john hawks weblog

paleoanthropology, genetics and evolution

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primates

  • Group life in primates

    Mon, 2013-02-04 00:48 -- John Hawks
    Synopsis: 
    A discussion of some factors affecting group size and dynamics in primates

    Primates form different kinds of groups. While there is variation within every species, each species has its own typical range of group sizes. Primate groups also vary in their structures. The structure of a group involves details about what kinds of individuals live in a group together. The group structure includes the number of adult males compared to females, called the sex ratio. It also includes the pattern of interactions among individuals, such as whether the entire group spends all its time together, or whether different individuals break into smaller parties for substantial amounts of time. The size and structure of groups have an adaptive role --- some kinds of groups are better suited to certain ecological or social contexts than others.

    Group size and structure are products of adaptation.

    The size and structure of groups in a species is a product of selection on individuals. Individuals always have choices: they can stay in a group, or they can leave; they can permit others to join their group or they can confront newcomers aggressively; they can join together to form a new group, or they can go it alone. Some choices will be more adaptive than others, and individuals who make the right choices about what kind of group to live in will tend to survive or reproduce more. This is how group sizes and structures evolve.

    In an evolutionary context, the kinds of groups found in different primate species are products of their ecology and social interactions. Typical group sizes can vary greatly among even closely related species or primates. The typical social group in orangutans, for example, consists of a single mother and her offspring. In contrast, chimpanzees --- who are relatively close in body size and diet to orangutans --- live in complex communities averaging over fifty individuals. The differences between these species' social groups must be explained by differences in their habitat, movement, mating system, and history. By studying primate groups, primatologists attempt to understand how these factors result in certain kinds of groups but not others.

    Predation

    Most primates are small to medium-sized mammals, and they are vulnerable to many kinds of predators. Carnivores, including leopards and hyenas in the Old World, and jaguars in the Americas pose a serious threat, even to large primates. Other natural predators include hawks and eagles, snakes, humans and other primates. Indeed, other primates are the most significant predators of some species. For example, chimpanzees are the most common predator of red colobus monkeys, as described below.

    Small primates who live in exposed habitat are most vulnerable to predation.

    Most monkeys suffer high rates of predation by carnivores and other predators, but these rates are highest for those species that live in relatively open, exposed habitats. In vervet monkeys, which live in relatively open savanna, predators account for up to 70 percent of deaths (Cheney and Wrangham 1986). Baboons are much larger primates than vervet monkeys, but the rate of predation on savanna baboons is very high as well. At these levels, predation exerts a strong selective force on these populations.

    For these primates, large groups tend to minimize predation risk. A large group of primates has more eyes, and is therefore more likely to notice the approach of a predator. Another strategy to resist predators is for individuals to raise a cry or alarm when they see a predator approach. Alarm calls can be a benefit to such groups, but an individual raising an alarm also raises its own risk --- predators will notice its noisy alarm. But in larger groups, a sudden scatter of alarmed primates competes for attention with the alarm call itself. In large groups, individuals can spend less time scanning for predators, meaning that they can devote more time to foraging. Likewise, the collective action of some individuals may deter predators from attacking. The mere presence of a collection of large male baboons may deter a not-so-hungry jackal, when a lone female and infant would be more likely to suffer attack.

    Movement and food patches

    Some kinds of habitat are much harder to move through than others. Terrestrial primates can move very quickly on the ground. But if they are on the ground far from a tree, they can be highly exposed to terrestrial and aerial predators. Small primates may be highly mobile through dense forest, because they can move quickly from tree to tree. Large primates must move more deliberately through trees, since not every branch can easily support their weight.

    Primates who cannot move easily or quickly may be limited in group size.

    Size and difficulty of movement may partially underlie the differences in group size between chimpanzees and orangutans. Chimpanzees can move effectively across the forest floor, and this terrestrial movement is their major way of traveling from place to place. In this way, they can maintain relatively large groups, as sets of individuals can forage or patrol over relatively long distances and still return regularly to other group members. In contrast, much of the forest habitat of orangutans has a swampy or closed forest floor through which it is very difficult to move. In some parts of the orangutan range, tigers add to the danger of the forest floor. As a result, orangutans spend the overwhelming majority of their time in the forest canopy, where movement is slow but safer (Knott 2001).

    Another element of ecology interacts with movement costs to affect group size: the distribution of food. Foraging for food is one of the major activities that primates conduct every day. Whether a group of primates is large or small, every individual must eat. For a group of primates that finds a huge tree with ripe fruit, life is relatively easy --- they can stay there and eat as long as they are not driven off by competitors or predators. If such large patches of food are common enough, the primates can stay in a large group and move from one patch to another. But nature does not only consist of large patches of ripe fruit and other primate foods. Sometimes fruits ripen a few at a time, so any tree can only feed a few primates. The availability of food can also vary from season to season, so that sometimes there may be plenty of food to eat, but at other times there is little.

    The availability of food limits group size. If patches of food are small, then individuals within a large group may compete intensely for each bite. Being in a large group is a serious cost when resources are slim. This kind of limit to group size also affects orangutans. Orangutans eat a large amount of ripe fruit, but once they find a patch the amount of ripe fruit is usually relatively small for such a large-bodied primate (Terborgh and van Schaik 1987). Larger groups are generally impossible to maintain --- the orangutans would just go hungry unless they could find their own food sources, and it is too costly to move quickly between small patches of fruit. Indeed, in some forests where food is more plentiful, orangutans do tend to spend more time in groups of multiple individuals (Delgado and van Schaik 2000).

