A comparative study of incisor procumbency and mandibular morphology in vampire bats.

The three species of vampire bats (Phyllostomidae: Desmodontinae), Desmodus rotundus, Diaemus youngi, and Diphylla ecaudata, are the only mammals that obtain all nutrition from vertebrate blood (sanguinivory). Because of the unique challenges of this dietary niche, vampire bats possess a suite of behavioral, physiological, and morphological specializations. Morphological specializations include a dentition characterized by small, bladelike, non-occlusive cheek teeth, large canines, and extremely large, procumbent, sickle-shaped upper central incisors. The tips of these incisors rest in cuplike pits in the mandible behind the lower incisors (mandibular pits). Here, we use microCT scanning and high-resolution radiography to describe the morphology of the mandible and anterior dentition in vampire bats, focusing on the relationship between symphyseal fusion, mandibular pit size, incisor size, and procumbency. In Desmodus and Diaemus, highly procumbent upper incisors are associated with relatively small mandibular pits, an unfused mandibular symphysis with substantial bony interdigitations linking the dentaries, and a diastema between the lower central incisors that helps to facilitate the lapping of blood from a wound. In Diphylla, less procumbent upper incisors are associated with relatively large mandibular pits, a completely fused mandibular symphysis, and a continuous lower toothrow lacking a central diastema. We hypothesize that symphyseal morphology and the presence or absence of the diastema are associated with the angle of upper incisor procumbency and mandibular pit development, and that spatial constraints influence the morphology of the symphysis. Finally, this morphological variation suggests that Diphylla utilizes a different feeding strategy as compared to Desmodus and Diaemus, possibly resulting from the functional demands of specialization on avian, rather than mammalian, blood.

Phenotypic plasticity and functional asymmetry in response to grip forces exerted by intercollegiate tennis players.

This study examined phenotypic and functional responses to extreme asymmetry in hand use, and recorded the grip forces that occur during the tennis serve to assess the forces that contribute to asymmetry. Compressive grip performance was measured for Division I collegiate tennis players (24 female, 24 male) and nonathlete college students (18 females, 17 males) during three experiments: single-repetition maximum voluntary contraction, 30 consecutive repetitions, and a 30-s static hold. Tennis players had significant asymmetry in both forearm circumference and grip strength. The dominant hand of female tennis players produced 25% more force than the opposite hand, while the difference between hands of male tennis players was 18%. However, endurance over 30-repetitions and during the 30-s hold did not significantly differ between the limbs of tennis players. No significant asymmetry in forearm measurements, grip strength, or endurance was detected between the limbs of nonathletes. Grip forces generated during the tennis serve were recorded for 4 male tennis players, using a racket with a dynamometer incorporated into the handle. Peak handgrip forces generated during the serve (150-250 N) averaged 31-44% of the maximum grip force of the participant. Tennis players have increased muscle mass and strength of the dominant limb. Moderate forces, occurring hundreds of times each match and practice, are likely the primary forces contributing to this morphological and functional asymmetry.

Cranial suture complexity in white-tailed deer (Odocoileus virginianus).

Neurocranial expansion and mastication are commonly implicated as the two major biomechanical factors affecting suture morphology. In deer the antlers provide an additional source of biomechanical stress acting on the skull. Equivalent stresses are not found in females, who lack antlers. We analyzed the complexity and interdigitation of the interfrontal and coronal sutures that surround the antler-bearing frontal bones of (n = 67) white-tailed deer (Odocoileus virginianus) to 1) evaluate changes in suture morphology throughout ontogeny, and 2) test the hypothesis that male deer have more complex sutures than females. Two methods were used to quantify suture morphology: fractal analysis and length-ratios (actual suture length divided by direct straightline length). Both techniques produced similar results, although the two methods cannot be considered equivalent. Suture complexity increases markedly throughout ontogeny, but appears to level off after animals have reached adulthood. Cranial size in males, but not females, continues to increase in adults. No significant increase in suture complexity with age in the adult cohort was detected. While deer are highly dimorphic in size and the presence of antlers, no significant differences existed between males and females for any measure of suture complexity. No consistent patterns emerged between suture complexity and skull size or antler characteristics. The presence of antlers appears to have a minimal effect on suture complexity in white-tailed deer. Factors that may contribute to the lack of dimorphism in suture complexity are discussed.

Cross-sectional geometry of the dentary in bats.

Bats exhibit remarkable diversity in dietary habits, with species specializing on insects, fruit, nectar, vertebrates and blood. Studies of larger mammals have shown that structural differences in dentary cross-sectional properties exist among species with different diets. Unfortunately, few of these studies have considered the role of phylogeny in shaping these apparent form-function associations. Here we ask whether a relationship exists between diet and dentary structure in bats when phylogenetic history is factored into the analysis. To answer this question, we compared results from phylogenetic generalized least squares (PGLS) and traditional (nonphylogenetic) regression analyses of dentary cross-sectional shape in frugivorous, nectarivorous, and insectivorous bats (253 individuals representing 72 species). Cross-sectional moments of inertia of the dentary between M(1) and M(2) were computed from bone densitometry scans of skeletal specimens. Traditional regressions of cross-sectional parameters against dentary length detected significant departures from isometry among frugivores. In contrast, PGLS analyses indicated that cross-sectional variables for each dietary group scaled with isometry. Thus, the allometric patterns illuminated by traditional statistics are linked to the phylogenetic structure of the sample. Identical patterns of significant differences in slopes and intercepts between frugivores and nectarivores emerged from both traditional and PGLS analyses. As predicted, the cross-sectional shape of the dentary in frugivores is consistent with increased resistance to torsion and bending, while that of nectarivores suggested a less resistant dentary. Although traditional and PGLS analyses yielded some similar results, the phylogenetic structure of a sample can drive apparent patterns of scaling and should be considered in comparative functional analyses.