A novel herpesvirus in a white stork associated with splenic and hepatic necrosis.

We identified a novel herpesvirus in a captive juvenile white stork (Ciconia ciconia) that experienced progressive weight loss followed by death. Histologic findings included severe splenic and hepatic necrosis, and intranuclear inclusion bodies in hepatocytes and unidentified splenic cells. The virus was amplified, sequenced, and subsequently accessioned as Ciconiid alphaherpesvirus 1. Phylogenetic analysis was performed and revealed that this virus is more closely related to mammalian herpesviruses than those within the genus Mardivirus. Additional sequence of viruses in this area may elucidate the ancestral virus that jumped from reptilian to mammalian hosts.

Comparative dental anatomy in newborn primates: Cusp mineralization.

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.

Nasal airflow in the pygmy slow loris (Nycticebus pygmaeus) based on a combined histological, computed tomographic and computational fluid dynamics methodology.

'Macrosmatic' mammals have dedicated olfactory regions within their nasal cavity and segregated airstreams for olfaction and respiratory air-conditioning. Here, we examined the 3D distribution of olfactory surface area (SA) and nasal airflow patterns in the pygmy slow loris (Nycticebus pygmaeus), a primate with primitive nasal cavities, except for enlarged eyes that converge upon the posterodorsal nasal region. Using the head of an adult loris cadaver, we co-registered micro-computed tomography (CT) slices and histology sections to create a 3D reconstruction of the olfactory mucosa distribution. Histological sections were used to measure olfactory surface area and to annotate CT reconstructions. The loris has a complex olfactory recess (∼19% of total nasal SA) with multiple olfactory turbinals. However, the first ethmoturbinal has a rostral projection that extends far anterior to the olfactory recess, lined by ∼90% non-olfactory epithelium. Only one (of three) frontoturbinals bears olfactory mucosa. Computational fluid dynamics simulations of nasal airflow and odorant deposition revealed that there is some segregation of respiratory and olfactory flow in the loris nose, but that it is not as distinct as in well-studied 'macrosmats' (e.g. the dog). In the loris, airflow is segregated medially and laterally to vertically elongated, plate-like first ethmoturbinals. Thus, lorises may be said to have certain macrosmatic anatomical characteristics (e.g. olfactory recess), but not segregated nasal airflow patterns that are optimized for olfaction, as in canids. These results imply that a binary 'microsmatic/macrosmatic' dichotomy does not exist. Rather, mammals appear to exhibit complex trends with respect to specialization of the turbinals and recesses.

Relative tooth size at birth in primates: Life history correlates.

OBJECTIVES: Dental eruption schedules have been closely linked to life history variables. Here we examine a sample of 50 perinatal primates (28 species) to determine whether life history traits correlate with relative tooth size at birth. MATERIALS AND METHODS: Newborn primates were studied using serial histological sectioning. Volumes of deciduous premolars (dp2 -dp4 ), replacement teeth (if any), and permanent molars (M1-2/3 ) of the upper jaw were measured and residuals from cranial length were calculated with least squares regressions to obtain relative dental volumes (RDVs). RESULTS: Relative dental volumes of deciduous or permanent teeth have an unclear relationship with relative neonatal mass in all primates. Relative palatal length (RPL), used as a proxy for midfacial size, is significantly, positively correlated with larger deciduous and permanent postcanine teeth. However, when strepsirrhines alone are examined, larger RPL is correlated with smaller RDV of permanent teeth. In the full sample, RDVs of deciduous premolars are significantly negatively correlated with relative gestation length (RGL), but have no clear relationship with relative weaning age. RDVs of molars lack a clear relationship with RGL; later weaning is associated with larger molar RDV, although correlations are not significant. When strepsirrhines alone are analyzed, clearer trends are present: longer gestations or later weaning are associated with smaller deciduous and larger permanent postcanine teeth (only gestational length correlations are significant). DISCUSSION: Our results indicate a broad trend that primates with the shortest RGLs precociously develop deciduous teeth; in strepsirrhines, the opposite trend is seen for permanent molars. Anthropoids delay growth of permanent teeth, while strepsirrhines with short RGLs are growing replacement teeth concurrently. A comparison of neonatal volumes with existing information on extent of cusp mineralization indicates that growth of tooth germs and cusp mineralization may be selected for independently.

