Variability and constraint of vertebral formulae and proportions in colugos, tree shrews, and rodents, with special reference to vertebral modification by aerodynamic adaptation.

BACKGROUND: The aim of the present study is to provide the first large data set on vertebral formulae and proportions, and examine their relationship with different locomotive modes in colugos (Dermoptera), tree shrews (Scandentia), and rodents (Rodentia), which have been considered less variable because they were thought to have a plesiomorphic number of 19 thoracolumbar vertebrae. MATERIALS AND METHODS: The data included 33 colugos and 112 tree shrews, which are phylogenetically sister taxa, and 288 additional skeletons from 29 other mammalian species adapted to different locomotive modes, flying, gliding, arboreal, terrestrial, digging, and semi-aquatic habitats. RESULTS: The following results were obtained: (1) intra-/interspecies variability and geographical variation in thoracic, lumbar, and thoracolumbar counts were present in two gliding colugo species and 12 terrestrial/arboreal tree shrew species; (2) in our examined mammals, some aerodynamic mammals, such as colugos, southern flying squirrels, scaly-tailed squirrels, and bats, showed exceptionally high amounts of intraspecific variation of thoracic, lumbar, and thoracolumbar counts, and sugar gliders and some semi-aquatic rodents also showed some variation; (3) longer thoracic and shorter lumbar vertebrae were typically shared traits among the examined mammals, except for flying squirrels (Pteromyini) and scaly-tailed squirrels (Anomaluridae). CONCLUSIONS: Our study reveals that aerodynamic adaptation could potentially lead to strong selection and modification of vertebral formulae and/or proportions based on locomotive mode despite evolutionary and developmental constraints. (Folia Morphol 2018; 77, 1: 44-56) Background: The aim of the present study is to provide the first large data set on vertebral formulae and proportions, and examine their relationship with different locomotive modes in colugos (Dermoptera), tree shrews (Scandentia), and rodents (Rodentia), which have been considered less variable because they were thought to have a plesiomorphic number of 19 thoracolumbar vertebrae. MATERIALS AND METHODS: The data included 33 colugos and 112 tree shrews, which are phylogenetically sister taxa, and 288 additional skeletons from 29 other mammalian species adapted to different locomotive modes, flying, gliding, arboreal, terrestrial, digging, and semi-aquatic habitats. RESULTS: The following results were obtained: (1) intra-/interspecies variability and geographical variation in thoracic, lumbar, and thoracolumbar counts were present in two gliding colugo species and 12 terrestrial/arboreal tree shrew species; (2) in our examined mammals, some aerodynamic mammals, such as colugos, southern flying squirrels, scaly-tailed squirrels, and bats, showed exceptionally high amounts of intraspecific variation of thoracic, lumbar, and thoracolumbar counts, and sugar gliders and some semi-aquatic rodents also showed some variation; (3) longer thoracic and shorter lumbar vertebrae were typically shared traits among the examined mammals, except for flying squirrels (Pteromyini) and scaly-tailed squirrels (Anomaluridae). CONCLUSIONS: Our study reveals that aerodynamic adaptation could potentially lead to strong selection and modification of vertebral formulae and/or proportions based on locomotive mode despite evolutionary and developmental constraints. (Folia Morphol 2018; 77, 1: 44-56).

Evolutionary acceleration in the most endangered mammal of Canada: speciation and divergence in the Vancouver Island marmot (Rodentia, Sciuridae).

The Vancouver Island marmot is the most endangered mammal of Canada. Factors which have brought this population to the verge of extinction have not yet been fully elucidated, but the effects of deforestation and habitat fragmentation on survival rates, as well as those of variation in rainfall, temperature, snowpack depth and snowmelt strongly suggest that marmots on the island are struggling to keep pace with environmental changes. Genetic analyses, however, seem to indicate that the Vancouver Island marmot may merely represent a melanistic population of its parental species on the mainland. Were it not for its black pelage colour, it is unlikely that it would have attracted much attention as a conservation priority. Our study uses three-dimensional coordinates of cranial landmarks to further assess phenotypic differentiation of the Vancouver Island marmot. A pattern of strong interspecific divergence and low intraspecific variation was found which is consistent with aspects of drift-driven models of speciation. However, the magnitude of shape differences relative to the putatively neutral substitutions in synonymous sites of cytochrome b is too large for being compatible with a simple neutral model. A combination of bottlenecks and selective pressures due to natural and human-induced changes in the environment may offer a parsimonious explanation for the large phenotypic differentiation observed in the species. Our study exemplifies the usefulness of a multidisciplinary approach to the study of biological diversity for a better understanding of evolutionary models and to discover aspects of diversity that may be undetected by using only a few genetic markers to characterize population divergence and uniqueness.

Anatomy of the squirrel wrist: bones, ligaments, and muscles.

