Sexual dimorphism is a significant aspect influencing morphological variability and evolution in primates. All African papionins, which include the genera Cercocebus and Lophocebus (mangabeys), Mandrillus (mandrills and drills), Papio (baboons), Rungwecebus (kipunji) and Theropithecus (geladas) (Strasser and Delson, 1987; Zinner et al., 2013), are characterized by being sexually dimorphic, resulting in significantly different morphologies between males and females (Delson et al., 2000; Smith and Jungers, 1997). In the case of Mandrillus, females account for 30%-40% of male size, while in the others, they correspond to 50%-70% of male size (Swedell, 2011). Large-bodied papionins, Mandrillus, Papio and Theropithecus, share a series of morphological characteristics such as a high degree of terrestriality, elongated faces with prominent snout, and being some of the most visually striking with varied colorations and showy manes, hypothesized to have evolved through sexual selection (Setchell and Dixson, 2001; Setchell, 2005; Swedell, 2011). From this, they are considered a good model for evaluating the dynamics of sexual selection and its mechanisms, because closely related species exhibit pronounced variations in sexually selected traits (Petersdorf et al., 2019).

At the phylogenetic level, molecular data indicate that large-bodied papionins do not form a monophyletic clade, despite sharing many morphological characteristics. Instead, mandrills are phylogenetically grouped with Cercocebus, while baboons cluster with geladas, arboreal mangabeys (Lophocebus) and kipunji (Liedigk et al., 2014; Pugh and Gilbert, 2018; Craig et al., 2024). It has been suggested that, in addition to this closeness, episodes of intergeneric introgression have occurred between baboons and geladas during their evolutionary history (Walker et al., 2019; Caldon et al., 2024), with interbreeding reported in extant wild and captive populations (Dunbar and Dunbar, 1974; Harris and Disotell, 1998; Zinner et al., 2011).

Baboons (Papio sp.) have successfully adapted to diverse habitats, and currently inhabit the coastal areas of the Red Sea in the Arabian Peninsula and sub-Saharan Africa (Fleagle, 2013). Six species of Papio are presently recognized under the phylogenetic species concept: P. anubis (olive), P. cynocephalus (yellow), P. hamadryas (hamadryas), P. kindae (kinda), P. papio (Guinea), and P. ursinus (chacma) (Zinner et al., 2009, 2013). Papio species exhibit a wide range of behavioral patterns and are phenotypically different (Elton and Dunn, 2020). They show differences in body size, cranial morphology, color and texture of adult coat, and sexually selected characteristics such as sexually dimorphic canines (Fischer et al., 2019; Martinez et al., 2019).

The genus Theropithecus has only one extant species, T. gelada, with three apparently distinct evolutionary units inhabiting the highlands of Ethiopia (Gippoliti, 2010; Shotake et al., 2016; Zinner et al., 2018; Snyder-Mackler et al., 2014; Trede et al., 2020; Caldon et al., 2024). They live in complex, multi-level societies with units composed of a reproductive leader male, several adult females and their young, which then group with other units forming bands, which in turn group into communities or herds (Snyder-Mackler et al., 2012). This species exhibits a high degree of sexual dimorphism in body size (males weigh on average 16.5-20.5 kg, and females 8.3-13.8 kg) and appearance, with males having long, shaggy hair, pronounced facial whiskers, and large canines, while females have shorter coat and more uniform color (Bergman and Beehner 2013; Fleagle, 2013).

Geladas, hamadryas and Guinea baboons maintain a complex multilevel social organization system, where the primary level of organization of their society is the one-male unit, which corresponds to the reproductive unit composed of a sexually active male leader, a variable number of adult females (1–12 in geladas; 1–6 in Guinea), their offspring and, in some cases, 1 or 2 follower males (Fleagle, 2013; Kummer, 1968). When several units join together, they form clans, which in turn to constitute bands, and finally, these gather into communities or troops around resources such as sleeping sites (Fleagle, 2013; Schreier and Swedell, 2009). However, they differ in that both baboon species exhibit male philopatry with limited female dispersal from groups, whereas geladas display female philopatry with male dispersal from the natal unit (Jolly, 2020; Fischer et al., 2017; Snyder-Mackler et al., 2014). They are characterized by polygyny, with intense competition for female mates, as the group’s leader male aggressively excludes others and monopolizes females, protecting his access to mating and offspring (Chowdhury et al., 2015; Kummer, 1968; Pines et al., 2011).

