Primates belong to the order Primates. Members of this order include prosimians, monkeys, apes, and humans.
The primates are divided into two suborders: Prosimii and Anthropoidea. Prosimians are the more primitive members of our order, i.e., they more closely resemble the earliest primates, whereas members of Anthropoidea (i.e., monkeys, apes, and humans) are more derived, that is, they exhibit evolved characteristics not present in the ancestral primates.
The majority of primates are found within 20 degrees north and south latitude of the equator. They primarily inhabit the continents and surrounding islands of Africa, Asia, and South America. One species of monkey, the Barbary macaque, is native to Gibraltar; however, it is believed to have been introduced by the Romans and, in modern times, restocked by the English. Primates, while largely confined to the tropics, also inhabit more temperate regions.
Forests can be divided and categorized according to their stage of development and/or regeneration (e.g., primary forest, secondary forest, forest edge, disturbed areas). While primates have specific habitat requirements, some species’ requirements are more stringent than others. For example, those species that locomote via brachiation (i.e., swinging by their arms) typically require primary forest. Other species are more adaptable and can inhabit a variety of forest types and possibly even disturbed areas. In the area of Costa Rica where I conduct research on howler monkey-feeding ecology, spider monkeys are found only in areas where the forest is continuous. Howler monkeys, conversely, can be seen in trees alongside roads in beach communities and traveling along electrical wires. Other species that do well in disturbed areas include species of langurs, macaques, and marmosets.
Most of the world’s primates inhabit tropical forests. Primarily, forest-dwelling species are arboreal in nature. Tropical forests can be classified by the amount of rainfall they receive, ranging from rain forests to dry forests. Rain forests experience little seasonal variability and rain falls year round. There is high species diversity but low species density. Thus, in a given unit of land, for example, many tree species will be present but at low numbers. Tropical forests that are not categorized as rain forests exhibit a greater degree of seasonal variability, in having both wet and dry seasons, and have lower species diversity but higher species density.
Primates are, to varying degrees, insectivorous and/or herbivorous. The degree of seasonality in a given area thus affects what is available as food at any point in time, since the life cycles of many insect and plant species are correlated with climatic conditions. Even in rain forests, where conditions are more stable, plant production cycles and the patchy nature of food resources affect food abundance and availability for primates.
Tropical forests contain a variety of habitats where primates are found. Some species have very specific habitat requirements. For example, Allen’s swamp monkey inhabits swamp forests of central Africa; proboscis monkeys are found adjacent to water on the island of Borneo; mountain gorillas inhabit cloud forest areas of central Africa; and certain genera of sakis and uakaris are found only in the seasonally flooded Amazonian forests.
There are several genera of primates that are found in temperate regions (e.g., areas of China, Japan, India, and Argentina). A few species are even found in high altitude/latitude areas where it snows seasonally. Several species of macaques live in regions that experience relatively high amounts of snow (for example, the snow monkeys of Japan, the Tibetan macaques of China, and the rhesus macaques of northern India). The Yunnan snub-nosed monkeys (genus: Pygathrix) of the Himalayan Mountains of China endure the coldest temperatures and longest winters of any nonhuman primate. At certain times of the year, their diet consists primarily of hanging lichens.
Primates found in more open areas tend to be semiterrestrial quadrupeds, for the obvious reason that the forest is not continuous. Several species of primates inhabit open woodland/scrub areas of sub-Saharan Africa (for example, savanna baboons, vervet monkeys, patas monkeys, and chimps). Some of these species are also found in more heavily forested regions, e.g., baboons and chimps. Two of these open-habitat species (the baboons and patas monkeys) exhibit cursory adaptations due to the greater risk of predation in an open environment. While most species sleep in trees, baboons in open, dry areas sleep in large congregations on “sleeping” cliffs.
