High-ranking Males Research Articles

High-ranking alleviates male local competition in lek mating systems

Territoriality entails demanding social interactions with competing individuals, typically males. Variation in quality of males can be predicted to affect the spatial arrangement of territories. We present a model aimed at understanding the spatial properties of territories on leks, where the presence of a hierarchy in a population of males leads to the clustering of individuals around high-ranking ‘hotshot’ males. The hierarchy results in a decrease in the number of nearest neighbors interacting directly with high-ranking males, with potential socio-sexual benefits for such males.

Assessment of Male Reproductive Skew via Highly Polymorphic STR Markers in Wild Vervet Monkeys, Chlorocebus pygerythrus

Male reproductive strategies have been well studied in primate species where the ability of males to monopolize reproductive access is high. Less is known about species where males cannot monopolize mating access. Vervet monkeys (Chlorocebus pygerythrus) are interesting in this regard as female codominance reduces the potential for male monopolization. Under this condition, we assessed whether male dominance rank still influences male mating and reproductive success, by assigning paternities to infants in a population of wild vervets in the Eastern Cape, South Africa. To determine paternity, we established microsatellite markers from noninvasive fecal samples via cross-species amplification. In addition, we evaluated male mating and reproductive success for 3 groups over 4 mating seasons. We identified 21 highly polymorphic microsatellites (number of alleles = 7.5 ± 3.1 [mean ± SD], observed heterozygosity = 0.691 ± 0.138 [mean ± SD]) and assigned paternity to 94 of 97 sampled infants (96.9%) with high confidence. Matings pooled over 4 seasons were significantly skewed across 3 groups, although skew indices were low (B index = 0.023-0.030) and mating success did not correlate with male dominance. Paternities pooled over 4 seasons were not consistently significantly skewed (B index = 0.005-0.062), with high-ranking males siring more offspring than subordinates only in some seasons. We detected 6 cases of extra-group paternity (6.4%) and 4 cases of natal breeding (4.3%). Our results suggest that alternative reproductive strategies besides priority of access for dominant males are likely to affect paternity success, warranting further investigation into the determinants of paternity among species with limited male monopolization potential.

Sperm concentration, coagulum weight, and testosterone levels differences according to social rank in male stump-tail macaques (Macaca arctoides)

In multi-female multi-male group-living species, both sexes can copulate with diverse partners. According to the priority of access model, high-ranking males have higher access to females during their fertile phases than the lower-ranking competitors. However, when females’ ovarian cycles are synchronized, dominant males are unable to monopolize all females, which gives a chance to lower ranking males to sporadically copulate and increase their likelihood to fertilize a female. Therefore, subordinate males perhaps show physiological characteristics allowing them to contend with the advantage that the dominant males already have. It was investigated if sperm counts, weight of the seminal coagulum, and testosterone levels showed any differences according to their social rank in male stump-tailed macaques. Testosterone levels increased linearly in relation to social rank (P = 0.004). No evidence was found that in the first ejaculation dominant rank was positively related to sperm counts and weight of the seminal coagulum. In the second ejaculation a positive correlation between dominant rank and sperm counts and weight of the seminal coagulum was found (P < 0.001). By the third ejaculation there was a negative correlation between sperm counts and dominant rank (P < 0.001), but no variability in the weight of the seminal coagulum. The results showed that high-rank is related to better physiological conditions to sustain multiple ejaculations, thanks to having higher testosterone levels.

Male resource defense mating system in primates? An experimental test in wild capuchin monkeys.

