Saguinus Bicolor

All marmosets and tamarins live in groups. Group sizes range between 3-15 individuals, but most species are typically found in groups of 4-9. Groups of two animals usually represent migrating individuals without a home range or incipient groups. On average, group size is smaller in tamarins and lion tamarins compared to Amazonian and eastern Brazilian marmosets. Large groups of 20 or more individuals reported in the literature most likely represent the temporal association of two or more neighboring groups.

Based on captive findings where keeping an adult pair together with their non-breeding offspring was the most stable situation, marmosets and tamarins have traditionally been classified as living in family groups. However, callitrichid social organization is much more complex in the wild. In the best-studied species—that of the saddleback tamarin—social organization varies within populations, with groups being composed of one adult female plus two or more adult males, one adult female plus one adult male, one adult male plus two or more adult females, and two or more adults of both sexes plus immature individuals. During its existence, a single group may pass through these different forms of organization. Other species for which long-term data are available—like the common marmoset (Callithrix jacchus) and the moustached tamarin (Saguinus mystax)—have also been reported to be highly flexible in their social organization.

All tamarins and marmosets are diurnal and usually leave their sleeping sites at sunrise. The length of their daily activity period is, however, variable between species. Moustached tamarins and saddleback tamarins are active for ten hours and retire on average two hours before sunset, while pygmy marmosets (Cebuella pygmaea) and common marmosets are active for 11-12 hours, almost until dusk. Many species sleep in dense tangles of vines and epiphytes, but the base of palm fronds or the forking of trunks and large branches are also used. Saddleback tamarins and lion tamarins make intensive use of tree hollows, while moustached tamarins have never been observed in this kind of shelter.

Home-range size is highly variable both within and between species. The smallest home ranges are found in pygmy marmosets, which occupy areas of 0.25-1.24 acres (0.1-0.5 hectares). They are centered upon one or a few major sources of plant exudates; once these sources have been exhausted, pygmy marmoset groups shift to another area. In marmosets of the genera Callithrix and Mico, home-range size is related to the relative importance of exudates in their diet: highly exuda-tivorous species usually occupy much smaller ranges (1.2-16.1 acres [0.5-6.5 hectares] in common marmosets, 3.2-11.1 acres [1.3-4.5 hectares] in black tufted-ear marmosets, Callithrix peni-cillata) compared to the less exudativorous species (27-40 acres [11-16 hectares] in buffy tufted-ear marmosets, Callithrix aurita, and about 69 acres [28 hectares] in the Aripuana marmoset, Mico intermedius). In tamarins, home-range size varies between 20 acres [8 hectares] and almost 495 acres [200 hectares], but reasons underlying this variation are less well known; it is likely to be related to the productivity of the habitat. The largest home ranges for any callitrichid have been reported for golden-rumped lion tamarins (Leontopithecus chrysopygus) who may range over 682 acres [276 hectares].

The overlap between home ranges of neighbors also varies between species and populations. Home ranges of pygmy marmosets and Goeldi's monkey do not overlap and may not even be contagious, while in golden lion tamarins overlaps of up to 60% have been observed. Neighboring groups meet and interact in these overlap areas regularly, normally during the first hours of the day. Interactions may be aggressive, including chasing and fighting, but may also include friendly interactions like grooming and playing; in common marmosets, saddleback tamarins, and mustached tamarins, individuals from neighboring groups have been observed to be involved in sexual interactions. While the function of these between-group encounters is often seen as resource defense, it also provides the opportunity for immature individuals and non-breeding adults to familiarize with potential mates from neighboring groups. Additionally, during between-group encounters animals may migrate from one group to another without an intervening solitary phase.

Within-group social behavior is characterized by much affiliation. Social grooming is the most frequent interaction and

Saguinus Imperator Infant
The emperor tamarin (Saguinus imperator) may be threatened due to destruction of its environment. (Photo by J-C Carton. Bruce Coleman, Inc. Reproduced by permission.)

is usually performed during prolonged resting periods. It is usually the adult and subadult group members who are involved in grooming, and the breeding female of a group can be the primary focus of the grooming activity. One individual laying down in front of another one and presenting the body in a posture not taken during solitary resting often initiates a grooming session. Apart from grooming, animals also huddle together for extended periods. While adults and subadults are grooming, youngsters are involved in social play, which involves chasing each other to-and-fro, grabbling, wrestling, and smooth, inhibited biting. Aggressive behavior between group members is generally rare and limited to threats and displacements. It may occur in small food resources that cannot accommodate several individuals; severe aggression with fighting is extremely rare in the wild.

