Yasuhiro Kikuchi

Relationship between humeral geometry and shoulder muscle power among suspensory, knuckle‐walking, and digitigrade/palmigrade quadrupedal primates

Shoulder morphology is functionally related to different patterns of locomotion in primates. To investigate this we performed a quantitative analysis of the relationship between cortical bone thickness (Cbt) of the muscle/tendon attachment site on the humerus and physiological cross‐sectional area (PCSA) of the shoulder muscle in primates with different locomotory habits. The deltoid, subscapularis, supraspinatus, and infraspinatus were investigated. A chimpanzee, a gibbon, a baboon, two species of macaque, a lutong, a capuchin, and a squirrel monkey were included in the study. The total length of the humerus was measured and the values were converted into three‐dimensional reconstructed data on a computer by computed tomography. The Cbt values were obtained from the volumes divided by the areas of the muscle/tendon attachment sites of the humerus by computer analysis. Muscle mass, muscle fascicle length, and muscle pennation angle were measured and PCSA was calculated using these parameters. A relatively high Cbt and small PCSA were characteristic of the gibbon. The gibbon’s high Cbt suggests that passive tension in the muscle/tendon attachment site of suspensory primates (brachiators) may be greater than that of quadrupedal primates, whereas the relatively small PCSA indicates an association with a large amount of internal muscle fascia to endure the passive stress of brachiation. Although chimpanzees undertake some suspensory locomotion, the results for this species resemble those of the digitigrade/palmigrade quadrupedal primates rather than those of the suspensory primate. However, the deltoid and subscapularis in chimpanzee differ from those of the other primates and appear to be affected by the peculiar locomotion of knuckle‐walking, i.e. the moment arm of forelimb in chimpanzees is relatively longer than that of digitigrade/palmigrade quadrupedal primates. Hence, a large PCSA in the deltoid and subscapularis may contribute to sustaining the body weight during locomotion. On the other hand, a thin cortical bone relative to a large PCSA was a feature of the cercopithecids, indicating that digitigrade/palmigrade quadrupedal locomotion produces less tension at the muscle/tendon attachment sites compared with that produced by brachiators.

Morphology of Brachial Plexus and Axillary Artery in Bonobo (Pan paniscus)

With 1 figure and 1 table

Functional morphology and anatomy of cervical vertebrae in Nacholapithecus kerioi, a middle Miocene hominoid from Kenya.

This paper describes the morphology of cervical vertebrae in Nacholapithecus kerioi, a middle Miocene primate species excavated from Nachola, Kenya in 1999-2002. The cervical vertebrae in Nacholapithecus are larger than those of Papio cynocephalus. They are more robust relative to more caudal vertebral bones. Since Nacholapithecus had large forelimbs, it is assumed that strong cervical vertebrae would have been required to resist muscle reaction forces during locomotion. On the other hand, the vertebral foramen of the lower cervical vertebrae in Nacholapithecus is almost the same size as or smaller than that of P.xa0cynocephalus. Atlas specimens of Nacholapithecus resemble those of extant great apes with regard to the superior articular facet, and they have an anterior tubercle trait intermediate between that of extant apes and other primate species. Nacholapithecus has a relatively short and thick dens on the axis, similar to those of extant great apes and the axis body shape is intermediate between that of extant apes and other primates. Moreover, an intermediate trait between extant great apes and other primate species has been indicated with regard to the angle between the prezygapophyseal articular facets of the axis in Nacholapithecus. Although the atlas of Nacholapithecus is inferred as having a primitive morphology (i.e., possessing a lateral bridge), the shape of the atlas and axis leads to speculation that locomotion or posture in Nacholapithecus involved more orthograde behavior similar to that of extant apes, and, in so far as cervical vertebral morphology is concerned, it is thought that Nacholapithecus was incipiently specialized toward the characteristics of extant hominoids.

Morphology of the thoracolumbar spine of the middle Miocene hominoid Nacholapithecus kerioi from northern Kenya.

A new caudal thoracic and a new lumbar vertebra of Nacholapithecus kerioi, a middle Miocene hominoid from northern Kenya, are reported. The caudal thoracic vertebral body of N.xa0kerioi has a rounded median ventral keel and its lateral sides are moderately concave. The lumbar vertebral body has an obvious median ventral keel. Based on a comparison of vertebral body cranial articular surface size between the caudal thoracic vertebrae in the present study and one discussed in a previous study (KNM-BG 35250BO, a diaphragmatic vertebra), N.xa0kerioi has at least two post-diaphragmatic vertebrae (rib-bearing lumbar-type thoracic vertebrae), unlike extant hominoids. It also has thick, rounded, and moderately long metapophyses on the lumbar vertebra that project dorsolaterally. The spinous process bases of its caudal thoracic and lumbar vertebrae originate caudally between the postzygapophyses, as described previously in the KNM-BG 35250 holotype specimen. In other words, the postzygapophyses of N.xa0kerioi do not project below the caudal border of the spinous processes, similar to those of extant great apes, and unlike small apes and monkeys, which have more caudally projecting postzygapophyses. Nacholapithecusxa0kerioi has a craniocaudally expanded spinous process in relation to vertebral body length, also similar to extant great apes. Both these spinous process features of N.xa0kerioi differ from those of Proconsul nyanzae. The caudal thoracic vertebra of N.xa0kerioi has a caudally-directed spinous process, whose tip is tear-drop shaped. These features resemble those of extant apes. The morphology of the spinous process tips presumably helps vertebral stability by closely stacking adjacent spinous process tips as seen in extant hominoids. The morphology of the spinous process and postzygapophyses limits the intervertebral space and contributes to the stability of the functional lumbar region as seen in extant great apes, suggesting that antipronograde activity was included in the positional behavior of N.xa0kerioi.

