Orangutans are the only extant great apes in Asia, where today they are restricted to Borneo (Pongo pygmaeus) and northern Sumatra (Pongo abelii and Pongo tapanuliensis; Groves, 2001; Nater et al., 2017). During the Pleistocene, however, they were widely distributed across mainland Southeast Asia, including southern China, and Southeast Asia during the Pleistocene (Fig. 1; Hooijer, 1948; Kahlke, 1972; Bacon and Long, 2001, 2002; Zhao et al., 2009a; Ibrahim et al., 2013). Genomic data for extant Pongo reveal a complex evolutionary history (Locke et al., 2011; Nater et al., 2015). Based on current genomic evidence, the ancestor of extant orangutans colonized Sundaland from the Southeast Asian mainland during the Pliocene (Mattle-Greminger et al., 2018). The subsequent divergence between P. abelii and P. tapanuliensis occurred in Sumatra at ∼3.4 Ma, representing the oldest split on Sundaland (Nater et al., 2017). Pongo pygmaeus separated from P. abelii much later, at about 674 ka (Nater et al., 2017).

Although orangutans are not present on mainland Asia today, the region is important for documenting the evolutionary history and distribution of Pongo during the Pleistocene, particularly in terms of first and last appearance datums. Although the Nanling Mountains in northern Guangxi appear to have served as a geographical barrier to the northern extent of Pongo weidenreichi (Liang et al., 2020; also known as the ‘Liang Hua Line’, Demeter and Bae, 2020), Pongo fossils are known from more than 30 cave sites throughout southern China, including those in Guangxi, Guizhou, Yunnan, Guangdong, and Hainan (Zhao et al., 2009a; Ibrahim et al., 2013; Harrison et al., 2014). As such, these new fossil discoveries are providing important evidence to document the early evolution of Pongo during the Pleistocene (Zhao et al., 2009a; Harrison et al., 2014; Wang et al., 2014). The Pongo fossils from Baikong Cave in Guangxi have been dated to ∼2.0 Ma based on biostratigraphy and paleomagnetic dating (Jin et al., 2014; Sun et al., 2014), and thus currently represent the earliest appearance of Pongo. The Pongo fossils from Baikong Cave, Juyuan Cave (∼1.8 Ma), Sanhe Cave (∼1.2 Ma), and Queque Cave (∼1.0 Ma) have been attributed to P. weidenreichi (Harrison et al., 2014; Wang et al., 2014).

In general, P. weidenreichi has been distinguished from later-appearing Pongo taxa by its overall larger teeth, lower crowned canines, and considerably larger second and third molars relative to the first molars (Hooijer, 1948; Harrison et al., 2014). Pongo specimens from Ganxian Cave, dated to between 168.9 ± 2.4 ka and 362 ± 78 ka by Uranium-series (U-series) and electron spin resonance dating, have also been classified as P. weidenreichi (Liang et al., submitted). Pongo weidenreichi from the Middle Pleistocene of Yixiantian Cave and Daxin Hei Cave, retain the same morphological features of P. weidenreichi from the Early Pleistocene in southern China, but have undergone a major reduction in dental size (Harrison et al., 2021).

A recent study has shown that P. weidenreichi may have survived to the early Late Pleistocene (Harrison et al., 2021), as suggested by evidence from Zhiren Cave that dates to ∼116–110 ka (Jin et al., 2009; Liu et al., 2010; Cai et al., 2017). Based on mammalian faunal comparisons, the ages of Pongo fossils from Shuangtan Cave, Baxian Cave, and Quzai Cave in southern China are considered comparable to those of Zhiren Cave. However, more recent geochronological research at Zhiren Cave indicates that the deposits may date to MIS 6, around 190–130 ka (Ge et al., 2020). In sum, Pongo fossils with definitive dates from the Late Pleistocene of southern China are relatively scarce. Additional Pongo fossils with more precise absolute age determinations are needed to document the taxonomic and phylogenetic status of Pongo assemblages in southern China during the Late Pleistocene, and in particular, the later period of the Late Pleistocene.

Long-term field studies reveal that extant orangutan populations on Sumatra and Borneo are heavily influenced by habitat and ecological conditions (Wich et al., 2009, 2016; Manduell et al., 2012; Vogel et al., 2015; Pandong et al., 2019; Seaman et al., 2019; Meijaard et al., 2021). Sumatran forests are of higher productivity than Bornean forests, as represented by a higher proportion of fruit-bearing trees and shorter periods of low-fruit availability (Wich et al., 2009, 2011; Pandong et al., 2019; Seaman et al., 2019). Because orangutan diets consist primarily of fruits supplemented by leaves and young shoots, bark, flowers, and insects, habitat productivity and fruit availability directly impact orangutan ecology (Wich et al., 2009). Habitat variation is thus likely to have shaped differences between extant Sumatran and Bornean orangutans in terms of population densities (van Schaik et al., 1995; Russon et al., 2001; Morrogh-Bernard et al., 2003; Husson et al., 2009; Vogel et al., 2015; Seaman et al., 2019), morphology (Taylor, 2006, 2009; Taylor and van Schaik, 2007), reproductive strategies, and life history (Wich et al., 2004; van Noordwijk and van Schaik, 2005; Wich et al., 2009).

However, no study has examined the relationship between morphological variation in fossil orangutan assemblages and environmental change during the Pleistocene. A well-documented feature of Pongo evolution during the Pleistocene is a general reduction in overall crown size (Hooijer, 1948; Gu et al., 1987; Ho et al., 1995; Schwartz et al., 1995; Harrison et al., 2021). Against this backdrop of a general reduction in tooth size, Pongo assemblages from mainland Southeast Asia exhibit significant regional variation in dental size during the Middle and Late Pleistocene. At the same time, stable carbon isotope data derived from fossil mammals from mainland Southeast Asia reveal substantial environmental changes through time as represented by variation in the forest and woodland-grassland distribution during the Pleistocene (Louys and Roberts, 2020). Thus, correlations between orangutan dental size and stable carbon isotope composition of associated fossil mammals may provide evidence to assess the impact of habitat change on Pongo during the Pleistocene.

The present study has three aims. First, we use U-series dating to constrain the age of a Pongo assemblage from Yincun Cave in Guangxi Zhuang Autonomous Region, southern China. Second, we present a detailed description of the Yincun Pongo teeth and conduct metric and morphological comparisons with extant and fossil Pongo assemblages from other sites in mainland and island Southeast Asia. Lastly, we attempt to correlate the dental size of fossil Pongo assemblages to a large-scale data set of stable carbon isotope data of fossil mammals from mainland Southeast Asia to explore Pongo dental size variation and the potential impact of environmental change on Pongo assemblages through time (i.e., from Early to Late Pleistocene). The results of our study contribute to a better understanding of Late Pleistocene Pongo assemblages in southern China and the relationship between environmental change and dental size variation of Pongo from mainland Southeast Asian sites during the Quaternary.