The social group of wild chimpanzees in the Mahali Mountains

There are more than six large groups of wild chimpanzees in the study area, which is in the north-eastern part of the Mahali Mountains of Western Tanzainia. One of these groups was provisionized, that is, customarily fed sugar cane and bananas. The characteristics of the social group of wild chimpanzees are clarified by long-term observation of the baited population. The chimpanzees live in a clear-cut social unit which consists of adult males, adult females, and immature animals. The permanency, stable membership, and integrative nature of the unit-group were confirmed during the course of this study. The size of unit-groups ranges from 30 to 80 head.The unit-group generally splits up into temporary subgroups that repeat joining and parting. The size of the subgroups of the baited population ranges from one to 28 head, the mean being 8.1 head. The centralization of a unitgroup is mainly sustained by the high sociability of adult males. The random nature of the membership of subgroups is emphasized in this paper, although subgroups are usually composed by social bonds on the basis of similar age, sex, blood relationship, and/or sexual attraction.The inter-unit-group interaction is peaceful; the subordinate unit-group avoids the dominant one. The home ranges of unit-groups overlap each other extensively, the overlapping areas being used flexibly by both unit-groups on the basis of dominance-subordination relationship. The member-exchange among unit-groups may sometimes occur, but the extent of openness or closedness of a unit-group has not been well elucidated.The research has been financed by the Scientific Research Fund of the Ministry of Education and in part by the Wenner-Gren Foundation.     

Class structure in a rhesus monkey group: the interplay between dominance and tolerance

Tests of competitive dominance are usually designed to eliminate social tolerance and variations in motivation. The present study applied a more naturalistic test paradigm to a large captive group of rhesus monkeys (Macaca mulatta). The monkeys were deprived of water for only 3 h, after which a shareable resource was presented. Twenty such tests were videotaped over a 6-month period. Drinking priority and the nature and duration of interactions around the water basin were compared to the group's clear-cut formal hierarchy, as defined by the direction of submissive teeth-baring. The adult hierarchy could be subdivided into two classes: precedence in drinking tests was as expected from formal dominance between classes but was not clear-cut within classes. This seemed due to variations in tolerance. Kin always belonged to the same class and showed high tolerance. Tolerant behaviour was also more common among unrelated adult females of the same class than of different classes. An alternative hypothesis to class structure, explaining the differences in tolerance on the basis of rank-distance effects, was rejected. The class division may be maintained through special intolerance between females ranking at the class borders.     

The role of relatedness in structuring the social network of a wild guppy population

Parasite avoidance is increasingly considered to be a potential driving factor in animal migrations. In many marine and freshwater benthic fish, migration into a pelagic environment by developing larvae is a common life history trait that could reduce exposure to parasites during a critical window of developmental susceptibility. We tested this hypothesis on congeneric fish (family Galaxiidae, genus Galaxias) belonging to a closely related species complex sampled from coastal streams in southeastern New Zealand. Migratory Galaxias have larvae that migrate to pelagic marine environments, whereas the larvae of non-migratory species rear close to adult habitats with no pelagic larval phase. Both migratory and non-migratory fish are hosts to two species of skin-penetrating trematodes that cause spinal malformations and high mortality in young fish. Using generalized linear models within an Akaike information criterion and model averaging framework, we compared infection levels between migratory and non-migratory fish while taking into account body size and several other local factors likely to influence infection levels. For one trematode species, we found a significant effect of migration: for any given body length, migratory fish harboured fewer parasites than non-migratory fish. Also, no parasites of any kind were found in juvenile migratory fish sampled in spring shortly after their return to stream habitats. Our results demonstrate that migration spares juvenile fish from the debilitating parasites to which they would be exposed in adult stream habitats. Therefore, either the historical adoption of a migratory strategy in some Galaxias was an adaptation against parasitism, or it evolved for other reasons and now provides protection from infection as a coincidental side-effect.     