Ecological models of mating systems provide a theoretical framework to predict the effect of the defendability of both breeding resources and mating partners on mating patterns. In resource-based mating systems, male control over breeding resources is tightly linked to female mating preference. To date, few field studies have experimentally investigated the relationship between male resource control and female mating preference in mammals due to difficulties in manipulating ecological factors (e.g., food contestability). We tested the within-group male resource defense hypothesis experimentally in a wild population of black capuchin monkeys (Sapajus nigritus) in Iguazú National Park, Argentina. Sapajus spp. represent an ideal study model as, in contrast to most primates, they have been previously argued to be characterized by female mate choice and a resource-based mating system in which within-group resource monopolization by high-ranking males drives female mating preference for those males. Here, we examined whether females (N = 12) showed a weaker preference for alpha males during mating seasons in which food distribution was experimentally manipulated to be less defendable relative to those in which it was highly defendable. Results did not support the within-group male resource defense hypothesis, as female sexual preferences for alpha males did not vary based on food defendability. We discuss possible reasons for our results, including the possibility of other direct and indirect benefits females receive in exercising mate choice, the potential lack of tolerance over food directed towards females by alpha males, and phylogenetic constraints.

The timing and causes of a unique chimpanzee community fission preceding Gombe's "Four-Year War".

While permanent group fissions are documented in humans and other primate species, they are relatively rare in male philopatric primates. One of the few apparent cases occurred in 1973 in Gombe National Park, Tanzania, when a community of chimpanzees split into two separate groups, preceding the famous "Four-Year War." We tested the hypothesis that the original group was a single cohesive community that experienced permanent fission, and investigated several potential catalysts. We calculated association, grooming, and ranging metrics from historical data and used community detection algorithms and matrix permutation tests to determine the timing, dynamics, and causes of changes in social network subgrouping structure. We found that the two communities indeed split from one cohesive community, albeit one with incipient subgrouping. The degree of subgrouping in grooming and association networks increased sharply in 1971 and 1972, a period characterized by a dominance struggle between three high-ranking males and unusually high male:female sex ratios. Finally, we found a relationship between post-split community membership and previous association, grooming and ranging patterns in most periods of analysis, one that became more pronounced as the fission approached. Our analysis suggests that the community began to split during a time of an unusually male-biased sex ratio and a protracted dominance struggle, and that adult males remained with those with whom they had preferentially associated prior to the split. We discuss the costs and benefits of group membership in chimpanzees and contrast these results with group fissions in humans and other taxa.

Group social rank is associated with performance on a spatial learning task.

Dominant individuals differ from subordinates in their performances on cognitive tasks across a suite of taxa. Previous studies often only consider dyadic relationships, rather than the more ecologically relevant social hierarchies or networks, hence failing to account for how dyadic relationships may be adjusted within larger social groups. We used a novel statistical method: randomized Elo-ratings, to infer the social hierarchy of 18 male pheasants, Phasianus colchicus, while in a captive, mixed-sex group with a linear hierarchy. We assayed individual learning performance of these males on a binary spatial discrimination task to investigate whether inter-individual variation in performance is associated with group social rank. Task performance improved with increasing trial number and was positively related to social rank, with higher ranking males showing greater levels of success. Motivation to participate in the task was not related to social rank or task performance, thus indicating that these rank-related differences are not a consequence of differences in motivation to complete the task. Our results provide important information about how variation in cognitive performance relates to an individual's social rank within a group. Whether the social environment causes differences in learning performance or instead, inherent differences in learning ability predetermine rank remains to be tested.

Consistency in social network position over changing environments in a seasonally breeding primate

Social network analysis has become increasingly used to investigate the consequences of individuals’ social tendencies in recent years; however, the question of whether individuals are consistent in their social network position over time or situations has received comparatively scant attention. Knowledge of the degree to which individuals are stable in their social behavior can improve our understanding the fitness consequences of processes such as the transmission of disease and social information through networks, as well as the evolution of social behavior and social structure in general. Here, I investigated individual consistency in social behavior in wild vervet monkeys (Chlorocebus pygerthrus) residing in a highly seasonal habitat. I constructed grooming networks for three time periods characterized by diverse environmental conditions and explored the repeatability of seven network metrics as well as the effects of sex and dominance rank on these metrics. Both female and male vervets were highly consistent in three correlated, weighted measures of social connectedness—grooming in-strength, out-strength, and eigenvector centrality, and in-degree was moderately repeatable in both sexes. In contrast, only males were consistent in their out-degree and clustering coefficient, and there was no evidence of consistency in betweenness for either sex. High-ranking females had higher in-degree and in-strength scores, and high-ranking males had lower out-degree and out-strength, but controlling for dominance rank did not affect the findings regarding the repeatability of any metric. These results add to the small number of studies that have explored the temporal and situational stability of social network phenotypes in wild animals. Relatively few animal social network studies have examined whether individuals are consistent in the positions that they occupy within networks. Determining the degree to which individuals are consistent in their social network characteristics over time and across naturally changing conditions is necessary for a better understanding of the ecology and evolution of social behavior. In this study, I examined individual consistency in several components of social network position in vervet monkeys residing in a highly seasonal environment. I also evaluated the effects of sex and dominance rank on these measures of sociality. I found that both male and female vervets were consistent in most measures of direct connectedness in grooming networks across seasons, but metrics capturing indirect social connections were overall less repeatable. Controlling for the effects of dominance rank on sociality measures did not change these results.