Two species of primate live in very dry habitats. The Hamadryas baboons inhabit the dry plains of Ethiopia, the driest habitat of any nonhuman primate. Gelada baboons are also grassland animals of Ethiopia. They are found on the high Ethiopian plateaus. They are adapted to eating a diet that consists, primarily, of grass. They creep along in a sitting position, picking grass blades, seeds, and corms. Both of these species rely on cliffs and gorges for sleeping sites.
Primate species vary in the size of the home range they occupy. For example the smallest of the primates, the mouse lemurs, are reported to have home ranges as small as 50 meters in diameter, whereas chimps can have home ranges of up to 40 kilometers. This variability is due to body and group size or to diet. Small animals tend not to range as far, and hence have smaller home ranges as compared with larger animals. Solitary foragers may not have to range as far as group-living primates because they do not have to compete with other group members for access to food. Finally, the dietary pattern of a species affects how large a supplying area, or home range, it needs. Thus, when comparing two group-living species that ingest a high quantity of leaves (which are relatively more abundant than fruit), gorillas with their larger body size would need larger home ranges than howler monkeys. When comparing ripe fruit specialists with more folivorous species, it is apparent that due to the patchy nature of fruit sources, the more frugivorous species need larger home ranges. Chimps and spider monkeys are considered to be ripe fruit specialists. They are closely related to gorillas and howler monkeys, respectively. Chimps have much larger home ranges than do gorillas, and, likewise, spider monkeys have much larger home ranges than howler monkeys. In the dry forest in Costa Rica, a spider monkey group’s home range includes howler monkeys’ home range as well as multiple other howler groups’ home ranges.
Some species defend their home range from con-specifics (i.e., other members of their species) and are thus termed territorial. They actively ward off intruders, either individuals or groups of conspecifics, from their home range and especially from their core area. The core area is that area of a group’s home range that is used and defended more intensively. Even those species that are considered to be nonterritorial will often defend their core area, whereas there may be a degree of overlap with surrounding groups of con-specifics in more peripheral areas of their home range.
While the distance traveled in a day, termed day range or daily path length, is often correlated with home range size, it is variable, and it too is often related to diet. Once again, those species that ingest a relatively higher percentage of fruit in their diet tend to range farther in search of fruit. For example, chimps can range up to 15 kilometers in a day. In addition, both chimps and spider monkeys fission into smaller foraging parties to avoid competing with other group members for this highly ephemeral resource.
Many researchers have recorded data on how primate species spend their time during the day and night as engaged in the following activity categories:
Feed: time spent actively engaged in feeding and foraging activities.
Rest: time spent resting during the day or, in the case of nocturnal species, at night.
Move: time spent moving through the home range.
Social: time spent engaged in social activities with family/group members (for example, grooming, playing, sexual behavior).
Species vary in the amount of time they spend in these various activities. In addition, activity cycles may vary both within and between seasons. For example, some species spend more time moving during the wet versus the dry season. Likewise, during the wet season, when rain is intense, or during the dry season, when sun and heat are intense, animals may move less. During the six-month dry season at one Costa Rican study site, the howler monkeys ranged from 7 to 10 hours resting time per day in 11 hours of observation. As you can imagine, this varies considerably from usual human patterns.
Diet, social organization, and group size and composition influence activity cycles. Because primate species can be categorized by their diet as well as type of social organization, and, to a lesser extent, by group size and composition, species tend to exhibit less within-species variability than between-species variability in activity cycles. Thus, activity cycles can be thought of as characterizing a species or being species-specific. Howler monkey activity cycles are thus skewed toward resting and less so toward social activity. My study group spent less than 1% of its time engaged in social activities. Conversely, the rhesus macaques I observed on the island of Cayo Santiago, Puerto Rico, were highly social and spent much less time resting than my howler subjects.