The social component of dominance relationships in rhesus monkeys (Macaca mulatta)

An all-male rhesus group was added to a second group, one animal at a time, at weekly intervals, in reverse order of their dominance ranks. Significant positive correlations prevailed for the host group dominance ranks throughout the study, but the dominance ranks of the introduced males were random with respect to their original ranks. A second experiment repeatedly introduced the alpha and beta males of a multi-male heterosexual group into a second group where they were the lowest-ranking animals. On each return they reclaimed alpha and beta positions. When the alpha and beta males of the second group were introduced to the first group, they became the lowest-ranking animals in the group. Reversal of the direction of introduction again demonstrated that relative ranks were dependent on the social context.     

The role of carbon dioxide in the initiation and maintenance of blue-green dominance in lakes

1. In 1993 the south basin of Squaw Lake, Wisconsin, U.S.A., was artificially circulated and injected with CO₂ in an attempt to eliminate the massive quantities of blue-greens normally present during summer. The unmixed, uninjected north basin was the control. Despite a great difference in CO₂ concentration and in pH between the two basins, their blue-green maxima began simultaneously and eventually reached the same size. The predominant algae in both basins were Aphanizomenon flos-aquae and Anabaena flos-aquae. 2. From 13 May to 9 September, experiments were done to determine the responses of photosynthesis (oxygen production) of the phytoplankton communities in both basins to changes in pH over the range 7–9. Both the north and south basins underwent two distinct shifts in the slopes of their photosynthetic responses to pH. These shifts paralleled changes in the proportion of blue-greens in their phytoplankton. 3. Experiments with additions of KHCO₃ or NaHCO₃ showed that the responses to pH were really responses to CO₂ concentration. Therefore, the data obtained in the photosynthesis experiments were used to calculate the kinetic parameters K_s* and V_max*. The K_s* values varied in relation to the proportion of blue-greens, with lower values during the period when blue-greens predominated. The V_max*values showed no such systematic changes. 4. CO₂ compensation and zero photosynthesis concentrations confirm that when the blue-greens dominated, CO₂ was taken up much more efficiently than when the phytoplankton comprised mostly non-blue-greens. No evidence appeared suggesting direct use of bicarbonate even at pH values of 10.0. 5. Experiments with water collected seasonally from four other lakes show that Squaw Lake is not unique. In each case the response slopes (and therefore K_s* values) were high during spring when non-blue-greens were dominant, but decreased when the blue-green maximum began. 6. Experiments with individual species of algae confirm that blue-greens generally have better CO₂ kinetics than do greens. 7. It is concluded that initiation of the blue-green maximum does not depend upon conditions of low CO₂ concentration or high pH. However, once the blue-greens become abundant they ensure their dominance by reducing concentrations of CO₂ to levels available only to themselves.     

The role of wild mammals in the maintenance of Rift Valley fever virus

Rift Valley fever virus (RVFV) is a zoonotic arbovirus affecting primarily domestic ruminants and humans. Numerous vector species are known or implicated in the transmission of RVFV. The role of mammals in the maintenance of RVFV, and the existence of a wild mammal reservoir in the epidemiologic cycle of RVFV, remain largely unknown. Our objective is to present a detailed review of studies undertaken on RVFV, often associated with wild mammals, with the aim of focusing future research on potential reservoirs of the virus. Natural and experimental infections related to RVFV in several mammalian orders, including Artiodactyla, Chiroptera, Rodentia, Primata (nonhuman), Perissodactyla, Carnivora, Proboscidea, Erinaceomorpha, and Lagomorpha, are reviewed; the first four orders have received the greatest attention. The possible role of wild ruminants, especially African buffalo (Syncerus caffer), is also discussed. Conflicting results have been published concerning rodents but, based on the literature, the likely candidate species include the African genera Arvicanthis and Micaelamys and the widely introduced roof rat (Rattus rattus). Members of the orders Chiroptera and Rodentia should receive greater attention associated with new research programs. For the other orders mentioned above, few data are available. We are unaware of any investigation concerning the orders Afrosoricida and Soricomorpha, which are represented in the geographic area of RVFV and can be abundant. As a first step to resolve the question of wild mammals as a reservoir of RVFV, serologic and virologic surveys should be promoted during epizootic periods to document infected wild animals and, in the case of positive results, extended to interepidemic periods to explore the role of wild animals as possible reservoirs.     

