The robust jaws and large, thick-enameled molars of the Plio-Pleistocene hominins Australopithecus and Paranthropus have long been interpreted as adaptations for hard-object feeding. Recent studies of dental microwear indicate that only Paranthropus robustus regularly ate hard items, suggesting that the dentognathic anatomy of other australopiths reflects rare, seasonal exploitation of hard fallback foods. Here, we show that hard-object feeding cannot explain the extreme morphology of Paranthropus boisei. Rather, analysis of long-term dietary plasticity in an animal model suggests year-round reliance on tough foods requiring prolonged postcanine processing in P. boisei.Increased consumption of such items may have marked the earlier transition from Ardipithecus to Australopithecus, with routine hard-object feeding in P. robustus representing a novel behaviour.

Experimental analyses directly inform how an anatomical feature or complex functions during an organism's lifetime, which serves to increase the efficacy of comparative studies of living and fossil taxa. In the mammalian skull, food material properties and feeding behaviour have a pronounced influence on the development of the masticatory apparatus. Diet-related variation in loading magnitude and frequency induce a cascade of changes at the gross, tissue, cellular, protein and genetic levels, with such modelling and remodelling maintaining the integrity of oral structures vis-à-vis routine masticatory stresses. Ongoing integrative research using rabbit and rat models of long-term masticatory plasticity offers unique insight into the limitations of functional interpretations of fossilised remains. Given the general restriction of the palaeontological record to bony elements, we argue that failure to account for the disparity in the hierarchical network of responses of hard versus soft tissues may overestimate the magnitude of the adaptive divergence that is inferred from phenotypic differences. Second, we note that the developmental onset and duration of a loading stimulus associated with a given feeding behaviour can impart large effects on patterns of intraspecific variation that can mirror differences observed among taxa. Indeed, plasticity data are relevant to understanding evolutionary transformations because rabbits raised on different diets exhibit levels of morphological disparity comparable to those found between closely related primate species that vary in diet. Lastly, pronounced variation in joint form, and even joint function, can also characterise adult conspecifics that differ solely in age. In sum, our analyses emphasise the importance of a multi-site and hierarchical approach to understanding determinants of morphological variation, one which incorporates critical data on performance.

The mandibular symphyseal joint is remarkably variable across major mammalian clades, ranging in adults from unfused (amphiarthrosis) to partially fused (synarthrosis) to completely ossified (synostosis). Experimental work conducted on primates suggests that greater ossification of the symphysis is a response to increased recruitment of the balancing‐side (i.e. nonchewing side) jaw‐adductor muscles during forceful unilateral biting and chewing, with increased fusion strengthening the symphysis against correspondingly elevated joint stresses. It is thus expected that species with diets composed primarily of foods that require high‐magnitude bite forces and/or repetitive loading to process will be characterized by greater degrees of symphyseal ossification than species with relatively easy‐to‐process diets (i.e. food items typified by low toughness and/or low stiffness). However, comparative support for this idea is limited. We tested this hypothesis in four dietarily diverse mammalian clades characterized by variation in symphyseal fusion – the Strepsirrhini, Marsupialia, Feliformia, and Caniformia. We scored fusion in adult specimens of 292 species, assigned each to a dietary category based on literature accounts, and tested for an association between these two variables using Pagel’s test for the correlated evolution of binary characters. Results indicate that greater fusion is associated with diets composed of resistant items in strepsirrhines, marsupials, and feliforms, providing some support for the hypothesis. However, no such relationship was detected in caniforms, suggesting that factors other than dietary mechanical properties influence symphyseal ossification. Future work should focus on such factors, as well as those that favour an unfused mandibular symphysis.

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Many organisms exhibit a decrease in the ability to modify their phenotypes in response to shifts in environmental conditions as they mature. Such age-dependent plasticity has important implications in a variety of evolutionary and ecological contexts, particularly with respect to understanding adaptive responses to heterogeneous environments. In this study we used experimental diet manipulation to examine the life-history trajectory of plasticity in the feeding complex of a model organism, the white rabbit (Oryctolagus cuniculus). We demonstrate that, contrary to expectations derived from previous cross-sectional studies of skeletal plasticity, the jaws of weanlings and young adults exhibit similar increases in relative bone cross-sectional areas in response to the introduction of mechanically challenging foods into their diets. Furthermore, we present evidence that sensitivity to loading patterns persists well into adulthood in some regions of the masticatory apparatus in rabbits, indicating that there is an extended window of opportunity to respond to changes in dietary properties during an animal's life span. We conclude that certain aspects of the facial skeleton of rabbits, and perhaps mammals in general, are sensitive to environmental stimuli long after skeletal maturity is achieved, highlighting the importance of plasticity as a source of adaptive variation at later life-history stages.

Using a model organism (rabbits) that resembles a number of mammalian herbivores in key aspects of its chewing behaviors, we examined how variation in dietary mechanical properties affects food breakdown during mastication. Such data have implications for understanding phenotypic variation in the mammalian feeding apparatus, particularly with respect to linking jaw form to diet-induced repetitive loading. Results indicate that chewing frequency (chews/s) is independent of food properties, whereas chewing investment (chews/g) and chewing duration(s), which are proportional to repetitive loading of the jaws, are positively related to food stiffness and toughness. In comparisons of displacement-limited and stress-limited fragmentation indices, which respectively characterize the intraoral breakdown of tough and stiff foods, increases in chewing investment and duration are linked solely to stiffness. This suggests that stiffer foods engender higher peak loads and increased cyclical loading. Our findings challenge conventional wisdom by demonstrating that toughness does not, by itself, underlie increases in cyclical loading and loading duration. Instead, tough foods may be associated with such jaw-loading patterns because they must be processed in greater volumes owing to their lower nutritive quality and for longer periods of time to increase oral exposure to salivary chemicals.

Sexual size dimorphism in the postcanine dentition of the late Miocene hominoid Lufengpithecus lufengensis exceeds that in Pongo pygmaeus, demonstrating that the maximum degree of molar size dimorphism in apes is not represented among the extant Hominoidea. It has not been established, however, that the molars of Pongo are more dimorphic than those of any other living primate. In this study, we used resampling-based methods to compare molar dimorphism in Gorilla, Pongo, and Lufengpithecus to that in the papionin Mandrillus leucophaeus to test two hypotheses: (1) Pongo possesses the most size-dimorphic molars among living primates and (2) molar size dimorphism in Lufengpithecus is greater than that in the most dimorphic living primates. Our results show that M. leucophaeus exceeds great apes in its overall level of dimorphism and that L. lufengensis is more dimorphic than the extant species. Using these samples, we also evaluated molar dimorphism and taxonomic composition in two other Miocene ape samples--Ouranopithecus macedoniensis from Greece, specimens of which can be sexed based on associated canines and P(3)s, and the Sivapithecus sample from Haritalyangar, India. Ouranopithecus is more dimorphic than the extant taxa but is similar to Lufengpithecus, demonstrating that the level of molar dimorphism required for the Greek fossil sample under the single-species taxonomy is not unprecedented when the comparative framework is expanded to include extinct primates. In contrast, the Haritalyangar Sivapithecus sample, if itrepresents a single species, exhibits substantially greater molar dimorphism than does Lufengpithecus. Given these results, the taxonomic status of this sample remains equivocal.

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