Ontogeny and Microanatomy of the Nasal Turbinals in Lemuriformes.

The nasal cavity of strepsirrhine primates (lemurs and lorises) has the most primitive arrangement of extant primates. In nocturnal species, the numerous turbinals of the ethmoid bear a large surface area of olfactory mucosa (OM). In this study, we examine turbinal development in four genera of diurnal or cathemeral lemuriformes. In addition, we examined an age series of each genus to detect whether structures bearing OM as opposed to respiratory mucosa (RM) develop differently, as has been observed in nocturnal strepsirrhines. In adults, the maxilloturbinal is covered by highly vascular respiratory mucosa throughout its entire length, with large sinusoidal vessels in the lamina propria; any parts of other turbinals that closely borders the maxilloturbinal has a similar mucosa. Posteriorly, the most vascular RM is restricted in the nasopharyngeal duct, which becomes partitioned from the dorsal olfactory region. A comparison of newborns to adults reveals that the first ethmoturbinal increases more in length in the parts that are covered with RM than OM, which supports the idea that ethmoturbinals can specialize in more than one function. Finally, we observe that the regions of turbinals that are ultimately covered with RM develop more accessory lamellae or additional surface area of existing scrolls compared to the regions covered with OM. Because such outgrowths of bone develop postnatally and without cartilaginous precursors, we hypothesize that the complexity of olfactory lamellae within the ethmoturbinal complex is primarily established at birth, while respiratory lamellae become elaborated due to the epigenetic influence of respiratory physiology. Anat Rec, 299:1492-1510, 2016. © 2016 Wiley Periodicals, Inc.

A RETROSPECTIVE STUDY OF PATHOLOGIC FINDINGS IN CAPTIVE ROCK HYRAX (PROCAVIA CAPENSIS) IN THE UNITED STATES.

A retrospective examination of gross and histologic reports was performed to find common lesions in young and adult captive rock hyrax ( Procavia capensis ) from multiple zoo populations. One hundred and thirty-seven reports were analyzed from specimens that were submitted to Northwest ZooPath from 1997 to 2013. Histologic findings from necropsy and biopsy reports and causes of mortality only from necropsy reports were compiled to determine the most common findings. Within the study population, 41 (30%) were male, 62 (45%) were female, and the remainder (34, 25%) were of undetermined sex. Of the 111 necropsies, 87 (78%) died naturally, and 24 (22%) were euthanatized. There were 26 (19%) biopsies with no known status of the animal. The most frequent causes of death or reason for euthanasia were bacterial septicemia (n = 29, 21%) and degenerative cardiomyopathy (n = 29, 21%). The other most prevalent lesions were hemosiderosis (n = 55, 40%), pancreatic islet and interstitial fibrosis (n = 36, 26%), pneumonia of undetermined cause (n = 26, 19%), enteritis/colitis (n = 24, 18%), and renal tubular necrosis (n = 20, 15%). In many animals of this study population (n = 115, 84%), multiple lesions affecting multiple organs were found.

Dental maturation, eruption, and gingival emergence in the upper jaw of newborn primates.

In this report we provide data on dental eruption and tooth germ maturation at birth in a large sample constituting the broadest array of non-human primates studied to date. Over 100 perinatal primates, obtained from natural captive deaths, were screened for characteristics indicating premature birth, and were subsequently studied using a combination of histology and micro-CT. Results reveal one probable unifying characteristic of living primates: relatively advanced maturation of deciduous teeth and M1 at birth. Beyond this, there is great diversity in the status of tooth eruption and maturation (dental stage) in the newborn primate. Contrasting strategies in producing a masticatory battery are already apparent at birth in strepsirrhines and anthropoids. Results show that dental maturation and eruption schedules are potentially independently co-opted as different strategies for attaining feeding independence. The most common strategy in strepsirrhines is accelerating eruption and the maturation of the permanent dentition, including replacement teeth. Anthropoids, with only few exceptions, accelerate mineralization of the deciduous teeth, while delaying development of all permanent teeth except M1. These results also show that no living primate resembles the altricial tree shrew (Tupaia) in dental development. Our preliminary observations suggest that ecological explanations, such as diet, provide an explanation for certain morphological variations at birth. These results confirm previous work on perinatal indriids indicating that these and other primates telegraph their feeding adaptations well before masticatory anatomy is functional. Quantitative analyses are required to decipher specific dietary and other influences on dental size and maturation in the newborn primate.

