Three-dimensional analysis of mandibular morphology in Otolemur

Euclidean distance matrix analysis (EDMA) of three-dimensional data is used here to examine mandibular morphology between two species of galagos. Otolemur crassicaudatus consumes primarily exudates, while O. garnettii is more frugivorous. Acquisition of exudates involves either gouging or scraping tree bark, and may involve different forces at the mandible than incising fruits. Previous studies of mandibular morphology in exudate-feeding primates produced mixed results, some suggesting that morphological specializations reflect adaptations for greater force at the anterior dentition, while others suggest specializations for producing a large gape. This study addresses these controversies by testing predictions associated with O. crassicaudatus generating greater force at the anterior dentition or producing a larger gape relative to O. garnettii. In addition, this study tests predictions associated with specializations of the anterior dentition in O. crassicaudatus as related to exudate-feeding. Crania and mandibles from 28 O. crassicaudatus and 17 O. garnettii were digitized in three dimensions, using 18 landmarks that summarize the shape of the jaw. Two-dimensional measurements were taken to assess incisor robusticity. All three-dimensional data were analyzed using EDMA, and bootstrap tests were executed to identify specific interlandmark differences that were driving any significant (P < 0.05) overall shape differences. Two-dimensional data were analyzed using Student's t-test for independent measures. Results revealed that there was a significant shape difference in mandibles between species, and that mandibles of O. crassicaudatus showed higher condyles, longer mandibles, decreased incisor procumbency, and greater incisor robusticity relative to O. garnettii. It is suggested that the results of the present study reflect adaptations for scraping in O. crassicaudatus rather than gouging.     

Tree-gouging by marmosets (Primates: Callitrichidae) enhances tree turnover

We tested whether gouging by Callithrix jacchus affects tree survival. The proportion of dead gouged trees was higher than the proportion of dead non-gouged trees, with larger effects on smaller trees. The number of holes did not affect tree survival. Tree-gouging by marmosets may enhance forest turnover.     

Cross-sectional Bone Distribution in the Mandibles of Gouging and Non-gouging Platyrrhini

Recent morphometric analyses have led to dissimilar conclusions about whether the jaws of tree-gouging primates are designed to resist the purportedly large forces generated during this biting behavior. We further address this question by comparing the cross-sectional geometry of the mandibular corpus and symphysis in tree-gouging common marmosets (Callithrix jacchus) to nongouging saddleback tamarins (Saguinus fuscicollis) and squirrel monkeys (Saimiri sciureus). As might be expected, based on size, squirrel monkeys tend to have absolutely larger cross-sectional areas at each tooth location sampled, while saddleback tamarins are intermediate, followed by the smaller common marmosets. Similarly, the amount and distribution of cortical bone in squirrel monkey jaws provides them with increased ability to resist sagittal bending (I _xx ) and torsion (K) in the corpus as well as coronal bending (I _xx ) and shearing in the symphysis. However, when the biomechanical parameters are scaled to respective load arm estimates, there are few significant differences in relative resistance abilities among the 3 species. A power analysis indicates that we cannot statistically rule out subtle changes in marmoset jaw form linked to resisting loads during gouging. Nevertheless, our results correspond to studies in vivo of jaw loading, field data, and other comparative analyses suggesting that common marmosets do not generate relatively large bite forces during tree gouging. The 3 species are like most other anthropoids in having thinner bone on the lingual than on the buccal side of the mandibular corpus at M₁. The similarity in corporal shape across anthropoids supports a hypothesized stereotypical pattern of jaw loading during chewing and may indicate a conserved pattern of mandibular growth for the suborder. Despite the overall similarity, platyrrhines may differ slightly from catarrhines in the details of their cortical bone distribution.

Does nonnutritive tree gouging in a rainforest‐dwelling lemur convey resource ownership as does loud calling in a dry forest‐dwelling lemur?

Nonhuman primates may defend crucial resources using acoustic or chemical signals. When essential resources are limited, ownership display for a resource may be enhanced. Defending resources may depend on population density and habitat characteristics. Using the Milne Edwards' sportive lemurs (Lepilemur edwardsi) and weasel sportive lemurs (L. mustelinus) as models, we tested whether two cryptic nocturnal lemur species differing in population density and habitat show differences in their vocal and chemical communication for signaling ownership of resources. L. edwardsi inhabits a western dry deciduous forest in a high‐density population, whereas L. mustelinus is found in an eastern rainforest in low density. We followed ten L. edwardsi (six males and four females) and nine L. mustelinus (four males and five females) for 215 hr during the early evening (06:00–10:00 p.m.) and the early morning (02:00–05:00 a.m.) and recorded their behavior using focal animal sampling. We found that both species differed in their vocal and chemical communication. L. edwardsi was highly vocal and displayed loud calling in the mornings and evenings while feeding or in the vicinity of resting places. In contrast, L. mustelinus never vocalized during observations, but displayed tree‐gouging behavior that was never observed in L. edwardsi. Tree gouging occurred more often during early evening sessions than early morning sessions. Subjects gouged trees after leaving their sleeping hole and before moving around. We suggest that, in weasel sportive lemurs, non‐nutritive tree gouging is used as a scent‐marking behavior in order to display ownership of sleeping sites. Altogether, our findings provide first empirical evidence on the evolution of different communication systems in two cryptic nocturnal primate species contrasting in habitat quality and population density. Further investigations are needed to provide more insight into the underlying mechanisms. Am. J. Primatol. 72:1062–1072, 2010. © 2010 Wiley‐Liss, Inc.     

