Demography, range use, and behavior in black lemurs (Eulemur macaco macaco) at Ampasikely, northwest Madagascar

We studied a black lemur population over a 2-year period (1992-1993) and 8 years later (2000) in a 50-ha secondary forest in northwest Madagascar. All of the animals were marked to investigate population dynamics and seasonal variation in ranging and behavior, and new data on black lemurs were obtained. Our data on demographic characteristics were expanded to include other forest sites and contrasted with those collected in other Eulemur macaco macaco field studies, in relation to human activity and the presence of introduced and cultivated plant species. Density is affected by deforestation and hunting. Group size and home range depend on the composition of the forest and probably food patches. Sex ratio at birth varies according to the number of females per group, a result that fits the local resource competition model. Groups are multimale-multifemale, and adult females form the core of the groups. Reproductive parameters indicate sharply defined seasonal breeding, a high female reproductive rate, and birth synchrony. Changes in group composition reveal male and female juvenile dispersal, male transfer between groups at the time of mating, and adult female transfer and group fission when groups exceed a critical size. At mating and birth, intergroup agonistic encounters occurred at home-range boundaries, and larger groups were dominant over smaller groups. Patterns of intragroup interactions suggest that males compete for access to groups of females during the mating season, and that females may compete for food resources during the birth season. Our study also reports female social dominance and lack of sexual weight dimorphism in this species.     

Sexual segregation in western grey kangaroos: testing alternative evolutionary hypotheses

In sexually dimorphic ungulates, sexual segregation is hypothesized to have evolved because of sex-specific differences in body size and/or reproductive strategies. We tested these alternative hypotheses in kangaroos, which are ecological analogues of ungulates. Kangaroos exhibit a wide range of body sizes, particularly among mature males, and so the effects of body size and sex can be distinguished. We tested predictions derived from these hypotheses by comparing the distribution of three sex–sex size classes of western grey kangaroos Macropus fuliginosus , in different habitats, and the composition of groups of kangaroos, across seasons. In accordance with the predation risk-reproductive strategy hypothesis, during the non-breeding season, females, which were more susceptible to predation than larger males, and were accompanied by vulnerable young-at-foot, were over-represented in secure habitats. Large males, which were essentially immune to predation, occurred more often than expected in nutrient-rich habitat, and small males, which faced competing demands of predator avoidance and feeding, were intermediate between females and large males in their distribution across habitats. During the breeding season, females continued to be over-represented in secure habitats when their newly emerged pouch young were most vulnerable to predation. All males occupied these same habitats to maximize their chances of securing mates. Consistent with the social hypotheses, groups composed of individuals of the same sex, irrespective of body size, were over-represented in the population during the non-breeding season, while during the breeding season all males sought females so that mixed-sex groups predominated. These results indicate that body size and reproductive strategies are both important, yet independent, factors influencing segregation in western grey kangaroos.     

Social Influences on Group Membership in <Emphasis Type="Italic">Propithecus verreauxi verreauxi</Emphasis>

The size and composition of a social group can influence the reproductive success of its members. I examined the hypothesis that residents actively try to manipulate group size and composition in Verreaux’s sifaka (Propithecus verreauxi verreauxi). I collected behavioral data from 21 individuals in 5 social groups from November 2000 to March 2002 in the Kirindy Forest (C.F.P.F.) of western Madagascar. I investigated the affiliative and agonistic behaviors of resident males and females toward other group members to determine whether residents attempt to manipulate the costs and benefits of group membership. I also examined 3 cases of immigration attempts and 1 case of emigration. As expected, group members facilitated the residency of subordinate members of the opposite sex. For example, females sought out the subordinate males, responded to their calls, and even groomed them more. However, intrasexual relationships were also important for establishing or maintaining residency. The data, in conjunction with the observations of immigration and emigration events, suggest that resident individuals of both sexes attempt to influence group size and composition. I suggest that single–male groups are not the norm in Propithecus verreauxi verreauxi because females encourage, and males do not discourage, multiple males to reside in the group to increase group stability and to reduce the risk of infanticide.     

