Rotation of the otic capsule in bipedal rats

Skull Shape Diversity in Pet Rabbits and the Applicability of Anatomical Reference Lines for Objective Interpretation of Dental Disease

Acquired dental problems are among the most frequently encountered diseases in pet rabbits. However, early symptoms are often overlooked because the affected animals first appear completely asymptomatic. Alterations from anatomical reference lines according to Böhmer and Crossley applied to standard skull X-ray images, have been shown to be indicative of tooth health problems in pet rabbits. Despite its proven usefulness, there are exceptions in which the anatomical reference lines appear not to be suitable for application. We addressed this issue by quantifying the cranial morphology of a large data set of pet rabbit patients (N = 80). The results of the morphometric analyses revealed considerable diversity in skull shape among the typical pet rabbits, but variance in only a few parameters influences the applicability of the anatomical reference lines. The most substantial parameter is the palatal angle. Specimens in which the anatomical reference lines could not be applied, have a rather large angle between the skull base and the palatal bone. We recommend to measure the palatal angle before applying the anatomical reference lines for objective interpretation of dental disease. Pet rabbits with a palatal angle larger than 18.8° are not strictly suitable for the successful application of the anatomical reference lines.

The impact of bipedal mechanical loading history on longitudinal long bone growth

Longitudinal bone growth is accomplished through a process where proliferating chondrocytes produce cartilage in the growth plate, which ultimately ossifies. Environmental influences, like mechanical loading, can moderate the growth of this cartilage, which can alter bone length. However, little is known about how specific behaviors like bipedalism, which is characterized by a shift in body mass (mechanical load), to the lower limbs, may impact bone growth. This study uses an experimental approach to induce bipedal behaviors in a rodent model (Rattus norvegicus) over a 12-week period using a treadmill-mounted harness system to test how rat hindlimbs respond to the following loading conditions: 1) fully loaded bipedal walking, 2) partially loaded bipedal walking, 3) standing, 4) quadrupedal walking, and 5) no exercise control. These experimental conditions test whether mechanical loading from 1) locomotor or postural behaviors, and 2) a change in the magnitude of load can moderate longitudinal bone growth in the femur and tibia, relative to controls. The results demonstrate that fully loaded bipedal walking and bipedal standing groups showed significant differences in the percentage change in length for the tibia and femur. When comparing the change from baseline, which control for body mass, all bipedal groups showed significant differences in tibia length compared to control groups. However, there were no absolute differences in bone length, which suggests that mechanical loads from bipedal behaviors may instead be moderating changes in growth velocity. Implications for the relationship between bipedal behaviors and longitudinal bone growth are discussed.

Response of the Axial Skeleton to Bipedal Loading Behaviors in an Experimental Animal Model

Many derived aspects of modern human axial skeletal morphology reflect our reliance on obligate bipedal locomotion. Insight into the adaptive significance of features, particularly in the spine, has been gained through experimental studies that induce bipedal standing or walking in quadrupedal mammals. Using an experimental animal model (Rattus norvegicus), the present study builds on earlier work by incorporating additional metrics of the cranium, employing quantitative methods established in the paleoanthropological literature, and exploring how variation in mechanical loading regimes impacts axial anatomy. Rats were assigned to one of five experimental groups, including "fully loaded bipedal walking," "partially loaded bipedal walking," "standing bipedally," "quadrupedal walking," and "no exercise control," and engaged in the behavior over 12-weeks. From μCT data obtained at the beginning and end of the experiment, we measured foramen magnum position and orientation, lumbar vertebral body wedging, cranial surface area of the lumbar and first sacral vertebral bodies, and sacral mediolateral width. Results demonstrate that bipedal rodents generally have more anteriorly positioned foramina magna, more dorsally wedged lumbar vertebrae, greater articular surface areas of lumbar and first sacral vertebral bodies, and sacra that exhibit greater mediolateral widths, compared to quadrupedal rodents. We further document variation among bipedal loading behavior groups (e.g., bipedal standing vs. walking). Our experimental animal model reveals how loading behaviors and adaptations may be specifically linked, and implicates a potential role for developmental plasticity in the evolutionary acquisition of bipedal adaptations in the hominin lineage. Anat Rec, 2018. © 2018 American Association for Anatomy.

