The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella ( N = 3), Cercocebus atys ( N = 4), Macaca fascicularis ( N = 3), Gorilla gorilla ( N = 1) and Pan troglodytes ( N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Consistency of Social Interactions in Sooty Mangabeys and Chimpanzees

Predictability of social interactions can be an important measure for the social complexity of an animal group. Predictability is partially dependent on how consistent interaction patterns are over time: does the behavior on 1 day explain the behavior on another? We developed a consistency measure that serves two functions: detecting which interaction types in a dataset are so inconsistent that including them in further analyses risks introducing unexplained error; and comparatively quantifying differences in consistency within and between animal groups. We applied the consistency measure to simulated data and field data for one group of sooty mangabeys (Cercocebus atys atys) and to groups of Western chimpanzees (Pan troglodytes verus) in the Taï National Park, Côte d'Ivoire, to test its properties and compare consistency across groups. The consistency measures successfully identified interaction types whose low internal consistency would likely create analytical problems. Species-level differences in consistency were less pronounced than differences within groups: in all groups, aggression and dominance interactions were the most consistent, followed by grooming; spatial proximity at different levels was much less consistent than directed interactions. Our consistency measure can facilitate decision making of researchers wondering whether to include interaction types in their analyses or social networks and allows us to compare interaction types within and between species regarding their predictability.

Consistency of social interactions in sooty mangabeys and chimpanzees

ABSTRACTPredictability of social interactions can be an important measure for the social complexity of an animal group. Predictability is partially dependent on how consistent interaction patterns are over time: does the behaviour on one day explain the behaviour on another? We developed a consistency measure that serves two functions: detecting which interaction types in a data set are so inconsistent that including them in further analyses risks introducing unexplained error; and comparatively quantifying differences in consistency within and between animal groups. We applied the consistency measure to simulated data and field data for one group of sooty mangabeys (Cercocebus atys atys) and to groups of Western chimpanzees (Pan troglodytes verus) in the Taï National Park, Côte d’Ivoire, to test its properties and compare consistency across groups. The consistency measures successfully identified interaction types whose low internal consistency would likely create analytical problems. Species-level differences in consistency were less pronounced than differences within groups: in all groups, aggression and dominance interactions were the most consistent, followed by grooming; spatial proximity at different levels was much less consistent than directed interactions. Our consistency measure can facilitate decision making of researchers wondering whether to include interaction types in their analyses or social networks and allows us to compare interaction types within and between species regarding their predictability.

Within-group spatial position and activity budget of wild sooty mangabeys (Cercocebus atys) in Taï National Park, Côte d’Ivoire

Within social groups, feeding competition and predation pressure affect individual spatial position. The costs and benefits associated to each position are likely to influence the time that individuals allocate to different activities. Whether the effect of spatial positioning on activity budget differs between individuals of different sex or dominance rank remains unclear. This study aimed at investigating the effect of within-group spatial position on the activity budget of male and female sooty mangabeys. Focal behavioral observations was used to collect the individual location and behavior every 15 minutes (N=5115 locations) on 29 individuals from a wild group of sooty mangabeys (Cercocebus atys) in the Taï National Park. The joint effect of rank, sex and spatial position on individual‟s activity budget was investigated. Females were more central in the group and both fed and rested more than males, independently of their rank. High-ranking individuals from both sexes were more likely to be central and both fed and rested longer than low-ranking ones. Females and high-ranking individuals from both sexes benefit from their social status by adopting spatial positions in the community that could influence their fitness positively. These results are discussed to improve our understanding of social dynamics in wild primates.Keywords: Spatial position, primates, socio-ecology, social dynamics.