Male-female affiliative relationships in naturally occurring ringtailed lemurs (Lemur catta) at the Beza-Mahafaly Reserve,Madagascar

Affiliative behavior between adult male and female ringtailed lemurs was examined as part of a project concerning male affiliation with conspecifics of all age/sex classes. Males in three social groups were studied over a 12 month period. Male-female preferred partnerships existed, and were variable according to reproductive season. Dominance rank, age, or tenure of the male did not appear to affect either the number of partnerships or frequency of affiliative behaviors that males had with females. However, males residing in groups with fewer males exhibited both higher frequencies of affiliative interactions with females and were nearest neighbors to females more often than males living in a group containing more males. Females were found to be responsible for proximity maintenance of male-female dyads in the majority of cases. Neither reproductive season nor seasonal availability of food resources strongly affected the frequency of affiliative interactions between males and females, it is proposed that an important aspect of successful group membership for male ringtailed lemurs relates to the development of social relationships with adult females. Males can benefit from such relationships in terms of greater centrality to the spatial core of the group, which can result in enhanced predator protection, greater opportunities for social contact, and potentially greater access to estrous females. © 1996 Wiley-Liss, Inc.     

Two girls for every boy: The effects of group size and composition on the reproductive success of male and female white-faced capuchins

Many factors have been hypothesized to affect the size and adult sex ratios of primate groups and these, in turn, have been argued to influence birth rates. Using park-wide census data collected on a population of capuchins over a 25-year period, we examined whether group size and adult sex ratio affect the per capita reproductive success of male and female white-faced capuchins (Cebus capucinus) in Santa Rosa National Park, Costa Rica. We found that the reproductive success of females (measured as the observed minus the expected ratio of immatures to adult females in the group) decreased with increasing group size, whereas that of males was independent of group size. The proportion of adult males residing in groups had significant, yet contrasting effects on males and females. Male reproductive success was negatively associated with the proportion of males residing in groups whereas female reproductive success increased with the proportion of males. The latter finding supports the intersexual conflict hypothesis, which suggests that a conflict of interest occurs between males and females over adult sex ratios. The effects of group size and composition on the reproductive success of capuchins, a male-dispersed omnivorous species, are similar to those reported for howlers, a bisexually-dispersed folivorous species. One common factor between these taxa is that groups with low ratios of males to females are at greater risk of takeovers and resultant infanticide. Our results suggest that regardless of dietary preference and dispersal pattern, the threat of infanticide can constrain primate group size and composition.     

Mate choice by males of the hermit crab Pagurus filholi: Do males assess ripeness and/or fecundity of females?

Males of the hermit crab, Pagurusfilholi, often grasp the edges of shells occupied by females and drag them during the mating season. This behavior was experimentally confirmed to be a precopulatory guarding behavior displayed by males for ripe females, and males were found to recognize females which were within about 5 days of spawning. Most theoretical models for mating preference assume the choosing sex (the male in the present case) has complete reproductive information about potential mates, and predict that males will preferably choose more fecund females and/or females that will require less guarding time (i.e. that will spawn sooner) as partners. Several male-choice experiments between two ripe females, both previously guarded by other males, were carried out to examine the above predictions. Males did not prefer females of larger size, higher fecundity or with less time remaining until spawning. These results suggest that males may not have complete information about potential partners, rather that male hermit crabs may adopt a mating strategy of pairing with the first ripe female they encounter. Even with such incomplete mate assessment, males may enhance their reproductive success by recognizing ripe females that will spawn within a given time (about 5 days in the present case).     

