their diet to these changes, and several studies have demonstrated the importance of fallback resources for this. Bonobos, like chimpanzees, might be particularly vulnerable to habitat fragmentation because of their high dependence on fruit availability. Little is known, however, about bonobo feeding ecology in fragmented habitats and their use of fallback resources. In this study, we investigate diet seasonal variation and the exploitation of preferred and fallback foods in a bonobo population living in forest‐savannah mosaics. Results show that bonobos have adapted to this fragmented habitat by feeding on only a few fruit species, including an important number of non‐tree species (liana, herb and savannah shrub), in comparison to populations living in dense forests. These non‐tree plants have been defined as fallback and non‐preferred foods, which are most probably consumed to maintain high frugivory. Interestingly, we identified that preferred foods are all typical of mature forests while fallback resources are mainly found in forest edges or disturbed areas. This finding indicates that bonobos prefer to use mature forests when feeding, as they do for nesting, but extend their range use to forest areas in close proximity to humans when the availability of preferred fruits is low. Finally, we show that bonobo diet relies heavily on two abundant fallback fruits: Musanga cecropioides and Marantochloa leucantha. Other studies have demonstrated that the selection of abundant fallback resources enables primates to subsist at high densities and to maintain cohesive groups, as observed at this study site. Our findings suggest that bonobos living in forest‐savannah mosaics can be considered as staple fallback food consumers. Am. J. Primatol. 77:948–962, 2015. © 2015 Wiley Periodicals, Inc.

     

Telomere dynamics in HIV-1 infected and uninfected chimpanzees measured by an improved method based on high-resolution two-dimensional calibration of DNA sizes

We developed an improved method for accurately measuring telomere lengths based on two-dimensional calibration of DNA sizes combined with pulsed field electrophoresis and quantitative analysis of high-resolution gel images. This method was used to quantify the length of telomeres in longitudinal samples of peripheral blood mononuclear cells (PBMCs) from five chimpanzees infected with human immunodeficiency virus type 1 (HIV-1) and three uninfected animals, 14 to 27 years of age. The average length of the telomere restriction fragments (TRF) of infected and uninfected chimpanzees were 11.7 ± 0.25 kbp, and 11.6 ± 0.61 kbp, respectively, and were about 1 kbp and 3 kbp longer than those of human infants and 30 year old adults, respectively. There was a trend of a slight decrease (30–60 bp per year) in the TRF of two HIV infected chimpanzees over 30–35 months, while the TRF of one naive chimpanzee slightly increased over 20 months. Although the number of chimpanzees in this study is small and no statistically significant linear dependencies on time were observed, it appears that in chimpanzees, rates of shortening of the TRF are comparable or smaller than in adult humans and are not significantly affected by HIV-1 infection, which may be related to the inability of HIV-1 to cause disease in these animals.     

Life in the slow lane revisited: ontogenetic separation between chimpanzees and humans

This study investigates the evolution of human growth by analyzing differences in body mass growth trajectories among three populations: the Ache of eastern Paraguay, the US (NHANES, 1999-2000), and captive chimpanzees. The relative growth statistic "A" from the mammalian growth law is allowed to vary with age and proves useful for comparing growth across different ages, populations, and species. We demonstrate ontogenetic separation between chimpanzees and humans, and show that interspecific differences are robust to variable environmental conditions. The human pattern of slow growth during the lengthened period from weaning to the beginning of the adolescent growth spurt is found among the Ache (low energy availability and high disease load) and also in the US (high energy availability and low disease load). The human growth pattern contrasts with that of the chimpanzee, where absolute growth rates and relative "A" values are faster and less prolonged. We suggest that selection has acted to decrease human growth rates to allow more time for increased cognitive development with lower body-maintenance costs.     

Behavioral and physiological response to inequity in bonobos (Pan paniscus)

Inequity aversion (IA), the affective, cognitive, and behavioral response to inequitable outcomes, allows individuals to avoid exploitation and therefore stabilizes cooperation. The presence of IA varies across animal species, which has stimulated research to investigate factors that might explain this variation, and to investigate underlying affective responses. Among great apes, IA is most often studied in chimpanzees. Here, we investigate IA in bonobos, a reputedly tolerant and cooperative species for which few IA studies are available. We describe how bonobos respond to receiving less preferred rewards than a partner in a token exchange task. We show that bonobos respond to receiving less preferred rewards by refusing tokens and rewards, and by leaving the experimental area. Bonobos never refused a trial when receiving preferred rewards, and thus showed no advantageous IA. We also investigate the variability in the disadvantageous IA response on a dyadic level, because the level of IA is expected to vary, depending on characteristics of the dyad. Like in humans and chimpanzees, we show that the tolerance towards inequity was higher in bonobo dyads with more valuable relationships. To study the affective component of IA, we included behavioral and physiological measures of arousal: a displacement behavior (rough self-scratching) and changes in salivary cortisol levels. Both measures of arousal showed large variability, and while analyses on rough self-scratching showed no significant effects, salivary cortisol levels seemed to be lower in subjects that received less than their partner, but higher in subjects that received more than their partner, albeit that both were not significantly different from the equity condition. This suggests that although overcompensated bonobos showed no behavioral response, they might be more aroused. Our data support the cooperation hypothesis on an interspecific and intraspecific level. They show inequity aversion in bonobos, a reputedly cooperative species, and suggest that the variability in IA in bonobos can be explained by their socioecology. Most successful cooperative interactions happen between mothers and their sons and among closely bonded females. The limited need to monitor the partners' investment within these dyads can result in a higher tolerance towards inequity. We therefore suggest future studies to consider relevant socioecological characteristics of the species when designing and analyzing IA studies.     