Comparative functional analysis of skull morphology of tree-gouging primates

Many primates habitually feed on tree exudates such as gums and saps. Among these exudate feeders, Cebuella pygmaea, Callithrix spp., Phaner furcifer, and most likely Euoticus elegantulus elicit exudate flow by biting into trees with their anterior dentition. We define this behavior as gouging. Beyond the recent publication by Dumont ([1997] Am J Phys Anthropol 102:187-202), there have been few attempts to address whether any aspect of skull form in gouging primates relates to this specialized feeding behavior. However, many researchers have proposed that tree gouging results in larger bite force, larger internal skull loads, and larger jaw gapes in comparison to other chewing and biting behaviors. If true, then we might expect primate gougers to exhibit skull modifications that provide increased abilities to produce bite forces at the incisors, withstand loads in the skull, and/or generate large gapes for gouging.We develop 13 morphological predictions based on the expectation that gouging involves relatively large jaw forces and/or jaw gapes. We compare skull shapes for P. furcifer to five cheirogaleid taxa, E. elegantulus to six galagid species, and C. jacchus to two tamarin species, so as to assess whether gouging primates exhibit these predicted morphological shapes. Our results show little morphological evidence for increased force-production or load-resistance abilities in the skulls of these gouging primates. Conversely, these gougers tend to have skull shapes that are advantageous for creating large gapes. For example, all three gouging species have significantly lower condylar heights relative to the toothrow at a given mandibular length in comparison with closely related, nongouging taxa. Lowering the height of the condyle relative to the mandibular toothrow should reduce the stretching of the masseters and medial pterygoids during jaw opening, as well as position the mandibular incisors more anteriorly at wide jaw gapes. In other words, the lower incisors will follow a more vertical trajectory during both jaw opening and closing.We predict, based on these findings, that tree-gouging primates do not generate unusually large forces, but that they do use relatively large gapes during gouging. Of course, in vivo data on jaw forces and jaw gapes are required to reliably assess skull functions during gouging.     

Comparative analysis of masseter fiber architecture in tree-gouging (Callithrix jacchus) and nongouging (Saguinus oedipus) callitrichids

Common marmosets (Callithrix jacchus) and cotton-top tamarins (Saguinus oedipus) (Callitrichidae, Primates) share a broadly similar diet of fruits, insects, and tree exudates. Common marmosets, however, differ from tamarins by actively gouging trees with their anterior teeth to elicit tree exudate flow. During tree gouging, marmosets produce relatively large jaw gapes, but do not necessarily produce relatively large bite forces at the anterior teeth. We compared the fiber architecture of the masseter muscle in tree-gouging Callithrix jacchus (n = 10) to nongouging Saguinus oedipus (n = 8) to determine whether the marmoset masseter facilitates producing these large gapes during tree gouging. We predict that the marmoset masseter has relatively longer fibers and, hence, greater potential muscle excursion (i.e., a greater range of motion through increased muscle stretch). Conversely, because of the expected trade-off between excursion and force production in muscle architecture, we predict that the cotton-top tamarin masseter has more pinnate fibers and increased physiological cross-sectional area (PCSA) as compared to common marmosets. Likewise, the S. oedipus masseter is predicted to have a greater proportion of tendon relative to muscle fiber as compared to the common marmoset masseter. Common marmosets have absolutely and relatively longer masseter fibers than cotton-top tamarins. Given that fiber length is directly proportional to muscle excursion and by extension contraction velocity, this result suggests that marmosets have masseters designed for relatively greater stretching and, hence, larger gapes. Conversely, the cotton-top tamarin masseter has a greater angle of pinnation (but not significantly so), larger PCSA, and higher proportion of tendon. The significantly larger PCSA in the tamarin masseter suggests that their masseter has relatively greater force production capabilities as compared to marmosets. Collectively, these results suggest that the fiber architecture of the common marmoset masseter is part of a suite of features of the masticatory apparatus that facilitates the production of relatively large gapes during tree gouging.