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Market effects offset the reciprocation of grooming in free-ranging redfronted lemurs, Eulemur fulvus rufus

Social grooming is a commonly observed affiliative behaviour in primates. Grooming has been suggested to represent a service in a biological marketplace, exchanged either for grooming or for other social commodities or services. Accordingly, grooming is predicted to be approximately reciprocated within a dyad when no other services are being exchanged, but it should be more asymmetrical if partners have different quantities of other services to offer. We analysed 412 grooming bouts observed in four groups of free-ranging redfronted lemurs to test this prediction. Grooming in this species seems to take place in a highly reciprocal manner because partners usually alternate in the roles of groomer and gromee within a grooming bout. However, within dyads there were asymmetries in the duration of grooming given and received. In both sexes, more grooming was directed from low-ranking towards high-ranking individuals than vice versa, and in males this asymmetry became more pronounced as the number of subordinates per group increased. Grooming in bisexual dyads was generally skewed in favour of males, but patterns of grooming between the sexes were less clear than within the sexes. In addition, aggression occurred at high frequencies between classes of individuals that were characterized by nonreciprocal grooming, suggesting that grooming may serve as a means to reduce aggression in dyads with a high potential for conflicts. Taken together, our results indicate that a strict reciprocation of grooming can be offset by power differentials between partners, where grooming appears to be traded by subordinates in exchange for the tolerance of dominants.

Mutual tolerance or reproductive competition? Patterns of reproductive skew among male redfronted lemurs (Eulemur fulvus rufus)

The social organization of gregarious lemurs significantly deviates from predictions of the socioecological model, as they form small groups in which the number of males approximately equals the number of females. This study uses models of reproductive skew theory as a new approach to explain this unusual group composition, in particular the high number of males, in a representative of these lemurs, the redfronted lemur (Eulemur fulvus rufus). We tested two central predictions of “concession” models of reproductive skew theory, which assume that subordinates may be allowed limited reproduction by dominant group members as an incentive to remain in the group, thereby increasing the group’s overall productivity. Accordingly, relatives are predicted to receive less reproduction than non-relatives, and the overall amount of reproductive concessions given to subordinates is predicted to increase as the number of subordinates increases. In addition, we tested whether the number of females in a group, a variable not previously incorporated in reproductive skew theory, affected reproductive skew among males. Using microsatellite analyses of tissue DNA, we determined paternities of 49 offspring born into our study population in Kirindy forest (western Madagascar) since 1996 to determine patterns of male reproductive skew to test these predictions. Our analyses revealed remarkable reproductive skew, with 71% of all infants being sired by dominant males, but both predictions of reproductive skew models could not be supported. Instead, the number of females best predicted the apportionment of reproduction among the males in this species, suggesting that current reproductive skew models need to incorporate this factor to predict reproductive partitioning among male primates and perhaps other group-living mammals.

Communal Defense of Territories and the Evolution of Sociality

The evolution of group living has attracted considerable attention from behavioral ecologists working on a wide range of study species. However, theoretical research in this field has been largely focused on cooperative breeders. We extend this line of work to species that lack alloparental care (hereafter termed “noncooperative species”) but that may benefit from grouping by jointly defending a common territory. We adopt a demographically explicit approach in which the rates of births and deaths as well as the dispersal decisions of individuals in the population determine the turnover rates of territories and the competition for breeding vacancies thus arising. Our results reveal that some of the factors thought to affect the evolution of cooperative breeding also affect the evolution of group living in noncooperative species. Specifically, high fecundity and low mortality of resident individuals both increase the degree of habitat saturation and make joining an established group more profitable for nonresidents (floaters). Moreover, if floaters can forcefully take over territories, the degree of habitat saturation also affects the chance that residents become targets of takeovers. In this situation, communal defense of territories becomes an important benefit that further promotes the evolution of group living.

Even Adult Sex Ratios in Lemurs: Potential Costs and Benefits of Subordinate Males in Verreaux's Sifaka (Propithecus verreauxi) in the Kirindy Forest CFPF, Madagascar

Optimal group size and composition are determined by both the costs and benefits of group living for the groups members. Verreauxs sifakas (Propithecus verreauxi), a diurnal lemur, form multimale multifemale groups with the tendency toward even adult sex ratios despite a small average number of females per group. The unexpected presence of multiple adult males may be explained by tolerance of other group members if subordinate males provide benefits to the group that outweigh the costs associated with their presence. Results based on both demographic data collected over a 13-year period and behavioral observations suggest that subordinate males provide no benefits in terms of infant survival and defense against group takeover by outside males. Although groups with more males are more likely to win intergroup encounters, subordinate males do not participate in these encounters more often than expected. Subordinate males are not costly to other group members in terms of direct intragroup feeding competition, but aggression rates between dominant and immigrated subordinate males increase in the mating season. Even though subordinate males provide very few benefits to the group, they are not very costly either and thus may be tolerated by resident females and dominant males. This tolerance may help to partially explain the tendency towards their unusual adult sex ratio.

Costs and benefits of multi-male associations in redfronted lemurs (Eulemur fulvus rufus)

The evolution of group-living has fascinated but also puzzled researchers from the inception of behavioural ecology. We use a simple optimality approach to examine some of the costs and benefits of group-living in redfronted lemurs (Eulemur fulvus rufus). We show that dominant males profit from accepting subordinates within their groups, as the latter significantly decrease the likelihood that the group is taken over by intruders. This benefit is large enough to outweigh the costs of reproductive competition and may constitute the driving force behind the evolution of multi-male associations in this species.