In contrast, chacma, olive, kinda and yellow baboons, known as savannah baboons and dubbed “COKY” baboons (Jolly, 2020), live in multi-male, multi-female groups (parsimoniously hypothesized as ancestral state of the genus) with female philopatry and male dispersal, where females remain in the group, constituting its stable core, and males leave it to join a new one (Fischer et al., 2019). Their mating system is polygynandrous, meaning that both females and males have multiple mating partners (Fischer et al., 2017, 2019), nevertheless, high-ranking males tend to have greater mating and reproductive success, with this bias manifesting more in chacma baboons (Bulger, 1993; Alberts et al., 2003, 2006). However, there is great variation in their degree of sexual dimorphism, reflecting differences in the intensity of male competition and female mate choice (Petersdorf et al., 2019).

The genus Papio shows a pronounced variation in the degree and expression of other sexual traits (Dixson, 2012). An example of this is body mass, which can range between 17–30 kg in males and 9–16 kg in females, resulting in a ratio that varies between 1.56 and 2.00 (Delson et al., 2000; Dunbar, 1990; Fischer et al., 2017; Swedell, 2011). The kinda baboon is the smallest and most gracile baboon species, exhibiting the least sexual dimorphism in body size (Singleton et al., 2017; Petersdorf et al., 2019), while the chacma baboon is the largest baboon species with the greatest size difference between sexes (Jolly et al., 2011). Additionally, Papio species in multi-level/one-male unit (polygynous males) and multi-male-multi-female (polygynandrous) systems differ in their growth rates. Polygynous males in the former group (P. papio and P. hamadryas) tend to accelerate and shorten their growth (thus growing less). While polygynandrous males in the latter group of species (P. anubis, P. ursinus and P. cynocephalus) are more likely to reach a larger body size (Leigh et al., 2005). An exception to this is P. kindae, which despite belonging to the second group of baboons, their growth patterns differ due to differences in their allometry patterns and size related to shape dimorphism (Singleton et al., 2017). It can also be observed that adult males of savannah baboons either lack or have moderately fur capes, whereas hamadryas and Guinea baboons have prominent capes with multiple light and dark rings (Jolly, 2020; Petersdorf et al., 2019). Also, differences in relative testis size have been described, with polygynous species having smaller adult testis size compared to baboons living in multi-male, multi-female groups, with larger testis, probably being selected for in polygynandrous primates (Dixson, 2017).

In most primates and mammals, the observed male-biased sexual size dimorphism, such as in Papio and Theropithecus, mirrors the strength of sexual selection acting upon males. In the past, morphometrics studies of the Papio skull have shown that the variation in baboons is influenced by various factors that reflect adaptations to local selection (Leigh and Cheverud, 1991). However, sexual selection and male competition are the default hypotheses to explain the pronounced dimorphism of the craniofacial skeleton, in particular canines (Leigh, 2006). Sexual selection could be partly responsible for the evolution of other secondary traits in male baboons or to contribute to female size, thus driving specific morphological divergence within and between genera (Gilbert et al., 2009; Leigh, 2006). Moreover, all these male-biased traits are likely to be informative at the phylogenetic level if they correlate to shared mating systems (Gilbert and Rossie, 2007).