Social Behavior,Reproductive Strategies,and Population Genetic Structure of <Emphasis Type="Italic">Lagothrix poeppigii</Emphasis>

For species of primates in which females emigrate, we would expect males within groups to be related to one another. Kin selection theory suggests that these males should associate preferentially with one another, be more affiliative and cooperative with one another than females are, and compete less overtly with one another over reproductive opportunities than males in female philopatric taxa do. Precisely these patterns of social behavior characterize well-studied populations of 2 of the 3 atelin primate genera: spider monkeys (Ateles) and muriquis (Brachyteles). For the third atelin genus, Lagothrix, patterns of intragroup social behavior have been less well-documented. We studied the social and reproductive behavior of lowland woolly monkeys (Lagothrix lagotricha poeppigii) in Ecuador during a one-year observational study and subsequently used molecular techniques to investigate population genetic structure and dispersal patterns for this taxon. Among adult male woolly monkeys, both affiliative and agonistic interactions were rare, and males were seldom in close proximity to one another. Relationships among male woolly monkeys are best characterized as tolerant, especially in the context of mating wherein direct competition among males was minimal despite the fact that females mated with multiple males. Relationships among females were likewise generally tolerant but nonaffiliative, though females often directed harassment towards copulating pairs. Affiliative interactions that did occur among woolly monkeys tended to be directed either between the sexes—primarily from female to male—or from younger towards older males, and the proximity partners of females tended to be members of the opposite sex. These results suggest that bonds between the sexes may be more important than same-sex social relationships and that direct female-female competition is an important feature of woolly monkey reproductive biology. Our genetic results indicate that, as in other atelins, dispersal by females is common, but some male dispersal likely occurs as well. In some but not all groups we studied, nonjuvenile males within social groups were more closely related to one another on average than females were, which is consistent with greater male than female philopatry. However, differences in these patterns among our study groups may reflect local variation in dispersal behavior.     

Drosophila melanogaster females change mating behaviour and offspring production based on social context

In Drosophila melanogaster, biological rhythms, aggression and mating are modulated by group size and composition. However, the fitness significance of this group effect is unknown. By varying the composition of groups of males and females, we show that social context affects reproductive behaviour and offspring genetic diversity. Firstly, females mating with males from the same strain in the presence of males from a different strain are infecund, analogous to the Bruce effect in rodents, suggesting a social context-dependent inbreeding avoidance mechanism. Secondly, females mate more frequently in groups composed of males from more than one strain; this mitigates last male sperm precedence and increases offspring genetic diversity. However, smell-impaired Orco mutant females do not increase mating frequency according to group composition; this indicates that social context-dependent changes in reproductive behaviour depend on female olfaction, rather than direct male-male interactions. Further, variation in mating frequency in wild-type strains depends on females and not males. The data show that group composition can affect variance in the reproductive success of its members, and that females play a central role in this process. Social environment can thus influence the evolutionary process.     

Defence of Females by Dominant Males of Artibeus jamaicensis (Chiroptera: Phyllostomidae)

Defence of females by dominant males of the Jamaican fruit‐eating bat Artibeus jamaicensis was observed in two natural colonies over 2 yr. A log‐linear model was used to evaluate the frequency distribution of visits to harems by sex, season and agonistic interaction of dominant males. Harem group size varied from four to 18 females, with one adult male in the small and medium‐sized groups and two males in the large groups (> 14 females). A highly significant interaction was noted between the age and sex of the visitor and the response of the dominant male. Male visitors were attacked more often than female and juvenile visitors. Aggressive defence increased during the reproductive seasons, with dominant males showing more agonistic responses towards male visitors. An increase in the frequency of visits by male visitors was noted in harem groups that ranged in size from four to 12 females, but the frequency of male visits declined in harem groups that contained more than 14 females.     

Terminal phase males stimulate ovarian function and inhibit sex change in the protogynous wrasse Thalassoma duperrey

Under experimental conditions, the probability of sex change in the protogynous wrasse Thalassoma duperrey is determined largely by an individual's relative size within a social group. Natural populations, however, contain two distinct male phenotypes that may also play a role in regulating sex change. To investigate potential effects of male phenotype, the ability to change sex, ovarian histology and serum estradiol-17 beta levels were examined in females maintained under controlled social settings. Large females housed with smaller or larger terminal phase males had significantly larger gonadosomatic indices than females housed singly, with other females or with smaller initial phase males. Similarly, ovaries of females housed with terminal phase males showed no histological evidence of sex change, whereas large females from other social groupings were in advanced stages of sex change. These results demonstrate terminal phase males inhibit sex change regardless of their size relative to the female. Furthermore, gonadosomatic indices, ovarian histology, and serum estradiol-17 beta levels of females housed with terminal phase males indicate normal ovarian function whereas ovaries of other treatment groups appear quiescent or are undergoing sex change. Consequently, terminal phase males may be required for normal ovarian development which may, in turn, inhibit sex change in T. duperrey.     

