Christopher J. Vinyard

Objectives This study aims to quantify the areas of most concern in medical students in relation to their residency application in the setting of the COVID-19 pandemic and to identify risk factors for all that cause concern and specific areas of concern in a population with well-established high rates of anxiety at baseline. The COVID-19 pandemic introduced sweeping changes to medical education that had wide-ranging effects on medical students and their applications for medical residencies. Methods In August 2020, we utilized a cross-sectional study to quantify student’s areas of concern related to residency application related to the COVID-19 pandemic. We asked participants to rate their levels of concern in 15 different aspects related to medical residency applications and the perceived impact that COVID-19 had on each. Results The survey was distributed to 984 osteopathic medical students, with 255 complete responses. The three areas of greatest impact were shadowing opportunities (4.15), volunteer opportunities (4.09), and conferencing opportunities (4.09). The most salient demographic variables were year in school, sex, and locale. Females reported higher levels of concern across all categories in the study compared to males, with statistical significance across all categories (all p<0.05, Range d=0.16 to 0.43), except for letters of recommendation and sub-internships. Conclusions The areas of most concern identified in our study were consistent with prior studies and may implicate the pressures that female medical students may feel compared to their male counterparts. The underlying cause(s) may be subject to future research.

Skeletal muscle fibre architecture provides important insights into performance of vertebrate locomotor and feeding behaviours. Chemical digestion and in situ sectioning of muscle bellies along their lengths to expose fibres, fibre orientation and intramuscular tendon, are two classical methods for estimating architectural variables such as fibre length (L_f) and physiological cross‐sectional area (PCSA). It has recently been proposed that L_f estimates are systematically shorter and hence less accurate using in situ sectioning. Here we addressed this hypothesis by comparing L_f estimates between the two methods for the superficial masseter and temporalis muscles in a sample of strepsirrhine and platyrrhine primates. Means or single‐specimen L_f estimates using chemical digestion were greater in 17/32 comparisons (53.13%), indicating the probability of achieving longer fibres using chemical digestion is no greater than chance in these taxonomic samples. We further explored the impact of sampling on scaling of L_f and PCSA in platyrrhines applying a bootstrapping approach. We found that sampling—both numbers of individuals within species and representation of species across the clade significantly influence scaling results of L_f and PCSA in platyrrhines. We show that intraspecific and clade sampling strategies can account for differences between previously published platyrrhine scaling studies. We suggest that differences in these two methodological approaches to assessing muscle architecture are relatively less consequential when estimating L_f and PCSA for comparative studies, whereas achieving more reliable estimates within species through larger samples and representation of the full clade space are important considerations in comparative studies of fibre architecture and scaling.

Cranial synchondroses are cartilaginous joints between basicranial bones or between basicranial bones and septal cartilage, and have been implicated as having a potential active role in determining craniofacial form. However, few studies have examined them histologically. Using histological and immunohistochemical methods, we examined all basicranial joints in serial sagittal sections of newborn heads from nine genera of primates (five anthropoids, four strepsirrhines). Each synchondrosis was examined for characteristics of active growth centers, including a zonal distribution of proliferating and hypertrophic chondrocytes, as well as corresponding changes in matrix characteristics (i.e., density and organization of Type II collagen). Results reveal three midline and three bilateral synchondroses possess attributes of active growth centers in all species (sphenooccipital, intrasphenoidal, presphenoseptal). One midline synchondrosis (ethmoseptal) and one bilateral synchondrosis (alibasisphenoidal synchondrosis [ABS]) are active growth centers in some but not all newborn primates. ABS is oriented more anteriorly in monkeys compared to lemurs and bushbabies. The sphenoethmoidal synchondrosis (SES) varies at birth: in monkeys, it is a suture‐like joint (i.e., fibrous tissue between the two bones); however, in strepsirrhines, the jugum sphenoidale is ossified while the mesethmoid remains cartilaginous. No species possesses an SES that has the organization of a growth plate. Overall, our findings demonstrate that only four midline synchondroses have the potential to actively affect basicranial angularity and facial orientation during the perinatal timeframe, while the SES of anthropoids essentially transitions toward a “suture‐like” function, permitting passive growth postnatally. Loss of cartilaginous continuity at SES and reorientation of ABS distinguish monkeys from strepsirrhines.