    Study terms: 
    predation
    adaptation
    patches
    Tags: 
    Anthropology 105
    explainer
    predation
    primates

  • Primate mating patterns

    Mon, 2013-02-04 00:38 -- John Hawks
    Synopsis: 
    Primate groups are shaped by the pattern of mating competition and interactions

    Ecology, diet, competition, and ease of movement all affect the size of primate groups. The structure of primate groups is primarily affected by the mating system. There are several elements of primate mating systems. In most species, individuals of either one sex or the other disperse from their natal group --- the one they were born in --- when they reach adulthood. This dispersal affects the structure of the groups by breaking some kinds of relationships and preserving others.

    For example, in primate species where maturing females transfer to a new group, males are often left within the group where they are born. This means that males can form relationships as juveniles that last their entire lives. The long-lasting male coalitions in chimpanzees are a side-effect of female dispersal, conditioned by large group sizes and other social factors. In contrast, male savanna baboons transfer to new groups when they reach adulthood. In baboon groups, the female associations are highly structured by kin relations, since mothers and daughters live in the same group as adults.

    Mating conflicts

    Mating is essential to reproduction, and is the contest through which individuals pass their genes into future generations. From an evolutionary perspective, individuals do whatever they can to promote their own chances to reproduce. Sometimes, individuals can promote their own reproduction by inhibiting the chances of others. In other instances, it may be in their best interest to cooperate with other individuals, or to bide their time waiting for higher-ranking individuals to die or lose status.

    The most basic conflict of interest in mating is between males and females. Because of their biological role in carrying and providing nutrition for their offspring, both before and after birth, females must make a large investment in reproduction. Considering the large cost of reproduction, an adaptive mating strategy for a female is to mate with the male whose genes will contribute to the best possible offspring. For this reason, females are typically choosy about which males they will mate with. This element of female choice can give rise to sexual selection, in which males are advantaged by the possession of features that females value.

    In contrast, males may have extreme levels of competition for mates. A single male may be able, through fighting, threat, or intimidation, to prevent other males from having mating access to females, or even to expel all other adult males from the group. If he is successful, the reproductive opportunities for such a male are tremendous. On the other hand, the opportunities of other males fall to zero. This places a tremendous genetic payoff on social competition for mating.

    The intensity and form of mating competition vary from species to species. Some kinds of primates have a sparse diet that is simply unable to sustain the caloric requirements of huge male body sizes. Other primates or may modify the conditions of combat through coordination of activity with other individuals, emphasis on advertising the risks of combat rather than pursuing combat itself, or other means.

    Sexual dimorphism is a difference in size or form between males and females.

    Males and females within a species often differ in size or morphology. For example, male primates almost always have larger canine teeth than females. The canine teeth are important in male mating competition --- males can display their canines as a threat to other males, and at an extreme they can injure or kill other males with these teeth. One indication of the importance of the canines in displays is that they tend to be more dimorphic in species that are active during the day, as opposed to nocturnal species (Leutenegger and Cheverud 1982).

    Sexual dimorphism in body size is very pronounced in many primate species. For example, orangutan males average around twice the body mass of females. The body mass dimorphism is even more extreme for gorillas than for orangutans. Many different factors influence the body size dimorphism in a primate species (Hedrick and Temeles 1989). One of these is mating competition between males --- intense male competition increases the value of male body size. Another factor can be food competition between the sexes --- when males are larger, they may be able to dominate a larger share of valued food resources. Additionally, males can take on important reproductive roles beyond mating, such as protecting young juveniles from predation or infanticide.

    Territorial primate groups maintain their home ranges against incursions by other groups or individuals.

    Mating competition in many primates involves territoriality, when males defend a home range against incursions from other males. Primate groups may be territorial as a result of a single male's action, or the coordinated activity of multiple males. For example, Males of some primate species dominate access to females by preventing other males from coming into their home range. These primate males are said to be territorial. Even small social groups, like the monogamous male-female pairs of gibbons, may be highly territorial. But chimpanzees provide one of the strongest instances of territoriality among primates. Groups of male chimpanzees walk the approximate perimeter of their territory, engaging in violent conflicts with any members of neighboring groups they might encounter (Wrangham 1999). As observed by Jane Goodall (1986) at Gombe in Tanzania, the males of one group of chimpanzees killed all of the adult males and several females and juveniles in a neighboring group over the course of several months.

    Kinds of groups in primates

    Group size, dispersal, and mating competition all contribute to the proportion of males and females found in any given group. Sometimes a single male and female form a group; sometimes a single male and multiple females; sometimes multiple males and females; and occasionally a single female and multiple males.

    Monogamous, or pair-bonded, species have long-lasting mating relationships between a single male and a single female.

    Gibbons tend to form long-lasting associations between a single adult male and a single adult female. These pair-bonded primates occupy territories that they defend against incursions from other individuals. Both males and females make long vocalizations, called songs, to establish their shared territory. They often vocalize together in duets, although in different contexts based on whether threats come from males or female intruders (Geissmann 2000, Mitani 1987). These groups of a single adult male and female and their offspring are called monogamous groups.