Alpha male status and availability of conceptive females are associated with high glucocorticoid concentrations in high-ranking male rhesus macaques (Macaca mulatta) during the mating season

The relationship between male mating opportunities, stress, and glucocorticoid concentrations is complicated by the fact that physiological stress and glucocorticoid concentrations can be influenced by dominance rank, group size, and the stability of the male dominance hierarchy, along with ecological factors. We studied the three highest-ranking males in nine different social groups within the same free-ranging population of rhesus macaques on Cayo Santiago, Puerto Rico, during the mating season, to examine variation in glucocorticoid concentrations in relation to number of females that conceived each month, alpha status, number of adult males in a group, and male rank hierarchy stability. We found that glucocorticoid concentrations were highest in the early mating season period when more females conceived in each group and declined linearly as the mating season progressed and the number of conceptive females decreased. Alpha males had significantly higher mean monthly glucocorticoid concentrations than other high-ranking males throughout the study period. Male age, number of adult males in a group, and hierarchy stability were not significantly associated with glucocorticoid concentrations. Our findings suggest that alpha males may experience significantly higher levels of physiological stress than their immediate subordinates and that this stress coincides with the period of the mating season when most conceptions occur.

Formidable females and the power trajectories of socially integrated male vervet monkeys

Sexual conflict theory expects females to avoid nonoptimal mating attempts by males. Although female vervet monkeys, Chlorocebus pygerythrus, can resist direct mating attempts, higher-ranking males still have more mating opportunities than lower-ranking ones. We presume that rank-related access reflects male–male competition that may conflict with female reproductive objectives. We extend an earlier report of codominance in this species to show that powerful females can undermine the restrictions imposed through male–male competition by improving the dominance rank of preferred male associates. We found that the dominance hierarchies of the sexes were comprehensively interdigitated and that males who had more female spatial associates, and who groomed with well-connected females, were more likely improve their Elo-ratings, which we use as an index of male power. The effects of partner number and integration, which predicted the probability of the initiation of aggression by lower-ranking males, suggest that association with females offered the prospect of protected threat if this likelihood increased. Although female rank and aggression were not directly consequential for males, we argue that female power and influence are intertwined and that both stem from the strength of female reproductive control.

A mixed strategy of counter-singing behavior in male rock hyrax vocal competitions