Facing the crowd: intruder pressure, within‐group competition, and the resolution of conflicts over group‐membership

Recent theory in social evolution has been mainly concerned with competition and cooperation within social groups of animals and their impact on the stability of those groups. Much less attention has been paid to conflicts arising as a result of solitary floaters (outsiders) attempting to join groups of established residents (insiders). We model such conflicts over group-membership using a demographically explicit approach in which the rates of births and deaths in a population determine the availability of group-vacancies and the number of floaters competing over these vacancies. We find that the outcome of within-group competition, reflected in the partitioning of reproduction among group members, exerts surprisingly little influence on the resolution of insider-outsider conflict. The outcome of such conflict is also largely unaffected by differences in resource holding potential between insiders and outsiders. By contrast, whether or not groups form is mainly determined by demographic factors (variation in vital rates such as fecundity and mortality) and the resulting population dynamics. In particular, at high floater densities territory defense becomes too costly, and groups form because insiders give in to the intruder pressure imposed on them by outsiders. We emphasize the importance of insider-outsider conflicts in social evolution theory and highlight avenues for future research.

Reproductive Competition Among Males in Multimale Groups of Primates: Modeling the Costs and Effectiveness of Conflict

Multimale groups of primates are characterized by strong reproductive competition among males, generally resulting in an uneven division of male reproductive success (reproductive skew). The observed patterns of conflict and reproductive skew have often been attributed to the so-called tug-of-war model. We show, however, that two important assumptions of this model are not met in male primates. First, the tug-of-war model assumes that reproductive conflict reduces overall group productivity, but in male primates (and most other vertebrates) conflict likely involves mortality rather than fecundity costs. Second, the tug-of-war model does not account for the possibility that male primates can achieve some reproductive success without engagement in open conflict, such as when a single male cannot guard several receptive females at the same time. We therefore develop a dynamic version of the tug-of-war model, in which reproductive competition causes mortality costs, and in which individuals can gain uncontested shares of reproduction dependent on the degree of female receptive overlap. This model differs substantially from the original tug-of-war model, and derives a new and rich set of comparative predictions. For instance, it predicts that the level of conflict among males declines as the queuing success of subordinate males increases (as survival increases), and also, as their uncontested share of reproduction increases, e.g., as female receptive overlap increases. Our model shows how male–male conflict and female receptive overlap collectively determine the level of reproductive skew among male primates, and illustrates that this relationship is more complex than previously thought.

Transparency, usability, and reproducibility: Guiding principles for improving comparative databases using primates as examples

Recent decades have seen rapid development of new analytical methods to investigate patterns of interspecific variation. Yet these cutting‐edge statistical analyses often rely on data of questionable origin, varying accuracy, and weak comparability, which seem to have reduced the reproducibility of studies. It is time to improve the transparency of comparative data while also making these improved data more widely available. We, the authors, met to discuss how transparency, usability, and reproducibility of comparative data can best be achieved. We propose four guiding principles: 1) data identification with explicit operational definitions and complete descriptions of methods; 2) inclusion of metadata that capture key characteristics of the data, such as sample size, geographic coordinates, and nutrient availability (for example, captive versus wild animals); 3) documentation of the original reference for each datum; and 4) facilitation of effective interactions with the data via user friendly and transparent interfaces. We urge reviewers, editors, publishers, database developers and users, funding agencies, researchers publishing their primary data, and those performing comparative analyses to embrace these standards to increase the transparency, usability, and reproducibility of comparative studies.

Longevity suppresses conflict in animal societies

Models of social conflict in animal societies generally assume that within-group conflict reduces the value of a communal resource. For many animals, however, the primary cost of conflict is increased mortality. We develop a simple inclusive fitness model of social conflict that takes this cost into account. We show that longevity substantially reduces the level of within-group conflict, which can lead to the evolution of peaceful animal societies if relatedness among group members is high. By contrast, peaceful outcomes are never possible in models where the primary cost of social conflict is resource depletion. Incorporating mortality costs into models of social conflict can explain why many animal societies are so remarkably peaceful despite great potential for conflict.

From Individual to Group Territoriality: Competitive Environments Promote the Evolution of Sociality

In many animal species individuals compete for the sole possession of a breeding territory, whereas in other species communal territories are shared among same-sex conspecifics. Under what conditions does natural selection favor the evolution of individual territoriality, and under what conditions does it favor the evolution of sociality? We develop a self-consistent game-theoretic model that allows for feedbacks between evolutionary and population dynamics. In this model, nonresident floaters can chose between three strategies: they can wait for a territory vacancy to arise, they can try to forcefully take over an already-occupied territory, or they can share a territory with an established resident. We show that competitive environments initially favor the evolution of an aggressive (territorial) strategy. Yet as competition increases further, a shift occurs from aggressive to social strategies. Moreover, territory owners (residents) respond to the behavior of floaters, such that a feedback occurs in which residents adjust their degree of tolerance to the level of floater aggression and vice versa. This feedback resembles the dynamics of a biological market and eventually leads to the coexistence of aggressive and social floater strategies in the population. Such mixed equilibria commonly occur in nature.