Research demonstrates the role that reproductive condition plays in female rhesus macaque activity cycles on the island of Cayo Santiago, Puerto Rica. The time that females spent in the various activity categories did not differ before or after an infant was born. However, as the infant increased in size, lactational demands increased to the point that females were spending almost no time resting or involved in social activities. This is especially surprising because subjects were provisioned, meaning that they were supplied with food. They supplemented their intake of monkey chow with plant parts. As the infants began to feed on their own, the females spent less time feeding/moving and more time in rest and social activities.
The following general definitions pertain to dietary categories that are relevant to the primates:
Herbivory: consumption of plant matter. The following are sub-categories of herbivory:
Frugivory: consumption of fruits.
Folivory: consumption of leaves.
Gramnivory: consumption of grass.
Nectivory: consumption of nectar.
Gumnivory: consumption of plant exudates (for example, sap, latex).
Faunivory: consumption of animal matter. The following is a sub-category of faunivory:
Insectivory: consumption of insects.
Omnivory: consumption of a combination of plant and animal matter.
All but one species of primate (the tarsier) are herbivorous to some degree. Most species consume fruit for carbohydrates, with the remainder of the diet made up of either leaves or insects to fill their protein requirements. Smaller primates usually consume a combination of fruits and insects (frugivore-insectivores), whereas larger species usually consume a combination of fruit, flowers, and leaves (frugivore-folivores). This size differential is due to the fact that large animals are not able to meet their dietary needs by foraging for insects. This is not to say that larger animals never consume insects. The largest primates—chimps, orangs, and some gorillas— consume tiny ants and termites.
Some of the primates are relatively unique in terms of their diet and foraging strategies. The tarsier is the only extant primate that is completely faunivorous, ingesting a variety of insects and small animals (for example, snakes, birds). The smallest of the New World monkeys (i.e., the marmosets and tamarins) are partly gumnivorous. These animals have modified nails that resemble claws. They are used to cling to upright trunks/branches to consume exudates. The marmosets have specialized front teeth that are used for gouging holes in trees to access exudates. Exudates are a very important aspect of their diet, and they defend those trees that are used for their consumption from neighboring monkey groups. The tamarins do not rely as extensively on exudates, and they lack the dental specializations seen in the marmosets. Some of the Madagascar lemurs are nectivorous and serve as important pollination agents. Bamboo lemurs feed primarily on bamboo shoots, which contain cyanogenic compounds that are highly toxic to most mammals. The gelada is the only primate that is considered to be gramnivorous, with as much as 90% of their diet coming from grass consumption. They occupy the high treeless plains of Ethiopia. While several species of primate are known to hunt small-to medium-sized mammals (for example, capuchins, baboons, orangutans, and chimps), only the chimps are known to hunt cooperatively. However, Strum observed her savanna baboon subjects to change their hunting strategy to more of a relay style, so that when one male was tiring, another male would take up the chase. This is the only non-chimp case that I know of that suggests a modicum of cooperation.
As mentioned, most primates consume fruit to varying degrees. In general, these fruits are ripe. However, some species can consume unripe fruit, and thus avoid competition with other species. I have often observed howler monkey subjects feeding in fig trees (figs are a preferred fruit of primates) for 1 week or more before the other two resident species (spider and capuchin monkeys) began to feed on them.
Because primate olfactory capabilities are reduced relative to other mammals, there is speculation that some groups of primates have derived visual capabilities for detecting when fruit is ripe. All of the Old World anthropoid primates possess trichromatic color vision, which allows us to distinguish the red-green hues of the spectrum. Two species of New World monkeys—the howler and capuchin monkeys—also possess this adaptation, but it evolved independently from the Old World species. All of the remaining species possess dichromatic vision. An alternate hypothesis to the ripe fruit scenario is that trichromacy may have evolved in response to leaf consumption. Young leaves are preferred over mature leaves in most species of primates. They are higher in water and lower in structural carbohydrates, and, in some cases, may be higher in protein and lower in chemical defenses. Some plant species produce red pigments, termed anthocyanins, in their young leaves. These anthocyanins act to “disguise” the leaves from those animals possessing dichromatic vision because they perceive them as being dark and hence mature.