    Polygynous groups have a single dominant male and multiple females.

    In many kinds of primates, a single male may dominate a group with multiple females. This is a polygynous group --- a multifemale, single-male group. Polygyny results from strong mating competition among males. Only if a single adult male can repel other males from the group can he reap the powerful reward of mating with many females.

    A well-known species with polygynous groups is the gorilla. Gorilla groups generally have one adult male, up to eight or more females, and their dependent offspring. Solitary males live outside of these polygynous groups and sometimes manage extragroup matings with females. These groups are maintained by strong mating competition --- a dominant male in a group repels other males by threats, intimidation, or violence.

    Even so, there is variability in gorilla societies. In mountain gorillas, multimale groups are common (Robbins 1999). In such groups, the dominant males have a majority of matings, and often harass subordinate males that attempt to mate. But subordinates do have many mating opportunities, demonstrating a social flexibility among gorillas.

    Polyandrous groups have a single dominant female and multiple males.

    Marmosets and tamarins, called callitrichids, are the smallest of the New World monkeys. These monkeys are among the few primates for which twin births --- and sometimes triplets --- are common. This means that females tend to have high energetic requirements in pregnancy, lactation, and caring for young. Callitrichids therefore face unique challenges compared to other primates.

    One way that these monkeys adapt to caring for more young is that older offspring of a female may stay with her for a longer time instead of quickly going off on their own. These older offspring help to watch and sometimes provide food for their younger siblings (Bales et al. 2000).

    Another behavioral adaptation is for a single female to mate with and coexist with multiple males. This kind of mating system is called polyandry. Mating with multiple males reduces the paternity certainty of the males --- a male cannot know if a female conceived her offspring with him or another male. As long as mating opportunities are limited, males may cooperate in a group with a single female on the chance of having offspring with her. These males help to provide food and defense for the young juveniles in polyandrous groups. Polyandry is not universal among callitrichids; it is adopted more when resources and mating opportunities are rare (Goldizen 1988).

    Fission-fusion societies are large multimale, multifemale communities that spend much of their time divided into smaller units that combine in different combinations.

    Some primates coexist in large groups numbering 50 individuals or more. A group this large is always a multimale group --- there is no way for a single male to deter other males from twenty or more adult females. But multiple males sometimes coordinate their behavior to deter neighboring groups. A large multimale group may occupy and defend a large territory, especially where movement costs are relatively low.

    Large primates who live in large groups have a problem: there are very few food patches large enough to feed them all. So even though a large multimale, multifemale group may occupy a substantial territory, it may not be possible for them to feed together much of the time. Chimpanzees live in such large multimale, multifemale groups. Even though members of the group share a dominance hierarchy, social interactions, and relationships, they spend much of their time apart. Smaller groups of individuals --- sometimes a single female and her young, sometimes male-female pairs, and sometimes small groups of either sex --- split apart in order to forage for food. These small groups recombine and split in different combinations, and sometimes all of them come together, especially when food is plentiful. This kind of social organization is called a fission-fusion society. Individuals divide into small foraging groups and come back together into the full community for social interactions.

    Study terms: 
    polygynous
    fission-fusion
    monogamous
    polyandrous
    Tags: 
    Anthropology 105
    primates
    behavior
    mating

  • How do primates move around?

    Fri, 2013-02-01 09:29 -- John Hawks
    Synopsis: 
    Exploring the way that primate locomotion influences body plan and behavior.

    The diversification of the first primates from other early mammals took place partly because the ancestors of the primates came to inhabit a unique environment --- the trees. These early primates developed many features to allow them to move quickly in this arboreal habitat. Early primates evolved the ability to direct and focus both eyes on objects, called binocular vision, which allowed them to accurately judge distances to branches and other objects. They also developed grasping hands and feet, with opposable digits --- thumbs and big toes. The fingertips were broader in these early primates, to apply greater grip strength to branches, and instead of claws primates developed wide nails. All of these features helped primates to succeed as arboreal specialists.

    • Primate adaptations to arboreal environments include binocular vision, opposable digits, enlarged fingertips and nails.

    Many primates today continue this arboreal existence, spending large proportions of their time off the ground and in the trees. Moving in an arboreal environment has the obvious risk of falling out of the tree if everything does not go exactly right. Yet primates are virtual trapeze artists, masters of the arboreal environment. Smaller monkeys and lemurs can rush headlong from branch to branch, seemingly mindless of any risk of falling, because of their unrivaled arboreal skills. These primates bridge immense gaps by leaping from one tree to another, limited only by the acceleration of gravity. Even large primates like chimpanzees and orangutans can rapidly scale trees and move effectively from one tree to another. These arboreal skills are made possible by a large suite of adaptations, many of which originated very early in primate evolution.

    But even though all primates are climbers, some of them have developed more specialized adaptations to other kinds of movement, or locomotion. These specialized forms of movement are adapted to different kinds of environments. Some primates are excellent at moving terrestrially, on the ground. Others are good at climbing tall vertical trunks and branches, or at swinging from one branch to another. Species who use these special forms of locomotion have consequences in their skeletons and muscle configurations.