The vomeronasal organ of Lemur catta.

The vomeronasal organ (VNO), also known as the Jacobson's organ, is a bilateral chemosensory organ found at the base of the nasal cavity specialized for the detection of higher-molecular weight (non-volatile) chemostimuli. It has been linked to pheromone detection. The VNO has been well studied in nocturnal lemurs and lorises, but poorly studied in diurnal/cathemeral species despite the large repertoire of olfactory behaviors noted in species such as Lemur catta. Here, the VNO and associated structures were studied microanatomically in one adult female and one adult male L. catta. Traditional and immunohistochemical procedures demonstrate the VNO epithelium consists of multiple rows of sensory neurons. Immunoreactivity to Growth-associated protein 43 (GAP43) indicates the VNO is postnatally neurogenic. In volume, the VNO neuroepithelium scales similarly to palatal length compared to nocturnal strepsirrhines. Numerous taste buds present at the oral opening to the nasopalatine duct, with which the VNO communicates, provide an additional (or alternative) explanation for the flehmen behavior that has been observed in this species. The VNO of L. catta is shown to be microanatomically comparable to that of nocturnal strepsirrhines. Like nocturnal strepsirrhines, the VNO of L. catta may be functional in the reception of high-molecular weight secretions.

Nasal morphometry in marmosets: loss and redistribution of olfactory surface area.

The two major groups of primates differ in internal nasal anatomy. Strepsirrhines (e.g., lemurs) have more numerous turbinals and recesses compared with haplorhines (e.g., monkeys). Since detailed quantitative comparisons of nasal surface area (SA) have not been made, we measured mucosa in serially sectioned monkeys (Callithrix jacchus, Cebuella pygmaea). Data were compared with previously published findings on the mouse lemur, Microcebus murinus. The nasal airways were digitally reconstructed using computed tomography scanned heads of Cebuella and Microcebus. In addition, morphometric and functional analyses were carried out using segmented photographs of the histological sections of Cebuella and Microcebus. The SA of the ethmoturbinal complex is about half as large in marmosets compared with Microcebus, and is covered with less olfactory mucosa (18%-24% in marmosets, compared with ∼ 50% in Microcebus). Whereas the ethmoturbinal complex of Microcebus bears half of the total olfactory mucosa in the nasal airway, most (∼ 80%) olfactory mucosa is distributed on other surfaces in the marmosets (e.g., nasal septum). A comparison to previously published data suggests all primate species have less olfactory surface area (OSA) compared with other similar-sized mammals, but this is especially true of marmosets. Taken together, these findings support the hypothesis that there is a reduced OSA in at least some haplorhines, and this can be linked to diminished posterosuperior dimensions of the nasal fossae. However, haplorhines may have minimized their olfactory loss by redistributing olfactory mucosa on non-turbinal surfaces. Our findings also imply that airflow patterns in the olfactory region differ among primates.

Mapping bone cell distributions to assess ontogenetic origin of primate midfacial form.

Midfacial reduction in primates has been explained as a byproduct of other growth patterns, especially the convergent orbits. This is at once an evolutionary and developmental explanation for relatively short snouts in most modern primates. Here, we use histological sections of perinatal nonhuman primates (tamarin, tarsier, loris) to investigate how orbital morphology emerges during ontogeny in selected primates compared to another euarchontan (Tupaia glis). We annotated serial histological sections for location of osteoclasts or osteoblasts, and used these to create three-dimensional "modeling maps" showing perinatal growth patterns of the facial skeleton. In addition, in one specimen we transferred annotations from histological sections to CT slices, to create a rotatable 3D volume that shows orbital modeling. Our findings suggest that growth in the competing orbital and neurocranial functional matrices differs among species, influencing modeling patterns. Distinctions among species are observed in the frontal bone, at a shared interface between the endocranial fossa and the orbit. The medial orbital wall is extensively resorptive in primates, whereas the medial orbit is generally depositional in Tupaia. As hypothesized, the orbital soft tissues encroach on available interorbital space. However, eye size cannot, by itself, explain the extent of reduction of the olfactory recess. In Loris, the posterior portion of medial orbit differed from the other primates. It showed evidence of outward drift where the olfactory bulb increased in cross-sectional area. We suggest the olfactory bulbs are significant to orbit position in strepsirrhines, influencing an expanded interorbital breadth at early stages of development.