Trichromatic individuals can discern the color red and can possibly even be attracted to the leaves or at least be able to “see past” the red streaking and detect the underlying yellow-green color that is characteristic of young leaves.
While the majority of larger primates are frugivore-folivores that prefer fruit to leaves, the degree to which they depend on fruit varies. As mentioned, some species are ripe fruit specialists and they range far and wide in their search for fruit sources. Alternatively, though, are those species that rely on leaves to a greater extent, especially when fruit is unavailable. Three groups of primates exhibit this strategy: (1) members of the Old World monkey subfamily Colobinae, (2) gorillas, and (3) two species of New World monkeys, the howler monkeys and the muriquis. The Colobinae (i.e., the colobus monkeys of Africa and the langurs and leaf monkeys of Asia), have specialized stomachs for digesting leaves. The microflora in their foregut are capable of breaking down a larger proportion of the plant structural carbohydrates, hemi-cellulose and cellulose, relative to the other large anthropoid primates. This is termed foregut fermentation. They can thus derive energy from this more ubiquitous food source. The other two groups—the gorillas and the two species of New World monkeys— are hindgut fermenters. They lack the gastrointestinal specializations seen in the colobines but are still capable of deriving energy from leaves via an elongated large intestine and slower throughput time (i.e., the time food takes to pass through the gut). These primate groups minimize energy expenditure because their gut activities are costly and leaves are a poorer source of energy relative to fruit.
Social Organization in Relation to Ecology
Social organization refers to the grouping patterns and inherent “rules” that determine group membership and conduct in social animals. Before a discussion of social organization can be attempted, however, several terms need to be clarified. There are several mating patterns that are relevant to primate social organization. They are as follows:
Monogamy: an individual having only one mate, often for life.
Serial monogamy: an individual having only one mate at a time. This is the mating pattern seen in humans and will not be discussed herein.
Promiscuous: an individual with multiple mating partners, especially females during a single estrus period.
Polygamy: an individual having more than one mate, regardless of the sex of the individual.
Polygyny: a male with more than one mate.
Polyandry: a female with more than one mate.
In addition, there are relevant membership patterns, especially those involved with who comes and goes within groups. They are:
Natal group: group into which an individual is born.
Female-philopatric: females stay in their natal group, also termed matrilineal.
Male-philopatric: males stay in their natal group, also termed patrilineal.
Emigration: an individual leaves its group, natal or otherwise.
Immigration: an individual enters a new group.
There are several genera of primates that are categorized as solitary foragers. For the most part, these animals are small nocturnal foragers that rely on crypsis (camouflage) to avoid detection by animal prey and predators (for example, the nocturnal prosimians of the families), Cheirogaleidae (mouse and dwarf lemurs), Galagonidae (galagos or bush babies), and Loridae (pottos and lorises). Females leave their young in nests or park them on branches while they are out foraging. Some of the Loridae have evolved a unique form of locomotion, termed slow climbing, that allows them to move very slowly and freeze for long periods of time so as to avoid detection.
In the past, these animals were categorized as having a solitary social organization wherein the largest grouping consisted of a female and her dependent young. Males and females came together for mating purposes. However, as more research has been conducted with these species, it has become apparent that their social organization is more complex. In many cases, it is now realized that male home ranges are large, relative to females’, and they overlap multiple female home ranges. This discovery has prompted a revised category of social organization for these species, termed dispersed polygyny since the male has multiple mates. In addition, females may house their young communally in tree holes or leaf tangles (for example, mouse lemurs), and these females may be related, i.e., matrilineal (for example, galagos).
