By:E. Christopher Kirk(Department of anthropology University of Texas at Austin)©2013stillproud.org Education

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What features identify primates from other mammals? This concern is of main importance come Biological anthropology - the research of primate adaptation and evolution.

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Why execute we, as humans, easily perceive near similarities between ourselves and also other primates? The simple answer come this question lies in the reality that humans and also all various other members the the stimulate Primates share a suite of attributes that castle inherited indigenous a common ancestor that lived 10s of millions of years ago. These features have been kept in every or many descendants of that last common ancestor of living primates, so it is feasible to enumerate a list of "primate hallmarks" that collectively distinguish members of the bespeak from various other mammals. Knowledge the role and evolutionary background of this shared characteristics is important due to the fact that they provide a great deal that information about the adaptations of the more quickly crown primates. Indeed, many of the functions shared in usual by crown primates loss into three categories: (1) adaptations for improved vision; (2) adaptations because that manual and pedal grasping; and also (3) adaptations because that a slow life history.

All living primates have forward-facing eyes (Figure 1; Johnson, 1901; Cartmill, 1992). In this respect, primates an ext closely resemble cats and owls than plenty of other mammals (e.g., squirrels or gazelles). Having actually forward-facing eyes provides primates a broad field the binocular vision (Heesy, 2004, 2009). In various other words, many of a primate"s visual field is viewed by both eyes simultaneously. By comparison, in varieties with laterally dealing with eyes, much of the visual field is monocular (Hughes, 1977). A huge binocular visual ar in primates is more than likely an adaptation for amplified depth perception, i beg your pardon is helped with by the binocular intuitive cues of vergence and stereopsis (Walls, 1942; McIlwain, 1996; Tovée, 1996). Return monocular cues (e.g., perspective and motion parallax) may also be offered to referee the street to a intuitive target, binocular cues are specifically useful because that fine depth late at fairly close ranges (Barlow & Mollon, 1982; McIlwain, 1996).


Comparison of a primate (a: sifaka, genus Propithecus) and non-primate mammal (b: jackrabbit, genus Lepus) come illustrate differences in eye orientation. The sifaka has actually forward-facing eyes that produce a broad field that binocular vision directly in front of that head. The jackrabbit has laterally dealing with eyes the only create a an extremely narrow field of binocular vision in prior of and above its head.
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As a group, primates additionally have high visual acuity (Figure 2). Acute vision in primates is the product of numerous discrete visual adaptations. First, primates have larger eyes than plenty of other mammals of similar body dimension (Ross & Kirk, 2007). Having huge eyes ensures the a big image is developed on the retina (Walls, 1942; land & Nilsson, 2002). This huge retinal image may climate be sampled by many photoreceptors, boosting visual resolution. Second, primates have actually a thick concentration of photoreceptors in the main retina (Kirk & Kay, 2004). Accordingly, once a primate directs its rigid toward things of interest, the retinal image of the object is sampled by a huge number of little and tightly pack photoreceptors. Third, primates have eyes that room more fully encircled through bone than in most other mammals (Cartmill, 1980, 1992). In strepsirrhines (i.e., lemurs and also lorises), this bony enclosure take away the type of a postorbital bar only, if haplorhines (i.e., tarsiers, monkeys, apes, and humans) also possess a postorbital key (Figure 3). This bony features aid stabilize the eye by insulating them from activities of chewing muscles nearby to the orbit (Cartmill, 1980; Heesy, 2005; Menegaz & Kirk, 2009).


In this plot, day-active haplorhines fall above the selection of all other mammals, while night-active haplorhines (i.e., owl monkeys and also tarsiers) and also all strepsirrhines overlap the upper selection of non-primate mammals. Intuitive acuity was measured in behavioral experiments or calculated based upon eye morphology and also ganglion cabinet density. Data bring away from: Calderone et al. (2003); Kirk & Kay (2004); Pettigrew & Manger (2008); Veilleux & Kirk (2009); Pettigrew et al. (2010); and Shinozaki et al. (2010).
Anterior views of the crania of a haplorhine primate (a – squirrel monkey, genus Saimiri), a strepsirrhine primate (b - bushbaby, genus Galago), and a non-primate mammal (c – raccoon, genus Procyon). The squirrel monkey orbit is enclosed posteriorly by a postorbital bowl (a, arrow). The bushbaby orbit has actually a postorbital bar (b, arrow) however lacks a postorbital plate, leave the orbit open posteriorly. The raccoon orbit lacks both a postorbital bar and postorbital plate, leave the orbit open superiorly (c, arrow) and also posteriorly. In both the bushbaby and also the raccoon, the braincase is clearly shows behind the orbit.