Maynard-Smith’s game-theoretical framework suggests that conflicts should be resolved through an exchange of agonistic displays which can be as effective as physical attack in securing contested resources. Hence, participants use signaling tactics that are governed by game-theoretical competition rules to maximize their benefit from the interaction. In rock hyraxes, males initiate singing in response to conspecific male songs, possibly to self-advertise and to facilitate comparison of performance by receivers. We hypothesized that hyrax counter-singing strategy can be explained by resident-intruder model and its outcome is determined by the identity and ability of the participants. Natural hyrax singing events and playback-induced events were classified according to the participants’ residence status—resident or bachelor (i.e., non-resident). As predicted by the resident-intruder model, resident listeners tended to reply to conspecific males at high rates (≈40 %), while bachelors showed low reply rates (17 %) to resident initiators. Unexpectedly, the lowest-ranking bachelors showed a tendency to reply to the highest-ranking residents. One possible explanation for this counter-intuitive behavior might be that weaker individuals exploit a situation in which it is unremunerated for a stronger opponent to retaliate (i.e., Napoleon strategy) and seize a self-advertisement opportunity in front of a large audience attending to the high-ranking males’ signal. Overall, we show that male hyraxes employ different counter-singing tactics depending on the residence and rank differences between the participants. This choice of competition tactics can be based on opponents’ individual recognition or/and on direct decoding of opponents’ traits encoded in the song. In nature, conflicts are usually resolved through signaling rather than by actual fighting. Rock hyrax males often participate in vocal competitions, possibly to reduce the need for physical confrontations. During our study on the dynamics of male hyrax vocal competition, we discovered that hyraxes employ mixed counter-singing strategy dependent on the residence status and social rank differences between the participants. A paradoxical “Napoleon-complex” tactic was observed in interactions between males, when low ranking bachelors were found to interact and challenge the highest-ranking residents. On the other hand, residents were found to interact with opponents regardless of residency and/or rank. On top of the cognitive abilities required to individually recognize an opponent and possibly decode its vocal signals, our results suggest that male hyraxes compare own individual traits to those of the opponent in a mutual signal assessment interaction.

Determinants of immigration strategies in male crested macaques (Macaca nigra).

Immigration into a new group can produce substantial costs due to resistance from residents, but also reproductive benefits. Whether or not individuals base their immigration strategy on prospective cost-benefit ratios remains unknown. We investigated individual immigration decisions in crested macaques, a primate species with a high reproductive skew in favour of high-ranking males. We found two different strategies. Males who achieved low rank in the new group usually immigrated after another male had immigrated within the previous 25 days and achieved high rank. They never got injured but also had low prospective reproductive success. We assume that these males benefitted from immigrating into a destabilized male hierarchy. Males who achieved high rank in the new group usually immigrated independent of previous immigrations. They recieved injuries more frequently and therefore bore immigration costs. They, however, also had higher reproductive success prospects. We conclude that male crested macaques base their immigration strategy on relative fighting ability and thus potential rank in the new group i.e. potential reproductive benefits, as well as potential costs of injury.

Mother-male bond, but not paternity, influences male-infant affiliation in wild crested macaques.

In promiscuous primates, interactions between adult males and infants have rarely been investigated. However, recent evidence suggests that male affiliation towards infants has an influence on several aspects of the infants’ life. Furthermore, affiliations may be associated with male reproductive strategy. In this study, we examined which social factors influenced male-infant affiliation initiated by either male or infant, in wild crested macaques (Macaca nigra). We combined behavioral data and genetic paternity analysis from 30 infants living in three wild groups in Tangkoko Reserve, Indonesia. Our results indicate that adult males and infants do not interact at random, but rather form preferential associations. The social factors with the highest influence on infant-initiated interactions were male rank and male association with the infant’s mother. While infants initiated affiliations with males more often in the absence of their mothers, adult males initiated more affiliations with infants when their mothers were present. Furthermore, males initiated affiliations more often when they were in the same group at the time the infant was conceived, when they held a high dominance rank, or when they had a close relationship with the mother. Interestingly, paternity did not affect male-infant affiliation despite being highly skewed in this species. Overall, our results suggest that adult males potentially associate with an infant to secure future mating with the mother. Infants are more likely to associate with a male to receive better support, suggesting a strategy to increase the chance of infant survival in a primate society with high infant mortality.Significance statementWe explore social relationships between males and infants in a promiscuous primate, the wild crested macaque. Our novel approach addresses the nature of affiliations both from males’ and infants’ perspectives. The results show that males and infants form preferential associations. Male-female affiliation, but not paternity, was a significant predictor of interactions initiated both by males and infants. Males initiated more interactions towards infants when the mother was in proximity, while infants initiated more interactions in her absence. Finally, high-ranking males were more likely to initiate interactions towards infants. We demonstrated that paternity is not a good predictor of male-infant affiliations, even in a species with a high reproductive skew and a relatively high confidence of paternity. Our paper is one of the first to show that infants are active agents in establishing and maintaining preferential relationships with males.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2116-0) contains supplementary material, which is available to authorized users.