Wild primates face grave conservation challenges, with habitat loss and climate change projected to cause mass extinctions in the coming decades. As large-bodied Neotropical primates, mantled howling monkeys (Alouatta palliata) are predicted to fare poorly under climate change, yet are also known for their resilience in a variety of environments, including highly disturbed habitats. We utilized ecophysiology research on this species to determine the morphological, physiological, and behavioral mechanisms howlers employ to overcome ecological challenges. Our data show that howlers at La Pacifica, Costa Rica are capable of modifying body size. Howlers displayed reduced mass in warmer, drier habitats, seasonal weight changes, frequent within-lifetime weight fluctuations, and gradual increases in body mass over the past four decades. These within-lifetime changes indicate a capacity to modify morphology in a way that can impact animals’ energetics and thermodynamics. Howlers are also able to consume foods with a wide variety of food material properties by altering oral processing during feeding. While this capability suggests some capacity to cope with the phenological shifts expected from climate change and increased habitat fragmentation, data on rates of dental microwear warns that these acclimations may also cost dental longevity. Lastly, we found that howlers are able to acclimate to changing thermal pressures. On shorter-term daily scales, howlers use behavioral mechanisms to thermoregulate, including timing activities to avoid heat stress and utilizing cool microhabitats. At the seasonal scale, animals employ hormonal pathways to influence heat production. These lines of evidence cumulatively indicate that howlers possess morphological, physiological, and behavioral mechanisms to acclimate to environmental challenges. As such, howlers’ plasticity may facilitate their resilience to climate change and habitat loss. While habitat loss in the tropics is unlikely to abate, our results point to a potential benefit of active management and selective cultivation to yield large, interconnected forest fragments with targeted phenology that provides both a complex physical structure and a diversity of food sources. These steps could assist howlers in using their natural acclimation potential to survive future conservation threats.

Many primates show responses to dead infants, yet testing explanations for these behaviors has been difficult. Callitrichids present a unique opportunity to delineate between hypotheses, since unlike most species, male caretakers form closer social bonds with infants than mothers. Callitrichids are also known to commit infanticide, leaving obvious wounds that may enable them to more readily recognize death. We present: (1) a case study of a wild common marmoset (Callithrix jacchus) group responding to an infant’s natural death, and (2) a review of published infant deaths across callitrichids (N = 16), testing for trends in the sex of reacting individuals and cause of death. In our case study, several group members frequently interacted with the dead infant, attempting to carry it. However, the strongest response was from a male that remained with the corpse for ~ 3 h, despite his group leaving the area. Across callitrichid species, corpse interactions were significantly sex-biased: 100% (N = 6) of accidental deaths involved corpse interaction by males (p = 0.007), compared to 60% (N = 3 of 5) by females (p = 0.095). Cause of death also played a significant role, with individuals attempting to carry dead infants in 100% (N = 6) of accidental deaths, but only 11.1% (N = 1 of 9) of infanticides (p = 0.001). Although the available literature is small and potentially subject to publication biases, these data support the idea that visually obvious wounds may influence callitrichids’ perception of dead conspecifics. Additionally, male-biased patterns of corpse interaction in callitrichids indicate that social bonds likely shape reactions to the dead, in addition to kinship. While published data on primate thanatology are limited, this study demonstrates quantitative approaches that can provide empirical insights into primates’ responses to dead conspecifics.

An ossified or ‘fused’ mandibular symphysis characterizes the origins of the Anthropoidea, a primate suborder that includes humans. Longstanding debate about the adaptive significance of variation in this jaw joint centers on whether a bony symphysis is stronger than an unfused one spanned by cartilage and ligaments. To provide essential information regarding mechanical performance, intact adult symphyses from representative primates and scandentians were loaded ex vivo to simulate stresses during biting and chewing – dorsoventral (DV) shear and lateral transverse bending (‘wishboning’). The anthropoid symphysis requires significantly more force to induce structural failure vs. strepsirrhines and scandentians with unfused joints. In wishboning, symphyseal breakage always occurs at the midline in taxa with unfused conditions, further indicating that an ossified symphysis is stronger than an unfused joint. Greater non-midline fractures among anthropoids suggest that fusion imposes unique constraints on masticatory function elsewhere along the mandible, a phenomenon likely to characterize the evolution of fusion and jaw form throughout Mammalia.

