Sexual dimorphism in primates has already been highlighted, among others, in body mass (Clutton-Brock et al., 1977; Zihlman and McFarland, 2000; Plavcan, 2001), dental structures (Greene, 1973; Wood et al., 1991; Plavcan, 2001), cranial (Wood, 1976; Wood et al., 1991; O’Higgins and Dryden, 1993; Plavcan, 2002; Schaefer et al., 2004; Balolia et al., 2017; Milella et al., 2021), and postcranial elements of the skeleton (Taylor, 1997; Plavcan, 2001; McFadden and Bracht, 2005; Balolia et al., 2013; Morris et al., 2019). To explain observed sexual dimorphism in primate species, the generally advocated hypothesis is a phenomenon of sexual selection (Plavcan, 2001; Isaac, 2005; Cassini, 2020) mainly driven by a competition between males for females and access to reproduction in male-biased species (Clutton-Brock and Huchard, 2013). This is thought to result in the evolution of sexual dimorphism for some morphological traits (Clutton-Brock, 2017). For example, the longer canine measured in males is assumed to be the result of a male-male competition (Thorén et al., 2006). In addition to the development of clear morphological differences, sexual dimorphism can also be found in social behavior (Clutton-Brock, 2017). Social differences between males and females already appear at an early age (Maestripieri and Ross, 2004) and require different cognitive abilities that may impact the brain architecture (Kotrschal et al., 2012; van Schaik et al., 2012). Indeed, while sexual selection on males seems to have favored neural structures involved in aggression and sensory motor functions, social selection on female has rather driven cerebral structures involved in sociocognitive skills (Lindenfors, 2005; Lindenfors et al., 2007). In particular, researchers have highlighted a difference between male and female neocortex size, which seems more developed in females (Lindenfors, 2005; Lindenfors et al., 2007). Similarly, the impact of sex differences on the cerebral asymmetry (i.e., the neuroanatomical differences between the two brain sides) has been extensively studied in Homo sapiens (Sowell et al., 2002; Toga and Thompson, 2003; Visser et al., 2014; Liang et al., 2021), but has also been observed in several groups of nonhuman primates (Cantalupo and Hopkins, 2001; Gilissen, 2001; Fears et al., 2011; Imai et al., 2011; Gómez-Robles et al., 2016; Marie et al., 2018). In addition, Bienvenu et al. (2011) demonstrated that primates with relatively large brains, such as modern humans and gorillas, display greater sexual dimorphism in endocranial size.

Within hominoids, gorilla species are especially known to have one of the most complex social lives within primates (Morrison et al., 2019), associated with an impressive sexual dimorphism, both in body size (Zihlman and McFarland, 2000) and skeletal elements (Taylor, 1997; Plavcan, 2001; Balolia et al., 2013; Morris et al., 2019). In particular, the eastern lowland gorillas (Gorilla beringei graueri) live in complex social groups that can reach 7 to 10 individuals (Tutin, 1996). Most of the groups are composed of a single male and several females, but one third of groups have been found to host up to two full-grown males (Robbins, 1995; Watts, 1996; Yamagiwa et al., 2003), and there is a clear hierarchical structure between males resulting in dominant and subordinate individuals associated with aggressive competitional behavior for females (Robbins, 1995; Watts, 1996; Doran and McNeilage, 1998). On the other hand, competition between females is rather food-driven (Watts, 1996; Sterck et al., 1997; Doran and McNeilage, 1998), although this fact mostly depends on the relatedness between females within a group (Watts, 1996). The presence of tight female-male associations in gorilla groups might seem disadvantageous for females due to food competition, but this association can be explained by the resulting reduction of infanticide (Watts, 1989, 1996; van Schaik, 1996; Doran and McNeilage, 1998).

The recent development of 3D imaging techniques has made it possible to explore many problematics related to primate brain evolution (Iurino et al., 2013; Balanoff and Bever, 2016). While the shape of the endocranium (i.e., three-dimensional [3D] reconstructions of the cavity of the endocranium; see Bruner et al., 2018) is not an exact reflection of the cerebral morphology, previous research on brain evolution showed that endocasts are a good proxy to study brain size and shape, especially within mammal taxa (Neubauer, 2014; Watanabe et al., 2018; Dumoncel et al., 2020; Melchionna et al., 2020). The endocranium is of particular interest for evolutionary inferences on fossil species whose cerebral structures are not directly accessible, but also in the case of rare and endangered species or populations for which only few museum specimens are accessible (e.g., Iwaniuk et al., 2020). The critically endangered eastern lowland gorillas (G. b. graueri) occupy an area of less than 20,000 km2 in the eastern Democratic Republic of the Congo (Plumptre et al., 2016). Due to the geopolitical context, little is known about this subspecies, whether on their ecology, anatomical differences with other gorillas, or sexual dimorphism within this population. In this study, we focused on the sexual dimorphism of the external and internal cranium of eastern lowland gorillas. The study of a single subspecies also has the advantage to avoid bias related to interspecific morphological variation (Stanford, 2001; Leigh et al., 2003). Sexual dimorphism in the primate brain and endocast has already been studied for different traits (Smaers et al., 2012; Kurth et al., 2018; Ritchie et al., 2018; Hecht et al., 2021; Liang et al., 2021). To this day, however, only a few researchers have explored sexual dimorphism in nonhuman primate brains through the use of 3D geometric morphometrics (but see Bienvenu et al., 2011). Here, we inferred the sexual dimorphism in gorilla external and internal cranium. We used virtual endocasts created from CT scanners with the aim to investigate differences in brain shape and size between male and female specimens. To do so, we used 3D geometric morphometrics and multivariate analyses to quantify the overall cranial and endocast shape and compare size, morphological diversity, and asymmetry in these two structures between males and females. Based on previous research, we expected male eastern lowland gorillas to show a larger mean size and more size variability than females for the external cranium (Schultz, 1962; O’Higgins and Dryden, 1993; Schaefer et al., 2004). We also hypothesized to find significant differences in external cranial shape between males and females, as found in O’Higgins and Dryden, 1993 and Schaefer et al. (2004). Concerning the internal cranium (i.e., endocast), Bienvenu et al. (2011) did not find any evidence between male and female endocast size in the gorilla. However, the authors hypothesized that their sampling was too small to reach statistical significance (i.e., only 16 adult gorillas). Therefore, using a larger sample size in our study, we expected to highlight a significant difference for the endocranial size related to the sex. Additionally, we expected to find clear significant differences in the endocranial shape between males and females (Bienvenu et al., 2011). However, our analyses differ from the latter by the use of sliding landmarks, which encompass areas that lack clear anatomical homologous landmarks. In addition, we studied the endocranium in parallel with the cranium since endocranial asymmetries could be the result of an asymmetry of the external structure of the cranium. The same holds true when investigating the endocranial size.