Phylogenetic community ecology and the role of social dominance in sponge-dwelling shrimp

When functional traits are evolutionarily conserved, phylogenetic relatedness can serve as a proxy for ecological similarity to examine whether functional differences among species mediate community assembly. Using phylogenetic- and trait-based analyses, we demonstrate that sponge-dwelling shrimp (Synalpheus) assemblages are structured by size-based habitat filtering, interacting with competitive exclusion mediated by social system. Most shrimp communities were more closely related and/or more similar in size than randomized communities, consistent with habitat filtering facilitated by phylogenetically conserved body size. Those sponges with greater space heterogeneity hosted shrimp communities with greater size diversity, corroborating the importance of size in niche use. However, communities containing eusocial shrimp - which cooperatively defend territories - were less phylogenetically related and less similar in size, suggesting that eusociality enhances competitive ability and drives competitive exclusion. Our analyses demonstrate that community assembly in this diverse system occurs via traits mediating niche use and differential competitive ability.     

Self-organizing dominance hierarchies in a wild primate population

Linear dominance hierarchies, which are common in social animals, can profoundly influence access to limited resources, reproductive opportunities and health. In spite of their importance, the mechanisms that govern the dynamics of such hierarchies remain unclear. Two hypotheses explain how linear hierarchies might emerge and change over time. The ‘prior attributes hypothesis’ posits that individual differences in fighting ability directly determine dominance ranks. By contrast, the ‘social dynamics hypothesis’ posits that dominance ranks emerge from social self-organization dynamics such as winner and loser effects. While the prior attributes hypothesis is well supported in the literature, current support for the social dynamics hypothesis is limited to experimental studies that artificially eliminate or minimize individual differences in fighting abilities. Here, we present the first evidence supporting the social dynamics hypothesis in a wild population. Specifically, we test for winner and loser effects on male hierarchy dynamics in wild baboons, using a novel statistical approach based on the Elo rating method for cardinal rank assignment, which enables the detection of winner and loser effects in uncontrolled group settings. Our results demonstrate (i) the presence of winner and loser effects, and (ii) that individual susceptibility to such effects may have a genetic basis. Taken together, our results show that both social self-organization dynamics and prior attributes can combine to influence hierarchy dynamics even when agonistic interactions are strongly influenced by differences in individual attributes. We hypothesize that, despite variation in individual attributes, winner and loser effects exist (i) because these effects could be particularly beneficial when fighting abilities in other group members change over time, and (ii) because the coevolution of prior attributes and winner and loser effects maintains a balance of both effects.     

The acquisition of dominance among free-ranging rhesus monkey siblings

In several species of Cercopithecine monkeys, younger sisters reverse dominance with older sisters during the course of maturation. Proximate hypotheses to account for the phenomenon are based on few and incomplete data, and tend to be unifactorial. This paper (1) presents systematic data on ‘youngest ascendancy’ among free-ranging rhesus monkey, Macaca mulatta, siblings; and (2) attempts to identify its proximate basis. Early social influences were inadequate to explain reversal. Older siblings resisted outranking, and only those younger siblings (including orphans) with powerful long-term support rebelled successfully. Supporters tended to be close relatives (such as the mother and other siblings) dominant to both siblings. Rebellion did not occur at some absolute age of the younger sibling. Its timing depended partly on the relative size of siblings, and partly on the extent of support the younger sibling had against the older. The study strongly suggests that the occurrence and timing of reversal depends on an interaction between factors predictive of individual fighting ability (such as body size) and those predictive of social power (such as alliances). Observed variation in the regularity with which ‘younges ascendancy’ occurs is discussed in relation to these findings.     