Callitrichids are highly vocal animals. In the wild, marmosets and tamarins can best be detected by listening to their calls. All species possess a long call, a vocalization of high intensity that is audible to the human ear over a distance of at least 500-650 ft (150-200 m). Long calls are composed of two

Callithrix Peni
A male tassel-eared marmoset (Callithrix humeralifera) peers out among the trees of the Brazilian Amazon. (Photo by Rod Williams. Bruce Coleman, Inc. Reproduced by permission.)

or more (up to about 30) notes, and are different between species, populations, and individuals; within an individual, the structure of long calls may also vary according to social context. Long calls function in intragroup cohesion and in the regulation of space between neighboring groups; in tamarin mixed-species troops they also function in the establishment of association. Long calls are most often heard in the early morning and usually precede and accompany encounters between neighboring groups.

Apart from long calls, all marmosets and tamarins possess a diversity of other calls. Specific alarm calls are emitted upon seeing a raptorial bird. Other group members (and even members of other species) "understand" these calls and may react in the appropriate way without seeing the raptor itself. Infant and juvenile tamarins utter squawks when soliciting and stealing food from another group member. Harsh and atonal calls are given during aggressive interactions. Many vocalizations are related to the cohesion of the group and coordination of activity. Pygmy marmosets vary the structure of their trills in relation to the distance to the nearest group member. In golden lion tamarins, different calls provide in formation about the sender's current location, activity, and intention.

Playback experiments with naturally recorded and with synthesized vocalizations revealed that pygmy marmosets show categorical perception, and that they respond to individual and contextual differences. Learning seems to be involved in the development of the vocal repertoire in marmosets and tamarins, since infants "babble" intensively and use vocalizations out of their proper context.

In contrast to most other simian primates, olfactory communication, that is communication by means of scent signals, plays an important role in the social life of marmosets and tamarins. Most callitrichids are equipped with scent glands in areas on and around the genitals (anogenital or circumgeni-tal glands), on the lower abdomen above the genitals (supra-pubic glands), and on the midline of the chest (sternal gland). The secretions of these glands consist of highly complex mixtures of fatty acids and their esters, proteins, and other organic compounds. Alone or mixed with urine, these secretions contain information about species and subspecies, sex, reproductive state, and individual identity of the sender. Scent gland secretions are applied to the environment through ritualized rubbing movements. Depending on the type of gland employed, this behavior is called anogenital, suprapubic, or sternal scent marking. The frequency with which the different types of scent marking behavior are employed varies between species. In most species, anogenital marking is the dominant mode, but in the Aripuana marmoset suprapubic marking prevails, and black-mantled tamarins, Saguinus ni-gricollis, most often use sternal marking.

Usually, scent gland secretions are applied to branches, trunks, and lianas, but some species also mark members of their group. In saddleback tamarins this behavior culminates in scent-marking "parties" where most or all group members mark each other and objects of the environment for 2-3 minutes. The sympatric moustached tamarin lacks this kind of performance. Reasons for such interspecific variation are suggested to rest in subtle differences in the social structure and mating strategies.

In tamarins, adult females scent mark significantly more than adult males, while in lion tamarins and marmosets, rates of scent marking are usually balanced between sexes or may be male-biased as in golden-headed lion tamarins, Leontop-ithecus chrysomelas. In common marmosets, Aripuana marmosets, saddle-back tamarins, and moustached tamarins scent marking is performed throughout the home range, and the spatial distribution of scent marks usually follows the patterns of home-range use. While this does not completely exclude a territorial function of scent marking, it is more consistent with a function in intragroup communication. In common marmosets scent marking by subordinate females may serve in the attraction of potential mates from neighboring groups.

In a few tamarin species urine washing has been observed, a behavior known from several other New World primates, particularly from squirrel monkeys (Saimiri). The behavior is, however, very rare and its function not known. Another behavior pattern related to olfactory communication is the

Sternal Gland Primates
A lion tamarin (Leontopithecus rosalia rosalia) vocalizing. (Photo by Animals Animals ©John Chellman. Reproduced by permission.)

anointing of the tail with scent-gland secretions and urine, as observed in Goeldi's monkey.

Marmosets and tamarins also employ visual signals in social communication. When they are aroused, the hair on the head, ears, shoulders, or the whole body is raised. This is particularly notable in species that possess hairy ornaments like the long crown hair of cotton-top tamarins (Saguinus oedipus) and the ear tufts of several marmoset species. Facial expressions like frowning, open mouth threats, and head shaking, are used in agonistic interactions. When threatening another individual—whether in their own group or in another group—several marmoset species turn around, lift their tail, and display their genitals. Lion tamarins and marmosets walk with an arched back and raised hair during agonistic interactions and in situations of social tension.