The exception to the solitary forager as nocturnal frugivore-insectivore is the case of the orangutan. While orangs are also an example of dispersed polygyny, it is not nocturnal insectivory that prompts their solitary ways, but rather that they are large arboreal frugivore-folivores that need a lot of food to satisfy their needs. Females cannot “afford” to share their home ranges with other females. They do depend on the resident polygynous male to protect them from non-resident males (often sub-adults) that may attempt to “rape” them. Orangs are the only nonhuman primates wherein rape has been reported. It is thought that forced copulation is a strategy whereby males may sire offspring before acquiring a home range and females of their own. It can thus be seen that this is a good example of an evolutionarily stable strategy that could have been selected for. Resident males respond to female screams and chase these “unwanted” males away.
Monogamy is seen in three primate families (Indriidae, Tarsiidae, Hylobatidae) and three genera of Atelidae (owl, titi, and saki monkeys). In addition, one species of the Colobinae, the Mentawai leaf monkey, is monogamous. This type of mating pattern is also termed “territorial pair,” in that bonded pairs defend their home ranges from other conspecifics. Because there is no male-male competition for females once pairs have become established, monogamous species are monomorphic (i.e., males and females do not differ in morphology, especially size). It is an exceptional type of mating pattern because male mammals are not reproductively limited as are females. It is thus a bit of a conundrum how this mating pattern could have been selected for since natural selection works to maximize reproductive success. There are, however, ecological characteristics that correlate with monogamy and hence are used to explain its occurrence. They are (1) the nature of the resource base, i.e., patchy distribution in time and space, and (2) the need for resource defense from con-specifics. These factors necessitate the partner’s cooperation in home range defense. Thus a lone female cannot be expected to defend a suitable supplying area to guarantee the successful rearing of her young.
Those males that cooperated with females to defend resources and raise young had a reproductive advantage given these environmental constraints and, thus, the trait(s) was/were selected.
Two genera-the indris (genus: Indri) and the gibbons and siamangs (genus: Hylobates)—practice duetting to aid in territorial defense. This involves the male and female “singing” coordinated duets to demonstrate to surrounding conspecifics that the home range is under bonded pair occupation. This behavior is believed to have evolved as a spacing mechanism, relieving pairs from constantly having to monitor all areas of their home range. In at least one species of primates—the owl monkey—pairs do not exhibit overt bonding behaviors of duetting and grooming. Yet, they are truly monogamous in that they do not practice extra-pair copulations whereas, for example, gibbons and siamangs do opportunistically mate with outside conspecifics.
Only one subfamily of primate—the Callitrichinae (i.e., tamarins and marmosets)—exhibits this extremely rare form of social organization. As with monogamy, it does not make adaptive sense for a male to limit his reproductive success by staying with, or in this case, sharing one female. Once again, however, there are extenuating circumstances that make this form of social organization adaptive for these primates. In addition, it may not always be the case that two males are sharing a female but rather that one male is “biding his time” awaiting the opportunity to mate with the resident female. In these species, the norm is for the female to birth twins. The newborns can weigh up to 24% of the female’s weight. The males are primarily responsible for carrying and caring for the twins. Independent offspring may also help care for their young siblings. The female’s primary role is nursing the offspring and she spends much of her time foraging to fulfill lactational demands. It is usually the case that only the dominant female breeds in the majority of these species due to reproductive suppression of subordinate females. The mechanism by which this occurs is either through pheromonal production by the dominant female or aggression toward the subordinates resulting in them being anovulatory, i.e., they do not ovulate.
Garber has shown that those groups with two resident males are more likely to successfully rear both offspring, thus demonstrating the adaptive significance of the “helper male” phenomenon. It is thought that a male (heretofore referred to as the primary male) mates with the female and that the other male (secondary male) is demonstrating his alloparental (care given to offspring by someone other than the parents) skills in the hopes that the female will mate with him when the primary male leaves. Transfer between groups is common in these species. Thus, the secondary male can move into position as the breeding male when the primary male leaves. Of interest is the fact that male hormone profiles change when they are actively caring for infants. They exhibit higher levels of prolactin, which is implicated as having a key role in maternal behavior.
