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Identifying ecological factors underlying primate group size has been a central theme in primate behavioral ecology. The ecological constraints model proposes that increased group size leads to enhanced within-group feeding competition, necessitating increased travel to encounter additional or more productive feeding sites. Over the course of three years, we studied the largest known group (>80 individuals) of Trachypithecus crepusculus (Indo-Chinese gray langur) for 1738 hours during 213 days (including 96 full day follows) in Wuliangshan, China. During this period, group size increased from 81 individuals to over 90 individuals. The group consumed plant parts from 27 ± 8 (range: 15-51) species per month, and a total 148 plant species during the study. Based on time spent feeding, the diet was similar (54.2% leaves and 32.1% fruit and seeds) to that reported for other colobines. Despite occupying a home range several times larger than other groups of Trachypithecus, we found no evidence of an increase in daily path length with increasing group size, and the group's time budget and daily path length remained relatively constant despite marked monthly changes in dietary pattern (e.g. time spent exploiting fruit and seeds vs. buds and young leaves, vs. mature leaves). These results are inconsistent with many of the predictions of the ecological constraints model. Rather, our data suggest that high dietary diversity and the ability to exploit difficult to digest foods enabled Indo-Chinese gray langurs to reduce opportunities for feeding competition associated with increased group size.

Identifying ecological factors underlying primate group size has been a central theme in primate behavioral ecology. The ecological constraints model proposes that increased group size leads to enhanced within-group feeding competition, necessitating increased travel to encounter additional or more productive feeding sites. Over the course of three years, we studied the largest known group (80 individuals) of Trachypithecus crepusculus (Indo-Chinese gray langur) for 1738 hours during 213 days (including 96 full day follows) in Wuliangshan, China. During this period, group size increased from 81 individuals to over 90 individuals. The group consumed plant parts from 27 ± 8 (range: 15-51) species per month, and a total 148 plant species during the study. Based on time spent feeding, the diet was similar (54.2% leaves and 32.1% fruit and seeds) to that reported for other colobines. Despite occupying a home range several times larger than other groups of Trachypithecus, we found no evidence of an increase in daily path length with increasing group size, and the group's time budget and daily path length remained relatively constant despite marked monthly changes in dietary pattern (e.g. time spent exploiting fruit and seeds vs. buds and young leaves, vs. mature leaves). These results are inconsistent with many of the predictions of the ecological constraints model. Rather, our data suggest that high dietary diversity and the ability to exploit difficult to digest foods enabled Indo-Chinese gray langurs to reduce opportunities for feeding competition associated with increased group size.