Geometric characters of the radius and tibia in Macaca mulatta and Macaca fascicularis.

We performed comparative analyses of four cross-sections of the distal radius and tibia in two species of macaque to clarify the relationships between bone morphology and locomotor type. The lengths of bones and five bone geometric properties in each section were examined and compared separately in both female and male Macacaxa0mulatta and Macacaxa0fascicularis. In M.xa0mulatta, there were no significant gender-specific differences in either the radius or the tibia. In contrast, the radius and tibia of male M.xa0fascicularis had greater geometric parameters in the 20% and 40% positions relative to the 5% and 10% positions from the distal end than those of their female counterparts. The radius and tibia of M.xa0mulatta were relatively longer than those of M.xa0fascicularis, and the sectional parameters of the tibia of M.xa0mulatta were relatively larger than those of M.xa0fascicularis. Standardization of the log-transformed bone length between the species revealed larger radial cortical bone areas in M.xa0fascicularis. In contrast, there were minimal differences in the tibial cortical bone areas between the two species. This study suggests that the observed distinctions in bone geometry in female and male M.xa0fascicularis may be due to gender-specific differences in the muscle weights of the forearm and calf, which may underlie the divergence in the leaping abilities of females and males of this species. Taken together, these results of interspecies comparisons may be related to the fact that arboreal primates such as M.xa0fascicularis undergo compressive mechanical stress due to the forelimb lead that occurs as the animal descends a sloping trunk or bridges a tree gap downward, while terrestrial primates such as M.xa0mulatta move on nearly flat substrates. Differences in fore- and hind-limb bone properties between the two species are discussed with regard to functional morphology and locomotor type.

Quantitative analyses of cross-sectional shape of the distal radius in three species of macaques

I conducted quantitative analyses of the cross-sectional shape of the distal radial shaft in three species of macaques, which differ in locomotor behavior: semi-terrestrial Japanese macaques (Macaca fuscata), arboreal long tailed macaques (M. fascicularis), and relatively terrestrial rhesus macaques (M. mulatta). I took CT scans of the distal radial shafts of a total of 180xa0specimens at the level of the inferior radio-ulnar articulation. From each CT image, the periosteal outline of the radius was traced automatically by a digital imaging technique. I determined five points (landmarks) on the outline by developing a standardized morphometric technique. Bone surface lengths were measured by using these landmarks and their soft tissue correlates were investigated. The results of this study were as follows: (1) Semi-terrestrial M. fuscata has features that are approximately intermediate between those of the other two species. M. fuscata has a relatively small groove for M. abductor pollicis longus and a large groove for Mm. extensor carpi radialis longus et brevis. These characters resemble those of M. fascicularis. On the other hand, the ulnar notch of M. fuscata is relatively large, a character which is similar to that of M. mulatta. Moreover, compared to the other two macaques, the surface of the flexor muscles of M. fuscata is intermediate in size. (2) The more terrestrial M. mulatta has a relatively large groove for M. abductor pollicis longus and a small groove for Mm. extensor carpi radialis longus et brevis. Moreover, M. mulatta has a relatively large ulnar notch and a small surface for the flexor muscles. (3) The arboreal M. fascicularis has similar features to those of M. fuscata for the first and second relative size index. However, in the ulnar notch, M. fascicularis has a peculiar character and the surface for the flexor muscles is relatively large compared to those of the other two species. These results can be interpreted in terms of positional habits and presumed functional demands. A form-functional study by Lemelin and Schmitt also corroborates the interpretations of the present study. Thus, the distal region of the forearm strongly reflects muscular development and joint resultant force, and is an important region for investigating locomotor adaptations in primates. The present study reveals the possibility of using this type of morphometric analysis for reconstructing the positional behavior of fossil primates.

Carpal bones of Nacholapithecus kerioi, a Middle Miocene Hominoid From Northern Kenya.

OBJECTIVESnThe carpal bones of the middle Miocene hominoid Nacholapithecus kerioi are described based on new materials.nnnMATERIALS AND METHODSnThe materials comprise a trapezoid, three capitates, two hamates, a centrale, a lunate, a triquetrum, and a pisiform, collected during the 2001 and 2002 field seasons from Nachola, Kenya. We also describe a pisiform recently assigned to the type specimen of N. kerioi, KNM-BG 35250.nnnRESULTSnIn the Nacholapithecus wrist, the ulnar styloid process articulates with both the triquetrum and pisiform, and the triquetrum facet on the hamate is relatively proximodistally oriented in dorsal view. The Nacholapithecus capitate possesses a moderate distopalmar hook-like process and separated radial articular facets for the trapezoid and the second metacarpal due to the carpometacarpal ligament attachment that is absent in the Proconsul capitate.nnnDISCUSSIONnThe carpal anatomy of Nacholapithecus is similar to that of the early Miocene hominoid Proconsul. However, Nacholapithecus wrist anatomy appears to exhibit slightly more emphasized stability. Am J Phys Anthropol 160:469-482, 2016.