Male and female baboons follow a common ontogenetic trajectory during their developmental stages, and in some cases juvenile cranial forms are indistinguishable between sexes (Collard and O’Higgins, 2001; Simons et al., 2018). Thus, craniofacial sexual dimorphism is related to size in African cercopithecines (including Papio and Theropithecus), and in baboons allometric scaling plays an important role (Frost et al., 2003; Leigh and Cheverud, 1991; Leigh, 2006; Singleton, 2002). Allometry plays a significant role in structuring adult morphological diversity, with variables such as the rate and/or duration of cranial growth being particularly important as they substantially contribute to skull shape variation in baboons (Frost et al., 2003; Frost, 2013). This is because males grow at a faster rate and for a longer period than females to reach their final adult size, while females slow their growth much earlier (Leigh and Bernstein, 2006). In relation to canine’s sexual dimorphism, it has been found that the male teeth erupts at a later age, at a faster rate and for a longer duration than in females, while in females, it erupts with extraordinary speed (Leigh et al., 2005).

Furthermore, there are species-level differences in the patterns of craniodental sexual dimorphism in Papio, as in the case of P. kindae (Singleton et al., 2017). Kinda baboons deviate from a common ontogenetic pathway, showing less pronounced size and shape dimorphism than other baboon species. It has even been observed that P. kindae adult cranial size and shape are like those of subadults in other baboon species (Petersdorf et al., 2019). Therefore, it has been proposed that less intense sexual selection may contribute to limit dimorphism and size increase in P. kindae (Leigh, 2006). However, the pattern of canine dimorphism in P. kindae is common to the mean of other Papio species (Singleton et al., 2017). On the one hand, P. ursinus exhibits higher canine dimorphism, which reflects the larger size of males (Plavcan, 1998). On the other hand, P. anubis presents lower canine dimorphism, which would be linked to increased canine height in females and low dimorphism in cranial shape (Singleton et al., 2017).

Within this context, this study aims to quantitatively evaluate and compare cranial sexual dimorphism between the genera Papio and Theropithecus by means of geometric morphometrics tools. We aim to describe the size and shape patterns of craniofacial dimorphism, and whether they manifest in the same way or not in these genera. Among the large-bodied papionins, these two genera are the most closely related phylogenetically, enabling new comparative assessments of cranial morphology and the evolution of sexual dimorphism in highly dimorphic and related species.

This study aimed to compare cranial sexual dimorphism between the genera Papio and Theropithecus. Using geometric morphometric methods: PCA, regression, centroid size and angular comparisons, we described the patterns of size and shape variation in craniofacial dimorphism and assessed their relationship to phylogeny and social structure. Our findings corroborate previous studies that highlighted the importance of allometry in structuring adult craniofacial morphology in papionins (Singleton, 2002; Frost et al., 2003; Dunn et al., 2013). However, by integrating data across all extant Papio species and Theropithecus gelada, this study reveals critical insights into the evolutionary dynamics of sexual dimorphism that transcend the simple documentation of morphological differences.

The PCA plots before adjusting for size ( Figures 2 – 4 ) show distinct morphological clusters for each genus, although some degree of overlap is observed. After adjusting for size ( Figures 7 , 8 ), a clearer separation emerges, indicating that non-size-related aspects of craniofacial morphology, particularly in midfacial projection and neurocranial proportions, retain genus-specific configurations (Singleton, 2002; Frost et al., 2003). These findings demonstrate that while size strongly influences craniofacial shape, certain structural features are conserved and diagnostic, reflecting deeper phylogenetic divergence rather than solely functional or ecological convergence. This pattern suggests that the evolutionary history of these lineages has produced distinct morphological signatures that persist despite similar selective pressures, a finding consistent with previous research on papionin cranial morphology (Frost et al., 2003; Singleton, 2002).

The position of T. gelada relative to Papio species provides particularly novel insights into the evolution of craniofacial dimorphism. Our results indicate that Theropithecus gelada represents more than simply another case of sexual dimorphism already documented in Papio species. Despite cranial size values comparable to P. papio males and P. cynocephalus females, T. gelada exhibits a degree of sexual dimorphism in craniofacial size that is closer to the smaller P. kindae ( Figure 9 ). This unexpected convergence in dimorphism magnitude between T. gelada and P. kindae represents a distinct pattern worth highlighting, as it challenges the assumption that body size alone predicts the magnitude of craniofacial dimorphism.