In addition to having actually adaptations for enhanced depth perception and also visual acuity, primates additionally have large visual regions of the brain (Allman, 1999; Kaas, 2005, 2008). Macaque monkeys, for example, have actually at least 32 functionally distinct areas of the cerebral cortex the are specialized primarily to processing visual information (Van Essen et al., 1992; valve Essen, 2004). These visual locations comprise 50% the the complete macaque cerebral cortex, whereas only 3% the the macaque cerebral cortex is devoted primarily come auditory functions. Numerous non-primate mammals have proportionately smaller sized visual areas of the mind and fewer functionally unique cortical visual areas (Kaas, 2005, 2008). The comparatively huge area of the primate mind devoted come vision is partly defined by the truth that primates must process a bigger amount of intuitive "input" to the mind than numerous other mammals (Kirk, 2006).


Most mammals (e.g., dogs, deer) are unable to choose up and hold an item with a solitary hand or foot. By contrast, many living primates share a variety of adaptations that make their hands and feet prehensile (Figure 4). First, the fingers and toes of primates room usually long contrasted to the length of the palm or sole, allow the digits to partly or completely encircle an object being grasped (Lemelin, 1999; Jungers et al., 2005; Kirk et al., 2008). Second, the hallux is frequently abducted and may therefore be opposed versus the sole and also the various other pedal digits in order to grasp an object (Cartmill, 1985). Third, the palms that the hand and the soles the the feet space hairless, permitting this surfaces to much better adhere come objects gift grasped. Fourth, primates have actually fingers and toes tipped by hairless apical pads. These apical pads have friction ridges and are richly innervated through mechanoreceptors (Cartmill, 1974, 1985; Hoffman et al., 2004). Fifth, the fingers and toes the primates usually have actually flattened nails rather of claws (Soligo & Müller, 1999). Together a an outcome of these characteristics, primates tend to use their hands and also feet in ways that differ fundamentally from many other mammals (Cartmill, 1985). For instance, while an arboreal squirrel might use its spicy claws come cling to a tree stems or big branch, arboreal primates commonly use your prehensile hands and feet to master smaller-diameter supports.


