3.1 Yòu as a Cave-Dwelling Species

There are several historical texts that describe the cave-dwelling habits of yòu. A tale about the swordswomen Nie Yinniang (聂隐娘) tells readers about her huge living cave where many yuán yòu dwell (vol. 194, Tài-píng-guǎng-jì (太平广记)). The poet Yang Shen (杨慎) (1488–1559) writes that “left and right are all the caves of yuán yòu (左右猿狖穴).” Liang Youzhen (梁有贞) of the Ming dynasty (1368–1644) heard “howls of the yuán yòu in the Yellow Dragon cave (黄龙洞深猿狖号)” at Mt. Luofu in Guangdong Province (Figure 1). Describing the mountains and rivers in Taiping Fu (太平府, which mainly corresponds to modern time Fanchang District, Wuhu City, Anhui Province, Figure 1), Gǔ-jīn-tú-shū-jī-chéng (Fāng-yú-huì-biān, vol. 811 (古今图书集成·方舆汇编, 第八百十一卷)) states that the yuán yòu lodge in caves on high cliffs (山椒石壁数十仞…猿狖穴之).

Based on the evidence above, the cave-dwelling habit of yuán yòu or yòu is consistent with the biological habit of François' langurs and the structure of the character yòu (狖). In the Dà-qīng-yī-tǒng-zhì (大清一統志), “Yuán Ridge (猿岭)” in Fengchuan the area (modern Fengkai County in Guangdong Province, Figure 1) is named because of “yuán yòu in the hill (猿狖所居)”. The yuán in this hill dwell in caves (山猿穴处泻飞泉, see Meng Yaozuo's Yuán-lǐng-fēi-quán in the Ming dynasty). All these historical records related to yòu are consistent with the cave-dwelling habits of François' langurs or yuán (Grueter and Ding 2006). Conversely, modern primatological studies have not reported findings of gibbons or snub-nosed monkeys in caves. In line with van Gulik (1967) inference, (yuán) yòu represents the same animal as yuán (Trachypithecus francoisi).

3.2 Yòu Fur Color

In terms of the fur color, the black yòu is often recorded in ancient works. Zhang Qi (张琦) of the Ming dynasty observed that the black yòu raided fruits (玄狖入偷杮霜). Tang Xianzu (汤显祖) (1550–1616) recorded black yòu occurring in the Zhenyang Gorge (玄狖接峰间) where yuán were often seen (Chen Yongyuan's Zhenyang Gorge (陈用原-浈阳峡); Qu Dajun's Guǎng-dōng-xīn-yǔ (屈大均-广东新语), Figure 1). Yǒng-lè-dà-diǎn (永乐大典) (vol. 2537) states, “the black yòu hang in trees (乔木挂玄狖).” However, one poem written by Guan Xiu (贯休) (832–912 C. E.) describes that “the yellow yòu is little (黄狖小)” (see “Pass by the Old House of Meng Haoran (经孟浩然鹿门旧居)”). This is consistent with the expectation that “yellow yòu” refers to the infant François' langur (Figure 2a). The emphasis on yòu as a type of black mammal (yòu: black yuán. see Yù-piān (玉篇), Zhèng-zì-tōng (正字通), Kāng-xī dictionary (康熙字典)) in texts above might stem from a temporary occurrence of yellow infants in langur groups, as the yellow individuals typically turn black at around 6 months of age. This biological variation may have influenced Li Shan's understanding of the color of yuán (Niu et al. 2022). Adult Sichuan snub-nosed monkeys are predominantly bright golden-orange, while newborns up to 1 year of age have light brownish-gray or light brown fur (Figure 2b). This color difference does not match that of yòu, further indicating that yòu is more likely to represent a François' langur rather than a Sichuan snub-nosed monkey.

In the Yún-nán-tōng-zhì (云南通志) (vol. 3), “Black yuán and yellow yòu (黑猿黄狖)” were recorded concurrently at Jijiao hill (犄角山) (which mainly corresponds to modern time Bijia hill (笔架山) in Daguan County (大关县), Zhaotong City) in northeast Yunnan Province, adjacent to present-day Guizhou Province (Figure 1). At first glance, this might appear consistent with the color differences between adult male and adult female gibbons (Figure 2c). But as mentioned earlier, “yellow yòu” likely refers to the golden infant of the black Francois' langur (yuán). In the poem Mountain Spirit (山鬼), Qu Yuan mentioned that “yòu chirp in the evening (狖夜鸣).” This vocalization pattern corresponds precisely to the latest time of day when François' langurs have been observed vocalizing. In contrast, scientists have not documented gibbons in China singing at sunset or after nightfall (Niu et al. 2022).

