4.2 Dietary response to phenology change
The quantity and quality of food resources are not always evenly distributed in space and time (Fryxell, 1991). Seasonal changes in temperature and precipitation often result in changes in primate foraging strategies by altering the availability of food resources (Oates, 1987). In this study, François’ langurs expanded their food intake in the dry season (dry season: 44 species; rainy season: 33 species), which effectively alleviated the foraging pressure caused by food scarcity. This may be a coping strategy for François’ langurs when there is food scarcity. Therefore, François’ langurs broaden the richness of their diet by increasing the food diversity and food variety to get enough food. This finding is consistent with other studies (Li et al., 2003). For instance, Luo (2000) found that in the Mayanghe area of Guizhou, the François’ langurs fed on more food species in the spring and autumn (42 species) than in the winter and summer (24 species). In the Nonggang area of Guangxi, François’ langurs also increased the number of foraging plant species during the dry season to cope with the lack of their preferred food (Zhou et al., 2006). Additionally, in the Chongzuo region of Guangxi, white-headed langurs increased their intake of plant species during the dry season (Zhou & Huang, 2021), while in Malaysia, Presbytis femoralis fed on more types of food during the food shortage season (Bennett, 1983). This trait was also seen in other food-eating primates, such as Lophocebus albigena , which increased their food diversity during the months when the fruit abundance was lower (John et al., 2001). These studies all supported the suggestion that primates can survive periods of food scarcity by adjusting the food diversity of their diet.
François’ langurs also survive long dry seasons by adjusting their food composition (Zhou et al., 2018). The results of this study showed that the François’ langurs increased their feeding on mature leaves when the feed intake of the young leaves decreased during the dry season when compared with that during the rainy season, and most of the seed and bark records occurred in the dry season (seed: 100%, bark: 85.6%). This was supported by other studies. According to Zhou et al. (2006), during the dry season when the food was scarce, the Nonggang François’ langur consumed mature and young leaves as an alternative food to solely young leaves. In Fusui, Li et al. (2009) found that François’ langurs also increased their intake of mature leaves in the dry season. In Chongzuo, Guangxi, white-headed langurs fed on more young leaves during the dry season but increased their intake of mature leaves (Li & Rogers, 2006). On the island of Kaba in Vietnam, Kaba langurs (Trachypithecus delacouri (Osgood, 1932)) consumed more leaves during the dry season and correspondingly reduced their fruit intake (Workman, 2010). Moreover, the long-tailed langur (Semnopithecus entellus (Dufresne, 1797)) in the Kanha region of India will eat mature leaves in large quantities when there is a shortage of young leaves, fruits, and flowers (Newton, 1992).