Variation in Dominance Relationships in Macaques from Phylogenetic and Socioecological Perspectives PDF Download

Author: Balasubramaniam Krishna-Natarajan
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Languages : en
Pages : 194

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Book Description
Understanding the evolution of sociality and social systems has been a major endeavor among evolutionary anthropologists and animal behaviorists. During the last two decades in particular, behavioral ecologists have found it a major challenge to explain broad inter- and intraspecific variation in primate social systems and in particular, variation in aspects of social structure related to dominance relationships, conflict management, and cooperative exchanges. One reason has been the lack of general consensus regarding the reliability of one or more operational behavioral measures related to dominance asymmetry and social power, despite these traits being central or core aspects of social structure. Although operational measures of both dominance asymmetry (Directional Consistency Indices, or DCI, and levels of counter-aggression) and social power (hierarchical steepness) are currently available, it is unclear whether one or more measures are (1) consistent with each other, and/or (2) are more suitable for groups with certain sets of characteristics. Another reason has been the proposal of apparently conflicting models to explain this variation in social structure. For instance, models based on phylogenetic constraints posit that several aspects of social structure are highly integrated, inherent species-specific characteristics that co-vary with each other and with species' evolutionary relationships. In contrast, models based on current socioecological factors (the Ecological Model of Female Social Relationships, or the EMFSR) posit that social structure varies widely between species with current ecological conditions, and possibly across and within groups of the same species of varying sizes. My dissertation research addresses both the methodological issue and the two models described above, focusing on the primate genus Macaca. All macaques show broadly similar patterns of social organization (e. g. female philopatry, male dispersal, and linear dominance hierarchies) and yet, broad inter- and intraspecific variation in several aspects of social structure, particularly in the extent to which dominance relationships are re-enforced. They are therefore uniquely appropriate for pursuing the overall aim of my thesis, i.e. to examine variation in dominance asymmetry and social power in macaques as a means to gain a greater insight into the roles of internal (phylogenetic) constraints and/or external (socioecological) factors in shaping variation in primate social structure. First, using six natural and 90 artificially generated datasets that varied broadly in group-characteristics, I confirm that dominance ranking methods used to compute both hierarchical steepness (David's score) and DCI (the I&SI method) yield rank orders consistent with each other and more crucially, with parametric Bayesian most-probable rank orders (Chapter 2). I also confirm, using multiple regression analyses, that the magnitude of the correlation coefficients of dominance ranks are unaffected by two out of three group characteristics (group size and interaction frequencies). That said, they are strongly and positively related to the extent to which groups show asymmetries in agonistic interactions, suggesting that correlations of rank orders using different methods are somewhat weaker in groups showing tolerant social styles. Finally, a series of t-tests for different subsets of artificial datasets confirm that two popularly used dyadic dominance indices (Dij-and Pij-scores) used to compute David's scores are equally consistent in yielding reliable rank orders, independently of group characteristics. Crucially, these findings justify my use in later chapters (3-5) of Dij-, Pij-based hierarchical steepness, and/or DCI as reliable indicators of dominance relationships in comparative analyses of natural macaque groups with a wide range of characteristics. Second, in Chapters 2 & 3, I test key predictions related to the phylogenetic constraints model. In particular, using a comparative dataset of dyadic dominance interactions for a range of macaques - 14 groups representing nine species - I ask whether measures of hierarchical steepness measures and counter-aggression vary more broadly between species than within species, and show strong phylogenetic signals. The dataset was assembled both from the literature and, in one case (a Celebes crested macaque group) collected by myself at the Buffalo Zoo. ANCOVA and multiple mixed-model regression analyses show that behavioral measures varied broadly between species but relatively less so within species. Further, randomization tests and test statistics show they were more similar among more closely related species, i.e. they displayed strong phylogenetic signals. To test some versions of phylogenetic models, I also examine (Chapter 3) whether these behavioral variations were consistent with the hypothesized placement of macaques into one of four `social style' categories. As predicted, steepness measures correlate negatively, and counter-aggression positively with scale. However, independent contrasts analyses show that these co-variations disappear after controlling for phylogenetic distances, suggesting that phylogeny largely explains observed co-variation in traits. Results of all phylogenetic tests and comparative analyses were highly consistent across nine phylogenetic trees, five extracted from the literature (the 10ktrees website) and four that I reconstruct from assembling available genetic datasets. Moreover, they were independent of variation in group size and current living condition. Overall, these findings strongly support the phylogenetic model. As such, they point to the need to expand similar analyses to include more macaque (and other primate) species, other aspects of social structure, and to control for within-species differences using intraspecific phylogenies. Finally, in Chapter 5, I test the predictions of intraspecific versions of the EMFSR, i.e. that when resources are clumped, larger groups face higher levels of within-group contest competition (WGC) and hence, show higher levels of despotism in social structure relative to smaller groups, that face higher levels of Between-group competition (BGC). To do so, I analyze data collected by myself and three co-investigators during a six-month field-study of three groups of free-ranging rhesus macaques of varying sizes at Cayo Santiago, Puerto Rico, controlling for the effects of several other potentially confounding socioecological variables, either because they were uniform (e. g. resource monopolizability) or absent (e. g. predation pressure). Using Kruskal-Wallis ANOVAs, Mann Whitney-U tests, and non-parametric MANOVAs, I establish that larger groups indeed show greater degrees of exposure to contexts of within-group contest competition (WGC) relative to smaller groups, at each of two independent sets of potentially monopolizable, clumped resources (feeders and drinking stations). Likewise, smaller groups are exposed more to contexts of between-group competition, or BGC. Further, the largest group shows the greatest behavioral indicators of exposure to WGC (highest propensities for dominant individuals to monopolize resources), and the least behavioral reactions (lowest proportions of snatching food, highest proportions of time spent feeding and drinking at resources) to exposure to BGC. This said, four aspects of social structure (DCI, counter-aggression, aggression intensity, and post-conflict affiliation between nonkin) did not vary across groups in predicted directions.^In particular, the largest group showed similar levels of despotism (characterized by higher DCI, more intense aggression, and low levels of counter-aggression and/or post-conflict affiliation) compared to the medium-sized or the smallest group. In summary, these findings establish strong links between group size and competitive regime in directions predicted by the EMFSR, but none in turn between these factors and aspects of social structure. As such, they are consistent with findings from the majority of empirical assessments of the model that have led to its recent scrutiny/ criticism. Finally, they establish the clear effects of group size on resource competition in a free-ranging primate population that resembles the conditions under which wild commensal rhesus macaques live on the Indian subcontinent. My dissertation research thus uniquely uses genetic and behavioral datasets to assess the evolution of macaque social structure, particularly aspects of social power and dominance asymmetry. In the Discussion, I speculate that, in addition to hierarchical steepness and DCI, Bayesian posterior probability may offer an additional measure to quantify dominance asymmetry and/or social power. Future research assessing the phylogenetic model should test for phylogenetic signals in additional aspects of social structure, and consider using phylogeographic approaches to examine whether transitions in aspects of social structure or style correspond to major, historic dispersal patterns into novel habitats, both in macaques and across a broad range of primate taxa. Finally, I suggest that future studies testing the EMFSR on both the Cayo Santiago macaques and other primate populations control for genetic/ phylogenetic constraints, individual health, and seasonal variation in datasets. I end by suggesting that a non-dichotomous, highly integrated approach be undertaken that uses a framework that encompasses `behavioral syndromes' or `social reaction norms' and combines both internal constraints and external factors to explain variation in primate social structure.