Primates generally live in well-developed social networks and this can be regarded as a defining characteristic of the order. Although species that are active by night (nocturnal) have commonly been described as solitary, field studies have revealed that there are intimate social links between individuals, maintained by intermittent contacts during the night and by sharing of nests during the daytime. Nevertheless, there is a major distinction between day-active (diurnal) primates and nocturnal species in that the former typically live in obvious cohesive social groups, whereas the latter usually move around and feed alone at night. In sum, while all primates have intricate social systems, as a general rule diurnal species are gregarious whereas in nocturnal species individuals are dispersed. Among nocturnal primates, the only exceptions to solitary behavior are found in a few species that are monogamous (pair-living), such as the avahis (Avahi) in Madagascar and the owl monkeys (Aotus) in the New World. Among diurnal primates, the only representative that is almost solitary like most nocturnal primates is the orangutan (Pongo) of Southeast Asia. Otherwise, the groups of gregarious diurnal primates can be classified into three main categories according to the composition of their groups: monogamous family units, one-male groups and multi-male groups. Monogamous groups typically consist of an adult pair living together with their immature offspring. Clear-cut examples of monogamy are found among lemurs (e.g., avahis, mongoose lemurs, red-bellied lemurs, and indri), among New World monkeys (e.g., owl monkeys, mar mosets, tamarins and Goeldi's monkey), in a few Old World monkeys (e.g., Mentawai langur) and in all gibbons. Such groups are necessarily relatively small and may contain between two and a dozen individuals. One-male groups, also known as harem groups, contain a single adult male, several adult females and a variable number of immature individuals. The best-known examples of one-male groups are found among such Old World monkeys as Hamadryas baboons (Pa-pio hamadryas), geladas (Theropithecus), guenons (Cercopithecus species), patas monkeys (Erythrocebus patas), and the majority of leaf-monkeys (e.g., black-and-white colobus and several langur species). Among the apes, gorillas also live in one-male groups. In many species that are characterized by harem groups, the surplus males join together in bachelor groups. Furthermore, in some cases several harem groups and bachelor male groups may move together in large herds that may contain over a hundred individuals, as is the case with Hamadryas baboons and geladas. Multi-male groups contain several adult males along with several adult females and a variable number of immature individuals. Examples of such social groups are widespread among primates and found in various diurnal lemurs like ringtails (Lemur catta) and some sifakas (e.g., Propithecus verreauxi); in most New World monkeys (e.g., capuchins, howler monkeys, spider monkeys, and woolly monkeys); in several Old World monkeys (e.g., plains baboons, vervet monkeys, and red colobus); and in chimpanzees. Various attempts have been made to reconstruct the evolutionary history of primate social systems. One key finding is that, although individuals are typically dispersed, nocturnal primates show social networks that exhibit parallels to the array of monogamous, one-male, and multi-male patterns found among diurnal primates. Reconstruction in comparison with other mammals suggests that the ancestral primates were nocturnal and lived in multi-male social networks similar to those found in most modern nocturnal prosimians.