    Vertical clinging and leaping

    Many prosimians, including tarsiers and many lemurs, use a form of locomotion called vertical clinging and leaping. These kinds of primates live in habitats where they climb relatively small tree trunks or bamboo. Sometimes they leap between these vertical supports. Other times, especially for the larger lemurs like sifakas, they leap along the ground. Their legs are relatively long compared to their arms, so that terrestrial movement can be more effective by leaping than by running on all fours.

    This form of locomotion gives their bodies a very distinctive shape --- especially tarsiers, who are masters of this pattern. Tarsier legs are longer than their bodies and forelimbs put together, and they especially have very long foot bones. The name for these bones are tarsals, giving these unique little primates their name.

    Below-branch locomotion

    Some monkeys and all apes are adapted more to hanging beneath branches than running atop them. This suspensory style of locomotion is essential for larger arboreal primates, which can move above only the largest branches. Hanging below branches enables large primates to climb into the forest canopy, often on smaller branches than could support them from above.

    A skeletal adaptation to suspension includes relatively long arms with very mobile shoulder joints. In contrast to quadrupedal monkeys, whose arms are limited in their range of motion, apes can move their arms through nearly a complete 360 degree circle. Also, apes' trunks are relatively flat from front to back, with the shoulders mostly alongside rather than in front of the spine. Putting the arms at the side requires long and strong collarbones --- called clavicles --- that serve as a strut supporting the shoulder musculature. Also, apes have extremely long fingers, useful for hooking onto branches quickly. Their thumbs are quite small, as they do not usually grip with their thumbs onto branches while hanging from them.

    • Brachiation is arm-over-arm swinging.

    Sometimes apes swing arm over arm from one branch to the next, a locomotor pattern called brachiation. Gibbons and siamangs are a brachiation specialists, but all living hominoids have the skeletal anatomy to enable them to brachiate. Brachiation conserves the energy of forward movement by treating the body as a swinging pendulum. This is a very efficient way to travel for medium-sized primates, combining energetic conservation with speed, and working with gravity instead of against it.

    • Quadrumanous locomotion uses all four hands and feet to move among branches.

    Large-bodied hominoids brachate less than gibbons. Branches of sufficient size to support the entire weight of a large ape are rarely located close enough to brachiate between them. Also, the pendular motion is less efficient for larger primates, because their arms just cannot be as long in proportion to the size of their bodies. Instead, when in the trees, the living great apes grip branches with three or four of their hands and feet at once. This allows large apes to support their weight on relatively small branches in the canopy. The masters of this kind of locomotion are orangutans, who move through the forest canopy moving one hand or foot at a time to a new support branch. This kind of locomotion, as if an animal was using four hands equally, is called quadrumanous locomotion.

    Knuckle-walking and fist-walking

    • Knuckle-walking allows quadrupedal walking in chimpanzees and gorillas, whose hands are adapted to suspension.

    Chimpanzees and gorillas, when they are on the ground, walk quadrupedally using the proximal finger joints of their hands instead of the palms of their hands. They use this style of locomotion, called knuckle-walking, because of their unique forelimbs. These apes have long forelimbs relative to their hindlimbs, and their hands are very long with strong tendons for curling the long fingers into a powerful hook. These hands are very well adapted to suspension in the trees, but they are not capable of being extended with the palms toward the ground, which is the way most other primates walk quadrupedally. So instead of walking palm-down, they use their knuckles.

    A few skeletal features of living chimpanzees and gorillas appear to be adaptations to knuckle-walking. Because the arms are held in a locked position while supporting the body, unlike the somewhat flexed position usual in quadrupeds, the proximal end of the ulna is more U-shaped, built for supporting the humerus mainly from below. Likewise, the joints of the hands are more limited in motion, and relatively incapable of being extended backward at the wrist and knuckles. It is not clear whether these unique anatomical consequences of knuckle-walking evolved in parallel in chimpanzees and gorillas, or whether they represent homologies inherited from a knuckle-walking common ancestor of gorillas, chimpanzees, and humans.

    Although many orangutans spend little time on the ground in their natural habitat, they can walk quadrupedally. But unlike chimpanzees and gorillas, orangutans do not walk on their knuckles when they are on the ground. Instead, they curl their hands inward, walking on the outside of their fists. This fist-walking accomplishes the same goal as knuckle-walking --- it allows walking on all fours without facing the palms of the hands toward the ground.

    Study terms: 
    brachiation
    knuckle-walking
    quadrumanous
    vertical clinging and leaping
    Tags: 
    Anthropology 105
    primates
    anatomy
    behavior

  • Anthropology 105, lecture 7: Eyes

    Sat, 2012-02-25 17:03 -- John Hawks
    Synopsis: 
    Illustrating phylogeny and evolutionary convergence using trichromacy and eye development

    Out of all the lectures in the course, this was one of my favorites to put together. I return to the topic of evolutionary developmental biology, first raised in the "Vertebrae" lecture, by extending from the Hox genes to toolkit genes, focusing on the role of Pax6 in eye development. Again, we see how model organisms like fruit flies and zebrafish are relevant to understanding human biology.

