Biomedical Engineering Reference
In-Depth Information
MORPHOLOGICAL DEFINITION OF
PRIMATE ORDER
Morphologically, the order Primates is set apart from other
mammals primarily by the many generalized features
retained by its member species. Both fore- and hindlimbs
have five digits and there is no reduction in the skeletal
elements as is true of more specialized mammals. For
example, all primates have a complete shoulder girdle,
including a well-developed clavicle and scapula, two
complete, distinct long bones in the distal segment of each
limb, and no reduction in the number of bones in either the
metacarpus or metatarsus.
In addition to this generalized mammalian morphology,
higher primates also possess a number of specializations.
The grasping hands and feet (together called the cheirodia)
which characterize the order have enhanced mobility of the
digits, particularly the first on each of the four extremities.
The digits usually end with a flattened distal phalanx which
has a nail on the dorsum and a tactile pad on the volar
(palmar/plantar) surface.
Cranial characteristics of higher primates generally
include a short snout in conjunction with a poorly devel-
oped olfactory apparatus. The number of teeth is reduced
from the generalized mammalian number of 44, although
all four classes of teeth (incisors, canines, premolars, and
molars) are present. The primate visual apparatus is well
developed and the eye is protected by a complete bony
orbit, including a complete postorbital bar. Vision is usually
binocular and the visual fields are overlapping. With rare
exception, e.g. the nocturnal NewWorld monkey Aotus, the
primate retina includes both rods and cones enabling color
vision. The cranial cavity is enlarged to accommodate the
large brain which shows considerable elaboration, partic-
ularly of the cerebral cortex. A posterior (occipital) lobe
and calcarine fissure are always present in the cerebral
cortex of Anthropoidea.
Throughout the order Primates morphology trends
towards an upright postural organization, culminating in
humans. All monkeys, apes, and humans have hemochorial
placentas. The progressively later age of weaning and the
lengthy social dependency between offspring and adults are
reflected in the prolonged postnatal life periods.
(
Coelho, 1985; Gavan, 1985; Larson, 1985; Turnquist,
1984b; Turnquist and Kessler, 1989a; Turner et al., 1997;
Zihlman et al., 2007
).
Sexual dimorphism is evident in a variety of morpho-
logical characteristics. The most obvious of these is overall
body size. Each species also has a variety of secondary sex
characteristics such as hair color of the face and shoulders
or the color of the perineum. In some species the intensity
of the difference between the sexes varies greatly depend-
ing on the breeding cycle of the animals. Marked sexual
dimorphism in body size is usually accompanied by
marked differences in dental morphology, particularly
canine size and the concurrent expansion of the maxilla and
mandible. Thus male faces are generally more prognathic
than females (
Figures 4.1. 4.2
). However, canine sexual
dimorphism is fairly widespread even in the absence of
body size dimorphism (
Nunn and van Schaik, 2002
).
GROWTH AND DEVELOPMENT
Prenatal Development, Congenital
Malformations, and Molecular Basis of
Primate Morphology
Prenatal development of all anthropoid primates closely
resembles that of humans and textbooks of human
embryology may be consulted for information relating to
the general sequence and pattern of developmental events.
However, gestational length and the precise duration and
timing of specific events varies considerably from species
to species.
In New World monkeys, gestation lasts approximately
20
25 weeks (
Ankel-Simons, 2007
). Old World monkeys,
including rhesus macaques and Hanuman langurs, have
a gestation of between 146 and 180 days (
Schultz, 1933
)
but Hanuman langurs' gestation has also been reported to
be slightly longer at 26 weeks and baboon gestation at 27
weeks (
Ankel-Simons, 2007
). The apes have the longest
gestational periods, with gibbons having a pregnancy of 30
weeks on average, bonobos (pygmy chimpanzees) 36
weeks, and 36
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38 weeks in orangutans, chimpanzees, and
gorillas, which is similar to the human average of 38 weeks
(
Table 4.1
).
Congenital defects, representing a similar array to those
reported in humans, are common in nonhuman primates.
Stills and Bullock (1981)
found congenital anomalies in
4.9% of all surviving births in squirrel monkeys. Among
the most common of these in both New and Old World
monkeys are limb deformities. For example, among
Japanese macaques (Macaca fuscata) 16.1% of infants
born over a 40-year period at the Awajishima Monkey
Center were known to have congenital limb defects (
Turner
et al., 2008
)
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SEXUAL DIMORPHISM
The degree of sexual dimorphism within primates varies
markedly among species. As a general rule, however,
arboreal species typically exhibit little sexual dimorphism,
whereas sexual dimorphism in terrestrial species is usually
more marked. The development of sexual dimorphism is
often related to different durations of growth periods in the
two sexes, although rates of growth during a given time
period or for a specific organ may also differ somewhat
a number similar to that found for the Cayo
Santiago population of Macaca mulatta (
Rawlins and
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