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.

Ardipithecus ramidus and the evolution of the human cranial base

The early Pliocene African hominoid Ardipithecus ramidus was diagnosed as a having a unique phylogenetic relationship with the Australopithecus + Homo clade based on nonhoning canine teeth, a foreshortened cranial base, and postcranial characters related to facultative bipedality. However, pedal and pelvic traits indicating substantial arboreality have raised arguments that this taxon may instead be an example of parallel evolution of human-like traits among apes around the time of the chimpanzee-human split. Here we investigated the basicranial morphology of Ar. ramidus for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus. Besides a relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element. These traits reflect a relative broadening of the central basicranium, a derived condition associated with changes in tympanic shape and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these human-like modifications. We used the preserved morphology of ARA-VP 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium, as in Australopithecus and Homo. Reorganization of the central cranial base is among the earliest morphological markers of the Ardipithecus + Australopithecus + Homo clade.

Evolutionary Developmental Biology and Human Language Evolution: Constraints on Adaptation

A tension has long existed between those biologists who emphasize the importance of adaptation by natural selection and those who highlight the role of phylogenetic and developmental constraints on organismal form and function. This contrast has been particularly noticeable in recent debates concerning the evolution of human language. Darwin himself acknowledged the existence and importance of both of these, and a long line of biologists have followed him in seeing, in the concept of "descent with modification", a framework naturally able to incorporate both adaptation and constraint. Today, the integrated perspective of modern evolutionary developmental biology ("evo-devo") allows a more subtle and pluralistic approach to these traditional questions, and has provided several examples where the traditional notion of "constraint" can be cashed out in specific, mechanistic terms. This integrated viewpoint is particularly relevant to the evolution of the multiple mechanisms underlying human language, because of the short time available for novel aspects of these mechanisms to evolve and be optimized. Comparative data indicate that many cognitive aspects of human language predate humans, suggesting that pre-adaptation and exaptation have played important roles in language evolution. Thus, substantial components of what many linguists call "Universal Grammar" predate language itself. However, at least some of these older mechanisms have been combined in ways that generate true novelty. I suggest that we can insightfully exploit major steps forward in our understanding of evolution and development, to gain a richer understanding of the principles that underlie human language evolution.

Variation of functional cranial components in six Anthropoidea species

Sixty male crania from three Platyrrhini and three Catarrhini genera were measured by means of the craniofunctional method. The aim was to analyze functional components of the skull and relate their function and the degree of encephalization to life history variables. We recognized two major and eight minor functional components. The objectives were to test (1) if within-taxa (Platyrrhini or Catarrhini) and/or between-taxa (Platyrrhini and Catarrhini) comparisons showed minor-component differentiation; and (2) if encephalization affects both primate groups differently. After standardization by size and scaling, 15 possible within-taxa and between-taxa comparisons were made. We found a strong phylogenetic signal, i.e., cranial differences were not randomly distributed, with the between-taxa variation being greater than within-taxa. Both hypotheses tested were accepted since: (1) There was no random variation between functional cranial components. They followed definite patterns for ancestral and derived traits. (2) Encephalization was present in all scaled comparisons, with Platyrrhini showing a higher degree of encephalization than Catarrhini. We conclude that major and minor craniofunctional components should be considered as correlated traits related to life history, because we found different patterns between platyrrhines and catarrhines, and within species of both taxa.