    Then, we zoom closer into the phylogeny of primates, considering the superfamilies and reminding students that New World monkeys, Old World monkeys and hominoids are all anthropoid primates. The anthropoids have a tremendously interesting difference with respect to color vision. Many New World monkey species have trichromacy in some individuals but many remain able only to see two colors. This is because one of the genes that codes for color-detecting pigments has different alleles. Heterozygotes can see three colors, homozygotes can see only two. By contrast, Old World monkeys and hominoids have trichromatic vision by virtue of a gene duplication in our ancestry, which generated two different genes that diverged in sequence to be sensitive to different wavelengths of light.

    The convergence of trichromatic vision reflects its adaptive value in anthropoids, which emerged from diurnal activity pattern, the need to detect young leaves for their protein content and low toxicity, and a coevolution of color vision with mating displays. At the same time, owl monkeys lost two-color vision in parallel with lorises and galagos, in this case reflecting the low adaptive value of color vision in nocturnal primates.

    Last, I discuss the polymorphism of eye color in living humans, which emerges due to the regulation of OCA2 in the surface layers of the iris.

    Study questions: 
    • Would you predict that the common ancestors of New World monkeys, Old World monkeys and hominoids had three-color or two-color vision?
    • Why is two-color vision so often lost in lineages that are active nocturnally?
    • Would it be possible to use zebrafish and fruit flies as models to understand human biology if we did not share common ancestors with these species? Why or why not?
    Study terms: 
    pigmentation
    melanin
    trichromatic
    monochromatic
    dichromatic
    toolkit gene
    Tags: 
    pigmentation
    phylogeny
    primates
    vision
    evo-devo
    development

  • Lost pregnancies in geladas after male takeovers

    Thu, 2012-02-23 19:21 -- John Hawks

    Ed Yong reports on new research from Eila Roberts, with Jacinta Beehner's research group at the University of Michigan, who was able to show that the rate of pregnancy loss among geladas (close baboon relatives) skyrockets when a new dominant male takes over a group "The Bruce effect – why some pregnant monkeys abort when new males arrive".

    Geladas live in units where a single dominant male lords over several related females, whom he monopolises as mates. It’s an enviable position, and males often have to fend off takeover bids by eager bachelors. If a newcomer ousts the chief monkey, it’s bad news for the group’s females. A wave of death sweeps through the unit, as the new male kills all the youngsters whom his predecessor fathered. Indeed, babies are 32 times more likely to die after a takeover than at any other time.

    But that’s not all. Eila Roberts from the University of Michigan has found that the new male’s arrival triggers a wave of spontaneous abortions. Within weeks, the vast majority of the local females terminate their pregnancies. It’s the first time that this strategy has been observed in the wild.

    It really adds a new perspective to the well-known examples of male infanticide in primates. Finding early enough evidence of pregnancies and tracking their progress takes painstaking work collecting and processing fecal samples for hormone levels -- where the hormone quantities may be known only long after the researcher returns from the field with observations. The study is in the early access section of Science, which makes it hard for me to give bibliographic information, but here's the abstract.

    Tags: 
    primates
    behavior
    infanticide
    geladas
    birth
    dominance

  • Incisors

    Mon, 2011-10-17 23:41 -- John Hawks
    Synopsis: 
    Laboratory exercise introducing incisors, including lemur tooth combs.

    The incisors are the front teeth. They are basically flat and have a blade-like occlusal surface. Each quadrant has two incisors.

    In humans and other primates, the upper central incisor (called the I1) is typically larger, the lateral (the I2) smaller.

    At this station you'll find casts of several primates, including some prosimians with tooth combs. Examine these mandibles. Some of the tooth combs include four teeth, and some six. The tooth combs with six teeth include the two incisors (I1 and I2) and the lower canines. The four-tooth combs are missing either the lateral incisor or the canine. Specialists disagree on this point. What do you think?

    Study terms: 
    incisor
    canine
    lemur
    Tags: 
    Anthropology 105
    explainer
    laboratory
    teeth
    primates

  • Primate classification and phylogeny

    Wed, 2011-10-12 13:07 -- John Hawks

    Our relationship to other kinds of primates is in part reflected by the pattern of similarities and differences we share with them. This pattern of similarities and differences is also used to classify different primate species into groups. There are six major branches of primates, classified as superfamilies. These include:

    Lemuroidea
    including the lemurs of Madagascar
    Lorisoidea
    including lorises and galagos
    Tarsioidea
    with the tarsiers
    Ceboidea
    or New World monkeys
    Cercopithecoidea
    or Old World monkeys
    Hominoidea
    including apes and humans

    The last three of these, hominoids, cercopithecoids and ceboids, share a common ancestor that lived sometime before 55 million years ago. These three superfamilies form a single branch, or clade, on the primate evolutionary tree.

    Scientists have often grouped these superfamilies into two major categories, called grades, which express the broad adaptations shared by different groups. Grades are not necessarily evolutionary lineages, but are meant to express the ways that different lineages share common sets of adaptations. One of these grades, the prosimians, includes lemurs, lorises, galagos, and tarsiers. Prosimians share a basic set of adaptations with other primates, including binocular vision, a slower rate of reproduction than many mammal groups, nails on the toes and fingers instead of claws, and other adaptations to life in the trees.

