R. brelichi
Wild and captive R. brelichi feed on different foods, and the ingestion of microorganisms via food may be one of the main sources of gut microbial colonization. In addition, these monkeys are exposed to different microorganisms through the environmental conditions in their habitat, including diet, water, soil, and social activities, which are potential sources of microbes in their gut (Diaz & Reese, 2021; Sun et al., 2023). Our study revealed strong similarities in the main gut microbiota between wild and captive R. brelichi . Regarding the top 10 most dominant taxa in the fecal microbiota of the two groups, Firmicutes, Bacteroides, and Spirochaetota were the dominant phyla common to the two groups. This is consistent with the results of Bornbusch et al. (2022) regarding the core dominant groups ofLemur catta in wild and captive populations. These results suggest that wild and captive R. brelichi share a potential core gut microbiota despite their different dietary habits and living environments. This core microbiota may be essential for maintaining body function and can be retained even in the face of environmental changes (Tian et al., 2020). In this study, Christensenellaceae_R_7_group, followed by Acinetobacter and Clostridia_UCG_014 , had the highest relative abundance in the gut microbiota of wild R. brelichi . In addition, LEfSe analysis showed that the abundance of these genera of wild R. brelichi was significantly higher than that of captive R. brelichi . Christensenellaceae_R_7 group and Clostridia_UCG_014 belong to the Firmicutes. The former primarily participates in cellulose and hemicellulose degradation (Wang et al., 2023), facilitating the host’s digestion of cellulosic substances and nutrient absorption, while the latter plays a crucial role in amino acid metabolism (Yang et al., 2021). As leaves rich in fibrous compounds dominate the diet of wild R. brelichi (Xiang et al., 2012), the significant increases in the abundance of both genera were also considered a response to changes in their diet. LEfSe analysis showed that the abundance of Fibrobacter , Ruminococcus ,UCG_005 , and Prevotellaceae_UCG_001 significantly increased in the gut microbiota of captive R. brelichi . Ruminococcus belongs to the Ruminococcaceae. When the energy intake is low, the increased abundance of Ruminococcaceae bacteria fermenting nonstructural carbohydrates can provide additional energy and nutrients to supplement the insufficient intake of energy from the diet (Amato et al., 2015). Meanwhile, Prevotellaceae_UCG_001 belongs to the genusPrevotella . Its enzymes can degrade cellulose and xylan, and its significant enrichment plays an important role in alleviating disordered glucose and lipid metabolism (Tang et al., 2020). The substantial increase in the abundance of these flora in the captive group may partially compensate for the significant decrease in the abundance of Christensenellaceae_R_7_group and Clostridia_UCG_014, thereby contributing to functional balance. Previous studies have also shown that R. brelichi has a greater capacity to use plant fiber as an energy source than R. bieti and R. roxellanae (Xi et al., 2023). This may be related to the wide distribution of cellulose-degrading bacteria such as UCG-005 ,Ruminococcus , Christensenellaceae_R-7_group, andFibrobacter in the intestine.
LEfSe analysis also showed that the abundance of Acinetobacter (belonging to Proteobacteria) was significantly increased in the gut of wild R. brelichi . This is consistent with the findings of Cabana et al. (2019), who showed that Acinetobacter was significantly enriched in the gut of wild Nycticebus javanicus , but contrasts with the findings of Sun et al. (2020), who revealed significant enrichment of Proteobacteria in the gut microbiota of captiveMoschus chrysogaster . Studies have shown that environment, soil, and animals are the natural habitats of Acinetobacter for its growth and reproduction (Zhai et al., 2020). In humans and animals, this is associated with diseases such as septicemia, pulmonary infections, meningitis, and diarrhea, with susceptibility to infection being associated with low host resistance, resulting in disease risk (Xu et al., 2014; He et al., 2023). The abundance of this bacterial genus in the gut of wild R. brelichi was significantly higher than that in captivity, with such infection being suggested to occur through contaminated food and water sources, and wild R. brelichi also have more chances to ingest soil. There is thus a need for further research on the imbalance of Acinetobacter in the gut microbiota of wild R. brelichi , in combination with analyses of the environment and feeding. LEfSe analysis showed that the potential pathogen Treponema showed significantly increased abundance in the gut of captive R. brelichi . Treponema is a spirochetal bacterium that can infect a wide range of hosts and tissues (Mamuad et al., 2020); for example, it is associated with porcine colonic spirochetosis, a diarrheal disease that can lead to reduced productivity. (Nguyen et al., 2023). A study by Zeng (2020) showed that an increase in the abundance of Treponema was conducive to the fermentation of cellulose and starch in R. roxellanae , but the role of Treponema in the gut of snub-nosed monkeys remains unclear. Therefore, further studies based on the physiological characteristics of R. brelichi should be conducted.
4.3 Functional differences of the gut microbiota between wild and captive R. brelichi
Under different environmental conditions, an increase or decrease of certain gut microbiota can be an adaptation to the changing environment. In this study, the potential functions of gut microbiota in R. brelichi were predicted using PICRUSt2. KEGG database analysis showed that functional genes of the gut microbiota in R. brelichi were mainly associated with metabolic pathways such as metabolism, genetic information processing, and cellular processes, which is consistent with the results of previous studies on this species (Huang et al., 2024). KEGG functions were also influenced by food provisioning, with wild foraging monkeys showing higher functions of metabolism and environment information processing, while captive food fed monkeys exhibited increases in genetic information processing, cellular processes, and organismal systems, which is contrary to the results of Li et al. (2024) on Yunnan snub-nosed monkey. At the KEGG Level 2 categories, gut microbial functions of R. brelichi were found to be mainly enriched in carbohydrate metabolism, amino acid metabolism, and lipid metabolism, which is consistent with the findings of most researchers on the metabolic functions of gut microbiota in non-human primates (Sun et al., 2016; Cheng et al., 2020; Wang et al., 2023). As a typical leaf-eating primate, R. brelichi in the wild mainly eat large leaves, while in captivity they ingest less cellulose and increase their intake of carbohydrates and fats. This is consistent with a significant increase in the abundance of carbohydrate metabolism pathways in the captive group; the ability to digest cellulose was decreased in captiveR. brelichi due to changes in the diet, which in turn increased its ability to digest simple carbohydrates (Wang et al., 2023); this also reflected the significant increases of Prevotellaceae_UCG_001 andRuminococcus in the captive group. Meanwhile, captive R. brelichi exhibited enhanced digestive and endocrine regulation capabilities. Additionally, a significant increase in the enrichment of metabolic pathways related to infectious diseases was identified, suggesting a heightened risk of infectious diseases in these monkeys. Wild R. brelichi exhibited greater ability of the xenobiotics biodegradation and metabolism pathway, xenobiotics are manmade refractory organic pollutants that are harmful to the health of living organisms. Most of them can readily be found in various components of the environment, such as soil, sediment, and water bodies (Zhang et al., 2021). Wild R. brelichi live in more complex environments and are more affected by exogenous pollutants than their captive counterparts; therefore, the enhancement of this function is understandable given the greater need to maintain gut health. In addition, the enrichment of the metabolic pathway of lipid metabolism was significantly increased in the gut of wild R. brelichi . This may be explained by the significant increase in the bacterial genus Christensenellaceae_R-7_group in the wild group, as a taxon mainly involved in host amino acid metabolism and lipid metabolism (Jiang et al., 2023). Overall, captivity altered the gut microbiota of R. brelichi , which in turn affected their functions, but these changes may have helped the host to adapt to captivity.