Hematologic and plasma biochemical reference values of the horned guan, Oreophasis derbianus.

The horned guan (Oreophasis derbianus) is an endangered species with small and seriously fragmented wild populations. Breeding efforts during the last decade have maintained a slowly increasing captive population with the potential to play an important role in the recovery of the species. Clinical hematology and biochemistries are powerful tools to diagnose and monitor diseases in captive birds. Therefore, establishing hematologic and plasma biochemistry reference values will improve the medical management of this species. This study determines the reference values for 9 hematologic and 15 plasma biochemical variables for 27 male and 12 female, apparently healthy, captive horned guans from three institutions. Differences related to age, sex, and husbandry were identified but were usually small and clinically insignificant. These results should improve veterinary care of captive individuals of this species and provide comparative data for other species of cracids.

The cerebral cortex of the pygmy hippopotamus, Hexaprotodon liberiensis (Cetartiodactyla, Hippopotamidae): MRI, cytoarchitecture, and neuronal morphology.

The structure of the hippopotamus brain is virtually unknown because few studies have examined more than its external morphology. In view of their semiaquatic lifestyle and phylogenetic relatedness to cetaceans, the brain of hippopotamuses represents a unique opportunity for better understanding the selective pressures that have shaped the organization of the brain during the evolutionary process of adaptation to an aquatic environment. Here we examined the histology of the cerebral cortex of the pygmy hippopotamus (Hexaprotodon liberiensis) by means of Nissl, Golgi, and calretinin (CR) immunostaining, and provide a magnetic resonance imaging (MRI) structural and volumetric dataset of the anatomy of its brain. We calculated the corpus callosum area/brain mass ratio (CCA/BM), the gyrencephalic index (GI), the cerebellar quotient (CQ), and the cerebellar index (CI). Results indicate that the cortex of H. liberiensis shares one feature exclusively with cetaceans (the lack of layer IV across the entire cerebral cortex), other features exclusively with artiodactyls (e.g., the morphologiy of CR-immunoreactive multipolar neurons in deep cortical layers, gyrencephalic index values, hippocampus and cerebellum volumetrics), and others with at least some species of cetartiodactyls (e.g., the presence of a thick layer I, the pattern of distribution of CR-immunoreactive neurons, the presence of von Economo neurons, clustering of layer II in the occipital cortex). The present study thus provides a comprehensive dataset of the neuroanatomy of H. liberiensis that sets the ground for future comparative studies including the larger Hippopotamus amphibius.

Variable temporoinsular cortex neuroanatomy in primates suggests a bottleneck effect in eastern gorillas.

We describe an atypical neuroanatomical feature present in several primate species that involves a fusion between the temporal lobe (often including Heschl's gyrus in great apes) and the posterior dorsal insula, such that a portion of insular cortex forms an isolated pocket medial to the Sylvian fissure. We assessed the frequency of this fusion in 56 primate species (including apes, Old World monkeys, New World monkeys, and strepsirrhines) by using either magnetic resonance images or histological sections. A fusion between temporal cortex and posterior insula was present in 22 species (seven apes, two Old World monkeys, four New World monkeys, and nine strepsirrhines). The temporoinsular fusion was observed in most eastern gorilla (Gorilla beringei beringei and G. b. graueri) specimens (62% and 100% of cases, respectively) but was seen less frequently in other great apes and was never found in humans. We further explored the histology of this fusion in eastern gorillas by examining the cyto- and myeloarchitecture within this region and observed that the degree to which deep cortical layers and white matter are incorporated into the fusion varies among individuals within a species. We suggest that fusion between temporal and insular cortex is an example of a relatively rare neuroanatomical feature that has become more common in eastern gorillas, possibly as the result of a population bottleneck effect. Characterizing the phylogenetic distribution of this morphology highlights a derived feature of these great apes.

