The common marmoset, Callithrix jacchus, belongs to the Callitrichidae family (New World Monkeys). Endemic in the Northeast forest of Brazil, they have conspicuous large white ear tufts and an annulled tail that changes between black and grey bars. The weight of an adult common marmoset vary between 300 gm to 450 gm, and the average body measurement of an adult common marmoset are neck to tail base 25 cm, tail 28 cm. Common marmosets normally live in extended family groups varying in size between 5-15 individuals. Typically, groups contain only one breeding pair, the dominant male and its mate. The breeding female suppresses reproduction in the other adult females. Breeding females show a high reproductive rate resulting from a biannual twinning and postpartum ovulation.
We have chosen the common marmoset (Callithrix jacchus) as model species, because it has been shown to be a promising model in which to study many aspects of social cognition. Marmosets, like all other callithrichids, show marked tolerance toward each other while feeding, and they frequently and closely attend to the foods others are eating. Callithrichids organize their behavior to a greater extent than do many other monkeys around the task of maintaining spatial and behavioral cohesion with their social partners. Due to their small size, these monkeys are more vulnerable to predation than are other monkeys, and, correspondingly, they are more cautious toward novel spaces and objects. As generalists, they consume fruits, snails, amphibians, lizards, spiders, small birds and nestlings in addition to insects and exudates. The predatory behavior of the genus Callithrix has been described as predominantly a stealthy, stalk and pounce, foliage-gleaning method. Nevertheless, extraction of hidden, embedded prey is a complex foraging technique and as such may not be readily mastered. Youngsters may therefore considerably benefit from skilled adults through observation.
We study marmosets both in captivity and in free-living conditions. Captive subjects are housed at the Biocenter (Althanstrasse) of the Faculty of Life Sciences (University of Vienna). Here we have two indoor cages (250 x 250 x 250 cm), attached to outdoor cages of the same size, in one observation room of the animal keeping facilities. All cages are equipped with branches, ropes and living plants. All animals are fed fruits, vegetables, monkey pellets and protein supplements. The temperature is 24-30°C during the day and 21-23°C at night. The humidity is ca. 50-70%. In summer daylight is the main source of lighting, while in winter additional UV-fluorescent tubes are used to maintain a 12:12 h light:dark cycle. We adhere to the guidelines for the Use of Animals in Research and the legal requirements in Austria. Due to necessary renovations we moved to a provisional keeping facility (in the faculty's greenhouse) in January 2005.
Our main Study site in the field is an area of 32 ha consisting of primary and secondary Atlantic Forest . The area is part of a condominium (a housing estate surrounded by a wall, which comprises about a hundred houses, greens, and several leisure facilities). It is situated 40km west of Recife in the state of Pernambuco, in the Northeast of Brazil (7°56'97''S, 35°1'23''W, 174m above sea level). The climate is seasonal. In the rainy season (June - August) temperatures range from 17 to 28°C and humidity from 90 to 100%. In the dry season temperatures can vary between 25 and 32°C with 70 to 80% humidity. Resources are more abundant in the dry season. Despite human impact, the area has preserved its richness and diversity regarding fauna and flora. The forest and its animal diversity are under law protection (IBAMA). The common marmosets are well-accustomed to the presence of humans.
We have selected marmosets to study several aspects of social cognition. Among them are imitation as an important mechanism of social learning, the function of social learning in the wild, the conditions for social learning during foraging (social relationships, dominance, food tolerance, cooperation), the development of social learning skill and the time course of its application. Furthermore, we want to adress in which form this primate species represent objects in its internal world, thereby studying object permanence and cross-modal representation.