Objectives: Variation in primate masticatory form and function has been extensively researched through both morphological and experimental studies. As a result, symphyseal fusion in different primate clades has been linked to either the recruitment of vertically directed balancing‐side muscle force, the timing and recruitment of transversely directed forces, or both. This study investigates the relationship between jaw muscle activity patterns and morphology in extant primates to make inferences about masticatory function in extinct primates, with implications for understanding the evolution of symphyseal fusion.Materials and methods: Three‐dimensional mandibular landmark data were collected for 31 extant primates and nine fossil anthropoids and subfossil lemur species. Published electromyography (EMG) data were available for nine of the extant primate species. Partial least squares analysis and phylogenetic partial least squares analysis were used to identify relationships between EMG and jaw shape data and evaluate variation in jaw morphology.Results: Primates with partial and complete symphyseal fusion exhibit shape‐function patterns associated with the wishboning motor pattern and loading regime, in contrast to shape‐function patterns of primates with unfused jaws. All fossil primates examined (except Apidium) exhibit jaw morphologies suggestive of the wishboning motor pattern demonstrated in living anthropoids and indriids.Discussion: Partial fusion in Catopithecus, similar to indriids and some subfossil lemurs, may be sufficient to resist, or transfer, some amounts of transversely directed balancing‐side muscle force at the symphysis, representing a transition to greater reliance on transverse jaw movement during mastication. Furthermore, possible functional convergences in physiological patterns during chewing (i.e., Archaeolemur) are identified.

Objectives: This study seeks to determine if (a) consumption of hard food items or a mixture of food items leads to the formation of premolar or molar microwear in laboratory capuchin monkeys (Sapajus apella) in one feeding session and (b) rates of microwear formation are associated with the number of food items consumed.Materials and methods: Five adult male capuchins were used in two experiments, one where they were fed unshelled Brazil nuts, and the other where they were fed a mixture of food items. Dental impressions were taken before and after each feeding session. Epoxy casts made from those impressions then were used in SEM analyses of rates of microwear formation. Upper and lower premolars and molars were analyzed. Qualitative comparisons were made and Spearman's rank‐order correlations used to examine the relationship between rates of microwear formation and number of Brazil nuts consumed.Results: Premolars and molars generally showed new microwear in the form of pits and scratches. However, the incidence of those features was low (0–6%). Rates of microwear formation were highest during the consumption of Brazil nuts.Discussion: Variations in the rate of microwear formation on the premolars likely reflected patterns of ingestion whereas consistency in the rate of microwear on the molars likely reflected patterns of chewing. While dental microwear formation seemed to be correlated with the number of hard objects consumed, rates did differ between individuals. Differences in results between the two experiments demonstrate some of the limitations in our knowledge of dental microwear formation.

Previous descriptive work on deciduous dentition of primates has focused disproportionately on great apes and humans. To address this bias in the literature, we studied 131 subadult nonhominoid specimens (including 110 newborns) describing deciduous tooth morphology and assessing maximum hydroxyapatite density (MHD). All specimens were CT scanned at 70 kVp and reconstructed at 20.5–39 μm voxels. Grayscale intensity from scans was converted to hydroxyapatite (HA) density (mg HA/cm³) using a linear conversion of grayscale values to calibration standards of known HA density (R² = .99). Using Amira software, mineralized dental tissues were captured by segmenting the tooth cusps first and then capturing the remainder of the teeth at descending thresholds of gray levels. We assessed the relationship of MHD of selected teeth to cranial length using Pearson correlation coefficients. In monkeys, anterior teeth are more mineralized than postcanine teeth. In tarsiers and most lemurs and lorises, postcanine teeth are the most highly mineralized. This suggests that monkeys have a more prolonged process of dental mineralization that begins with incisors and canines, while mineralization of postcanine teeth is delayed. This may in part be a result of relatively late weaning in most anthropoid primates. Results also reveal that in lemurs and lorises, MHD of the mandibular first permanent molar (M₁) negatively correlates with cranial length. In contrast, the MHD of M₁ positively correlates with cranial length in monkeys. This supports the hypothesis that natural selection acts independently on dental growth as opposed to mineralization and indicates clear phylogenetic differences among primates.