In 1935, von Koenigswald described a huge, high-crowned molar that he purchased from a Chinese drugstore in Hong Kong and found this molar to present distinctive features compared to other known primate teeth and erected the species Gigantopithecus blacki for it (Von Koenigswald, 1935). Evidence accumulated since then reveals that this extinct giant ape is the largest primate ever discovered (Zhang and Harrison, 2017). Most of the G. blacki specimens come from South China (Pei and Woo, 1956; Woo, 1962; Zhang, 1982; Wang, 2009; Jin et al., 2009, 2014; Zhao and Zhang, 2013; Wang et al., 2017; Zhang and Harrison, 2017), while a few isolated teeth from North Vietnam (Ciochon et al., 1996), North Thailand (Bocherens et al., 2017), and Java (Noerwidi et al., 2016; Zanolli et al., 2019) have also been reported. Over the past six decades, nearly two thousand isolated teeth as well as four partial mandibles have been unearthed from ongoing cave excavations across South China (Zhang and Harrison, 2017). The estimated age of G. blacki ranges from ∼2.0 Ma to ∼300 ka (initial Early Pleistocene to Middle Pleistocene), as indicated by associated faunal assemblages and direct dating methods (Sun et al., 2014; Shao et al., 2014, 2015, 2017). Since the initial discovery of G. blacki, its phylogenetic relationship has been questioned. Gigantopithecus blacki was once considered to be a hominin (Weidenreich, 1945; Broom and Schepers, 1946; Gelvin, 1980; Zhang and Zhao, 2013). However, some also considered it to be a specialized great ape, with a possible close affinity to the Sivapithecus–Indopithecus clade (Pei and Woo, 1956; Pilbeam, 1970; Miller et al., 2008; Olejniczak et al., 2008a; Begun, 2010). Recently, Welker et al. (2019) successfully extracted dental proteins from G. blacki molar enamel and demonstrated that this species was an early diverging pongine.

Attempts have been made to determine the dietary behavior of G. blacki through a variety of approaches, such as dentognathic morphology (e.g. enamel thickness, tooth root length, occlusal area, and mandibular corpus depth; Woo, 1962; Olejniczak et al., 2008a; Kupczik and Dean, 2008; Zhang and Zhao, 2013; Kono et al., 2014; Zhang and Harrison, 2017), stable isotope analysis (Nelson, 2014; Qu et al., 2014; Bocherens et al., 2017; Jiang et al., 2021; Hu et al., 2022), dental microwear (Daegling and Grine, 1994; Zhao and Zhang, 2013), incidence of dental caries (Han and Zhao, 2002; Wang, 2009), analysis of phytoliths (Ciochon et al., 1990), and analysis of starch grains (Qu, 2014). These studies provided indications—e.g. enhanced enamel stability reflected by high calcium (Ca) isotope values, possibly being adapted to the consumption of hard foods (Hu et al., 2022)—that G. blacki may have been capable of processing mechanically challenging foods (MCFs), i.e. tough foods (Dickson, 2003; Olejniczak et al., 2008a; Kono et al., 2014) and/or hard foods (Kupczik and Dean, 2008; Qu, 2014; Hu et al., 2022). Feeding on tough or hard foods may require high dental resistance to fracture associated with repetitive and/or intense loading, respectively, while feeding on hard foods may further require relatively large bite force (BF). However, relatively few quantitative studies have investigated these dental biomechanical properties for G. blacki (e.g. Kupczik and Dean, 2008). Here, we address the following questions: 1) can the teeth of G. blacki resist breakage when processing MCFs and 2) does G. blacki have absolutely larger BF than extant apes and australopiths?