Secondary transfer of adult mantled howlers (<Emphasis Type="Italic">Alouatta palliata</Emphasis>) on Hacienda La Pacifica,Costa Rica: 1975–2009

Natal emigration by male and female mantled howlers (Alouatta palliata), and subsequent immigration into breeding groups, is well documented for the free-ranging population on Hacienda La Pacifica, Costa Rica, but secondary transfer was considered rare (Glander in Int J Primatol 3:415–436, 1992). Population surveys in 1998 and 2006 caused us to question our assumptions and to re-evaluate our long-term data set from a post hoc perspective. We first identified all animals observed or captured as adults in more than one non-natal group anywhere in the population. We then systematically analyzed joining or leaving by adults in seven groups tracked for various times from 1975 to 2005 for patterns suggesting secondary transfer. Fourteen adults (nine females, five males) were found in two different non-natal groups as adults. In addition, one male and one female that became dominant and reproduced in their natal group later transferred to a second group, and one female was known to be a tertiary transfer. Data from the seven tracked social groups indicate that 35% of all the males and 29% of all the females were potential secondary transfers. In these groups, males leaving or joining was not associated with group size or absolute number of females. Females leaving or joining was not associated with group size or absolute number of males, but females left groups with more females and joined groups with fewer females. Both sexes left groups with unfavorable sex ratios for their sex and joined groups with sex ratios more favorable for their sex. Since a favorable sex ratio is associated with reproductive success in other howler populations, this suggests secondary transfer as a reproductive strategy. Other factors could also influence secondary transfer.     

Do humans show a “species-typical” group size?: Age,sex, and environmental differences in the size and composition of naturally-occurring casual groups

The size, age,and sex composition of 2277 casual groups from eight shopping malls and two sidewalk shopping areas, and 1086 groups from other public environments, were recorded and analyzed to reveal age, sex, and environmental components of human grouping tendencies. Groups containing grade school or teenage children were significantly larger than groups of other ages. Adult females were found in larger groups than males; although females readily formed groups with either sex, adult males formed few groups containing one or two other males. Group size showed a significant, inverse correlation with social distances between first, second, and third nearest neighboring group members in a photo sample, but group size showed no correlation with social distances between strangers. These results are interpreted in the light of an ethological hypothesis that contemporary grouping patterns had their origins in adaptive, inborn aggregative tendencies that were later incorporated into the repertoire of nonverbal human behaviors.     

It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp (Alpheus angulosus), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution.     

The building-up of social relationships: behavioural types, social networks and cooperative breeding in a cichlid

Consistent individual differences in behavioural types may not only cause variation in life-history decisions, but may also affect the choice of social partners and sociality in general. Here, we tested whether and how behavioural type influences the establishment of social ties using the cooperatively breeding cichlid, Neolamprologus pulcher. In a habitat saturation experiment with individuals pre-tested for behavioural type, we first analysed whether behavioural type affected the likelihood of settlement (i.e. social status), group sizes, and the types of dominant and subordinate individuals accepted as group members. Corrected for effects of body size and sex, the behavioural type did not affect settlement. However, bold dominant males only accepted smaller females, and grouped with bold subordinates, while shy dominant males accepted larger females than themselves, and grouped with shy subordinates. Second, we analysed the relationships between behavioural type and the aggressiveness or affiliation social network. Behavioural type significantly affected the number and quality of connections within the two networks. We show that behavioural types affect group composition, social networks and status achieved, in interaction with body size. Thus, the interactions within groups may depend not only on age, size and sex, but also on the behavioural type of the individuals involved.     