Meanwhile, in another gazetteer, Dà-qīng-yī-tǒng-zhì (vol. 183), the authors note that “there are numerous yuán yòu (多猿狖)” at Jijiao Hill. Therefore, it is more likely that yòu (or yuán yòu) refers to the François’ langur inhabiting Jijiao Hill in Yunnan Province. In addition, this area is part of Mt. Wumeng (乌蒙山). Today, the François' langur can still be found in a small area of Mt. Wumeng in the south of Guizhou (Deng and Zhou 2018). Guizhou has long been associated with the presence of yuán yòu, dating back to the Tang dynasty. The Older Book of the Tang Dynasty (旧唐书) records: “Nowadays Bozhou, in the southwest of the Tang Empire, is very remote (from Xi'an City, Shaanxi Province). Yuán yòu are found there, and few people can reach that area (今播州, 西南极远, 猿狖所居, 人迹罕至).” Bozhou refers to modern-day Zunyi City in the north of Guizhou Province, where François' Langurs are still found today (Niu et al. 2016; Zhou and Huang 2021, Figure 1).

Based on the appearance, behavior, and distribution of (yuán) yòu, I suggest that (yuán) yòu represents a François’ langur rather than a Sichuan snub-nosed monkey or gibbon.

3.3 Yuán Yòu and Li Bai's Yuán

In Anhui province, the name of Chizhou City (池州) during the Tang dynasty was Qiupu (秋浦) (Figure 1). There were many yuán in that area (see vol. 10 Guì-chí-xiàn-zhì (贵池县志), Guangxu edition (光绪版), 1883). Li Bai composed the poem series Qiupu Songs: Seventeen Poems (秋浦歌十七首) and described yuán in four of them: “There are white yuán in Qiupu. They look like pieces of falling snow when yuán jump (秋浦多白猿, 超腾若飞雪)”; and “Howls of yuán sound sorrowful in the night in Qiupu (秋浦猿夜愁),” for example. These lines became classics, widely recited by poets in later generations.

Subsequent poets often reminisced about Li Bai in their nostalgic poems when visiting or passing through Qiupu. Wang Anshi (王安石) (1021–1086) wrote a poem to honor the memory of two great Tang poets, Li Bai and Du Mu, after hearing yuán vocalizing near the river in Qiupu (see Hé-wáng-wēi-zhī-qiū-pǔ-wàng-qí-shān-gǎn-lǐ-tài-bái-dù-mù-zhī (和王微之秋浦望齐山感李太白杜牧之)). Guo Xiangzheng (郭祥正) (1035–1113), in Composing poems to reply Qiupu Songs: Seventeen Poems (3rd) (追和李白秋浦歌十七首其三) wrote: “It is so cold that yuán yòu can rarely be seen in the mountains (山寒猿狖稀).” Peng Sunyi (彭孙贻) (1615–1673) wrote: “The Yuán yòu are full of sorrow at the front of Li Bai's memorial temple (李白祠前猿狖愁)” (see Five Nostalgic Poems (3rd) (五怀诗其三)). Unlike Wang Anshi's usage, both Guo and Peng referred to Li Bai's yuán using the term “yuán yòu” rather than the monosyllabic “yuán.” Once again, this cultural tradition supports the idea that yuán yòu represents the same animal as yuán—the François' langur.

3.4 Summary

This study clearly showed that yòu is more likely to be a François' langur than a snub-nosed monkey or a gibbon. The cave-dwelling habit of yòu was supported by textual evidence and is also consistent with the formation of the character yòu (狖) and the behavioral ecology of the François' langur. Yòu are mainly black, though it may occasionally refer to the yellow infant of the François' langur. The yuán (T. francoisi) in Li Bai's Qiupu Songs (秋浦歌) represents the same animal as yuán yòu described by later poets. Each of these major biological and cultural characteristics aligns with those of François' langurs. Thus, the François’ langur is most likely to be the original referent of yòu.

