Volker Sommer

Dominance, age, and reproductive success in free-ranging female hanuman langurs (Presbytis entellus)

Correlations among female age, dominance, and reproduction were investigated for a 12-year period in free-ranging, provisioned Hanuman langurs (Presbytis entellus), living in one-male groups near Jodhpur in Rajasthan, India. Of 2940 displacement episodes, 27% occurred over natural food, 26% over provisioned food, 8% over grooming, 23% over position and shade, and 16% for other reasons. It was possible to reconstruct a displacement hierarchy that was linear and stable over short periods but fluctuated according to the age composition of the troop, resulting in an age inversed dominance structure. Females occupied top ranks as soon as they experienced menarche (around 2.4 years of age) and gradually declined thereafter, with postmenopausal females (≥30 years) being the lowest ranking individuals. Old females tended to be peripheral, while young females were highly social and active. Fertility peaked at about 7 years and gradually decreased thereafter, but infant mortality was much higher in young females than in old ones. During years when females gave birth, their ranks, especially those of old females, were higher than the average expectation for their age class, which suggests that females compete more vigorously if they have an infant. Reproductive success (i.e., infant survival to ≥2 years) declined significantly from high-over middle-to low-ranking females but did not differ for the three age classes investigated, because the higher fecundity of young females was balanced by better rearing success of older females. These results are discussed in light of the controversy over whether the langur social system is strongly influenced by kin selection (Hrdy and Hrdy, 1976; Dolhinow et al., 1979).

Infanticide among-free-ranging langurs (Presbytis entellus) at Jodhpur (Rajasthan/India): recent observations and a reconsideration of hypotheses

Considerable disagreement characterizes the debate concerning frequency, causation, and function of infanticide in connection with adult male replacements in bisexual one-male troops of hanuman langurs (Presbytis entellus). Detailed observations are presented about two noninfanticidal and three infanticidal male changes including six eye-witness and five presumed cases of infanticide within three langur troops during a long-term study at Jodhpur, Rajasthan, India. The results do not support any explanatory hypotheses focussing on social crowding, regulation of population density, social stress, sexual frustration, incest avoidance, or social bonding, but are in general though not total agreement with the reproductive advantage hypothesis: mainly unrelated infants were killed (one possible exception), the infanticidal male generally sired the subsequent offspring (one exception), and the mean interbirth interval subsequent to infanticide is by 2.1 months shortened. Likewise, several cases of stress induced abortions occurred. It is demonstrated that postconception estrous behaviour is by no means a female counterstrategy to infanticide in order to confuse males concerning the issue of paternity, since an infanticidal male did not spare the subsequent offspring of mothers who copulated with him during pregnancy and pregnant females did not discriminate between fathers and non-fathers.

Male reproductive success in harem troops of hanuman langurs (Presbytis entellus)

Based on cross-sectional and longitudinal data collected in 1967–1988 by various observers, male reproductive success was studied in the Hanuman langurs of Jodhpur, India. The harem-structured social organization ensures a high degree of paternity certainty. Births occur throughout the year, with significant peaks and minima in March and November, respectively (n =398).The interbirth interval averages 16.7 months (n = 114).The duration of harem residencies varies between 3 days and ≥ 74.0 months, with a mean of 26.5 (n = 64). Harem holder replacements occur during all months of the year. No male achieves residency in more than one troop, suggesting that residency is associated with a distinct peak in the resource holding potential of a given male. Reproductive success among males varies considerably. Male mortality is high due to migration and intrasexual competition, leading to an adult sex ratio of 1:4.9. It is estimated that one-quarter of all adult males will never gain harem residency. Conceptions achieved outside harem residencies are so rare (4.7%) that a viable low-risk strategy, opting for longevity instead of harem residency, is unlikely. Tenure length has a stronger influence on male reproductive success than harem size because interbirth intervals are significantly shorter in small harems than in larger ones. It is assumed that females in one-male breeding structures compete for sperm and that such competition is more intense in larger harems.

Play in chimpanzees of the Arnhem Zoo: Self-serving compromises.

