Large brains are a defining feature of primates, as is a clear allometric trend between body mass and brain size. However, important questions on the macroevolution of brain shape in primates remain unanswered. Here we address two: (i), does the relationship between the brain size and its shape follow allometric trends and (ii), is this relationship consistent over evolutionary time? We employ three-dimensional geometric morphometrics and phylogenetic comparative methods to answer these questions, based on a large sample representing 151 species and most primate families. We found two distinct trends regarding the relationship between brain shape and brain size. Hominoidea and Cercopithecinae showed significant evolutionary allometry, whereas no allometric trends were discernible for Strepsirrhini, Colobinae or Platyrrhini. Furthermore, we found that in the taxa characterized by significant allometry, brain shape evolution accelerated, whereas for taxa in which such allometry was absent, the evolution of brain shape decelerated. We conclude that although primates in general are typically described as large-brained, strong allometric effects on brain shape are largely confined to the order's representatives that display more complex behavioural repertoires.

Increasing body size is accompanied by facial elongation across a number of mammalian taxa. This trend forms the basis of a proposed evolutionary rule, cranial evolutionary allometry (CREA). However, facial length has also been widely associated with the varying mechanical resistance of foods. Here, we combine geometric morphometrics and computational biomechanical analyses to determine whether evolutionary allometry or feeding ecology have been dominant influences on facial elongation across 16 species of kangaroos and relatives (Macropodiformes). We found no support for an allometric trend. Nor was craniofacial morphology strictly defined by dietary categories, but rather associated with a combination of the mechanical properties of vegetation types and cropping behaviours used to access them. Among species examined here, shorter muzzles coincided with known diets of tough, resistant plant tissues, accessed via active slicing by the anterior dentition. This morphology consistently resulted in increased mechanical efficiency and decreased bone deformation during incisor biting. Longer muzzles, by contrast, aligned with softer foods or feeding behaviours invoking cervical musculature that circumvent the need for hard biting. These findings point to a potential for craniofacial morphology to predict feeding ecology in macropodiforms, which may be useful for species management planning and for inferring palaeoecology.

Interspecific variation in the craniofacial morphology of kangaroos and wallabies is associated with diet and feeding behaviors. Yet, to how fine a taxonomic scale this relationship might exist is unknown. Using a combination of established morphometric analyses and novel finite element approaches, we test the limits of these associations by examining three closely-related pademelon taxa: the red-necked pademelon (Thylogale thetis), and two subspecies of the red-legged pademelon (Thylogale stigmatica stigmatica and Thylogale stigmatica wilcoxi). All three taxa have distinct proportions of graze (grasses) and browse (leaves, stems, and branches of trees and shrubs) in their diets. We identified clear morphological differences in the crania between all three taxa and significant influences of geography and climate on cranial shape. We found significant differences in shape and strain magnitudes along the muzzle and cheek bones of each group that are consistent with the properties of their respective diets. These results suggest that feeding ecology influences craniofacial morphology down to the subspecies scale for at least some kangaroos and wallabies, which mirrors what is known at the macroevolutionary level for these species. This lends further weight to the predictive value of cranial morphology in determining feeding ecology among the Macropodiformes and may be of use in inferring feeding ecology of less accessible species for conservation and management.