Because they live in well-defined social networks, primates typically exhibit regular and relatively intense social interactions. One very common form of social interaction is grooming, which is frequently reciprocal. Even in nocturnal primate species that show dispersal of individuals at night, and in orangutans, which are usually dispersed by day, social grooming is a prominent feature of occasional encounters between familiar individuals. In prosimians, social grooming is usually carried out mainly with the teeth, and in lemurs and lorises (strepsirrhines) the tooth-comb is actively used. In higher primates, by contrast, the hands usually play a more intense role in social grooming, particularly in Old World monkeys and apes. Although the visual sense is highly developed in primates, olfactory signals continue to play a role in social interactions, particularly in prosimians and New World monkeys. Nocturnal lemurs and lorises still have relatively large olfactory bulbs in the brain, and marking with urine and/or feces and with secretions from special skin glands (e.g., on the chest) is prominent. For dispersed nocturnal prosimians, olfactory marking may be the primary means of communication between individuals while active. Visual displays are particularly important in diurnal primates, some of which have developed quite striking coloration patterns of the fur (e.g., certain lemurs, Old World monkeys, and gibbons). In fact, ringtailed lemurs show an interesting display pattern that
combines both olfactory and visual elements. During encounters between groups that have been labeled "stink fights," individuals anoint their tails with secretions from marking glands on the arms and then wave their tails in the air while strutting around. Perhaps the greatest diversity of color patterns on the face and elsewhere on the body is found in the African guenons, which often have characteristic head movements that emphasize any species-specific facial markings. Vocalizations are also generally important for social interactions among primates. Nocturnal primates usually have a relatively restricted vocal repertoire, but the calls that they do have are important for maintaining contact between dispersed individuals. Some of the smallest nocturnal primates (e.g., mouse lemurs, dwarf bushbabies) have calls that are in the ultrasonic range. Diurnal primates generally have richer vocal repertoires containing numerous calls in the audible range and their subtlety (e.g., through intergradation between call types) can be quite pronounced, particularly in certain Old World monkeys and chimpanzees. Many species like the lion tamarins and titi monkeys have long calls to maintain contact between neighboring groups.
Although it is often assumed that all primates show territorial behavior, defense of an exclusive territory is in fact comparatively rare among primates. Numerous nocturnal primates show range overlap between adults of both sexes, and diurnal primates that live in gregarious groups often show quite extensive overlap between group ranges. Some nocturnal prosimians, such as sportive lemurs (Lepilemur) in Madagascar and in a minority of diurnal primates, including some lemurs (e.g., certain populations of sifakas, Propithecus, and in the indri), show true territoriality in the sense of behavior shown to defend an exclusive area. There seems to be a general trend for primates that live in monogamous groups to show marked territorial behavior, and it has in fact been suggested that one of the factors promoting monogamy is joint defense of an area containing vital resources. Territorial behavior has been found in a variety of monogamous species, including such nocturnal lemurs as avahis (Avahi), such cath-emeral lemurs as the mongoose lemur (Eulemur mongoz), such diurnal lemurs as the indri (Indri), most marmosets and tamarins (Callitrichidae), and all gibbons (Hylobatidae). In fact, the indri, the gibbons, lion tamarins, and titi monkeys show conspicuous, often melodious vocalizations that carry over great distances in the forest and seem to play a part in territoriality. These "great calls" of the monogamous indri and gibbons provide one of the most striking examples of convergent evolution to be found among primates.
Most primate species are either exclusively nocturnal (active at night between dusk and dawn) or clearly diurnal (active by day between dawn and dusk). The majority of prosimian primates are nocturnal in habits, whereas simian primates are typically diurnal. Indeed, the only nocturnal representatives among simian primates are the owl monkeys of South and Central America (Aotus species); all the rest of the monkeys and apes, like humans, are diurnal. Of the three natural groups of prosimian primates, two contain only noctur-
nal species (loris group; tarsiers) while the third (lemurs) contains mainly nocturnal species but also some diurnal species. Among the lemurs, there is also an unusual pattern known as cathemerality in which there is a combination of nocturnal and diurnal activity. This is found in most or all brown lemurs (Eulemur species) and gentle lemurs (Hapalemur species). It has been found that in such species the proportions of nocturnal and diurnal activity vary over the annual cycle, and it seems that seasonal variation in ambient temperatures plays a part in this. Cathemeral activity has also been reported for some owl monkey populations in South America. Compared to other mammals, all primates have relatively large eyes, but in nocturnal primates the eyes are generally even larger. As a further adaptation to nocturnal life, lemurs and lorises typically possess a special reflecting layer behind the retina of the eye, known as a tapetum lucidum. Unique among mammals, the reflecting properties of this structure are derived from flat crystals of riboflavin. Although they are also nocturnal, both tarsiers and owl monkeys lack a reflecting layer behind the retina and they compensate for this by having even larger eyes than nocturnal lemurs and lorises. This is just one indication that tarsiers and owl monkeys are secondarily nocturnal and have adapted in a different way to night-time activity.
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