In a colony of 25 chimpanzees maintained at Arnhem (The Netherlands), play behavior was studied for four months, yielding 1,651 play bouts involving the 11 oldest immature chimpanzees on which detailed data were collected during 88 focal animal samples of 30 min each. Play frequency and object play were negatively correlated with increasing age. These findings potentially reflect changing motivations during development and increasingly perfected skills. Like-aged partners were preferred play partners and older partners initiated play more frequently, probably because younger individuals feared rough responses by older immatures and waited until the latter initiated play bouts in a self-handicapped manner. Moreover, play was frequently interrupted by short breaks. This, in conjunction with the preference for like-aged partners, may be mechanisms to achieve and maintain compromises between the play partners different interests in social play (which e.g. depend on sex and physical development) and to minimize the risk to being cheated. Males played significantly more often than females and played more chasing games. Male-male play was over-represented whereas female-female play was underrepresented. These facts potentially foreshadow sex differences in (wild) maturing chimpanzees, where males engage in more agonistic conflicts and tend to build coalitions.

Sex Differences in Mortality among Langurs (Presbytis entellus) of Jodhpur, Rajasthan

In a population of about 1,300 Hanuman langurs (Presbytis entellus) living in one-male breeding structures at Jodhpur, Rajasthan, India, census data resulted in male:female sex rati

Wanderers between harems and bachelor bands: male hanuman langurs (Presbytis entellus) at Jodhpur in Rajasthan

1. Life-histories of individually identified males of an Asian colobine monkey, the Hanuman langur, were followed in order to understand mechanisms and functions of social processes associated with the formation of male bands, especially the degree of cooperation and competition among males. 2. The monkeys belonged to a geographically isolated population which is subject of a field-study in Rajasthan, India, spanning more than 15 years. The population consists of 27-29 one-male / multi-female troops (harems; average 39 members, home ranges up to 1.3 km2) and 12-14 all-male bands (average 12 members, moving ranges up to 20 km2). Females are highly philopatric, whereas males transfer from bisexual troops into male bands. Harem holder residencies average 27 months (range 3 days to more than 74 months). No male achieved residency in more than one troop, suggesting that residency is associated with a distinct peak in a given males resource holding potential. The proportion of infants sired by extra-troop males is minimal. The dispersal pattern of langur males seem to reflect a long-lasting effort to achieve reproduction. This goal is tied to the necessity to rise through the ranks of a male band to the top of its dominance hierarchy, because only the highest ranking males can replace current harem holders during male band invasions into the home range of bisexual troops. 3. The age-class composition of seven male bands with a mean membership of 3-25 individuals was studied in detail. The longitudinal fluctuation in membership was less pronounced than the differences between bands. This reflects ecological conditions in the moving range of a given band which determine a ceiling in the number of members. The bands contained 27% juveniles, 23% subadults, 13% young adults, 30% prime adults and 9% old males. At any given time, in each band lived at least one individual who was of at least young adult age. Immatures often followed elder males during daily travels, but not

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.

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 bands 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.

Homosexual Mounting in Free-Ranging Female Hanuman Langurs (Presbytis Entellus)

Female—female mounting was studied for 3233 hr in a bisexual one-male troop of free-ranging Hanuman langurs in northwestern India over a period of 6 years. The population breeds throughout the year and female—female mounts (n = 524) occurred during all months. All 15 adult females mounted and were mounted, although only 20% of the individuals exhibited more than half of all mounter activity and 33% more than half of all mountee activity. Various similarities with heterosexual mounting suggest that sexual arousal is a stimulus of the activity. Cycling, pregnant, and lactating females acted as mounters as well as mountees. However, ovulating individuals were greatly overrepresented concerning their likelihood of mounting and being mounted. Females of any given rank in the dominance hierarchy showed mounter as well as mountee activity, although mounters were higher ranking than mountees in 84% of cases. Young mounters were overrepresented and middle- to old-aged mounters underrepresented, whereas no such differences existed for mountees. Mounting relations among four young adult paternal half-sisters who occupied top positions in the dominance hierarchy were significantly overrepresented. Although the ultimate function of homosexual female mounting is not fully understood, it is suspected to be connected with intrasexual competition: If “pseudocopulations” induce mountees to reduce the number of solicitations addressed to the male, the probability of insemination decreases and mounters thus reduce the number of future competitors.