Social interactions among adult male langurs (Presbytis entellus) at Rajaji Wildlife Sanctuary

A population of langurs (Presbytis entellus)at the Rajaji Wildlife Sanctuary in northern India was investigated for 1820 hr throughout a 10-month period in 1978. Data were collected from four bisexual troops and the adult males that ranged outside of bisexual troops. Most (60%) of the observation hours occurred with a main study troop from which social and ecological data were collected. The langur population at Rajaji shows pronounced birth and mating seasons. The population density is high (ca. 80/km ²), with about 75% of the adult males living outside of bisexual troops, which typically are large and multimale. Males outside of bisexual troops occur in small all-male bands or as isolates. Relations between bisexual troops and all-male bands are characterized by relatively low levels of aggression, and members of all-male bands are able to associate with bisexual troops for prolonged periods during the mating season. As a result of these associations, nontroop males are about as successful as troop males in achieving reproductive access to troop females. These associations between bisexual troops and all-male bands occurred with a minimal amount of agonistic behavior and without mortality or injury to troop females or immatures.     

On the dynamics of the hanuman langur populations of jodhpur (Rajasthan,India)

The city of Jodhpur (26°18′N, 73°8′E) supports a population of about 900 hanuman langurs (Presbytis entellus) divided into 24 bisexual troops and 12 all-male bands in an area of 60 km². This population has been censused from 1968 to 1978. Over this period the population of bisexual troops has remained stable around 700, while the population of all-male bands has increased from 160 to 230 individuals. The bisexual troops show a tendency towards a reduction in population growth rate with increasing troop size, with troops over 50–60 tending to split. Very small troops may grow by large scale immigration. Although a number of male changes and mortality through infanticide have been recorded, there is no evidence of a regular periodicity in the occurrence of initial, growth and mature phases in the life history of a bisexual troop. Unlike the bisexual troops, the all-male bands show no tendency towards a reduction in growth rate with the increase in band size, but show a continuous growth of band size over the study period. Langurs of Jodhpur rely heavily on cultivated fields for their sustenance. This cultivation has been on increase over the study period, and since the males invade cultivation more readily, they may have been able to take fuller advantage of these increasing resources and affect a population increase that has not been possible for the bisexual troops.     

The variable social organization of hanuman langurs(Presbytis entellus), infanticide,and the monopolization of females

Data from 24 wild populations of hanuman langurs (Presbytis entellus)in south Asia are used to test hypotheses seeking to explain variation in troop structure and the incidence of infanticide. The occurrence of infanticide is associated with a one-male troop structure and not with a high density. The density, predation, and economic-advantage hypotheses, as explanations for the occurrence of one-male and multimale troops, are not supported by the review. However, the monopolization hypothesis is not contradicted; the number of adult males per troop is significantly correlated with troop size and with the number of adult females per troop. Therefore it is suggested that a one-male troop structure will arise if a male is able to monopolize a group of females, a multimale troop if he cannot. One-male troops may predispose to infanticide because of high variance in male mating success and high intermale competition between groups rather than within troops. If female dispersion determines troop structure, it is speculated that females could manipulate males to form a multimale society if the advantages in terms of infant survival and intertroop conflict exceeded the costs in terms of not producing infanticidal “sexy sons.”     

Adult male replacement and social change in two troops of hanuman langurs (Presbytis entellus) at Jodhpur,India

I studied the process of adult male replacement and social change in two one- male troops (B20 and B21) of hanuman langurs (Presbytis entellus)at Jodhpur, India. Male-male competition lasted for about 6 months before the successful takeover of one troop (B20). During that period, five adult males from three neighboring bands (AMB7, AMB9, and AMB10) and a resident male of a neighboring troop (B21) were involved in taking over the troop. The latter male also copulated with six females during his interim residency, which suggests that he may have opportunistically maximized his mating chances with females of a neighboring group. During an intertroop interaction, a 14-month-old female infant of the other troop (B21) was fatally attacked by an adult female of the first troop and the infant eventually died. The attacker may have taken advantage of the disorganization created by male-male competition, perhaps to eliminate a future food competitor. In addition, the first troop gained an additional feeding area from the other troop’s range; it included a sleeping site and a waterhole, indicating that territorial fights during social instability may have led to the expansion of the winner’s resource area.     

