High dietary diversity supports large group size in Indo‐Chinese gray langurs in Wuliangshan, Yunnan, China

Abstract

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.

Northern pigtailed macaques rely on old growth plantations to offset low fruit availability in a degraded forest fragment

“…A study made on the largest known group of Trachypithecus crepusculus found similar results with larger total HR but no increase in DPL when compared with other groups. Instead, the authors suggested that high dietary diversity enables to support large group size by decreasing within‐group feeding competition (Pengfei et al, 2014). We suggest here that plantations with large clumps of high‐quality seeds provide enough food in quality, quantity, and for long enough time to sustain such a large troop in Sakaerat.…”

The foraging ecology of two neighbouring chimpanzee communities from Budongo Forest

“…Contrary to previous studies of chimpanzee dietary diversity (Wrangham et al, 1998;Fawcett, 2000), the diet of the Waibira community increased in diversity when food was more abundant, albeit not at a significant level. This might be a way of maintaining a larger overall community size as has been suggested for a group of Indo-Chinese grey langurs (Trachypithecus crepusculus) in Wuliangshan, China (Pengfei et al, 2015). The group of langurs studied was 3-10 times larger than other groups of this species, yet it did not travel greater distances per day or devote more time to travel.…”

“…Mean daily path length was also not correlated with group size in five groups of black-and-white colobus (Colobus guereza), although the largest group exhibited the longest mean daily path length (Fashing, 2001). More recently, Pengfei et al, (2015) reported that daily path length in Indo-Chinese grey langurs (Trachypithecus crepusculus) did not increase with group size but remained relatively constant even when explicit monthly changes in dietary pattern occurred.…”

“…Further, the temporal scale on which foraging decisions are usually analysed might be too large (see Asensio et al, 2009). The majority of studies use daily averages from primates foraging in cohesive groups and whose group size and/or daily path length is expected to only vary with seasonal changes in food abundance and/or the size of feeding patches (Bronikowski and Altmann, 1996;Busia et al, 2016;Fashing, 2001;Pengfei et al, 2015;Struhsaker and Leland, 1988;Wrangham et al, 1993). Daily averages, however, are unlikely to capture adjustments that foragers make throughout the day, especially not in species that live in more flexible social units such as fission-fusion species.…”

“…Given that the Waibira community ranges over an area that corresponds quite well to its larger overall community size (Jakob Villioth, unpublished data), chimpanzee densities of the two 63 communities are probably not substantially different. As outlined above, community size might be linked to diet diversity as well (Pengfei et al, 2015), thus the documentation of natural demographic variation within populations should receive more attention in discussions of within-species variation (Struhsaker, 2008).…”