This finding is especially significant when considering the contrasting social systems of these species. While P. kindae displays reduced sexual dimorphism associated with lower male-male competition and a polygynandrous mating system (Petersdorf et al., 2019), T. gelada lives in a multilevel society with intense male competition for reproductive access (Snyder-Mackler et al., 2012, 2014). The convergence in dimorphism magnitude, despite these contrasting social systems, suggests that multiple evolutionary pathways can produce similar morphological outcomes. This challenges simplistic models that directly link social organization to morphological dimorphism and indicates that the relationship between social structure and morphology is more complex than previously recognized.

The shared aspects of shape dimorphism between Papio and Theropithecus are primarily related to facial elongation, muzzle robusticity, and neurocranial proportions, consistent with previous studies (Singleton, 2002; Frost et al., 2003). However, our analysis reveals that these similarities manifest differently in each genus, with T. gelada showing distinctive patterns in the relative proportions of these features. This suggests that while the basic developmental and functional constraints shaping dimorphism are conserved across papionins, lineage-specific modifications have occurred during the separate evolutionary histories of these genera.

The comparison of allometric vectors and the resulting phenogram ( Figure 10 ) reveal a strong phylogenetic imprint on craniofacial sexual dimorphism. Species cluster in a manner broadly consistent with their molecular relationships (e.g., Rogers et al., 2019; Santander et al., 2022; Mutti et al., 2023), indicating that evolutionary history constrains morphological trajectories even across lineages with diverse ecological and social adaptations. This finding supports the hypothesis that phylogenetic history is a primary driver of sexual dimorphism patterns in papionins, providing a framework within which functional adaptations can evolve (Frost et al., 2003; Gilbert et al., 2009).

This observation is important for understanding baboon evolution and ecology because it demonstrates how ecological specialization can drive morphological divergence even within a framework of phylogenetic constraint (Fleagle, 2013). Our findings address a fundamental evolutionary question: is phylogenetic history the primary driver of sexual dimorphism patterns, or are functional factors, such as social organization and ecological specialization, equally or more influential? Previous studies in diverse primate groups have demonstrated that ecological specialization acts as an important driver of morphological divergence and speciation (Kamilar and Cooper, 2013; Cardini and Elton, 2008). Specific dietary adaptations have been shown to have profound effects on primate craniofacial morphology, as documented by Ross et al. (2012) and Ravosa et al. (2016), who found that dietary specializations produce significant changes in mandibular biomechanics and facial structure, even in closely related lineages. For example, Kamilar and Cooper (2013) documented how dietary adaptations in primates lead to specific craniofacial modifications that can evolve rapidly in response to ecological pressures, even when phylogenetic constraints exist. The evidence suggests a complex interplay between these factors, where the broad conservation of dimorphism patterns across papionins points to phylogenetic constraint, while the specific manifestations of dimorphism in each lineage reflect adaptations to particular ecological and social contexts.

Taken together, our findings demonstrate that craniofacial sexual dimorphism in these genera is broadly conserved in magnitude but varies in morphological expression between lineages. This highlights the importance of considering both phylogenetic history and functional/ecological context when interpreting patterns of sexual dimorphism. The case of Theropithecus gelada is particularly instructive, as it represents a distinct evolutionary experiment in which similar degrees of dimorphism to certain Papio species have evolved in the context of different ecological specializations and social structures.

Understanding how craniofacial features evolve under competing pressures from sexual selection, ecological adaptation, and phylogenetic inertia will require future research that integrates genomic, developmental, and behavioral data. Our study provides a foundation for these future investigations by demonstrating that the evolution of sexual dimorphism in closely related primates follows complex pathways that cannot be reduced to simple correlations with body size or social system. This complexity reflects the multifaceted nature of selection pressures acting on primate morphology and underscores the value of comparative studies in elucidating evolutionary processes.