Ecology and behavior ofPresbytis thomasi in Northern Sumatra

A field study of 23 bisexual troops, ranging in size from 3 to 21 members ( =8), and two all-male groups of the Thomas's leaf monkey (Presbytis thomasi) was conducted in North Sumatra from November 1981 to April 1984. Most troops (N=19 or 82.6%) contained only one adult male. Two troops and one group were most intensively studied. The home range was 12.3–15.7 ha for the two bisexual troops, and only 1.7 ha for the one, all-male, ten-member group. Fruits composed more than 50% of their diets. Vocalizations were classified into 13 types. Births occurred at any time of the year. Among three males of a bisexual troop, serious fights were observed: two males died of wounds and the former beta male became the new alpha male. After this social change, the home range area of this troop gradually shifted eastward. But, 3.5 months after the social change, a 9-month-old infant male stayed alone in the western part of its former range. Thereafter, he became a solitary male and sometimes went into the riverine area of the Bohorok river. The occurrence of male replacement suggests instability of multi-male organization in bisexual troops. Moreover, the different mortality rate between males and females and the unequal sex ratios forced by the formation of one-male troops, maintained high tension levels among males competing on females.     

Male competition and coalitions in langurs (Presbytis entellus) at Jodhpur,Rajasthan, India

During a 15 month study on free ranging langurs (Presbytis entellus) at Jodhpur, Rajasthan, India, 5 adult male replacements were observed as a result of nontroop male invasions into the home ranges of 3 neighbouring one-male troops comprising 16–28 members each. Jodhpur langurs have no breeding season. Periods of instability during resident male changes lasted 11–119 days. Linear dominance hierarchies could be detected within the 3 main rival male bands of 2, 5, and 28–35 members. The respective alphas drove their allies away after their bands succeeded cooperatively at occupying a troop. During gradual replacements interim residencies alternated with multi-male stages. A large band's alpha may have had better chances to win the competition, since adult and nonadult allies functioned as “buffers” in agonistic encounters. The role of kin selection in structuring the composition of male bands and male coalitional behaviour cannot yet be quantified. Tactical “deceit” of powerful males to cause unrealistic expectations and in this way agonistic engagement of less strong males can be ruled out. “Sneaking copulations” is a proximate advantage for subordinate supporters, since they participated in 61.9% of all sexual interactions. Female promiscuity might reflect a strategy to induce male-male competition and thus select for a strong resident.     

Intermale relations and troop male membership changes in langurs (Presbytis entellus) in Nepal

Data on intermale social relations and troop membership changes in one Nepalese high-altitude population of free-ranging langurs (Presbytis entellus)are reported here. Data were collected from six troops by three observers and cover 32 months of observations. The predominantly multi-male troops indicate an alternating pattern of exclusions and introductions with gradual adult male replacement. Takeovers and infant killing were not observed. Analysis of adult social behavior records show qualitative and quantitative differences in intrasexual relations, with primarily agonistic social contacts occurring between males. Agonistic encounters between females and between males differ in frequency of occurrence, types of be-haviors used, cause, and consistency in direction of threats between individuals. Individual adult male frequency of interaction with females and immatures varied significantly, with the majority of these interactions occurring between the dominant troop male and other troop members. Data indicate that intermale dominance is a major factor in determining male access to fertile females: This appears to be achieved by either directly excluding males from the troop or effectively “controlling” their inter-actions with troop females. Data from these studies are compared with data from other Presbytis entellusinvestigations. Review of these data suggests that intraspecific variability in intermale social dynamics and type of troop male membership change are correlated with the percentage of nontroop males. It is suggested that environmental pressures resulting in social crowding can be critical in determing the occurrence of takeovers in some populations of Presbytis entellus.     

Socio-sexual factors of troop fission in wild Japanese monkeys (Macaca fuscata yakui) on Yakushima Island,Japan