(a) two views that the hand that a primate (sifaka, genus Propithecus), mirroring the nails, hairless palmar skin with friction ridges, and big apical pads top top the digits. (b) comparison of the hand and also foot skeletons that a primate (bushbaby, genus Galago) and a non-primate mammal (tree shrew, genus Tupaia). From left come right: bushbaby hand; bushbaby foot; tree shrew hand; tree shrew foot. Keep in mind that the bushbaby, choose all strepsirrhine primates, has actually a claw on its second pedal digit the it offers for grooming. All various other digits of the bushbaby room tipped by nails. Unequal the bushbaby, the tree shrew has actually claws on all digits, much shorter fingers/toes, and also lacks one abducted hallux.
The vital events in an organism"s lifespan are carefully interrelated in their timing (Harvey and Clutton-Brock, 1985). Numerous non-primate mammals (e.g., shrews and also mice) screen the fast development, beforehand reproduction, large litter sizes, and also short lifespans characteristic of types with a "fast" life history. By comparison, primates typically exhibit a suite of developmental and physiological features connected with a "slow" life history (Charnov & Berrigan, 1993; Ross, 1998; Mumby & Vinicius, 2008). First, primates maturation slowly. They have long gestation lengths and long periods of infant dependency. Primates also require a long duration of time between birth and also adulthood, and thus have late periods of first reproduction. Second, primates generally give bear to little litters (usually solitary or twin births) and also have lengthy interbirth intervals. Together a result, the total variety of offspring created by an separation, personal, instance primate throughout its life time tends to be reasonably small. Third, primates often tend to be long-lived contrasted to other mammals. Because that example, the best lifespan of a computer mouse lemur is 15 years. Through contrast, the maximum lifespan the the similar-sized spiny pocket computer mouse or field mouse is just 2 years and also 4 years, respectively (Turbill & Ruf, 2010). Due to the fact that many behavioral, ecological, and physiological determinants are correlated with these life background variables, the concern of why primates as a team have evolved slow life histories is complex. However, it is likely that primate life backgrounds are influenced by the truth that most primates room social and have moderate- come large-sized brains (Smuts et al., 1987; Byrne & Whiten, 1988; Kappeler & Pereira, 2003). Unlike non-social mammals, primates may hold-up maturation in order to administer a longer duration of time for social discovering (Barrickman et al., 2008). The cognitive demands of sociality may also select for increased mind size and intelligence (Byrne & Whiten, 1988). Increased brain size, in turn, commonly only evolves in the paper definition of sluggish life history (van Schaik & Deaner, 2003).
The well-known literature often consists of misleading information about the characteristic functions shared by crown primates. Because that example, it is not uncommon to conference the assertion the all primates have actually very big brains. This declare is one overgeneralization. Together a group, haplorhine primates carry out tend to have very large brains relative to body size (Martin, 1990). In this respect, haplorhines are unusual however not unique, because some various other mammals (e.g., whales and dolphins) have equivalent or greater levels of encephalization (Allman, 1999). By contrast, strepsirrhine primates often tend to have mind sizes the are basically "average" for placental mammals of comparable body mass. Strepsirrhines together a group thus exhibit a variety of encephalization comparable to the of plenty of other mammalian teams (e.g., carnivores and also artiodactyls; Jerison, 1973; Passingham, 1982).

One may likewise encounter the twin assertions the all primates have actually a bad sense of smell and also that only primates have binocular vision. Although haplorhines carry out indeed have actually a decreased olfactory system, strepsirrhines have olfactory anatomy that is no fundamentally various from that of many other mammals (Cave, 1973; smith et al., 2001, 2007). Similarly, binocular vision shows up to be virtually universal among mammals (Hughes, 1977; Heesy, 2004). What differs between varieties is no the visibility of binocular vision, yet rather the size of the binocular intuitive field. Even rabbits, v laterally facing eyes and also 360˚ summary vision, have actually a narrow ar of binocular vision (Hughes, 1977).

With regard to claims of primate exceptionalism, it should be noted that over there is only one anatomical attribute that is well-known to be existing in all living primates but is lacking in all various other living teams that have actually been studied. In most mammals, the middle ear has a bony floor dubbed an auditory bulla (Figure 5). Primates are unique amongst living mammals in having actually an auditory bulla the is totally formed by the petrosal bone (MacPhee, 1981). When auditory bullae have actually probably progressed multiple times separately in mammals to defend the contents of the center ear, the certain identity that the bone that creates the bulla has no clear adaptive significance.


a: Inferior see of the basicranium of a primate (lemur, genus Eulemur) showing the listening bullae formed by the petrosal bones. The specimen’s appropriate auditory bulla (shown ~ above the left side) is opened up to reveal the center ear air an are and the ring-shaped bone (ectotympanic) the supports the eardrum. B: Inferior watch of the basicranium that a non-primate mammal (cat, genus Felis) reflecting auditory bullae that construct from the ectotympanic bones and independent centers the ossification (entotympanic bones). The specimen’s left bulla (shown top top the appropriate side) is opened up to disclose the center ear air space, i beg your pardon is divided into two surrounding chambers through a slim bony septum.
Although crown primates have actually many similar attributes because they re-publishing a usual ancestor, many of these features have not been universally maintained in living species. Humans, for example, no longer possess one abducted hallux due to the fact that of selection for effective terrestrial bipedalism. Similarly, callitrichine primates have re-evolved claws on their digits to facilitate clinging to tree trunks while feeding ~ above exudates. the is additionally important to identify that other groups of mammals have independently evolved plenty of of the functions shared through crown primates (a process called homoplasy; Futuyma, 1998). Because that instance, a variety of arboreal rodents and also marsupials have evolved prehensile feet v an abducted hallux and large apical pads (Cartmill, 1985; Nowak, 1991). Furthermore, the characteristic features of crown primates most likely did not evolve concurrently in the primate stem lineage. Of the plenty of features debated above, just one (long hands-on digits) is well-known with certainty come be existing the earliest feasible stem primates - the plesiadapiforms (Bloch & Boyer, 2003; Bloch et al., 2007). Accordingly, if plesiadapiforms space stem primates, climate the other characteristic features of crown primates must have advanced after long digits an initial appeared.