Anatomical differences among squirrels are usually most evident in the comparison of flying squirrels and nongliding squirrels. This is true of wrist anatomy, probably reflecting the specializations of flying squirrels for the extension of the wing tip and control of it during gliding. In the proximal row of carpals of most squirrels, the pisiform articulates only with the triquetrum, but in flying squirrels there is also a prominent articulation between the pisiform and the scapholunate, providing a more stable base for the styliform cartilage, which supports the wing tip. In the proximal wrist joint, between these carpals and the radius and ulna, differences in curvature of articular surfaces and in the location of ligaments also correlate with differences in degree and kind of movement occurring at this joint, principally reflecting the extreme dorsal flexion and radial deviation of the wrist in flying squirrels when gliding. The distal wrist joint, between the proximal and distal rows of carpals, also shows most variation among flying squirrels, principally in the articulations of the centrale with the other carpal bones, probably causing the distal row of carpal bones to function more like a single unit in some animals. There is little variation in wrist musculature, suggesting only minor evolutionary modification since the tribal radiation of squirrels, probably in the early Oligocene. Variation in the carpal bones, particularly the articulation of the pisiform with the triquetrum and the scapholunate, suggests a different suprageneric grouping of flying squirrels than previously proposed by McKenna (1962) and Mein (1970). J. Morphol. 246:85-102, 2000. Published 2000 Wiley-Liss, Inc.

Carpal development and morphology in archontan mammals.

Carpal morphology and development in bats, colugos, tree shrews, murids, and sciurids were studied in order to homologize carpal elements. Prenatal coalescence of discrete cartilaginous templates with a loss of a center of ossification appears to be the most common method of reducing carpal elements in these mammals. Only bats and colugos showed postnatal ossification between discrete elements as a method of reducing carpal elements. Carpal morphology of tree shrews is more diverse than previously reported. Ptilocercus shows a highly derived carpal morphology that may be related to its relatively greater arboreality. Dendrogale exhibits what is most likely the ancestral tupaiid carpal morphology. Carpal morphologies of Tupaia, Urogale, and Anathana are identical to each other. Carpal morphology differs between megachiropterans and microchiropterans. These differences may be related to different aerodynamic constraints between the suborders. The carpal morphology of microchiropterans is diverse and may reflect different adaptive regimes between microchiropteran families. Carpal morphology of the colugos shows both megachiropteran and microchiropteran characters. The function of these characters in colugos and bats (stabilization of the carpus in dorsiflexion) is proposed to be similar, although the locomotor roles may be quite different between these taxa.

Comparative myology of the forelimb of squirrels (Sciuridae).

The musculature of the shoulder, arm, and forearm was studied in 19 genera of squirrels, representing the Pteromyinae (flying squirrels) and all 7 tribes of the Sciurinae (tree and ground squirrels). The objective was to locate derived anatomical features of functional or phylogenetic significance and to determine how much morphological variation underlies the diverse locomotor behavior of squirrels, which includes terrestrial and arboreal bounding, climbing, digging, and gliding. The fossil evidence suggests that arboreality is primitive for squirrels, and in fact tree squirrels appear to represent the primitive sciurid morphology. Ground squirrels are less uniform and exhibit a few derived features, including a clavobrachialis muscle not seen in other squirrels. Pygmy tree squirrels, which have evolved independently in three tribes, exhibit convergence of forelimb anatomy, including the loss or reduction of several muscles in the shoulder and forearm. The forelimb anatomy of flying squirrels is the most derived and differs from that of tree squirrels in details of shoulder, arm, and forearm musculature. Some of these muscular differences among squirrels have phylogenetic significance, being shared by closely related genera, but none has significance above the tribal level. Many of the differences suggest a variety of changes in function that are amenable to further study.

Jaw muscles of Old World squirrels.

The jaw, suprahyoid, and extrinsic tongue muscles were studied in 11 genera, belonging to five tribes, of Old World squirrels. Significant variation in most of the adductor muscles is evident. The most primitive state of sciuromorphy is seen in the African tree squirrels Paraxerus and Funisciurus, especially as reflected in the anterior deep masseter. A derived state of sciuromorphy is found in five genera of Old World squirrels and perhaps evolved independently in each. Reduction of the temporalis muscle was observed in three genera, distantly related to one another. A unique arrangement of the superficial masseter is reported in the Asian giant tree squirrels, Ratufa. The arrangement of the masseter in the African pygmy squirrel, Myosciurus, is very similar to that of the South American pygmy squirrel, Sciurillus. We present hypotheses about the functional significance of these differences. In the derived state of sciuromorphy, which is found in three cases in squirrels that feed extensively on hard fruits, the anterior deep masseter is well positioned to increase the strength of the power stroke of the incisor bite. Among the pygmy squirrels, the position of the anterior deep masseter suggests that it plays a more significant role in molar chewing.

Radiographic estimation of long bone cross-sectional geometric properties.