The troop fissions which occurred in a wild population of Japanese monkeys (Macaca fuscata yakui) were observed from 1977 to 1979 on Yakushima Island. The fissions were initiated in the breeding season by non-troop males who established a consort relation with estrous females. In order to analyze the socio-sexual factors which accelerated the fissions, the male emigrations and immigrations before and after two successive fissions, and the copulation frequencies, competition among males and preferences of mating partners in both sexes in the 1977–78 breeding season after the first fission were examined. The results indicated that three factors (a large number of non-troop males, a shortage of troop males and the females' choice of mating partners) effectively influenced on the establishment of consort relationships between non-troop males and estrous females. It is suggested that these factors may exert different effects on the troop disorganization in relation to troop size. In small-sized troops, a large number of non-troop males and a shortage of troop males may lead to stronger competition between them, and the females' choice affected by prolonged intimate relations with the dominant TMs may reduce their priority of access to estrous females. This situation possibly stimulates fission or male emigration in small-sized troops under the natural conditions on Yakushima Island. In contrast, in large-sized troops under isolated conditions, a surplus rather than a shortage of troop males may contribute to troop disorganization, as most former studies have suggested. A higher socionomic sex ratio may decrease the mating activities of subordinate troop males and increase the competition among them. This situation possibly accelerates the fission of large-sized troops through prolonged interactions between females and subordinate or peripheral troop males. A lower ratio and the females' choice, however, raise the mating chances of subordinate troop males and may not promote the fission of large-sized troops under isolated conditions. This study was financed in part by a Grant-in-Aid for Special Project Research on Biological Aspects of Optimal Strategy and Social Structure from the Japan Ministry of Education, Science and Culture, and by the Cooperative Research Fund of the Primate Research Institute, Kyoto University.     

Intertroop Transfer and Dominance Rank Structure of Nonnatal Male Japanese Macaques in Yakushima, Japan

We examined the interaction between intertroop transfer and male dominance ranks in a wild population of Japanese macaques (Macaca fuscata yakui) in Yakushima using data collected over 15 years. Intertroop transfer tended to maintain a linear, stable, and age-graded dominance rank order among nonnatal males irrespective of variation in troop size or composition. All males that joined a troop at the top of the rank order were prime adults. Among males joining at lower ranks, entry at the most subordinate position in the hierarchy was common. Males joining at lower ranks tended to join troops in which all other resident males were the same age or older. Adult males tended to join troops with few or no males. Young males tended to join troops with many resident males, and in which a relatively large proportion of males was other young ones. Intertroop transfer was responsible for most rank changes of resident males. The most common cause of males rising in rank was the emigration or death of a higher-ranking male. Males fell in rank most frequently as a result of a new male joining the troop at the top of the hierarchy. Rank reversals among resident males were rare. The cumulative effects of male transfers produce sociodemographic variation within a troop over time and sociodemographic diversity among troops in a local population. A key feature of intertroop diversity is that larger troops have a significantly greater proportion of young males than smaller troops. This diversity also creates the potential for intertroop variation in the severity of male competition and provides a range of options for transferring males.     

Population and social dynamics changes in ring-tailed lemur troops at Berenty,Madagascar between 1989 - 1999

In the present study, we recorded all births, immigrations, deaths, and emigrations for a population of ring-tailed lemurs at Berenty Reserve, Madagascar, between September 1989 and August 1999. In September 1989, three troops (C, B, and T) inhabited the study area of 14.2 ha. During the 10-year period, eight troop divisions, six evictions of females, and three troop takeovers of ranges by other troops occurred in and around the study area. Consequently, in August 1999, the number of troops in the same area increased to six (CX, C1, C2A, C2B, T1, and T2). The number of lemurs aged >1 year increased from 63 to 82, which resulted from 204 births, 58 immigrations, 125 deaths, and 118 emigrations. Of the 204 newborn lemurs during the study period, 103 died, 44 emigrated outside the study area, and 57 remained within the study area. The total number of lemurs that emigrated from natal troops was 69 (54 males and 15 females). Natal males left their troops around the age of 3. Non-natal males changed troops after a tenure varying from 1 to 7 years. Survival curves showed a fall in survival rates of both sexes to < 0.5 between the ages of 2 and 3. For females, the survival rate gradually decreased to < 0.2 at the age of 9. On the other hand, due to emigration, the survival rate of males could not be determined after the age of 5 yr. Since some males attained high-rank at the age of 6 – 10 yr, the prime age for male ring-tailed lemurs is thought to be around 7 – 10 yr. Ring-tailed lemurs are essentially female philopatric, because all cases of females leaving natal troops resulted from troop divisions or forced evictions. Such social changes may have resulted from competition among females. All cases of troop divisions or evictions occurred in larger troops consisting of ≥20 lemurs, and only a few females could rejoin their troops. When males joined such a female-group, a new troop was formed. Although promoted by an increase in population, frequent emigrations of females from original troops are the characteristics of ring-tailed lemurs at Berenty.     