4.1 Animal Name and Identity

The act of naming is among the most fundamental functions of language (Borkfelt 2011). Language shapes how scientists conceive of scientific activity (Gonzalez 2021). Therefore, it is necessary to clarify the identity of traditional Chinese wildlife names to avoid further confusion. This study concurs with the inference that yòu refers to yuán (van Gulik 1967; Jiang Liangfu's view in Cui and Li 2003). Yuán yòu is a disyllable that refers to the same animal as yuán or yòu. The original identity of yuán yòu is most likely the François’ langur. With the help of traditional animal names and their referents, historical data concerning specific animals from archival documents can contribute to an understanding of traditional culture and historical zoogeography.

4.2 Reinterpretation of Traditional Culture

François' langurs held cultural and religious significance in traditional Chinese society (Niu et al. 2022). Many written records in Chinese refer to yuán yòu or yòu. Unfortunately, this disyllabic name (yuán yòu) has generally been misinterpreted in modern China as two distinct monosyllabic names, each referring to a separate taxon (e.g., Liu 1980; Li et al. 1989; Wen 2009; Zhang 2015). For instance, Qu Yuan writes: “Mysterious is the deep forest, the place where yuán yòu dwell (深林杳以冥冥兮, 乃猿狖之所居).” In contemporary academic literature, yuán yòu in this sentence has often been interpreted as yuán (gibbons) and yòu (snub-nosed monkeys), rather than as a compound name (Liu 1980; Li et al. 1989; Wen 2009; Zhang 2015). However, a more accurate interpretation of this line is: “Mysterious is the deep forest, the place where François' langurs dwell.”

Yi Yuanji (易元吉) (ca. 1100), renowned for his paintings of yuán in ancient China, is also significant in this context. Some art historians record that the primate species observed by Yi Yuanji was referred to as yuán yòu (see Table S1 4–6 in Niu 2023). This study shows that yuán yòu is a compound word referring to François’ langurs in such texts. Meanwhile, the reinterpretation of yòu as the François' langur helps resolve van Gulik's (1967) question regarding why yòu came to mean “black monkey” in modern dictionaries (Ci-hai 辞海 1989).

Niu et al. (2022) argued that the primary referent of the word yuán shifted from François' langurs to gibbons in early modern China (1890s–1950s). In a similar way, the mainstream referent of the name yuán yòu (or yòu) changed from the François' langur to the snub-nosed monkeys recognized today, coinciding with the period when Chinese scholars began studying biology as a modern scientific discipline (Liu 1954; Niu et al. 2022). This study supports the idea that the corresponding relationships between traditional primate names and their mainstream referents may shift over time in China.

Looking ahead, it is essential to clarify the referents of other traditional animal names. Doing so will allow for more accurate reinterpretation of traditional species knowledge, which could potentially play an important role in supporting the biocultural conservation of wildlife in the future (Burton 2002).

4.3 Historical Distribution of Primates: Implication for Future Conservation

Historical records are a vital data source for comprehending the geographical distribution of mammals (Nüchel et al. 2018; Wan et al. 2019; Huang et al. 2021). For instance, several authors have focused on the role of historical records in mapping the past habitat of snub-nosed monkeys (Li et al. 2002; Zhao et al. 2018). This is crucial for formulating effective strategies to safeguard their survival and promote conservation efforts. Nevertheless, the correspondence between traditional animal names and the actual species they refer to can evolve over time. Therefore, accurately identifying the species represented by names in historical texts is of utmost importance (Niu et al. 2022; Niu 2023). Currently, the referent of the traditional name yòu is often misinterpreted as snub-nosed monkeys. This misunderstanding has had a lasting impact on the understanding of the historical distribution of primates in China among modern researchers. I provide three case studies to illustrate how the identification of yòu changed has influenced authors’ interpretations of historical species distributions (in Chongqing City, Guangxi Province, and Zhejiang and Fujian Provinces; see Figure 1), and I discuss the implications for species conservation in China.

First, due to the misrepresentation of yòu as referring to snub-nosed monkeys, some researchers have suggested that snub-nosed monkeys once occurred in Mt. Jinfo (金佛山), Chongqing City (e.g., Liu 1959; Wen 2009, 2018, 2019; Figure 1). However, only François’ langurs were found there during field investigations between 1980 and 1991 (Zhang et al. 1992).

Second, a similar confusion occurred when researchers interpreted yòu in gazetteers from Guangxi province. For example, He (1993) described yòu as snub-nosed monkeys in Longshan County (now Mashan County; Lóng-shān-xiàn-zhì (隆山县志), 1938; Figure 1). However, the primate species that actually inhabits this karst region is the François' langur (Zhou and Huang 2021).

