Dust in the wind: How climate variables and volcanic dust affect rates of tooth wear in Central American howling monkeys.

OBJECTIVES: Two factors have been considered important contributors to tooth wear: dietary abrasives in plant foods themselves and mineral particles adhering to ingested food. Each factor limits the functional life of teeth. Cross-population studies of wear rates in a single species living in different habitats may point to the relative contributions of each factor. MATERIALS AND METHODS: We examine macroscopic dental wear in populations of Alouatta palliata (Gray, 1849) from Costa Rica (115 specimens), Panama (19), and Nicaragua (56). The sites differ in mean annual precipitation, with the Panamanian sites receiving more than twice the precipitation of those in Costa Rica or Nicaragua (∼3,500 mm vs. ∼1,500 mm). Additionally, many of the Nicaraguan specimens were collected downwind of active plinian volcanoes. Molar wear is expressed as the ratio of exposed dentin area to tooth area; premolar wear was scored using a ranking system. RESULTS: Despite substantial variation in environmental variables and the added presence of ash in some environments, molar wear rates do not differ significantly among the populations. Premolar wear, however, is greater in individuals collected downwind from active volcanoes compared with those living in environments that did not experience ash-fall. DISCUSSION: Volcanic ash seems to be an important contributor to anterior tooth wear but less so in molar wear. That wear is not found uniformly across the tooth row may be related to malformation in the premolars due to fluorosis. A surge of fluoride accompanying the volcanic ash may differentially affect the premolars as the molars fully mineralize early in the life of Alouatta.

Mechanical defenses in leaves eaten by Costa Rican howling monkeys (Alouatta palliata).

Primate species often eat foods of different physical properties. This may have implications for tooth structure and wear in those species. The purpose of this study was to examine the mechanical defenses of leaves eaten by Alouatta palliata from different social groups at Hacienda La Pacifica in Costa Rica. Leaves were sampled from the home-ranges of groups living in different microhabitats. Specimens were collected during the wet and dry seasons from the same tree, same plant part, and same degree of development as those eaten by the monkeys. The toughness of over 300 leaves was estimated using a scissors test on a Darvell mechanical tester. Toughness values were compared between social groups, seasons, and locations on the leaves using ANOVA. Representative samples of leaves were also sun-dried for subsequent scanning electron microscopy and energy dispersive x-ray (EDX) analyses in an attempt to locate silica on the leaves. Both forms of mechanical defense (toughness and silica) were found to be at work in the plants at La Pacifica. Fracture toughness varied significantly by location within single leaves, indicating that measures of fracture toughness must be standardized by location on food items. Monkeys made some food choices based on fracture toughness by avoiding the toughest parts of leaves and consuming the least tough portions. Intergroup and seasonal differences in the toughness of foods suggest that subtle differences in resource availability can have a significant impact on diet and feeding in Alouatta palliata. Intergroup differences in the incidence of silica on leaves raise the possibility of matching differences in the rates and patterns of tooth wear.

Dental topography and molar wear in Alouatta palliata from Costa Rica.

Paleoprimatologists depend on relationships between form and function of teeth to reconstruct the diets of fossil species. Most of this work has been limited to studies of unworn teeth. A new approach, dental topographic analysis, allows the characterization and comparison of worn primate teeth. Variably worn museum specimens have been used to construct species-specific wear sequences so that measurements can be compared by wear stage among taxa with known differences in diet. This assumes that individuals in a species tend to wear their molar teeth in similar ways, a supposition that has yet to be tested. Here we evaluate this assumption with a longitudinal study of changes in tooth form over time in primates. Fourteen individual mantled howling monkeys (Alouatta palliata) were captured and then recaptured after 2, 4, and 7 years when possible at Hacienda La Pacifica in Costa Rica between 1989-1999. Dental impressions were taken each time, and molar casts were produced and analyzed using dental topographic analysis. Results showed consistent decreases in crown slope and occlusal relief. In contrast, crown angularity, a measure of surface jaggedness, remained fairly constant except with extreme wear. There were no evident differences between specimens collected in different microhabitats. These results suggest that different individual mantled howling monkeys wear their teeth down in similar ways, evidently following a species-specific wear sequence. Dental topographic analysis may therefore be used to compare morphology among similarly worn individuals from different species.

Dental microwear in live, wild-trapped Alouatta palliata from Costa Rica.

One problem with dental microwear analyses of museum material is that investigators can never be sure of the diets of the animals in question. An obvious solution to this problem is to work with live animals. Recent work with laboratory primates has shown that high resolution dental impressions can be obtained from live animals. The purpose of this study was to use similar methods to begin to document rates and patterns of dental microwear for primates in the wild. Thirty-three Alouatta palliata were captured during the wet season at Hacienda La Pacifica near Canas, Costa Rica. Dental impressions were taken and epoxy casts of the teeth were prepared using the methods of Teaford and Oyen (1989a). Scanning electron micrographs were taken of the left mandibular second molars at magnifications of 200x and 500x. Lower magnification images were used to calculate rates of wear, and higher magnification images were used to measure the size and shape of microwear features. Results indicate that, while basic patterns of dental microwear are similar in museum samples and samples of live, wild-trapped animals of the same species, ecological differences between collection locales may lead to significant intraspecific differences in dental microwear. More importantly, rates of microwear provide the first direct evidence of differences in molar use between monkeys and humans.

Dust accumulation in the canopy: a potential cause of dental microwear in primates.

Dental microwear researchers consider exogenous grit or dust to be an important cause of microscopic wear on primate teeth. No study to date has examined the accumulation of such abrasives on foods eaten by primates in the forest. This investigation introduces a method to collect dust at various heights in the canopy. Results from dust collection studies conducted at the primate research stations at Ketambe in Indonesia, and Hacienda La Pacifica in Costa Rica indicate that 1) grit collects throughout the canopy in both open country and tropical rain forest environments; and 2) the sizes and concentrations of dust particles accumulated over a fixed period of time differ depending on site location and season of investigation. These results may hold important implications for the interpretation of microwear on primate teeth.