Life history of Eulemur fulvus rufus from 1988-1998 in southeastern Madagascar

In this study, we compare the life-history patterns of male and female Eulemur fulvus rufus based on longitudinal data collected on individuals from two study groups from 1988-1998 in southeastern Madagascar. Mean group size was 9.5 individuals, and groups either contained more adult males than females or equal numbers of both sexes. Females reproduced for the first time between 2 and 4 years of age and reproduced each year, although the mean interbirth interval between surviving offspring was 2.1 years. An average of two adult females reproduced annually in each social group, and age and body weight may positively influence reproductive success. Females also appear to be philopatric but not female-bonded. Young natal males immigrated between 3 and 4.5 years of age and may join a new group within 612 months based on the age of emigrants. Once in a social group, they remained until old age, although a male's spatial position in the social group varied with age. Young nonnatal males were members of the social core and had the first opportunity to mate with all estrous females. Older males were peripheral to the social group and mated with females later in their cycle. We hypothesize that group size, the number of females in the group, and individual variation in reproductive success is influenced by several ecological conditions at this site: extreme variability in food availability during reproductive periods, the lack of large food patches, low plant species diversity, and small numbers of important aseasonal food sources such as Ficus species.     

Behavioral Tactics of Male Sockeye Salmon (Oncorhynchus nerka) under Varying Operational Sex Ratios

Previous studies have demonstrated several reproductive-behavior patterns in male salmon, including competitive and sneaking tactics, the formation of hierarchies, and non-hierarchical aggregations around ripe females. Through behavioral observations at varying spatial and temporal scales, we examined the hypothesis that operational sex ratio (OSR) determines male sockeye salmon (Oncorhynchus nerka) distribution and breeding tactics. Patterns of male distribution and behavior varied over both coarse and fine scales, associated with apparent shifts in reproductive opportunities, the physical characteristics of the breeding sites, and the deterioration of the fish as they approached death. Females spawned completely within a few days of arriving on the spawning grounds, whereas males courted the available ripe females from the date of their arrival on the spawning ground until their death. This difference in reproductive lifespans tended to elevate late-season OSRs but was partially counterbalanced by male departures and the arrival of other ripe females. The proportion of males able to dominate access to ripe females decreased and the number of large courting groups increased over the course of the season, apparently related to both increasing OSR and the deteriorating physical condition of males. However, great variation in OSR was observed within the spawning sites on a given day. OSRs were generally higher in shallow than in deep water, perhaps because larger females or more desirable breeding sites were concentrated in shallow water. The aggregations of males courting females were not stable (i.e. many arrivals and departures took place) and male aggression varied with group size. Aggression was most frequent at low OSRs and in groups of intermediate size (2–4 males per female), and much less frequent in larger groups, consistent with the needs of maximizing reproductive opportunities while minimizing unproductive energy expenditure. These results indicate that, while OSR strongly influences male distribution and behavior, other factors such as physical condition, limited movement and habitat choice may also affect male reproductive opportunities.     

Mate-Guarding as a Male Reproductive Tactic in <Emphasis Type="Italic">Propithecus verreauxi</Emphasis>

Sexual selection theory predicts that in group-living mammals, male reproductive tactics can lead to high reproductive skew in favor of dominant individuals. In sifakas (Propithecus verreauxi), a group-living primate with extremely seasonal reproduction, male reproductive success is highly skewed because dominant males sire almost all offspring despite a tendency toward an even adult group sex ratio. To understand the underlying behavioral mechanism resulting in this rank-related reproductive skew in male sifakas, we studied mate-guarding as a potential reproductive tactic. Behavioral observations of dominant males and adult females in combination with hormonal determination of timing of female receptivity in 9 groups at Kirindy Forest revealed that dominant males spent more time in proximity to females when they were receptive and were responsible for the maintenance of this proximity. Results also indicated that monopolization of receptive females was facilitated by both estrous asynchrony within groups and by the ability of dominant males to obtain olfactory cues as to the timing of female receptivity. Although dominant males engaging in mate-guarding are expected to experience various costs, there was no evidence for decreased foraging behavior and only a trend toward increased aggression between dominant and subordinate non-natal males within groups. Our results are in accordance with the hypothesis that dominant males use mate-guarding to monopolize receptive females and that it is one proximate mechanism that contributes to the high reproductive skew observed within the population of male sifakas at Kirindy.     