    Other primates include the monkeys, apes, and humans, who are grouped as anthropoids. Unlike the prosimians, the anthropoids form a single evolutionary lineage, or clade, because monkeys, apes, and humans are more closely related to each other than to any other living group. The evolutionary relationship of the anthropoids leads them to share many derived features, including

    Classification and phylogeny of living primate superfamilies

    Lemuroidea

    The lemurs are living and fossil primates of Madagascar. Lemurs share several derived features with lorises, including a set of closely-spaced and projecting lower incisors and canines, called a tooth comb, and a claw rather than a nail on the second toe. Both these features are used for grooming, and are so specialized relative to other primates, that lemurs and lorises may be classified as sister groups in a single clade, called the Strepsirrhini, a relationship supported by the close genetic relationship of the two superfamilies. Lemurs and lorises also share general features that were most likely present in the earliest primates, including a single postorbital bar instead of a bony enclosure for the eye orbits, and an external nose membrane connected to the upper lip.

    Today, the lemurs include five distinct families, all limited to Madagascar. These range in size from the mouse lemurs, which are the smallest living primates at only 60 grams, to the indri, approaching 10 kg. The most divergent living lemuroid is the aye-aye, a nocturnal creature with long bony clawed fingers and perpetually growing incisors, both supporting an adaptation to finding and eating grubs inside of wooden branches. The other lemuroids show a broad range of dietary and locomotor adaptations. Some species, like the sifaka, primarily leap using long hindlimbs and cling to vertical branches. Others are arboreal quadrupeds, or spend substantial time on the ground.

    In the recent past, a greater diversity of lemurs existed on Madagascar than remain today. Large extinct lemurs such as Megaladapis reached over a hundred kilograms at their largest, exceeding the size of female gorillas. Some large extinct lemurs appear to have had a sloth-like adaptation for below-branch suspension and feeding, and most of the larger forms primarily ate leaves. These lemur species existed within the past fifteen hundred years, and were likely driven to extinction by humans, who reached the island within that time period. The discovery of lemur skeletal remains in association with human archaeological sites confirms this extinction hypothesis.

    Lorisoidea

    The living lorisoids include galagos and lorises. Galagos, or bushbabies, are small prosimians weighing for the most part a fraction of a kilogram. Nocturnal creatures with large eyes and ears, and long tails, galagos are found across West Africa and into the central and southern portions of the continent. Galagos eat mainly fruits, insects, and gums, and some species are mostly quadrupedal, while others are adapted to leaping and a vertical posture. Lorises share a similar diet and a nocturnal activity pattern with the galagos, but differ in their relatively slow and deliberate style of foraging. Several species of lorises are distributed across Africa, South Asia, and Southeast Asia.

    Tarsioidea

    Tarsiers are small primates from the islands of Southeast Asia: Java, Borneo, Sulawesi, and the Philippines. Averaging slightly greater than 100 grams, tarsiers have several distinctive skeletal adaptations, including very long legs and ankles, immense eyes, and large hands and feet. These features support the adaptation of tarsiers of clinging to vertical branches and leaping between them, as well as their nocturnal activity pattern. Tarsiers eat insects, lizards, and other small vertebrates.

    The tarsiers share several features with anthropoid primates that may indicate a close phylogenetic relationship between the two. Unlike other prosimians, tarsiers lack a moist external nose, and they have a wide postorbital plate instead of a narrow bar. These and several more subtle cranial features link tarsiers with monkeys and apes. Many paleontologists believe that anthropoids may have originated from an Eocene group, called the omomyids. This group shares many features with living tarsiers and which may represent the common ancestor of both groups of living primates. If true, then the tarsiers are the closest primate relatives to the anthropoids, and the two groups form a clade called the Haplorrhini.

    Ceboidea

    Ceboidea includes the American, or New World, monkeys. Much of the earliest record of primate evolution, dating to greater than 40 million years ago during the Paleocene and Eocene, is from North America. These lineages apparently became extinct in North America by Oligocene times, and the New World monkeys that appear in the Oligocene of South America derive from an early Old World anthropoid lineage. The earliest anthropoid primates now known are from East Asia, which may have been the original source location for the anthropoids. Present-day New World monkeys possess three upper and lower premolars on each side, like most prosimians and the earliest anthropoids. This primitive dental formula distinguishes the New World monkeys from both the Old World monkeys and apes, which have only two premolars rather than three. Because of their round, forward-facing nostrils, the New World monkeys are called Platyrrhini, or flat-nosed, in contrast to the more narrow noses of the cercopithecoids and hominoids, which together are called Catarrhini, or downward-nosed. These differences imply that the platyrrhines existed as a lineage apart from catarrhines at least as early as 35 million years ago, when the first fossil catarrhines lived.

    South America was an island continent until around 5 million years ago, when the connection to North America arose. The first fossil New World monkeys date to the Late Oligocene, some 30 million years ago, meaning that these primates reached South America by an ocean crossing, probably an accidental journey on rafting vegetation from Africa. Once they reached South America, these monkeys underwent an impressive diversification. There are five living subfamilies of New World monkeys, with a total of sixteen genera. These vary from the relatively tiny callitrichines, including the marmosets and tamarins, to the relatively large atelines, including spider monkeys and howler monkeys, but most species range from one to five kilograms in mass. Several New World monkeys, including spider monkeys, howlers, and capuchin monkeys, have prehensile tails.