East–West cranial differentiation in pre-Columbian human populations of South America

South Amerindians are frequently thought of as a rather biologically homogeneous megapopulation. However, when native South Americans are assessed by information coming from DNA variability analysis, they resolve into two, major distinct entities of Eastern and Western zones. The purpose of this study is to investigate if the same dual pattern emerges from craniometric data. We approached this question by means of functional craniometric variables. We found strong evidence that Westerners and Easterners constitute two distinct and independent microevolutionary universes when cranial morphology is assessed. The existence of a third universe, Northwest, cannot be completely ruled out, but needs further investigation. We also discovered that Westerners and Easterners present similar degrees of internal variation, contrary to the findings of geneticists and molecular biologists. Palaeoamericans seem to be more similar to Easterners than to Westerners and North-Westerners. Our results suggest that this East-West cranial differentiation is more probably the result of differential rates of genetic drift and gene flow acting on each side of the Cordillera. However, different intensities of gene flow between Palaeoamericans and Amerindians in the highlands and in the lowlands cannot be completely dismissed as a possible explanation for the differentiation found.

Orthocephalization in the postweaning squirrel monkey

Twenty male squirrel monkeys (Saimiri sciureus boliviensis) raised in captivity were allotted to one of the following groups: weanling control (C6) sampled at 6 months of age; young control (C24) fed ad libitum on a control diet and killed at 24 months of age; and malnourished (M24) fed ad libitum on a low-protein diet and sampled at 24 months of age. Cranial points and the lateral semicircular canals were marked. On each skull, a strict lateral teleradiograph was taken, and the lengths of the midsagittal chords and their angles with respect to the vestibular line were measured. Age changed the lengths in about 70% of the chords and more than 50% of the angles. Malnutrition arrested about 50% of the lengths, but the angles were practically not affected. It is concluded that the postweaning Saimiri sciureus undergoes orthocephalization according to a general pattern already observed in rodents and suggested for pongids. Postweaning malnutrition affected growth in size but not shape changes related to the orthocephalization of the Saimiri skull.

Effect of masticatory muscle fatigue on cranio-vertical head posture and rest position of the mandible

The aim of this study was to investigate whether induced fatigue of the masticatory muscles had any influence on the head posture, and whether this influence is related to the rest position and the movement characteristics of the mandible. The sample consisted of 13 female individuals, aged 23-34 yr. For the evaluation of possible changes in the natural cranio-vertical head posture, standardized facial profile photographs were used. Photographs were also used for the study of the facial characteristics. The freeway space and the opening and closing velocity of the mandibular displacement, as well as the duration of the masticatory cycles, were monitored with an optoelectronic method. A controlled dynamic fatigue was induced by a specially constructed spring-loaded device placed in the premolar region. No significant changes in the mean cranio-vertical postural position of the head were found during the various recording stages, while the freeway space was found to increase significantly after the fatigue test. No significant differences were observed concerning the average values of the mandibular movement characteristics. The analysis of the association between the individual changes showed an increase in the freeway space after the fatigue test in the subjects which exhibited an increase in the duration of the masticatory cycle in that period. No significant associations could be found between the changes in the head posture and the mandibular movement characteristics. Also, no significant correlation could be found between the facial type of the subjects and the variables studied.

Drift barriers in the postcartilaginous development of the mammalian otic capsule

Postcartilaginous development of the otic capsule was studied in undecalcified histological material from rabbits following sequential fluorochromic time labeling of mineralizing tissues, using combined microradiography, fluorescence microscopy and osteoid staining. Early fetal bone formation was monitored by labelling of the experimental animals in utero. In contrast to the normal pattern of drift movements displayed by long bones and extracapsular cranial bone, capsular bone inside a zone immediately surrounding the perilymphatic space developed as a separate functional unit in which growth and modeling was absent. Bone tissue behavior appeared to depend on its spatial relation to the membranous labyrinth rather than histological characteristics. These findings suggest the role of inner ear tissues as a functional matrix in control of capsular bone dynamics beyond fetal life.