Influences of Mating Group Composition on the Behavioral Time-Budget of Male and Female Alpine Ibex (Capra ibex) during the Rut

During the rut, polygynous ungulates gather in mixed groups of individuals of different sex and age. Group social composition, which may vary on a daily basis, is likely to have strong influences on individual’s time-budget, with emerging properties at the group-level. To date, few studies have considered the influence of group composition on male and female behavioral time budget in mating groups. Focusing on a wild population of Alpine ibex, we investigated the influence of group composition (adult sex ratio, the proportion of dominant to subordinate males, and group size) on three behavioral axes obtained by Principal Components Analysis, describing male and female group time-budget. For both sexes, the first behavioral axis discerned a trade-off between grazing and standing/vigilance behavior. In females, group vigilance behavior increased with increasingly male-biased sex ratio, whereas in males, the effect of adult sex ratio on standing/vigilance behavior depended on the relative proportion of dominant males in the mating group. The second axis characterized courtship and male-male agonistic behavior in males, and moving and male-directed agonistic behavior in females. Mating group composition did not substantially influence this axis in males. However, moving and male-directed agonistic behavior increased at highly biased sex ratios (quadratic effect) in females. Finally, the third axis highlighted a trade-off between moving and lying behavior in males, and distinguished moving and female-female agonistic behavior from lying behavior in females. For males, those behaviors were influenced by a complex interaction between group size and adult sex ratio, whereas in females, moving and female-female agonistic behaviors increased in a quadratic fashion at highly biased sex ratios, and also increased with increasing group size. Our results reveal complex behavioral trade-offs depending on group composition in the Alpine ibex, and emphasize the importance of social factors in influencing behavioral time-budgets of wild ungulates during the rut.     

Boys will be boys: social affinity among males drives social segregation in western grey kangaroos

In many sexually dimorphic and polygynous species, individuals exhibit social segregation by grouping with others of their own sex. Several proximate mechanisms have been proposed to explain social segregation: female avoidance of males, male avoidance of females, avoidance of harassment, male social affinity, female social affinity and activity budget asynchrony. Some of these have been tested in ungulate species, but few previous studies have been able to rigorously test, or distinguish between, these mechanisms because they have failed to examine the fission/fusion dynamics of groups. We tested these proximate mechanisms simultaneously in western grey kangaroos Macropus fuliginosus , by examining whether females, small males or large males instigated segregation by leaving mixed-sex groups or joining individuals of their own sex or size class. Females, small males and large males left mixed-sex groups as if leaving were independent of sex–size class. In contrast, large males joined male-only groups more frequently than expected. These results suggest that the need for males to maintain contact with other males can contribute to the cohesion of male-only groups and promote segregation. As male–male competition occurs in many polygynous species that sexually segregate, a comparable mechanism might be operating in other taxa, and should be examined further.     

Mating Group Size and Stability in Reindeer Rangifer tarandus: The Effects of Male Characteristics,Sex Ratio and Male Age Structure

In polygynous mating systems, males compete intensely for mates and may mate several females during a single reproductive season. Accordingly, factors influencing the ability of males to control a larger number of females during the breeding season can provide information on the processes underlying sexual selection. In ungulates, age, body mass and social rank are considered good predictors of the reproductive success of males, but how male age structure and sex ratio in the population influence mating group (MG) dynamics has received little empirical testing. Between 1996 and 2005, we manipulated male age‐ and sex structure and monitored MG dynamics in a reindeer (Rangifer tarandus) population. We investigated the influence of male characteristics, percentage of males and male age structure on MG size and stability. We found that males with higher social rank (that were also older and heavier) controlled larger MGs (therefore had greater mating opportunities) and had more stable MGs (corresponding to a higher ability to maintain and control females) than males of lower social rank. Moreover, MG size and MG stability decreased as the percentage of males in the population increased, most likely resulting from greater male–male competition and increased female movements. Male age structure did not influence MG stability. Given the positive relationship between mating success and MG size (and likely MG stability), frequent female movements and intense competition among males to control females seem to be the principal components of reindeer MGs dynamic.