    Cercopithecoidea

    The cercopithecoids, or Old World monkeys, include two major groups. The cercopithecines, including macaques, baboons, guenons, and mangabeys, have a broad diet based on fruits, leaves, seeds, nuts, and insects. The colobines, including African colobus monkeys and Asian langurs and proboscis monkeys, tend to specialize to a greater degree on leaves in their diets. Old World monkey species have a diversity of anatomical adaptations to support these general patterns. The dietary diversity of cercopithecines is supported by broader incisors and low-crowned molars, typical of fruit eaters, and pouch-like cheeks for stashing food. Colobines have high-crowned molars for shearing leaves, and a large specialized gut for digesting leafy matter.

    A key adaptation shared by all living cercopithecoids is the distinctive shape of their molars. These teeth have cusps aligned into two high ridges extending across the tooth from side to side, a pattern called bilophodont. The ridges on the top and the bottom teeth interlock in opposing sawtooth patterns, creating a strong shearing action, ideal for reducing leaves and other fibrous plant matter, such as fruit rinds. This dental pattern is first found in fossil monkeys from the Early to Middle Miocene of East Africa. The living varieties of cercopithecoids arose later and underwent a major adaptive radiation during the Pliocene. Today the cercopithecoids include some of the most successful varieties of primates because of their geographic extent, the number of their species, and their dense populations.

    Hominoidea

    The hominoids include the living apes and humans, and their fossil relatives. The living great apes belong to four species, including orangutans, gorillas, chimpanzees, and bonobos. The gibbons and siamangs are in the hylobatid family, and include several different species sometimes called lesser apes. Humans and their extinct relatives are the hominins.

    Tags: 
    primates
    taxonomy
    phylogeny

  • Primate extractive foraging and tool use

    Tue, 2011-09-20 17:08 -- John Hawks
    Synopsis: 
    Many kinds of primates make and use tools, or find other ways to defeat the natural defenses of their foods.

    An important difference among some primate species is their ability to get foods that are hidden or protected by natural defenses. A little cleverness may yield foods that are inaccessible to other animals.

    For example, gorillas eat a high proportion of leaves and stems of terrestrial plants, especially in mountainous habitat where fruits are scarce. These herbaceous plant parts often have defenses such as stinging hairs or thorns. Such defenses are meant to deter animals like gorillas from eating the plants, and they are effective — it hurts to eat plants that sting! But gorillas can make use of these plants by following special methods to neutralize the defenses. One kind of sting-covered nettle leaves is commonly eaten by mountain gorillas, which carefully roll stacks of leaves in a way that encapsulates the stings inside a single leaf where they do not hurt so much to chew [1].

    Some primates make and use tools for extractive foraging, including chimpanzees, bonobos, orangutans and capuchin monkeys. A tool can be any kind of natural object that is altered by an individual and used for a purpose. Capuchins use and alter sticks to probe holes for insects [2]. Some groups of capuchins have developed a way of cracking nuts by using large stones [3]. Capuchins are small monkeys, so it is quite impressive to see one lift a stone bigger than his head, then toss it down forcefully to break open a nut. Other capuchins gather around to watch and pick up the shattered fragments of nutmeats. Younger capuchins seem to choose to watch the most skilled nutcrackers, which gives them a basis for learning through this social event [4].

    Chimpanzees use both simple and complex tools. The most celebrated chimpanzee tool is the termite stick. This is simply a stick or leaf stem that has been stripped by the chimpanzee, forming a long probe. This is inserted into termite or ant nests where the insects crawl onto the stick. Then, the chimpanzee pulls the stick out and licks off the termites [5].

    A more elaborate version of this behavior, probing into holes for a hidden resource, can be used to obtain honey. Honey is an important resource for chimpanzees in many parts of their range, and is produced both by bees that live in trees or hollow logs, and by bees who live in burrows underground. Finding the entrance to an underground hive is a simple matter of watching where the bees go. But the brood and honey chambers of these burrows may be a meter or more underground, and removed some distance from the entrance. Chimpanzees must dig quite a long tunnel in some cases to get the honey, and for this they use several different wooden tools to probe, soften and break up the ground, and dig [6].

    Chimpanzees also crack nuts across some parts of their habitat, and this is one of their most complex tool-using behaviors [7]. Different groups use different techniques for cracking nuts. Generally, a chimpanzee puts a nut on a large stone or log. Then, the chimpanzee uses a hammerstone or log to strike the nut. This may take several blows, and the effectiveness depends on the orientation of both the nut and hammer. Chimpanzees return to favored stone platforms or tree roots over many years, so that this technological element is a persistent feature of chimpanzee societies. Archaeologists have studied this behavior to try to see what traces may remain from using stone in this way, and have even found evidence of chimpanzee nutcracking from thousands of years ago [8]. Some chimpanzees do not crack nuts at all, even those who have nuts in their environment. For example, the chimpanzees at Loango, Gabon, do not crack nuts but use complex sets of tools to probe underground bee hives for honey [9].

