Demography and reproductive output in langurs of the Western Ghats, India

Life history traits evolve such that the reproductive output of an organism is maximized. Demographic characteristics, a consequence of life history traits, indicate the reproductive output per individual in group-living species. Both phylogenetic and ecological factors influence demographic traits. In the forests of the Western Ghats, India, we studied the demography of three langur species: Semnopithecus johnii, a wet forest-dwelling species; Semnopithecus hypoleucos, largely a wet forest-dwelling species; and Semnopithecus priam, a primarily dry forest-dwelling species. S. hypoleucos and S. priam are genetically closer to each other than to S. johnii. We sampled a total of 193 groups of the three species of langurs. The group size was smaller in the two wet forest-dwelling species, S. johnii (median = 10) and S. hypoleucos (nine), than in the dry forest-dwelling species, S. priam (18). The number of adult females per group was higher in S. priam (seven) and S. johnii (six) than in S. hypoleucos (four). On the other hand, the adult female:immature ratio, indicating reproductive output and life history, was highest in S. johnii (1:0.33) followed by S. hypoleucos (1:1) and S. priam (1:1.09). Our results suggest that reproductive output is lowest in the arboreal wet forest species and increases as the species become somewhat dry deciduous forest dwellers, or almost facultative dry forest dwellers, and relatively more terrestrial. Some traits, such as group size, appear to be more sensitive to ecological factors, and some other traits such as age-sex ratios and reproductive output appear to be more conservative.