Caitlin Schrein

The Structural Rigidity of the Cranium of Australopithecus africanus: Implications for Diet, Dietary Adaptations, and the Allometry of Feeding Biomechanics

Australopithecus africanus is an early hominin (i.e., human relative) believed to exhibit stress‐reducing adaptations in its craniofacial skeleton that may be related to the consumption of resistant food items using its premolar teeth. Finite element analyses simulating molar and premolar biting were used to test the hypothesis that the cranium of A. africanus is structurally more rigid than that of Macaca fascicularis, an Old World monkey that lacks derived australopith facial features. Previously generated finite element models of crania of these species were subjected to isometrically scaled loads, permitting a direct comparison of strain magnitudes. Moreover, strain energy (SE) in the models was compared after results were scaled to account for differences in bone volume and muscle forces. Results indicate that strains in certain skeletal regions below the orbits are higher in M. fascicularis than in A. africanus. Moreover, although premolar bites produce von Mises strains in the rostrum that are elevated relative to those produced by molar biting in both species, rostral strains are much higher in the macaque than in the australopith. These data suggest that at least the midface of A. africanus is more rigid than that of M. fascicularis. Comparisons of SE reveal that the A. africanus cranium is, overall, more rigid than that of M. fascicularis during premolar biting. This is consistent with the hypothesis that this hominin may have periodically consumed large, hard food items. However, the SE data suggest that the A. africanus cranium is marginally less rigid than that of the macaque during molar biting. It is hypothesized that the SE results are being influenced by the allometric scaling of cranial cortical bone thickness. Anat Rec, 293:583–593, 2010.

Beyond Gorilla and Pongo: Alternative Models for Evaluating Variation and Sexual Dimorphism in Fossil Hominoid Samples

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.

Evolution Acceptance Among Undergraduates in the South

Over the last two decades, studies and opinion polls have sought to quantify and qualify acceptance of the facts and theory of evolution by Americans in the public and at schools and universities. These studies and polls typically examine associations between evolution acceptance and factors such as age, religiosity, or education experience. Fewer studies have considered how evolution acceptance is associated with types of variables such as personal interests, behaviors, and decision-making. This chapter reviews research on populations of undergraduates attending large public universities in the southeastern and southwestern United States. I consider how exposure to the theory and facts of evolution during K-12 and undergraduate schooling is associated with undergraduate students’ interests, behaviors, and decision-making about social issues that have a scientific basis.

Facial biomechanics in Australopithecus africanus: implications for feeding ecology

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.