Changes in skull components of the squirrel monkey evoked by growth and nutrition: an experimental study

Twenty weanling 6-month-old male squirrel monkeys were allotted to the following treatments: 1) first control animals were killed at weaning; 2) second control animals were killed when 24 months old; and 3) malnourished animals were fed on a low-protein diet and killed at age 24 months. Lateral and vertical teleradiographies were taken. Growth of the neurocranial and splanchnocranial components were measured by volumetric (size estimators) and morphometric (shape estimators) indices. All facial components grew. The neurocranial components showed a heterogeneous behavior: The anteroneural component remained stable, and the increase of the midneural component was compensated by a decrease in the posteroneural component. Malnutrition affected the growths of 1) the craniofacial complex, 2) the splanchnocranium, and 3) the respiratory and midneural components. Growth influenced skull shape through 1) increases of the splanchnocranium and the midneural component relative to the neurocranium; 2) decreases of the masticatory and optic components relative to the splanchnocranium, and 3) decreases of the anteroneural and posteroneural components relative to the neurocranium. Malnutrition influenced skull shape through the relationship between the anteroneural component and the neurocranium. These results confirmed the existence of functional interrelationships among the cranial components. A new approach to craniological studies is suggested.

An assessment of posture in bipedal rats

Alteration in posture was examined following foreleg amputation in rats. After the surgical induction of bipedal stance, a detailed, quantified, and longitudinal assessment of rat posture and the amount of time spent in the upright stance was undertaken. An experimental group of rats had both forelimbs amputated. A group of unaltered quadrupedal rats served as controls. A standardized series of photographs of altered and control rats was taken. Postural differences between the two groups of rats were analyzed through photographic analysis of their profiles in upright and in crouched positions. Analysis of the photographs showed that the percentage of rats assuming upright posture did not differ between the experimental and control groups. Foreleg amputation forced the rats to adopt an altered posture, but did not result in habitual assumption of upright stance to a greater degree than that of controls. Adaptation to bipedalism was marked by a tucking of the hindlimbs under the body, and a flexure of the vertebral column and the head.

The role of function in the development of human craniofacial form--a perspective

As an anatomical region the head combines great diversity of function with close integration of structure. Consequently no structural component has autonomy of form. There is a sequence of maturation of functions and their related structural components, and in this sequence the nervous system and its supportive structures mature first. The nasal airway matures next in response to increasing body mass, and the masticatory system constitutes the last major functional system to reach maturity. The later the maturation of the function, the greater is the requirement for its related morphology to adapt to preceding skeletal templates. These matters of developmental sequence, and extrinsic as well as intrinsic craniofacial functions, are paramount considerations in interpreting the form of any component of head anatomy.

Skeletal changes in the hindlimbs of bipedal rats

Experimental bipedalism in rats has been proposed as a model for studying evolutionary and orthopedic change. This study quantified the skeletal changes which occur in the hindlimbs of these bipedal rats. Bipedalism was produced in 16 male rats by forelimb amputation during the neonatal period. The rats were sacrificed at maturity and the femurs and tibia-fibulas from randomly selected hindlimbs were weighed and measured. Out of a total of 25 parameters, only one, anteroposterior diameter of the proximal femur, was found to differ statistically between the bipedal and control groups. No changes were found in bone length, weight, or curvature. The results of other bipedal studies were reviewed, in light of these findings; and it is concluded that rats do not achieve true bipedalism, or increased hindlimb loading.