Consequences of a male takeover on mating skew in wild Sanje mangabeys.

Among primate species living in multimale-multifemale groups, the number of receptive females may determine the rank of the lowest male that copulates (priority of access model, or PoA [Altmann SA. 1962. A field study of the sociobiology of rhesus monkeys, Macaca mulatta. Annals of the New York Academy of Sciences 102:338-435]). Factors, such as temporary instability in the hierarchy and female behavior can, however, affect high-ranking males' ability to monopolize females, reducing mating skew and causing the hierarchy to depart from predictions of PoA. Here, I use behavioral data collected over a 22-month period on a wild group of Sanje mangabeys (Cercocebus sanjei) to examine the effect of a takeover by two immigrating males who became α and β in the hierarchy, and of female behavior on male mating skew. Data on male agonistic interactions were collected on nine males using ad libitum observations, while information on male mating success (i.e., daily proportion of ejaculatory copulations with the focal female) was collected through focal follows of 12 sexually receptive females. Before the takeover, the hierarchy was stable and highly skewed, with the α-male monopolizing up to 75% of copulations. At this time, however, mating skew did not follow the predictions of PoA. During the takeover, from the time one of the newly immigrant males replaced the α until the second immigrant male became the new β, the stability of the hierarchy dropped significantly and mating was no longer skewed. Accordingly, the top two ranking males at that time (i.e., the new α and former β) accounted for only 33% of copulations. After the takeover, rank stability increased, and mating skew followed the PoA. Female mating solicitations also had a significant positive effect on male mating success, and may have contributed to the deviation from the PoA. This study demonstrates that temporary rank instability decreases high-ranking males' ability to monopolize copulations, causing a departure from the predictions of the PoA. Am. J. Primatol. 79:e22532, 2017. © 2016 Wiley Periodicals, Inc.

Production of grooming-associated sounds by chimpanzees (Pan troglodytes) at Ngogo: variation, social learning, and possible functions.

Chimpanzees (Pan troglodytes) use some communicative signals flexibly and voluntarily, with use influenced by learning. These signals include some vocalizations and also sounds made using the lips, oral cavity, and/or teeth, but not the vocal tract, such as "attention-getting" sounds directed at humans by captive chimpanzees and lip smacking during social grooming. Chimpanzees at Ngogo, in Kibale National Park, Uganda, make four distinct sounds while grooming others. Here, I present data on two of these ("splutters" and "teeth chomps") and consider whether social learning contributes to variation in their production and whether they serve social functions. Higher congruence in the use of these two sounds between dyads of maternal relatives than dyads of non-relatives implies that social learning occurs and mostly involves vertical transmission, but the results are not conclusive and it is unclear which learning mechanisms may be involved. In grooming between adult males, tooth chomps and splutters were more likely in long than in short bouts; in bouts that were bidirectional rather than unidirectional; in grooming directed toward high-ranking males than toward low-ranking males; and in bouts between allies than in those between non-allies. Males were also more likely to make these sounds while they were grooming other males than while they were grooming females. These results are expected if the sounds promote social bonds and induce tolerance of proximity and of grooming by high-ranking males. However, the alternative hypothesis that the sounds are merely associated with motivation to groom, with no additional social function, cannot be ruled out. Limited data showing that bouts accompanied by teeth chomping or spluttering at their initiation were longer than bouts for which this was not the case point toward a social function, but more data are needed for a definitive test. Comparison to other research sites shows that the possible existence of grooming-specific sound dialects in chimpanzees deserves further investigation.