Geographically structured genomic diversity of non-human primate-infecting Treponema pallidum subsp. pertenue

BackgroundIncreasing evidence suggests many non-human primate (NHP) species in sub-Saharan Africa are infected with Treponema pallidum subsp. pertenue (TPE), the bacterium causing yaws in humans. In humans, yaws is characterized by lesions of the extremities and face, while Treponema pallidum subsp. pallidum (TPA) causes venereal syphilis and is characterized by primary lesions on the genital, anal or oral mucosae, and has not been detected in NHPs. Due to a paucity of genetic data, it remains unclear whether other Treponema pallidum (TP) subspecies found in humans also occur in NHP and how the genomic diversity of TPE lineages that do occur in NHPs is distributed across hosts and space.MethodologyWe observed a combination of yaws- and syphilis-like symptoms in sooty mangabeys (Cercocebus atys atys) in Taï National Park (TNP), Côte d’Ivoire and collected swabs and biopsies from symptomatic animals. We also collected NHP bones from eight species from TNP, as well as from 19 species at 12 field sites across sub-Saharan Africa. Samples were screened for TP DNA using PCRs. In-solution hybridization capture coupled with high throughput sequencing was used to sequence TP genomes from positive samples.Principal findingsWe generated four nearly complete TP genomes from biopsies and swabs and five partial genomes from bones. Phylogenomic analyses revealed that both syphilis- and yaws-like lesions of sooty mangabeys within a single social group in TNP were caused by TPE. All TPE genomes determined from these sooty mangabeys were different and exhibited divergence levels not observed in TPE from a single species at any other field site where the disease seems to be epizootic. In general, simian TPE isolates did not form monophyletic clades based on host species or the type of symptoms caused by an isolate, but rather clustered based on geography.ConclusionsThere is a large diversity of TPE strains infecting NHPs in TNP. Our observations within a single social group of sooty mangabeys do not support the epizootic spread of a single clone, but rather points towards frequent independent introductions of the bacterium, which can cause syphilis- and yaws-like lesions. On a larger scale, the geographic clustering of TPE genomes might be compatible with cross-species transmission of TPE within ecosystems or environmental exposure leading to acquisition of closely related strains.Author’s summaryIndividuals in several populations of wild non-human primates (NHP) in sub-Saharan Africa are infected with Treponema pallidum subsp. pertenue (TPE), a pathogen causing yaws disease in humans. In humans, yaws is characterized by skin lesions of the extremities and face. In contrast, Treponema pallidum subsp. pallidum, which causes venereal syphilis in humans, has not been observed in NHPs. We describe a combination of yaws- and syphilis-like symptoms in a sooty mangabey (Cercocebus atys atys) social group in Taï National Park (TNP), Côte d’Ivoire. We sampled lesioned animals and collected and tested NHP bones from field sites across sub-Saharan Africa. We were able to reconstruct four genomes from swabs/biopsies and five partial genomes from bone samples. Phylogenomic analyses revealed that syphilis-like lesions and yaws-like lesions in TNP were caused by a large diversity of TPE strains. Additionally, simian TPE isolates did not form monophyletic clades based on the host species or the types of symptoms caused by an isolate, but rather clustered by geographic origin. This is suggestive of cross-species transmission of TPE within ecosystems or environmental exposure leading to acquisition of closely related strains.

Bystanders intervene to impede grooming in Western chimpanzees and sooty mangabeys

Grooming interactions benefit groomers, but may have negative consequences for bystanders. Grooming limits bystanders' grooming access and ensuing alliances could threaten the bystander's hierarchy rank or their previous investment in the groomers. To gain a competitive advantage, bystanders could intervene into a grooming bout to increase their own grooming access or to prevent the negative impact of others' grooming. We tested the impact of dominance rank and social relationships on grooming intervention likelihood and outcome in two sympatric primate species, Western chimpanzees ( Pan troglodytes verus ) and sooty mangabeys ( Cercocebus atys atys ). In both species, rather than increasing their own access to preferred partners, bystanders intervened mainly when an alliance between groomers could have a negative impact on them: when the lower-ranking groomer was close to the bystander in rank, when either groomer was an affiliation partner whose services they could lose, or the groomers were not yet strongly affiliated with each other. Thus, bystanders in both species appear to monitor grooming interactions and intervene based on their own dominance rank and social relationships, as well as triadic awareness of the relationship between groomers. While the motivation to intervene did not differ between species, mangabeys appeared to be more constrained by dominance rank than chimpanzees.