Third, a passage in the Geographic Records of Wu Kingdom (吴录·地理志) states: “There are a lot of yòu in Yang County, Jian'an City (now Jian'ou City (建瓯市) in Fujian Province, Figure 1). They resemble yuán but have visible nostrils, and when it rains, they use their tails to cover their nostrils. Yòu are also found in Linhai City (now Linhai County in Taizhou City, Figure 1) (建安阳县多狖, 似猿而露鼻, 雨则以尾反塞鼻孔。郡内及临海皆有之).” Based on this or similar descriptions, most researchers concluded that yòu referred to snub-nosed monkeys and believed that these monkeys once lived in Zhejiang and Fujian Provinces (Li et al. 2002; Nüchel et al. 2018; Wen 2018, 2019; Zhao et al. 2018). However, in a different source from adjacent Zhejiang Province (see Shen Yiji (沈翼机) and coauthors' Zhè-jiāng-tōng-zhì (浙江通志)), yòu is defined as a black yuán (Trachypithecus francoisi, Niu et al. 2022). Zhè-jiāng-tōng-zhì (vol. 104) states: “Yòu, black yuán, are abundant in the mountains (狖, 黑猿, 山中多有之)” in the Guiji (会稽or Kuaiji) region (now Shaoxing City; Figure 1). Between the two contradictory accounts discussed above, the historical evidence presented below supports the interpretation that yòu more accurately refers to François' langurs in both provinces.

According to the Bā-mǐn-tōng-zhì (八闽通志) and Fú-jiàn-tōng-zhì (福建通志) from the Qing dynasty, the François' langur (yuán), not the snub-nosed monkey, was the widely distributed primate species in Fujian Province (Niu et al. 2022). This is also supported by poetic records. During the Tang dynasty, several poets described the presence of yuán in Fujian Province. Dai Shulun (戴叔伦) (732–789 CE) wrote a poem titled “Listening to yuán with Cui Facao near the Jianxi River (和崔法曹建溪闻猿),” recalling hearing loud primate calls when traveling with a friend. Luo Luoxian (骆罗宪) of the Song dynasty wrote: “Yuán's calls from mountains of Fujian echoed in the moonlight (数声猿叫闽山月).” These accounts suggest that François’ langurs were widespread in Fujian province during the Tang and Song dynasties.

In northern Zhejiang Province, near Lingyin Monastery in Hangzhou City, Xianglin Cave (香林洞) and Huyuan Cave (呼猿洞) (Figure 3) are notable landmarks. According to a study by Niu et al. (2022), in ancient times, some poets observed François' langurs (yuán) in these caves. A field visit to the Lingyin Feilai Peak Scenic Area in January 2025 confirmed that Huyuan Cave, Xianglin Cave, Cold Spring, and Feilai Peak—as described in historical records—are authentic natural features (Figure 3a–c). These microhabitats are more suitable for François' langurs than for gibbons or snub-nosed monkeys. Regrettably, this once-thriving species has now vanished from the Feilai karst forests, leaving only historical echoes behind. Moreover, others wrote a few descriptive sentences about François' langurs in the south of Zhejiang Province. In Wenzhou City, Lu Ying (路应) (745–811 C. E.) wrote: “Yuáns' calls echoed from the deep cave (猿声响深洞)” (Figure 1). Lin Jingxi (林景熙) (1242–1310) recorded: “A thirsty yuán guided its young to drink from a stream (渴猿引子下饮涧).” In Taizhou City (台州市), yuán and golden yuán (金丝猿) were recorded in Zhè-jiāng-tōng-zhì (浙江通志) (vol. 105). The behavior and coloration of these animals align more closely with François' langurs. However, a few authors have misinterpreted yòu as referring to snub-nosed monkeys in these classical texts (see also Gao You's (高诱) Huái-nán-zǐ-zhù (淮南子注)). Such misinterpretations diminish the reliability of historical data, especially when trying to understand species extinction dynamics.

This particular case of misreading yòu suggests that even sincere interpretations by ancient scholars could stray from the original intent. Considering the value of these ancient texts, modern researchers should adopt a more cautious and analytical approach. Instead of accepting earlier readings uncritically, we must engage more deeply with the source material. Tan (1982) offers several valuable suggestions for handling historical documents in geographical research; for example, avoiding premature conclusions based on limited evidence and not blindly accepting traditional interpretations of ancient literature. These recommendations are not only relevant to historical zoogeography but are also highly beneficial for the study of the historical relationship between animal names and their identities.