Social and Ecological Correlates of Parasitic Infections in Adult Male Gray-Cheeked Mangabeys (Lophocebus albigena)

Intestinal parasites may constitute an important evolutionary and ecological force. We aimed to identify social, physiological, and environmental factors that correlate with intestinal parasite infections in adult male gray-cheeked mangabeys (Lophocebus albigena). We analyzed 102 fecal samples collected from 18 adult males over 19 mo for the incidence (proportions of samples with parasites) and intensity (total number of parasites per gram of feces) of infection of nematodes relative to social status, fecal glucocorticoid and testosterone metabolites, group size, and rainfall, all of which are factors that earlier studies suggested can be important mediators of parasite load. Parasite incidence was greater in immigrant males compared to low- and high-ranking males whereas parasite intensity was greater in immigrant males and low-ranking males compared to high-ranking males. Fecal samples with more parasites had higher concentrations of fecal glucocorticoid and testosterone metabolites than those with fewer parasites. As immigrant males had a greater incidence of parasites and higher concentrations of both metabolites than resident males, this profile appears to fit immigrant males best. We also found higher nematode intensities in mid-sized groups and during periods with more rainfall. Our results suggest that it will be fruitful for future studies to explore the role of immigrant males as spreaders of intestinal parasites.

Does Male Care, Provided to Immature Individuals, Influence Immature Fitness in Rhesus Macaques?

Among many mammals, maternal care strongly impacts infant survival; however, less is known about whether adult males also affect infant fitness. Paternal care is expected when providing care enhances offspring survival and reproduction, which likewise increases fathers’ fitness. Males might also care for unrelated immature individuals to increase their mating probability with the immature individuals’ mothers. Studies in multimale primate groups showed that sires enhance food access for offspring and provide protection in conflicts. Furthermore, fathers’ presence during infancy has been suggested to accelerate offspring sexual maturation. However, no study has yet directly linked the degree of father-offspring bonds to offspring fitness in primates. We previously reported father-offspring affiliation in rhesus macaques, pronounced during early infancy and independent of mothers’ presence. The present study aims at investigating whether affiliation with fathers or other males affects proxies of immature fitness (body mass gain, body fat and testis size). First, we combined behavioral, genetic and morphometric data from 55 subjects of one group. Second, using demographic and genetic data, we investigated for 92 individuals of the population whether mother- and father-offspring co-residence during immaturity influenced offspring lifetime reproductive success (LRS). Our results show that focal rank and higher amounts of affiliation with high-ranking males during infancy tend to positively impact body mass gain of female, but not male focal animals. In contrast, body mass gain of male focal individuals, but not females’, appeared to be higher when affiliation of male immature individuals was evenly distributed across their adult male partners. Moreover, we found mothers’, but not fathers’, presence during immaturity to predict offspring LRS. Our results suggest that male-immature affiliation, but not father-offspring co-residence, potentially impacts proxies of immature fitness. However, future studies should investigate the underlying mechanisms of male-immature relationships and their impact on immature fitness in more detail.

Audience effects in chimpanzee food calls and their potential for recruiting others

One proposed benefit for the seemingly costly behaviour of food calling is the recruitment of social allies and mates by the signaller. In chimpanzees, food calls are only produced for approximately half of all feeding events. Therefore, we investigated the influence of social and ecological context on the probability of making a food call upon arriving to a food patch in a group of wild chimpanzees. First, we tested whether feeding events where food calls had been uttered did in fact attract more individuals to join the caller. Secondly, we examined the influence of two sources of audience effects: those who were physically present with the caller and those who were presumed nearby but out of sight, and thirdly the effect of various ecological factors. We found that when feeding on fruit species, events where food calls had been produced had a higher probability of group mates arriving, even whilst controlling for the effect of pant hoots. Furthermore, the probability of uttering a food call was motivated by social more than ecological context. Specifically, high-ranking males were more likely to make food calls when estrous females were nearby, while low-ranking males and females generally called more when more females were nearby, irrespective of their reproductive state. These effects were independent of the increase in food call probability when male callers were accompanied by more males. Our findings support the recruitment function of food calls and suggest that high-ranking males call to attract estrous females to food patches to obtain mating opportunities.