The vomeronasal complex of nocturnal strepsirhines and implications for the ancestral condition in primates.

This study investigates the vomeronasal organ in extant nocturnal strepsirhines as a model for ancestral primates. Cadaveric samples from 10 strepsirhine species, ranging from fetal to adult ages, were studied histologically. Dimensions of structures in the vomeronasal complex, such as the vomeronasal neuroepithelium (VNNE) and vomeronasal cartilage (VNC) were measured in serial sections and selected specimens were studied immunohistochemically to determine physiological aspects of the vomeronasal sensory neurons (VSNs). Osteological features corresponding to vomeronasal structures were studied histologically and related to 3-D CT reconstructions. The VNC consistently rests in a depression on the palatal portion of the maxilla, which we refer to as the vomeronasal groove (VNG). Most age comparisons indicate that in adults VNNE is about twice the length compared with perinatal animals. In VNNE volume, adults are 2- to 3-fold larger compared with perinatal specimens. Across ages, a strong linear relationship exists between VNNE dimensions and body length, mass, and midfacial length. Results indicate that the VNNE of nocturnal strepsirhines is neurogenic postnatally based on GAP43 expression. In addition, based on Olfactory Marker Protein expression, terminally differentiated VSNs are present in the VNNE. Therefore, nocturnal strepsirhines have basic similarities to rodents in growth and maturational characteristics of VSNs. These results indicate that a functional vomeronasal system is likely present in all nocturnal strepsirhines. Finally, given that osteological features such as the VNG are visible on midfacial bones, primate fossils can be assessed to determine whether primate ancestors possessed a vomeronasal complex morphologically similar to that of modern nocturnal strepsirhines.

Metabolic bone disease in juvenile koalas (Phascolartcos cinereus).

Due to climate restrictions in parts of North America and Europe, koalas (Phascolarctos cinereus) are housed indoors. Koala young (joeys) raised indoors are susceptible to the development of metabolic bone disease (MBD) due to a lack of exposure to natural ultraviolet light to themselves and their female parents (dams). In this retrospective study, radiographs from 27 koala joeys born at four zoos in North America and two zoos in Europe were evaluated for signs of MBD. Eight of the joeys were radiographically diagnosed with MBD and four additional joeys were considered suspect MBD cases; in two joeys absence or presence of MBD could not be determined. All joeys had mild to severe hip and shoulder dysplasia. There were significant associations between a lack of exposure to UV light and MBD development and between MBD and the degree of severity of hip and shoulder dysplasia. It is recommended to house breeding female koalas and their joeys outdoors whenever possible.

Microanatomical variation of the nasal capsular cartilage in newborn primates.

The breakdown of nasal capsule cartilage precedes secondary pneumatic expansion of the paranasal sinuses. Recent work indicates the nasal capsule of monkeys undergoes different ontogenetic transformations regionally (i.e., ossification, persistence as cartilage, or resorption). This study assesses nasal capsule morphology at the perinatal age in a taxonomically broad sample of non-human primates. Using traditional histochemical methods, osteopontin immunohistochemistry and tartrate-resistant acid phosphatase procedure, the cartilage of the lateral nasal wall (LNC) was studied. At birth, matrix properties differ between portions of the LNC that ultimately form elements of the ethmoid bone and regions of the LNC that have no postnatal (descendant) structure. The extent of cartilage that remains in the paranasal parts of the LNC varies among species. It is fragmented in species with the greatest extent of maxillary and/or frontal pneumatic expansion. Conversely, greater continuity of the LNC is noted in newborns of species that lack maxillary and/or frontal sinuses as adults. Chondroclasts occur adjacent to elements of the ethmoid bone, along the margin of the nasal tectum, and/or along islands of cartilage that bear no signs of ossification. Chondroclasts are prevalent along remnants of the paranasal LNC in tamarin species (Leontopithecus, Saguinus), which have extensive frontal and maxillary bone pneumatization. Taken together, the morphological observations indicate that the localized loss of cartilage might be considered a critical event at the onset of secondary pneumatization, facilitated by rapid recruitment of chondro-/osteoclasts, possibly occurring simultaneously in cartilage and bone.