adaptations - attributes of an organism that boost the likelihood the it will efficiently survive and also produce healthy offspring

abducted - displaced far from the midline; one abducted hallux quarter from the other toes and facilitates the ability of the foot to grasp an object, such as a branch

arboreal - living mainly or solely in trees

artiodactyls - hoofed mammals with an even variety of toes; e.g., gazelle, deer, cows, pigs

auditory bulla - a bony prestige beneath the cranium of most mammals that forms the floor of the middle ear space

binocular - viewed by both eyes simultaneously

bipedalism - locomotion on two limbs

callitrichine - member of a family of small Neotropical monkeys, consisting of marmosets, pygmy marmosets, tamarins, and also lion tamarins

cerebral cortex - the external covering of the largest part of the mind (cerebrum) in many mammals; frequently referred to together the brain"s "grey matter"

crown primates - the team that consists of all the progeny (both living and also extinct) the the last common ancestor of life primates

depth perception - the ability to referee the distance to things using visual cues

encephalization - brain size measured relative to body size

exudates - substances, such together gums or resins, that drain from damaged tree tissues

friction ridges - fine ridges emerging on the surface of the hairless palms, soles, and digits of part mammals; this ridges rise adhesion between the skin and an object being grasped

gestation - the period of mammalian development from conception to birth

hallux - the good toe

haplorhines - a team of connected primates that includes tarsiers, monkeys, apes, and humans

homoplasy - the independent development of similar attributes in separate evolutionary lineages; these functions may evolve through comparable developmental pathways ("evolutionary parallelism") or various developmental pathways ("evolutionary convergence")

infant dependency - the duration of mammalian advance from birth to the allude at i m sorry the offspring may successfully survive live independence of that is mother

interbirth interval - the median time between successive births

laterally - encountering away native the midline that the body

life history - the series of successive transforms that an organism undergoes throughout its advance from conception to death

manual - of, or relating to, the hands

mechanoreceptors - cell in the skin regarded the feeling of touch that respond to mechanically stimuli (e.g., pressure, stretch, vibration)

middle ear - an air-filled an are between the ear drum and cochlea (organ the hearing) that consists of the three tiny skeletal (malleus, incus, and stapes) that transmit acoustic vibrations come the inner ear

monocular - perceived by only one eye

motion parallax - a monocular cue because that depth late in which near objects show up to relocate farther loved one to an observer in activity than do much objects

orbit - the bony eye socket

pedal - of or relating to the feet

perspective - a monocular cue for depth tardy in i beg your pardon parallel lines show up to converge towards the horizon

petrosal - a bone the surrounds the inside ear in all mammals and also forms the complete auditory bulla in crown primates

photoreceptors - light-sensitive cells (i.e., rods and also cones) in the retina

prehensile - referring to the capability to hold and also manipulate an object using a single appendage

Primates - a mammalian taxonomic group (order) that has the living lemurs, lorises, tarsiers, monkeys, apes, and humans

plesiadapiforms - a team of extinction mammals that may be stem primates; all recognized plesiadapiform varieties are an ext than 38 million year old

retinal image - the photo projected ~ above the retina through the eye"s cornea and lens

social - living in semi-stable groups characterized by created relationships between individuals that are based on repeated interactions

stem - describes the extinction members of an evolutionary lineage that branched native the lineage prior to the appearance of the last typical ancestor the the life members of the lineage

stereopsis - the late of depth and also three-dimensionality that is created when the brain combines the slightly various images from the left and right eye; one binocular cue for depth perception

strepsirrhines - a team of associated primates that consists of lemurs and lorises

vergence - the movement of the eyes towards the midline when concentrating on close to objects; one binocular cue for depth perception

visual acuity - the capability to detect visual details; likewise called "resolving power"

visual field - the total region of space viewed by an organism"s eye


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