Grizzled langurs, Presbytis comata , a largely sexually monomorphic species, are reported to occur in populations where either the majority of groups comprise 1 adult male with 1 adult female, or where groups comprise 1 adult male with multiple females. As such, they may have a monandrous mating system. I investigated whether 1-male/1-female groups indeed form a significant part of the species’ social system, and whether habitat variation (forest fragment size, distance to the forest edge, altitude) affects social
organization. I found the species from sea level to 2,565 m above sea level in groups from 1 to 13 individuals. I recorded mostly 1-male/multifemale groups with offspring or, alternatively, all-male groups. Two out of 55 groups comprised 1-male/1-female groups with offspring. Group size was negatively correlated with altitude and forest fragment size, and positively correlated with increasing distance from the forest edge. Altitudinal variation in group sizes was driven mainly by fewer adult females being present in
groups at higher elevations; the number of adult males (almost invariably 1), subadults, juveniles, and infants, as well as the infant/adult female ratio, showed little altitudinal variation. One-male/1-female groups have been recorded repeatedly over a 25-year period in a high-altitude population on Mt. Patuha, West Java, but even here, on average, three fifths of the groups comprise 1 adult male with multiple females. At high-altitude sites, P. comata may indeed have a monandrous mating system, but at lower elevations it seems similar to that of other Presbytis langurs