Schwartz et al. (2020) and Chai (2018) recently derived methods to estimate tooth resistance to fracture and BF, respectively. Schwartz et al. (2020) proposed a new metric, absolute crown strength (ACS), as a proxy for assessing tooth resistance to fracture. This parameter can be readily measured on the mesial section of a tooth. Estimating ACS involves two dental anatomical variables, i.e. average enamel thickness (AET) and bi-cervical diameter. In the Schwartz et al. (2020) study, tooth crown strength was reflected by two indices. One is the critical force (denoted by PRF) needed to fully propagate a radial-median fracture from cusp to cervix, and the other is the critical force (denoted by PMF) needed to fully propagate a margin fracture from cervix to cusp. Dental radial-median fractures can only be caused by biting on hard food items, while margin fractures can be induced by biting on hard or soft objects (Schwartz et al., 2020). The higher the tooth crown strength, the larger the critical force needed to cause radial-median fractures or margin fractures, and thus the larger the PRF and PMF. Both PRF (Supplementary Online Material [SOM] S1) and PMF (SOM S1) can be estimated according to the formulae derived by Lawn and Lee (2009). It is worth mentioning that in the biomechanical work of Lawn and Lee (2009), a tooth was modeled as a simplified dome-like structure with an equal-thickness enamel layer sitting atop a semicircular dentine base. To test whether ACS is a significant predictor of tooth crown strength (i.e. PRF and PMF), Schwartz et al. (2020) estimated ACS, PRF, and PMF for a sample of 139 extant hominoid and extinct hominin mandibular molars and then performed least squares regressions between ACS and PRF, and between ACS and PMF. Results showed that ACS exhibited significant linear correlations with both PRF and PMF (with r2 > 0.86), indicating that ACS is strongly correlated with the Lawn and Lee (2009) force estimates of tooth fracture.

In the numerical model proposed by Chai (2018), a tooth cusp was modeled as a truncated cone characterized by equal-thickness enamel layer that rests on conical-like dentine base (Chai, 2018: Fig. 4), which is different from the scheme for deriving ACS (Lawn and Lee, 2009; Schwartz et al., 2020). In addition, occlusal loading was simulated as a tooth cusp biting on a hard particle entrapped at the central fossa of the opposing tooth. With these simplifications on cusp structure and occlusal loading form, Chai (2018) conducted a series of finite element analyses (FEAs) to evaluate the conditions for cusp failure under occlusal loading. Simulation results showed that 1) tensile stresses were greater at the enamel–dentine junction (EDJ) and the peak stress occurred slightly below the dentine horn tip and 2) BF was correlated with two dental anatomical parameters: cusp enamel thickness and dentine horn angle (DHA). In parallel with the numerical simulations, Chai (2018) carried out tooth fracture tests to examine the reliability of the numerical model. Specifically, the polished cusps of human maxillary molars were subjected to axial occlusal force loaded by a tungsten carbide ball. After unloading, teeth were cut open to observe internal crack patterns. Internal cracking patterns showed general consistency with the numerical simulation results. For instance, in both cases, the enamel cracks extended from the EDJ and the peak stress occurred near the EDJ. Thus, despite underlying assumptions and limitations, the simplifications in the numerical model were deemed acceptable (Chai, 2018, 2020). Although there are other methods to estimate BF, most require cranial material (e.g. Demes and Creel, 1988; O'Connor et al., 2005; Wroe et al., 2010; Constantino et al., 2010; Eng et al., 2013) and are not suitable for the current study because no cranial remains of G. blacki have been found.

To test the hypothesis that G. blacki was capable of processing MCFs, we adopted the methods of Schwartz et al. (2020) and Chai (2018) to estimate ACS and BF, respectively. We then compared ACS and BF between G. blacki and a sample of extant great apes and extinct australopiths. The dietary profiles of some of the extant great apes have been well documented (e.g. Elgart-Berry, 2004; Vogel et al., 2008, 2009, 2014; Yamagiwa and Basabose, 2009; Coiner-Collier et al., 2016). We thus use these profiles to determine whether G. blacki had processing capability comparable to those of extant great apes. For instance, if G. blacki has higher ACS and absolutely larger BF than Pongo, then G. blacki teeth were likely capable of withstanding the stresses associated with biting on hard foods, given that some Pongo species have been observed to consume hard seeds in the wild (i.e. Pongo abelii and Pongo pygmaeus wurmbii in the wild; Vogel et al., 2014) and in captivity (e.g. P. pygmaeus; Lucas et al., 1994). Australopiths and G. blacki have dentognathic traits that have been adaptively linked to processing MCFs, such as robust jaws and thickly-enameled cheek teeth (Teaford and Ungar, 2000; Olejniczak et al., 2008b), yet it is not clear how they differ in their resistance to tooth fracture and BF potential. Therefore, we also compared ACS and BF between G. blacki and australopiths.