Due to their small body size, callitrichids are susceptible to a wider array of predators than larger primates. Successful and unsuccessful attacks by raptorial birds—Guianan crested eagles (Morphnus guianensis), ornate hawk eagles (Spizaetus ornatus), bicolored hawks (Accipiter bicolor)—and snakes—anacondas (Eunectes murinus), rainbow boas (Boa constrictor), jararacas (Bothrops jararaca)—have been witnessed. Tayras (Eira barbara) have been seen carrying dead tamarins in their mouths and hairs of tamarins were found in feces of ocelots (Felis pardalis). Callitrichids respond to predators by alarm calling, escaping, and hiding. If a raptor attacks while animals are exposed in an open tree crown and no escape to dense vegetation is possible, they may let themselves drop as a last resort. Once a predator has been detected and can no longer hunt by surprise, marmosets and tamarins may approach and intensively mob the predator with specific vocalizations. Saddleback tamarins have been observed striking at a resting snake, and moustached tamarins were observed attacking a rainbow boa in order to rescue a group member.

As a counterstrategy against being surprised by a predator, marmosets and tamarins are constantly vigilant. They frequently interrupt their ongoing activity to scan the surroundings. Adult male marmosets and tamarins perform such vigilance more often than other group members.

In areas south of the Amazon, saddle-back tamarins form mixed-species troops (interspecific associations) with sym-patric congeners, either moustached tamarins, red-bellied tamarins (Saguinus labiatus), or emperor tamarins (Saguinus imperator). In these troops, one group of saddlebacks and one group of the other tamarin species spend much of their active time together and jointly exploit plant resources, and their home ranges overlap almost completely. The species separate for the night, and contact is often re-established the next morning by means of long calls. Although species participating in mixed-species troops use all layers of the forest from the ground to the emergent trees, the smaller saddle-back tamarins range on average at lower heights than the associated larger species. Species forming mixed-species troops also differ in their strategies of foraging for prey. In northern Bolivia, Goeldi's monkeys regularly participate as the third party in associations of saddleback tamarins and red-bellied tamarins. Members of the different species in a mixed-species troop rarely interact directly with each other. If such interactions occur, they are usually agonistic and involve displacements or mild aggression by a member of the larger species

Saguinus Leucopus
A juvenile white-lipped tamarin (Saguinus leucopus) rests on a tree branch. (Photo by Roy P. Fontaine/Photo Researchers, Inc. Reproduced by permission.)
Callithrix Jacchus Genitalia
A pygmy marmoset (Cebuella pygmaea) eats fruit in the Upper Amazon Basin. (Photo by Rod Williams. Bruce Coleman, Inc. Reproduced by permission.)

towards a member of the smaller species. Agonistic interactions mainly occur in food resources of limited size. There are also friendly interactions between members of the associated species, and particularly juveniles and subadults have been observed playing intensively.

Major benefits of mixed-species troops are seen in increased safety from predators and increased foraging efficiency. By associating with another species, group size is increased without compromising the social organization and mating patterns. Increased group size may reduce the risk for each individual of being the target of an attacking predator. Studies on associations of moustached and saddleback tamarins on the Urucu River in Brazilian Amazonia have revealed that the two species may also benefit from a "division of labor" with regard to their vigilance: moustached tamarins, which use higher strata of the forest, are more likely to detect aerial threats, while saddleback tamarins use lower strata of the forest and are more likely to detect threats coming from below. Saddleback tamarin may also derive a benefit from the association by capturing prey that has been flushed by and escaped from mustached tamarins.

Like most New World monkeys and unlike Old World monkeys and apes, all callitrichid males and one-third of the females are dichromats, that is, they can only distinguish two colors, while two-thirds of the females are trichromats, that is, they possess normal color vision. While the gene for the short wavelength is located on chromosome 7, the genes coding for the middle and long wavelength are located on chromosome X. Having only one chromosome X, males possess only two different color vision genes. Whether females are dichromat or a trichromat depends on whether they are homozygotous or het-erozygotous at the respective gene locus on the chromosome X. The reasons for the maintenance of this polymorphic color vision system are unknown. It has been suggested that trichro-mats are better at detecting ripe fruits, while dichromats may have an advantage in detecting camouflaged prey and predators.

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