Cortisol but not testosterone is repeatable and varies with reproductive effort in wild red deer stags

Although it is known that hormone concentrations vary considerably between individuals within a population, how they change across time and how they relate to an individual’s reproductive effort remains poorly quantified in wild animals. Using faecal samples collected from wild red deer stags, we examined sources of variation in faecal cortisol and androgen metabolites, and the potential relationship that these might have with an index of reproductive effort. We also biologically validated an assay for measuring androgen metabolites in red deer faeces.We show that variation in hormone concentrations between samples can be accounted for by the age of the individual and the season when the sample was collected. Faecal cortisol (but not androgen) metabolites also showed significant among-individual variation across the 10-year sampling time period, which accounted for 20% of the trait’s phenotypic variance after correcting for the age and season effects. Finally, we show that an index of male reproductive effort (cumulative harem size) during the mating season (rut) was positively correlated with male cortisol concentrations, both among and within individuals. We suggest that the highest ranking males have the largest cumulative harem sizes (i.e. invest the greatest reproductive effort), and that this social dominance may have associated behaviours such as increased frequency of agonistic interactions which are associated with corresponding high levels of faecal cortisol metabolites (FCM).

Characteristics of alpha males in Nepal gray langurs.

In species with strong male-male competition, access to females in multimale-multifemale groups is usually regulated via a dominance hierarchy. The highest ranking (alpha) male often has priority of access and sires most offspring. The alpha male can change in three basic ways: (i) a recent immigrant or a resident challenges and becomes the new alpha; (ii) formation of a new group; (iii) succession-becoming alpha after higher ranking males have left. When, in a given primate population, the alpha male changes in different ways, two questions arise: (a) which is the most successful tactic and (b) do male attributes, such as age, aggressiveness or propensity to commit infanticide, affect the outcome? We examined these questions in the seasonally breeding Nepal gray langurs (Semnopithecus schistaceus) at Ramnagar, where new alpha males were either recent immigrants or residents. Success was measured as alpha tenure, residency duration, and the number of offspring sired (paternity exclusion based on DNA analysis, 28 infants). We documented 12 alpha-male tenures across two multimale-multifemale groups between 1991 and 1997. The predominant mode of change was the immigrant tactic. Age had no effect perhaps because alpha males were among the youngest adult males in their group. As expected, infanticidal males performed similarly to non-infanticidal ones. Alpha tenure was highly variable and longer for immigrant alphas and hyper-aggressive ones. However, none of the tactics or attributes examined resulted in significantly longer residencies or more offspring, likely because of the timing of immigrations and stochastic effects (i.e., the number of conceptions occurring per alpha tenure). The influence of female mate choice on male reproductive success requires further investigation. Furthermore, it remains to be examined why resident alpha males-with their presumed better knowledge of their opponents -performed so poorly. Am. J. Primatol. 79:e22437, 2017. © 2015 Wiley Periodicals, Inc.

Life as a bachelor: quantifying the success of an alternative reproductive tactic in male blue monkeys.

In species that live in one-male groups, resident males monopolize access to a group of females and are assumed to have higher reproductive success than bachelors. We tested this assumption using genetic, demographic, and behavioral data from 8 groups of wild blue monkeys observed over 10 years to quantify reproduction by residents and bachelors and compare the success of the two tactics. We used maximum-likelihood methods to assign sires to 104 offspring born in the study groups, 36 of which were sired by extra-group males, i.e., residents of neighboring groups and bachelors. Among these extra-group males, high-ranking males (many of whom were neighboring residents) were more likely to sire offspring than low-ranking males, but the time these visiting males spent in the mother’s group when she conceived (male presence) did not predict their relative success. When bachelors competed for reproduction with other bachelors, neither rank nor male presence during the mother’s conceptive period affected the probability of siring an offspring, suggesting that highly opportunistic mating with conceptive females is important in bachelor reproduction. In a second analysis, we used long-term data to estimate resident and bachelor reproductive success over the long term, and particularly to determine if there are any circumstances in which a typical bachelor may sire as many offspring as a typical resident during one or two periods of residency. Our findings generally support the assumption of a resident reproductive advantage because in most circumstances, a lifelong bachelor would be unable to sire as many offspring as a resident. However, a bachelor who performs at the average rate in the average number of groups for several years may have similar lifetime reproductive success as a male whose reproduction is limited to one short period of residency, especially in a small group. Our findings suggest that one should not assume a resident reproductive advantage for males in one-male groups in all circumstances.