Multivariate analysis of variations in intrinsic foot musculature among hominoids

Comparative analysis of the foot muscle architecture among extant great apes is important for understanding the evolution of the human foot and, hence, human habitual bipedal walking. However, to our knowledge, there is no previous report of a quantitative comparison of hominoid intrinsic foot muscle dimensions. In the present study, we quantitatively compared muscle dimensions of the hominoid foot by means of multivariate analysis. The foot muscle mass and physiological cross‐sectional area (PCSA) of five chimpanzees, one bonobo, two gorillas, and six orangutans were obtained by our own dissections, and those of humans were taken from published accounts. The muscle mass and PCSA were respectively divided by the total mass and total PCSA of the intrinsic muscles of the entire foot for normalization. Variations in muscle architecture among human and extant great apes were quantified based on principal component analysis. Our results demonstrated that the muscle architecture of the orangutan was the most distinctive, having a larger first dorsal interosseous muscle and smaller abductor hallucis brevis muscle. On the other hand, the gorilla was found to be unique in having a larger abductor digiti minimi muscle. Humans were distinguished from extant great apes by a larger quadratus plantae muscle. The chimpanzee and the bonobo appeared to have very similar muscle architecture, with an intermediate position between the human and the orangutan. These differences (or similarities) in architecture of the intrinsic foot muscles among humans and great apes correspond well to the differences in phylogeny, positional behavior, and locomotion.

Sacral vertebral remains of the Middle Miocene hominoid Nacholapithecus kerioi from northern Kenya

This study describes two new sacral specimens of Nacholapithecus kerioi, KNM-BG 42753I and KNM-BG 47687A, from the Aka Aiteputh Formation in Nachola, northern Kenya, excavated in 2002. They are of roughly equal size and are considered to belong to males. When scaled by body mass, the lumbosacral articular surface area of the better preserved specimen, KNM-BG 42753I, is smaller than that in Old World monkeys but similar to that in extant great apes and New World monkeys, as well as Proconsul nyanzae. The relatively narrow dimensions of the first sacral vertebral body in the transverse and sagittal planes are characteristics of N.xa0kerioi and P.xa0nyanzae and similar to those of extant great apes. In N.xa0kerioi, lumbosacral surface area relative to body mass is small. This may simply be an extension of a trend from the previously reported small thoracolumbar vertebrae to the sacrum. u200bThe first sacral vertebrae of N. kerioi and Epipliopithecus vindobonensis have a higher craniocaudal vertebral body reduction (CVR; a higher CVR indicates a wider cranial width relative to a narrower caudal width), similar to that in Old World monkeys. Old World monkeys have a higher CVR, and usually have three sacral vertebrae, fewer than seen in extant great apes, which have a lower CVR and four to six (sometimes as many as eight) sacral vertebrae. New World monkeys have a lower CVR than Old World monkeys, but generally possess only three sacral vertebrae, and have a large caudal articular surface, which may be related, at least in the Atelidae, to the grasping ability of their tails. The possibility that N.xa0kerioi had only three sacral vertebrae cannot be ruled out, because E.xa0vindobonensis and Old World monkeys, with higher CVRs, have sacra consisting of three sacral vertebrae.

Sexual dimorphism of body size in an African fossil ape, Nacholapithecus kerioi

Sexual size dimorphism in the African fossil ape Proconsul nyanzae (18 million years ago, 18xa0Ma) has been previously documented. However, additional evidence for sexual dimorphism in Miocene hominoids can provide great insight into the history of extant hominoid mating systems. The present study focused on body mass (BM) sexual dimorphism in Nacholapithecus kerioi from the Middle Miocene (16-15xa0Ma) in Africa. Bootstrap analysis revealed that P.xa0nyanzae BM sexual dimorphism was lower than that in Pan troglodytes, which exhibits moderate sexual dimorphism, as reported previously. The same simulation revealed that BM sexual dimorphism of N.xa0kerioi was comparable with that in Gorilla spp.; i.e., the males were approximately twice as large as the females. High sexual dimorphism in extant apes is usually indicative of a polygynous social structure (gorilla) or solitary/fission-fusion social system (orangutan). However, because of the high proportion of adult males in this fossil assemblage, the magnitude of dimorphism inferred here cannot be associated with a gorilla-like polygynous or oranguran-like solitary/fission-fusion social structure, and may reflect either taphonomic bias, or some other social structure. Extant hominoids have a long evolutionary history owing to their deep branching, comprising only a few existing members of the original highly successful group. Therefore, it is not surprising that the mating systems of extant hominoids fail to provide fossil apes with a perfect model. The mating systems of extinct hominoids may have been more diverse than those of extant apes.