The vomeronasal organ of New World monkeys (platyrrhini).

Although all platyrrhine primates possess a vomeronasal organ (VNO), few species have been studied in detail. Here, we revisit the microanatomy of the VNO and related features in serially sectioned samples from 41 platyrrhine cadavers (14 species) of mixed age. Procedures to identify terminally differentiated vomeronasal sensory neurons (VSNs) via immunolabeling of olfactory marker protein (OMP) were used on selected specimens. The VNO varies from an elongated epithelial tube (e.g., Ateles fusciceps) to a dorsoventrally expanded sac (e.g., Saguinus spp.). The cartilage that surrounds the VNO is J-shaped or U-shaped in most species, and articulates with a groove on the bony palate. Preliminary results indicate a significant correlation between the length of this groove and length of the VNO neuroepithelium, indicating this feature may serve as a skeletal correlate. The VNO neuroepithelium could be identified in all adult primates except Alouatta, in which poor preservation prevented determination. The VNO of Ateles, described in detail for the first time, had several rows of VSNs and nerves in the surrounding lamina propria. Patterns of OMP-reactivity in the VNO of perinatal platyrrhines indicate that few or no terminally differentiated VSNs are present at birth, thus supporting the hypothesis that some platyrrhines may have delayed maturation of the VNO. From a functional perspective, all platyrrhines studied possess structures required for chemoreception (VSNs, vomeronasal nerves). However, some microanatomical findings, such as limited reactivity to OMP in some species, indicate that some lineages of New World monkeys may have a reduced or vestigial vomeronasal system.

Neocortical neuron morphology in Afrotheria: comparing the rock hyrax with the African elephant.

The mammalian neocortex contains a great variety of neuronal types. In particular, recent studies have shown substantial morphological diversity among spiny projecting neurons in species that diverged close to the base of the mammalian radiation (e.g., monotremes, afrotherians, and xenarthrans). Here, we used a Golgi technique to examine different neuronal morphologies in an afrotherian species, the rock hyrax (Procavia capensis), and provide a comparison with the related African elephant (Loxodonta africana). Results showed that spiny neurons in the rock hyrax neocortex exhibit less morphological variation than in elephants, displaying a higher frequency of relatively "typical" pyramidal neurons. A quantitative comparison of rock hyrax pyramidal neuron morphology between frontal and visual areas, moreover, revealed greater spine density of neurons in frontal cortex, but no differences in other morphological aspects. Regional variations in pyramidal structure have also been observed in the African elephant, as well as a number of primate species.

Evolution of the muscles of facial expression in a monogamous ape: evaluating the relative influences of ecological and phylogenetic factors in hylobatids.

Facial expression is a communication mode produced by facial (mimetic) musculature. Hylobatids (gibbons and siamangs) have a poorly documented facial display repertoire and little is known about their facial musculature. These lesser apes represent an opportunity to test hypotheses related to the evolution of primate facial musculature as they are the only hominoid with a monogamous social structure, and thus live in very small groups. Primate species living in large groups with numerous social relationships, such as chimpanzees and rhesus macaques, have been shown to have a complex facial display repertoire and a high number of discrete facial muscles. The present study was designed to examine the relative influence of social structure and phylogeny on facial musculature evolution by comparing facial musculature complexity among hylobatids, chimpanzees, and rhesus macaques. Four faces were dissected from four hylobatid species. Morphology, attachments, three-dimensional relationships, and variation among specimens were noted and compared to rhesus macaques and chimpanzees. Microanatomical characteristics of the orbicularis oris muscle were also compared. Facial muscles of hylobatids were generally gracile and less complex than both the rhesus macaque and chimpanzee. Microanatomically, the orbicularis oris muscle of hylobatids was relatively loosely packed with muscle fibers. These results indicate that environmental and social factors may have been important in determining morphology and complexity of facial musculature in the less social hylobatids and that they may not have experienced as strong selection pressure for mimetic muscle complexity as other, more social primates.