New methods for imaging tooth enamel microstructure in 3D: A case study in the enamel of Pan troglodytes and Homo sapiens
Chewing is one of the first steps in the process of digestion, during which forces are transmitted through tooth enamel and into particles of foods that are then broken down. Tooth enamel must therefore be capable of imparting forces large enough to break food apart without itself fracturing under the high stresses generated during chewing. Fred Foster’s research searches for evidence of evolved adaptations that reduce the likelihood of fracture within the small-scale structures that form enamel itself; i.e., “enamel microstructure.” Tooth enamel comprises many nanometer-scale crystallites that are glued together to form micrometer-scale prisms. These prisms are orientated in complex patterns that are hypothesized to resist the propagation of cracks that form within enamel due to crushing and grinding food. If a crack propagates entirely through the tooth, not only is chewing efficiency reduced through loss of tooth functionality, but risk of infection may increase when internal soft tissue is exposed. Consequently, there should be strong selective pressure on tooth structure to resist tooth fracture, which makes microstructural patterns in enamel prism orientation of special interest as a dietary adaptation.
A major confound in testing adaptive hypotheses about enamel microstructure lies in imaging small-scale features in three dimensions. Tooth enamel is formed from dense packed hydroxyapatite, which limits the use of conventional scanning technologies. Foster is using methods from material sciences to create 3D representations of enamel that can be used to assess the role of microstructure as a crack-stopping mechanism. Advances in X-Ray microCT technology have produced high-resolution scanners that can image tooth enamel in the Molecular Imaging Center at Rutgers University. Through a combination of serial scanning electron microscopy and ion beam milling, Foster will create a stack of images for 3D modeling, similar to how an MRI is used to create 3D models of soft tissue. Lower second molars from one human and one chimpanzee are used to evaluate how variation in enamel microstructure is related to diet. Chimpanzees have relatively thinner enamel than humans, despite generating more than twice the bite force and consuming hard foods more frequently. This incongruity between relative enamel thickness and dietary behavior provides Foster an opportunity to test how microstructure is related to dietary behavior in two closely related large-bodied hominoids. By comparing microstructure in 3D, he will be able to evaluate if and how differences in complexity affect the likelihood of tooth fracture that results from differences in dietary ecology.
The role of the gut microbiome in digestion and energy production in wild Bornean orangutans (Pongo pygmaeus wurmbii) across shifting nutritional landscapes
Orangutans are great apes whose dietary preference for fruits confronts them with serious nutritional challenges in the Asian forests they inhabit. Fruit availability fluctuations are both extreme and unpredictable, particularly for the orangutans of Borneo, where feeding ecology can be described as oscillating between “feast and famine" periods. During famine times, total caloric intake of orangutans falls significantly as they are forced to switch from dwindling supplies of fruits to fibrous food items that are more abundant but much more difficult to digest, such as tree bark, pith, and mature leaves. Gut microbes are known to be key players in fiber digestion, partly because the microbial fermentation process produces molecules that are rapidly absorbed by host organisms and thus serve as a direct source of energy. The relative importance of microbial fermentation in helping organisms meet their minimum energetic requirements, however, varies among organisms. Ruminants, for example, fulfill 80 to 95% of their daily energetic needs through microbial fermentation, while highly folivorous howler monkeys obtain at least 30% of their daily energy from fermentation. While this figure is still unknown for wild orangutans, Brittain expects that microbial fermentation will prove to be extremely important for meeting energy requirements and, thus, a key component of the nutritional strategy adapting orangs to “feast and famine” ecological conditions.
The objective of this research is to measure the energetic and digestive roles of the gut microbiome in a population of wild Bornean orangutans (Pongo pygmaeus wurmbii) across their shifting nutritional landscape. Brittain’s overarching research question is: during low fruiting periods, when orangutans consume proportionally more dietary fiber, will the relative abundance of fiber-degrading gut microbes mirror this increase and also produce a corresponding increase in microbial energy production that compensates for caloric intake deficits? She will examine environmental changes in food availability, the nutritional contents of food items, and the gut microbe composition and microbial energy production from fecal samples at the Tuanan Orangutan Research Station, in Indonesian Borneo, which is CHES faculty member Dr. Erin Vogel’s field site of nearly 16 years. This research will advance our understanding of the gut microbiome’s role in the survival of wild mammals faced with unpredictable or changing nutritional landscapes. Brittain’s project will also contribute directly to conservation efforts for these critically endangered primates, particularly in Borneo where only 38 viable metapopulations with more than 100 individuals remain. By enhancing our understanding of orangutan health and well-being, the data will also improve captive management practices, as well as rehabilitation and release efforts.