Miniaturization and its effects on cranial morphology in plethodontid salamanders, genus Thorius (Amphibia, Plethodontidae): II. The fate of the brain and sense organs and their role in skull morphogenesis and evolution

Relative size and arrangement of the brain and paired sense organs are examined in three species of Thorius, a genus of minute, terrestrial salamanders that are among the smallest extant tailed tetrapods. Analogous measurements of representative species of three related genera of larger tropical (Pseudoeurycea, Chiropterotriton) and temperate (Plethodon) salamanders are used to identify changes in gross morphology of the brain and sense organs that have accompanied the evolution of decreased head size in Thorius and their relation to associated changes in skull morphology. In adult Thorius, relative size (area measured in frontal plane, and length) of the eyes, otic capsules, and brain each is greater than in adults of all of the larger genera; relative size of the nasal capsules is unchanged or slightly smaller. Interspecific scaling phenomena--negative allometry of otic capsule, eye and brain size, isometry or slight positive allometry of nasal capsule size, all with respect to skull length--also are characteristic of intraspecific (ontogenetic) comparisons in both T. narisovalis and Pseudoeurycea goebeli. Predominance of the brain and eyes in Thorius results in greater contact and overlap among these structures and the nasal capsules in the anterior portion of the head. This is associated with anterior displacement of both the eyes and nasal capsules, which now protrude anterior to the skull proper; a change in eye shape; and medial deformation of anterior braincase walls. Posteriorly, predominance of the otic capsules has effected a reorientation of the jaw suspensorium to a fully vertical position that is correlated with the novel presence of a posteriorly directed squamosal process and shift in origin of the quadropectoralis muscle. Many of these changes in cranial morphology may be explained simply as results of mechanical (physical) interactions among the skeletal, nervous, and sensory components during head development at reduced size. This provides further evidence of the role of nervous, sensory, and other "soft" tissues in cranial skeletal morphogenesis, and reinforces the need to consider these tissues in analyses of skull evolution.

Adaptation of head posture in response to relocating the center of mass: a pilot study

Relationships have been established between head posture and craniofacial morphology in both adults and children. The determinants of head posture are not yet understood. Does posture dictate form, or vice versa? This pilot study investigates the short-term effect of artificial forward relocation of the center of the mass of the head on postural adaptation in the sagittal plane. The observed responses are not systematically uniform and, in fact, are characterized by a lack of consistency and a high degree of intra- and interindividual variability.

Spatial position of the lateral semicircular canal in 14--60-day-old rat heads

During growth the rat skull straightens considerably due to flexions between skull parts and individual bones. This straightening is defined as orthocephalization. Orthocephalization might also be caused by a change in head posture during growth, i.e. it may be "apparent". In order to study the significance of this phenomenon, radiographs of dried skulls of male rats, 14, 30 and 60 d of age were subjected to angular measurements between the plane of the lateral semicircular canal and the plane of foramen magnum and of the basisphenoid bone, respectively. When the plane of the lateral semicircular canal is placed horizontally, the head is claimed to be in natural position. No changes in the angulation between the lateral semicircular canal and the basisphenoid bone could be revealed, whereas the foramen magnum was shown to rotate upwards and backwards in relation to the plane of the canal. This implies that the orthocephalization which takes place between 14 and 60 d is caused only by angular changes, and that no apparent changes occur due to changes in head posture.

Head posture and craniofacial morphology

The associations between craniofacial morphology and the posture of the head and the cervical column were examined in a sample of 120 Danish male students aged 22-30 years. Two head positions were recorded on lateral cephalometric radiographs, one determined by the subject's own feeling of a natural head balance (self balance position), and the other by the subject looking straight into a mirror (mirror position). Craniofacial morphology was described by 42 linear and angular variables, and postural relationships by 18 angular variables. A comprehensive set of correlations was found between craniofacial morphology and head posture. The correlations were similar for both head positions investigated. Of the postural variables, the position of the head in relation to the cervical column showed the largest set of correlations with craniofacial morphology. Extension of the head in relation to the cervical column was found in connection with large anterior and small posterior facial heights, small antero-posterior craniofacial dimensions, large inclination of the mandible to the anterior cranial base and to the nasal plane, facial retrognathism, a large cranial base angle, and a small nasopharyngeal space. The possible role of functional factors in mediating the relationship between morphology and posture was discussed.