公平规范与自然资源保护——基于进化博弈的理论模型

传统理论认为社区对公共自然资源的管理必然导致"公共地悲剧".但新近研究证明个人行为往往受到社会规范的影响并遵循公平互惠等"非理性"原则,进而指出社群有可能自发实现对公共资源有效的合作保护.基于这一思路,综合运用进化博弈论与行为经济学理论构建数学模型模拟了社区居民参与对自然资源的家庭承包管护的策略演化动态,并分析公平规范在该过程中能够发挥的潜在作用.结果表明在经济补偿相对管护所需劳动成本较为有限的情况下,"理性"的参与者无法长期维持高水平的保护合作;相比之下,如果公平规范对当地居民有较强的约束力,则可以实现更高的保护合作水平,其最终均衡由群体中对公平有较高要求的参与者比例决定.这一结果从理论上预测了利用公平规范推动社区参与自然资源保护合作并提高生态补偿效率的可行性.     

Lack of gp120-induced anergy and apoptosis in chimpanzees is correlated with resistance to AIDS

Human immunodeficiency virus-1 (HIV-1) infects both humans and chimpanzees, but in the chimpanzee, HIV-1 infection leads only very rarely to loss of CD4 T cells or to AIDS-like disease. The pathogenetic basis for this difference in host range is not understood. In previous studies, using CD4 T cells from HIV-1 seronegative human donors, we demonstrated that crosslinking of CD4-bound gp120, followed by signaling through the T cell receptor for antigen (TCR), resulted in cell death by apoptosis. To determine whether activation-induced apoptosis correlates with progression to AIDS, we studied the chimpanzee. Our data suggest that, although human CD4 T cells respond to CD4 ligation with anergy and apoptosis upon activation, chimpanzee CD4 T cells do not undergo apoptosis after cross-linking of CD4-bound gp120, followed by signaling through the TCR. In addition, proliferation assays show that chimpanzee CD4 T cells do not become anergic after CD4 ligation. Thus, it is possible that, in the chimpanzee, the absence of cellular anergy and apoptotic cell death after CD4 ligation by HIV-1 gp120 protect this primate species from progression to AIDS-like disease.This investigation was supported by National Institute of Health grants AI-30575, AI-29903, AI-35513, and RR00015 (TH Finkel), AI-05060 (WC Satterfield), American Foundation for AIDS Research grants 02270-16-RG (TH Finkel) and 770188-11-PF (NK Banda), the Concerned Parents for AIDS Research, the UCHSC Cancer Center, the Eleanore and Michael Stobin Trust, and the Bender Foundation.     

Ficus sp. and Musanga leo‐errerae: coexistent keystone fruits for chimpanzees in Kalinzu forest

The composition of Ficus sp. and Musanga leo‐errerae in Chimpanzees’ diet was investigated by faecal analysis and direct observation in the medium altitude forest of Kalinzu, along the albertine rift, south‐western Uganda. The fruit availability of Ficus species showed significant variations while that of Musanga leo‐errerae was consistent and significantly higher than that of Ficus (P = 0.053; t = ?2.034) all year round. Their consumption was not opportunistic as no correlation existed between their fruit abundance and their occurrence in chimpanzee faecal samples/diet (Musanga leo‐errerae: r = 0.153, P = 0.456; Ficus sp.: r = 0.039, P = 0.848). Results showed that Musanga leo‐errerae and Ficus species seeds occurred in 80.2% and 67.2% respectively of the total 2635 chimpanzee faecal samples analyzed. Although there was no significant difference between chimpanzees party size that fed on Musanga leo‐errerae and Ficus tree species, the rate of consumption was significantly different in the low (t = 3.835; P = 0.031) than the high fruiting season (t = 2.379; P = 0.063). Ficus sp. and Musanga leo‐errerae genera function as coexistent keystone fruits for chimpanzees because they perfectly complement each other in terms of chimpanzees’ sustenance. This information has significant implications in the management of tropical forests like Kibale, Budongo, Bwindi Impenetrable, Gombe and Mahale inhabited by primate populations especially the endangered ones like the chimpanzee.