The social organization of forest hanuman langurs(Presbytis entellus)

The social organization of hanuman langurs (Presbytis entellus;Colobinae) was studied in Kanha Tiger Reserve, Central Indian Highlands, for 2300 hr (1980–1985), in a mosaic of moist deciduous forest and anthropogenic meadow. The langur population density was 46.15/km ² and the mean troop and band sizes were 21.7 and 14.0, respectively. Of 14 troops, 13 were one-male and 1 was trimale. The population adult sex ratio was 1:2.5. The majority of female sexual solicitations was directed toward the harem male. The birth season was December to May, with an estimated gestation of 171–224 days. A review of langur reproductive seasonality suggests that breeding throughout the year is confined to those populations able to exploit human food sources. Mortality during the first year of life was 40%, including infanticide. A significant positive correlation was found between the age of an infant at death or disappearance and the mother’s subsequent interbirth interval. Five cases of social change are described, including female transfer, one-male to multimale change, troop formation, and gradual and rapid replacement of troop males. Takeover-associated infant killing by band males, in an undisturbed moderate-density population, supported the sexual-selection/infanticide hypothesis but not the social-pathology hypothesis. However, it could not be directly confirmed that an invading infanticidal male gains a reproductive advantage. The male tenure of harems was estimated to be 45 months.     

Comparative study of grooming relationships among wild Japanese macaques in Kinkazan A troop and Yakushima M troop

The influences of socionomic sex ratio (SSR; adult males/adult female) and troop size upon male-male, female-female, and male-female grooming relationships were examined and compared between two wild Japanese macaque troops (Kinkazan A and Yakushima M troops) in Japan. The Yakushima M troop was smaller and had a higher-SSR than the Kinkazan A troop. Between the troops, (1) the male-male grooming frequency and number of partners were greater in the Yakushima M troop than in the Kinkazan A troop; (2) the female-female grooming frequency and number of partners were not different; and (3) the male-female grooming frequency and number of partners were not different. Based on these features, the patterns of female-female and male-female grooming relationships appear to be independent of SSR and troop size variations. In contrast, male-male grooming relationships are influenced by both factors, especially SSR. Frequent grooming interactions among males may be useful for the continued coexistence of relatively many males especially in a higher-SSR troop.     

Abortions in free ranging Hanuman langurs (Presbytis entellus) — a male induced strategy?

During a 10-year long term study, 6 eye-witnessed and 1 pressumed cases of abortions occurred in 3 one-male bisexual troops of free ranging langurs (Presbytis entellus) near Jodhpur, Rajasthan, India. The age of the unborns varied from 35 to about 200 days. The subsequent birth interval ranged between 7.1–21.1 months. 2 miscarriages occurred during stable periods of residency of a single male. 5 cases occurred prior to or after infanticide in connection with male changes. In one case a female was attacked by the new male before she aborted. In one case a female presumably aborted after attacks on her semiweaned infant. Most of the reproductive losses hence seem to be related to psychical and physical stress exerted by new males on pregnant females. As part of their reproductive strategy, males reduce their waiting time to insemination in this way. Abortions may likewise represent an adaptive reproductive strategy of females, who prefer to abort instead of investing in a foetus which is likely to be killed after birth.     

Social stability and change among forest hanuman langurs (Presbytis entellus)

The social organization of hanuman langurs (Presbytis entellus, Colobinae) was studied in Kanha Tiger Reserve, Central Indian Highlands between 1980 and 1983, followed by six brief return visits over ten years. Censuses of the 7 km² Kanha meadows demonstrated little change in population density and structure between 1982 and 1990; the population was consistently composed of one-male troops and all-male bands. During the return visits the focal C troop extended its known 74.5 ha range by only 5.6%. By 1993 two identified, habituated, adult females remained within the troop. The adult male was resident in C troop for ≥nine years and eight months.     

Take-over and infanticide in South Indian Hanuman langurs (Presbytis entellus)

A take-over and infanticide were observed in a South Indian population of Hanuman langurs (Presbytis entellus). A one-male, bisexual troop of langurs (Troop B1) was attacked by a band of two adult males and one subadult male. During the take-over, the resident male and one elderly female were driven from the troop. The youngest infant, about two months old, was fatally wounded by an attack from the dominant incoming male (S1). However, other young infants in Troop B1 were left unharmed. All three incoming males remained in Troop B1 but the troop appeared to remain functionally one-male, as S1 did not allow the other adult male to copulate. There was no sign that S1 attempted to eject the other two males from troop B1. © 1993 Wiley-Liss, Inc.     