Because of their biomechanical significance, cross-sectional geometric properties of long bone diaphyses (areas, second moments of area) have been increasingly used in a number of form/function studies, e.g., to reconstruct body mass or locomotor mode in fossil primates or to elucidate allometric scaling relationships among extant taxa. In the present study, we test whether these biomechanical section properties can be adequately estimated using biplanar radiographs, as compared to calculations of the same properties from computer digitization of cross-sectional images. We are particularly interested in smaller animals, since the limb bone cortices of these animals may not be resolvable using other alternative noninvasive techniques (computed tomography). The test sample includes limb bones of small (25-5,000 g) relatively generalized quadrupedal mammals--mice, six species of squirrels, and Macaca fascicularis. Results indicate that biplanar radiographs are reasonable substitutes for digitized cross-sectional images for deriving areas and second moments of area of midshaft femora and humeri of mammals in this size range. Potential application to a variety of questions relating to mechanical loading patterns in such animals is diverse.

Taxonomic status of Saguinus tripartitus (Milne-Edwards, 1878).

A review of specimens and field notes of Carlos Olalla and his sons showed that they collected Saguinus tripartitus and Saguinus fuscicollis lagonotus at the same place on the right bank of the lower Rio Napo in Peru. Thus, these two forms are sympatric, although they probably occur in different habitats. Therefore, Saguinus tripartitus should be treated as a distinct species, not as a subspecies of Saguinus fuscicollis.

Forelimb anatomy of New World monkeys: myology and the interpretation of primitive anthropoid models.

The forelimbs of 12 genera of New World monkeys, two genera of Old World monkeys, and a gibbon were dissected. Of the 54 muscles examined, 19 exhibited significant intergeneric variation. We present arguments for which morphologies are primitive and which are derived within platyrrhines and within anthropoids. We conclude that the forelimbs of Cebus apella and Callicebus moloch represent good models of the ancestral anthropoid morphology. Thus among living anthropoids they are most appropriate for comparisons with early fossil anthropoids. They are also useful for determining whether myological anomalies of human aneuploids are atavistic. Wagner tree analyses were conducted to assess the value of these myological characters in phylogenetic studies of platyrrhines. In most respects the Wagner trees were consonant with phylogenies previously proposed, although some hypothesized trees are less parsimonious than others in explaining our data. There is an unexpected number of derived features shared by Aotus and the Atelines. There are marked dissimilarities in forelimb musculature between Aotus and Callicebus.

Flying squirrels are monophyletic.

Seven genera of flying squirrels share five characters of wrist anatomy, which form a functional complex associated with the support of the patagium. In these characters, they differ from all genera of tree and ground squirrels examined. Among mammals, gliding membranes have evolved independently in several other groups. The manner of attachment of the patagium to the forelimb is different in each and demonstrates five morphologies differing from that of flying squirrels. This complex wrist anatomy of flying squirrels provides evidence that gliding evolved only once among squirrels and that the flying squirrels are a monophyletic group.

A study of a black howling monkey (Alouatta caraya) population in northern Argentina.

A population of Alouatta caraya in northern Argentina had an ecological density of 130 animals per km2 . Mean troop size varied from 7.2 to 8.9 individuals, and the ratio of adult males to adult females from 0.58 to 0.51. Infants comprised from 6% to 14% of the population, juveniles from 16% to 21%. These percentages probably vary seasonally in response to a birth peak at the beginning of the dry season. Males were age-graded in multi-male troops. Sexual dimorphism was extreme in this species. Males were all black and averaged 6.7 kg; females were yellow-brown and averaged 4.4 kg. Juvenile males retained the pelage color of the female until approximately 4.5 yr of age and 5 kg in weight. No genital mimicry or exaggeration occurred in this species. Vocalizations of A. caraya were similar to those of A. seniculus, both of which tend to be lower pitched than those of A. palliata.

Blood groups of the mantled howler monkey (Alouatta palliata).

Fifty-two howler monkeys were tested for their human-type A-B-O blood groups. All were group B, as shown by the presence of B and H in their saliva, and anti-A in serum. The B-like agglutinogen of their red cells is common to all New World monkey species tested, and is of different origin and significance than their true A-B-O blood group. Differences among the B-like agglutinogens of the red cells of howler monkeys, marmosets, rabbits and humans group B were demonstrated, and limited tests have also been performed to study the biochemical basis of the anti-B reactions.

Observations on the geographic variation and skeletal development of Aotus.

The geographic pattern of coat color and craniometric variation were examined in Aotus. The coat color and pattern permitted recognition of the geographic origin of night monkeys from many parts of Latin America. The geographic pattern of craniometric variation differed from the pattern of phenotypic variation and led the authors to doubt the validity and usefulness of the currently recognized subspecific classification. Tooth eruption and bone ossification were studied in a sample of Aotus skeletons. Tentative criteria were presented for judging the ages of night monkeys by visual examination of their teeth or by x-ray of their teeth or bones.

Y-autosome translocation in the howler monkey, Alouatta palliata.

Two modal diploid chromosome numbers were encountered in a population sample of howler monkeys, Alouatta palliata. The modal diploid number of males was 53, females: 54. Karyotypes prepared by G-band and C-band techniques identified a Y-auto-some translocation in the males.