Marmosets and tamarins have well-developed visual, olfactory, and auditory perceptual skills. Saddle-back tamarins can recognize former social partners even after separations of several years (Epple and Niblick 1997). Formal tests of cognition have often failed to provide evidence of cognitive abilities, possibly because removal of animals from social groups for testing is highly disruptive. However, Savage and others (1987) successfully tested color discrimination in cotton-top tamarins by moving the test apparatus to the home cage. Eglash and Snowdon (1983) with pygmy marmosets and Hauser and others (1995) with cotton-top tamarins have found evidence of precursors of the mirror self-recognition of apes. Hauser (in press) and Hauser and Carey (1998) have studied expectations about objects and events, including numerosity and understanding of causality in cotton-top tamarins. When tested appropriately, the cognitive abilities of callitrichids are highly developed.
Callitrichids living in a naturalistic environment seem to maintain a ''cognitive map" of their environment and detect even minor changes. Under such conditions, neutral novel objects can stimulate little interest, and attempts to provide for environmental enrichment have been disappointing. Saddle-back tamarins living in a greenhouse habituated quickly to novel objects, usually in less than 15 min. Reintroducing the same objects 3 months later failed to produce any sign of interest (Menzel and Menzel 1979). Novel objects themselves might not be effective, but enrichment devices that allow animals to remain actively involved with searching for food items (Molzen and French 1989) do effectively encourage sustained activity and might be good enrichment devices (Box 1988; McGrew and others 1986; Molzen and French 1989).
Marmosets and tamarins recognize individual humans on the basis of odor (Cebul and others 1978), voice, and appearance. Callitrichids can develop strong likes and dislikes of individual humans. The monkeys appear to have long memories and respond with fearful behavior to hearing the voice or footsteps of someone who has captured them several months earlier. Curiously, personnel in several colonies have found that shoes might be an important element of an individual caregiver's appearance. On the basis of this anecdotal evidence, it might be wise for caregivers to wear the same shoes, lest the animals become agitated. Colony managers have observed that hand-raised animals socialized to people can become overaggressive, especially when the animals reach puberty.
Personnel whose primary experience is with other species need to know about the cooperative breeding, twinning, and extensive caretaking by fathers and other group members. They should learn some of the important vocalizations