Allometry and Evolution in the Galago Skull

Abstract

To probe the ontogenetic bases of morphological diversity across galagos, we performed the first clade-wide analyses of growth allometries in 564 adult and non-adult crania from 12 galagid taxa. In addition to evaluating if variation in galago skull form results from the differential extension/truncation of common ontogenetic patterns, scaling trajectories were employed as a criterion of subtraction to identify putative morphological adaptations in the feeding complex. A pervasive pattern of ontogenetic scaling is observed for facial dimensions across galagids, with 2 genera also sharing relative growth trajectories for masticatory proportions (Galago, Galagoides). As the facial growth series and adult data are largely coincidental, interspecific variation may result from character displacement and consequent selection for size differentiation among sister taxa. Derived configurations of the jaw joint and jaw muscle mechanical advantage in Otolemur and Euoticus appear to facilitate increased gape during scraping behaviors. Differences in aspects of masticatory growth and form characterizing these 2 genera highlight selection to uncouple shared ontogenetic patterns, which occurred via transpositions that retained ancestral scaling patterns. Due to the lack of increased robusticity of load-resisting mandibular elements in Otolemur and Euoticus, there is little evidence to suggest that exudativory in galagos results in correspondingly higher masticatory stresses.

Allometry of Masticatory Loading Parameters in Mammals

“…Further refinement of this model may benefit from analyses of the role of jaw‐adductor contractile properties in explaining variation in chewing rate. Moreover, given the presence of ontogenetic variability in jaw‐adductor fiber types (Maxwell et al, 1979; Langenbach et al, 2008; Ravosa et al, 2010a), architecture (Herring and Wineski, 1986; Herring et al, 1991; Taylor et al, 2006) and mechanical advantage (Ravosa, 1991b; Ravosa and Daniel, 2010; Ravosa et al, 2010b), the relative contribution of such factors to understanding postnatal variation in chewing frequency likewise remains to be fully explored.…”

Viewpoints: Feeding mechanics, diet, and dietary adaptations in early hominins

“…This example of genetic assimilation suggests that the neonatal disparity in skeletal robusticity between ancestor and descendant provides the functional benefit to ensure similar strains at homologous sites, and thus similar levels of bone formation later in ontogeny (Wallace et al, ). Comparative ontogenetic studies support this scenario, in that scaling between close relatives that differ in dietary properties or feeding ecology vary frequently with regard to transpositions of growth allometries for measures of mandibular robusticity and jaw‐adductor mechanical advantage (i.e., only y ‐intercept differences: Vinyard and Ravosa, ; Ravosa and Daniel, ; Ravosa et al, ). This represents the heterochronic phenomenon of predisplacement (Gould, 1966), where selectively advantageous ancestral size:shape covariation relationships are maintained in descendant taxa via an earlier ontogenetic shift in the onset of a given phenotype (Gould, 1971; Dodson, 1975a,b).…”

Exploiting a readily available but hard to digest resource: A review of exudativorous mammals identified thus far and how they cope in captivity

“…Obligate feeders must have the capability of inducing the production of gum, with a well-developed dentition adapted to damage a tree or liana severely enough for it to produce gum, a process called gouging. All exudativores have evolved specialized dentition to gouge and harvest gum (Coimbra-Filho & Mittermeier 1977;Ravosa et al 2010;Burrows et al 2016). The upper first premolars are used as an anchor for the teeth on the upper mandible to scrape the cambium away, stimulating gummosis (Nussinovitch 2009).…”