Survey of the black howler monkey,<Emphasis Type="Italic">Alouatta pigra</Emphasis>, population at the Mayan site of Palenque,Chiapas, Mexico

A survey of the population of the black howler monkey (Alouatta pigra) present at the Mayan site of Palenque was conducted during 2000. A total of 911 man/hours, spread over 112 days were spent surveying the 600 ha area of pristine forest at the site for howler troops. We detected the presence of 136 individuals of which 131 were members of 20 troops, the rest were 3 solitary adult males and 2 adult males travelling as a pair. Ecological density was estimated at 23 individuals/km². Mean troop size was 7.0 individuals and it ranged from 2–12 individuals; 60% of the troops were multimale. All sighting of howler monkeys were in evergreen rain forest and 75% were in trees ≥20 m in height. The reported densities and mean troop size are higher than those reported for the species in Guatemala and in central Quintana Roo, Mexico. The vegetation of the forest contains tree species reported to be used by species ofAlouatta in the Moraceae, Sapotaceae, Leguminosae, and Lauraceae plant families. Protection of a large perimeter area (ca 1700 ha) around the archeological site by the Mexican government ensures the conservation of the forest and of the black howler monkey population present at the site.     

Male life history in natural populations of Japanese macaques: Migration, dominance rank, and troop participation of males in two habitats

This paper compares male life history parameters of two populations of Japanese macaques (Macaca fuscata Blyth, 1875), studied without provisioning: Yakushima (M. f. yakui), a subtropical forest habitat in southwestern Japan, and Kinkazan (M. f. fuscata), a temperate, deciduous forest habitat in northeastern Japan. The males of the two sites experienced similar life histories with respect to several traits. Age at natal dispersal was at about 5 years. Average troop residence was about three years. Most males joined troops at the bottom of the rank order, although a few males joined troops at the top rank. Dominance ranks of males tended to rise with the death or departure of higher ranking males. Visiting males accounted for about 41% of observed mating at both sites. However, the two sites differed in the sex ratio of troops, partly because a larger proportion of males apparently lived outside of troops in the Kinkazan site compared to Yakushima. In particular, non-natal young males were absent from the main study troop at Kinkazan. Large within-species variation may exist in the degree to which males associate with troops.     

Departure of juvenile malePresbytis entellus from the natal group

Five bisexual groups (four unimale and one multimale) were studied at Ambagarh Reserve Forest from 1985 to 1987 in relation to factors influencing departures of male juveniles from their natal troops. Of 20 male juveniles observed, 11 left their natal groups, 4 died or disappeared, and 5 remained. The male juveniles left their natal groups only after a takeover by a new adult male. The weaned juvenile males either were chased away by the new adult male or left voluntarily.     

Adolescent male chimpanzees (Pan troglodytes) form social bonds with their brothers and others during the transition to adulthood

Social relationships play an important role in animal behavior. Bonds with kin provide indirect fitness benefits, and those with nonkin may furnish direct benefits. Adult male chimpanzees (Pan troglodytes) exhibit social bonds with maternal brothers as well as unrelated adult males, facilitating cooperative behavior, but it is unclear when these bonds develop. Prior studies suggest that social bonds emerge during adolescence. Alternatively, bonds may develop during adulthood when male chimpanzees can gain fitness benefits through alliances used to compete for dominance status. To investigate these possibilities and to determine who formed bonds, we studied the social relationships of adolescent and young adult male chimpanzees (N = 18) at Ngogo in Kibale National Park, Uganda. Adolescent male chimpanzees displayed social bonds with other males, and they did so as often as did young adult males. Adolescent and young adult males frequently joined subgroups with old males. They spent time in proximity to and grooming with old males, although they also did so with their age peers. Controlling for age and age difference, males formed strong association and proximity relationships with their maternal brothers and grooming relationships with their fathers. Grooming bonds between chimpanzee fathers and their adolescent and young adult sons have not been documented before and are unexpected because female chimpanzees mate with multiple males. How fathers recognize their sons and vice versa remains unclear but may be due to familiarity created by relationships earlier in development.