Traditionally, this type of social system was referred to as a harem, implying that one male had control over a group of females. Once again, however, as more research has been conducted with free-ranging groups of primates, the picture has changed. Only one species of primate exhibits a true harem social organization. The basic unit of Hamadryas baboon social organization is a male and his small group of females. The male maintains control of his females by herding and punishing them for straying, especially if they move too close to other males. The other species that live in one-male groups do not have the same degree of male control over the females, and the male’s tenure in the group is often of short duration. For example, in the gelada baboon, the male relies on the support of the females in order to remain in residence. If an outside male attempts to take over the group, he may very well do so if the resident females do not ally with the current male. In the case of the patas monkeys, the resident male does not even have exclusive reproductive rights to the resident females. During the mating season, outsider males mate with some of the females. While this seems not to be in the male’s best interest, the resident male, on average, impregnates a higher percentage of resident females than outsider males, and it is possible that those males that are associated with a group of females have differential access to resources such as food and water relative to bachelor males.
While some species of primates exhibiting this type of social organization are matrilineal (for example, Hanuman langurs), others, such as gorillas, are not. In addition, because male tenure may be short due to male takeovers, infanticide by the new males is seen in some species. This practice is interpreted as an evolutionarily stable strategy on the part of the males. Relative to females, males have a much lower probability of reproducing. Their options are limited because not all males are successful in acquiring a group of females and, as mentioned, tenure in a group is short due to the constant pressure from outside males. When a nursing infant dies, the mother ceases to lactate and becomes sexually receptive (i.e., “in estrus”). By killing off dependent offspring, the male has a greater probability of passing on his genes. While it may seem strange that a female would mate with a male that has killed her infant, it may be in her best interest, reproductively speaking. First, it would not be adaptive to delay becoming pregnant again. Second, if infanticide is under genetic control, a female may increase her own reproductive fitness by producing infanticidal sons that will have a better chance of reproducing.
The one-male grouping pattern is believed to have evolved where the resource base is insufficient to support larger groupings. In effect, females’ reproductive success would suffer beyond a given threshold of individuals.
Many species of New and Old World monkeys are multi-male, as are some of the lemurs of the Madagascar. There is a tendency for multi-male groups to be matrilineal. It is theorized that where resources are defensible, females have evolved the pattern whereby they remain in their natal group and cooperate with kin (i.e., matrilineal). With regard to spider monkeys and chimps, the males remain in their natal group and defend an area; this attracts females emigrating from other groups and is termed resource defense polygyny. Because both of these species are ripe fruit specialists and fruit is patchy in time and space, the group fissions into small foraging parties. While the group is comprised of multiple males and females and, hence, multi-male-multi-female, these species are categorized as having a fission-fusion or community type of social organization because of their unique pattern of intermittently fissioning and later, reuniting. In the case of the chimps, the core of the group is composed of groups of related males. These males actively defend the home range. Within this home range, females inhabit smaller individual home ranges. Thus it can be seen how the term community is appropriate for this species.
- Falk, D. (2000). Primate diversity. New York: W. W. Norton.
- Garber, P. (1997). One for all and breeding for one: cooperation and competition as a tamarin reproductive strategy. Evolutionary Anthropology, 5, 187-199.
- Lucas, P. W., Darvell, B. W., Lee, P. K. D., Yuen, T. D. B., & Choong, M. F. (1998). Colour cues for leaf food selection by long-tailed macaques (Macaca fascicularis) with a new suggestion for the evolution of trichromatic colour vision. Folia primatologica, 69, 139-152.
- Nowak, R. M. (1999). Walker’s primates of the world. Baltimore, MD: Johns Hopkins University Press.
- Rowe, N. (1996). The pictorial guide to the living primates. Charlestown, RI: Pogonias Press.
- Strum, S. C. (1987). Almost human. New York: W. W. Norton.