Ontogenetic differences in mandibular morphology of two related macaque species and its adaptive implications

Abstract

Mandibular morphology is determined not only by dietary habits, but also by sexual selection and allometry in primates. It is well-known that African papionins show intra-and interspecific variations through varied extensions of a common ontogenetic allometric trajectory in the face. Here, we used geometric morphometrics to compare the ontogenetic trajectories of large-bodied Japanese macaques and small-bodied long-tailed macaques in the sister clade of African papionins. The two species showed a major common allometric trend that was comparable to that of African papionins, but the allometric trajectory was transposed parallel to each other with few interspecies differences in mandibular shape. A minor allometric trend occurred before the eruption of the first molar. During extensino of this allometric trend in Japanese macaques, mandibular shape becomes mechanically suitable for processing tough food items in their dietary repertoire in winter. The decoupling of size and shape in the major allometric trend can allow for adaptive modifications in mandibular shape, which in turn may play a central role in speciation in macaques. Compared to other African papionins, macaques are widely distributed in temperate areas and have survived in fluctuating climates and habitats. Thus, evolutionary modifications that occur in different ontogenetic bases can result in variations in size and shape that are uniquely adaptive for a given clade.

Ontogenetic changes in jaw leverage and skull shape in tufted and untufted capuchins

“…These results correspond to previous comparisons of capuchin craniofacial ontogeny in which a number of shape variables differentiate before M 1 eruption (Cole, 1992; Masterson, 1997). In fact, a wider pattern has been observed across primates, one that suggests that interspecific and/or intergeneric variation in cranial morphology is distinct either in utero or neonatally (e.g., Cobb & O'Higgins, 2004; Marroig, 2007; Mitteroecker et al, 2004; Rogers Ackermann & Krovitz, 2002; Simons et al, 2018; Singleton, 2012; Terhune et al, 2014; Toyoda et al, 2022; Zumpano & Richtsmeier, 2003). The majority of these studies focused either solely on cranial or mandibular shape.…”

Hybridization and its impact on the ontogenetic allometry of skulls in macaques

“…It should be noted that the present finding of a limited contribution of hybridization-induced ontogenetic trajectory change to the increase in adult shape variation may be limited to the case of parental pairs with relatively small ontogenetic trajectory differences. Parallel ontogenetic trajectory differentiation between species has been widely observed in primate skull shape (Mouri 1996; Ponce de León and Zollikofer 2001; Toyoda et al 2022), while recent multivariate studies have shown that the direction of ontogenetic trajectories of cranial shape differs between species to varying degrees across primate taxa (Mitteroecker et al 2004; Vioarsdóttir and Cobb 2004; Singleton 2012; Simons and Frost 2021). The impacts of hybridization between species with greater differences in ontogenetic trajectories are of interest to future studies.…”

“…The parallel ontogenetic trajectory differentiation among species has been widely observed in primate skull shape (Mouri 1996;Ponce de León and Zollikofer 2001;Toyoda et al 2022), while recent multivariate studies have shown that the direction of ontogenetic trajectories of cranial shape differs among species in some primate taxa (Mitteroecker et al 2004;Vioarsdóttir and Cobb 2004;Singleton 2012;Simons and Frost 2021). How hybridization between species that show directional dissociation of ontogenetic trajectories affects ontogenetic direction is of interest for future studies.…”