    Chimpanzees and other apes use tools for purposes other than foraging. For example, some chimpanzees clip a leaf with their lips or teeth as a signal to other individuals---perhaps an invitation to groom or to play. Leaves and leaf stems are used extensively for wiping the body and probing teeth. Leaves are also used to soak up water and squeeze it into the mouth, like a sponge. These and other simple uses of natural objects vary among populations of chimpanzees extensively. Tool use therefore suggests that chimpanzees are interacting with some aspects of the material world in part through their mental adaptations for social behavior, as they absorb behavioral and technological knowledge from other individuals.

    Other hominoids use tools less extensively than chimpanzees but show similar abilities to perform complex tasks. Like chimpanzees, orangutans can be trained to use many kinds of human tools, even extending to complex tasks. But their natural use of tools is very limited, perhaps linked to the relative lack of extractive foraging opportunities in their arboreal existence [10]. Likewise, bonobos use leaves in some ways similar to chimpanzees, but extractive foraging is not common [11]. Experiments in naturalistic settings show that chimpanzees tend to use their existing cultural knowledge to solve new problems. For example, chimpanzee groups where sticks are a common solution to problems tend to use sticks to probe for novel foods, while those who use more leaves in other contexts will more likely probe with fingers than with sticks [12]. The familiarity with tool use may help develop new tool-using behaviors, even if the cognitive potential for tool use is widely shared among primates that don't use them.

    References

    1. Citekey Byrne:1993 not found
    2. Phillips PC. The Language of Gene Interaction. Genetics. 1998;149:1167–1171.
    3. Anderson JR. Use of objects as hammers to open nuts by capuchin monkeys (Cebus apella). Folia primatologica; international journal of primatology. 1990;54(3-4):138-45.
    4. Ottoni EB, de Resende BD, Izar P. Watching the best nutcrackers: what capuchin monkeys (Cebus apella) know about others' tool-using skills. Animal cognition. 2005;8(4):215-9.
    5. Goodall J. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Harvard University Press; 1986.
    6. Sanz CM, Morgan DB. Flexible and Persistent Tool-using Strategies in Honey-gathering by Wild Chimpanzees. International Journal of Primatology. 2009;30(3):411 - 427.
    7. Boesch C, Marchesi P, Marchesi N, Fruth B, Joulian édéric. Is nut cracking in wild chimpanzees a cultural behaviour?. Journal of Human Evolution. 1994;26(4):325 - 338.
    8. Citekey Mercader:2002 not found
    9. Boesch C, Head J, Robbins MM. Complex tool sets for honey extraction among chimpanzees in Loango National Park, Gabon. Journal of human evolution. 2009;56(6):560-9.
    10. van Schaik CP, Ancrenaz M, Borgen G, Galdikas B, Knott CD, Singleton I, Suzuki A, Utami SS, Merrill M. Orangutan cultures and the evolution of material culture. Science (New York, N.Y.). 2003;299(5603):102-5.
    11. Hohmann G, Fruth B. Culture in Bonobos? Between‐Species and Within‐Species Variation in Behavior. Current Anthropology. 2003;44(4):563 - 571.
    12. Gruber T, Muller MN, Strimling P, Wrangham R, Zuberbühler K. Wild chimpanzees rely on cultural knowledge to solve an experimental honey acquisition task. Current biology : CB. 2009;19(21):1806-10.
    Study terms: 
    social learning
    tool
    extractive foraging
    termite fishing
    capuchins
    chimpanzees
    Tags: 
    tool use
    primates
    behavior
    culture
    learning
    social learning

  • Primate vertebral numbers

    Sun, 2011-09-18 20:36 -- John Hawks
    Synopsis: 
    A laboratory exercise to explore the numbers of vertebrae in different primates.

    Between the skull and the sacrum, humans have 24 vertebrae. Well, most humans, anyway. Sometimes humans have a few more or less.

    Humans vary in the length of the lumbar region, the number of vertebrae between the lowest ribs and the pelvis. The typical number is five, but some people have only four. Rarely, people have six lumbar vertebrae.

    Non-human primates also vary in the number of lumbar vertebrae. This variation is connected to locomotion. Species with vertical, suspensory postures have relatively short lumbar columns. Chimpanzees, gorillas and orangutans have fewer lumbar vertebrae than humans. Quadrupedal primates, including most monkeys and prosimians, have longer lumbar columns than humans.

    What to do: This station has several skeletons of different kinds of primates — both New World and Old World monkeys and apes. Determine the number of lumbar vertebrae in each of these primates. Do these primates vary in the other segments? Do they ,for instance, have the same number of ribs?

    Anatomy of the vertebral column
    Study questions: 
    1. Consider the number of lumbar vertebrae in gorillas and orangutans. Explain how these apes each have relatively few lumbar vertebrae and humans have more than either. What do you suppose was the number of vertebrae in the common ancestors of these apes and humans?
    2. Why do quadrupeds have a longer lumber spine?
    3. Why do you think there is very little variation in the cervical spine?
    Study terms: 
    lumbar
    cervical
    thoracic
    posture
    locomotion
    New World monkeys
    Old World monkeys
    apes
    hominoids
    Tags: 
    Anthropology 105
    explainer
    laboratory
    anatomy
    primates
    vertebrae

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