Squirrel monkey dominance and social behavior as related to group size and group structure

Adult male and female squirrel monkeys were tested in nonsocial adaptation and pairwise and triad social situations differing in sex composition. Social behaviors, nonsocial behaviors, and dominance hierarchies were observed during social testing. Dominance hierarchies were similar in groups differing in size and social structure. Nonsocial behaviors decreased in females and submissive animals paired with males or dominant monkeys. Aggressiveness between females decreased and the beginnings of coalitions between females were observed in the presence of a male. The social behavior patterns, but not dominance hierarchies, are consistent with behaviors observed in larger groups of squirrel monkeys.     

Genealogical and cross-genealogical dominance relations in a group of free-ranging rhesus monkeys (Macaca mulatta) on Cayo Santiago

Observations of dominance relations in a large group of rhesus monkeys on Cayo Santiago were carried out over a period of 25 months. Dyadic interactions in which an aggressive gesture in one individual was followed by a submissive gesture in the other were recorded as fights and considered reliable indices of dominance. The analysis revealed the following characteristics: (1) Maternal dominance over female offspring; (2) maternal dominance over male offspring up the age six years at which time the son leaves his natal group or remains in the group and rises in rank over his mother; (3) dominance of older brothers over younger male siblings until the age of five years at which time the younger brother rises in rank; (4) rank reversal between sisters when the younger sister reaches the age of three to four years; (5) brother-sister relative rank dependency on age until the male sibling reaches three to four years at which time he rises in rank; (6) linear dominance relations in the crossgenealogical dominance hierarchy; and (7) linear, but unstable, dominance relations in the adult male hierarchy. With few exceptions, the pattern of genealogical, cross-genealogical, and adult male dominance relations in the group under study was consistent with data reported for a small social group (group F) on Cayo Santiago and for Japanese macaques.     

Sociometrics of Macaca mulatta: II. Decoupling centrality and dominance in rhesus monkey social networks

The concept of the linear dominance hierarchy and a much less precise notion of a central/peripheral ordering of individuals have been prominent among the ideas about social organization of monkey groups. Although the latter has seldom been quantified, the ranks of individuals in the two orders are usually assumed to be correlated. This paper reports on a longitudinal study of a free-ranging group of rhesus monkeys. The individual histories of progression of dominance rank and an independently determined measure of centrality in the social grooming network are compared among a set of males. Centrality is not a static correlate of dominance rank as implied by the findings of short-term studies. Rather, centrality is a more sensitive indicator of status than is dominance rank, to which it is related in a dynamic fashion. Small changes in dominance rank may be followed by large changes in centrality. An increase in centrality may facilitate rise in dominance rank. These findings suggest a complex psychology of status, rather than a simple causative relation between the two variables.     

Effects of increased testosterone secretion on the behavior of adult male rhesus living in a social group.

Selected members of an all-male social group of rhesus monkeys were treated with twice-weekly injections of human chorionic gonadotropin (HCG) to determine the effects of the consequently increased androgen secretion on social behavior. Radioimmunoassay of blood samples confirmed significant increases in serum testosterone levels in each of 4 subject, but only 3 of these showed significant behavioral changes. The basic social structure of the group, including the dominance hierarchy, was not disrupted by the treatment with HCG. Rather, the behavioral changes which did not occur appeared to be intensifications of previously existing social relationship.     

Electrical signalling of dominance in a wild population of electric fish

Animals often use signals to communicate their dominance status and avoid the costs of combat. We investigated whether the frequency of the electric organ discharge (EOD) of the weakly electric fish, Sternarchorhynchus sp., signals the dominance status of individuals. We correlated EOD frequency with body size and found a strong positive relationship. We then performed a competition experiment in which we found that higher frequency individuals were dominant over lower frequency ones. Finally, we conducted an electrical playback experiment and found that subjects more readily approached and attacked the stimulus electrodes when they played low-frequency signals than high-frequency ones. We propose that EOD frequency communicates dominance status in this gymnotiform species.