SOCIAL ORGANIZATION AND BEHAVIOR
Social status within the group is linked to breeding status, and while the breeding pair is usually codominant, if there is more than one breeding female, one of the breeding females is dominant over the other. For nonbreeding individuals, the dominance hierarchy is age-graded and sex is not a factor (Digby 1995). Dominant individuals displace others at feeding sites and exhibit a variety of postures, vocalizations, and behaviors that include open-mouth threats, nips, cuffs, lunges, grabs, ear-tuft flicks, genital presenting, chasing, and biting (Abbott 1984; Digby 1995). They are the center of social life in the group and subordinate animals favor being in proximity with dominant individuals and groom them preferentially (Digby 1995).
Photo: Ludwig Miller
Photo: Gustl Anzenberger
Group support for developing infants is necessary among common marmosets. Twins are often 20 to 27% of the mother's total body weight; this is the equivalent of a 135-pound woman giving birth to two 16-pound babies. A female would not be able to care for infants alone because of the high demands of pregnancy and lactation as well as the mechanical difficulties of carrying two heavy infants, therefore all age-sex classes contribute to infant survival and development among common marmosets (Stevenson & Rylands 1988; Rothe et al. 1993; Tardif et al. 1993; Kinzey 1997). As discussed previously, compliance with cooperative rearing is probably linked to kin selection and fitness. The non-reproductive helpers in the group are probably related to the infants because they are related to the breeding female. It is within their interest to ensure the survival of her offspring because this increases the helper's inclusive fitness (Rothe et al. 1993; Saltzman et al. 1997). Studies on captive marmosets indicate that survival rate increases as number of non-reproductive helpers increases, up to a point. The highest survival rate of infants (95.7%) is found in groups of 10 common marmosets (the breeding pair and eight helpers) (Rothe et al. 1993). Interestingly, the average group size in the wild is about nine members.
Photo: Gustl AnzenbergerFrom birth, common marmosets have a very strong cling reflex and do not voluntarily leave their carrier's back for the first two weeks of life. They are very active starting in the second week, crawling on their carrier's back and investigating their surroundings (Stevenson & Rylands 1988). Immediately after birth, the breeding male and presumptive father of the infants begins to carry the twins and caregiving by offered by the father, mother, or other members of the group (Yamamoto 1993). Over the following weeks, time off the backs of carriers gradually increases and the infants develop locomotory behaviors and coordination and begin to exhibit play behavior (Stevenson & Rylands 1988). By about three months of age, the infants are almost completely weaned and are capable of self-feeding, though they do not gnaw their own holes to feed on gum but rather lick the holes created by older individuals (Stevenson & Rylands 1988). The infancy stage lasts until about five months of age and is followed by the juvenile period, which lasts between five and 10 months (Stevenson & Rylands 1988; Yamamoto 1993). By five months, common marmoset juveniles are 75% of their adult weight. Interaction with other group members besides parents is emphasized and play becomes rougher as future status is worked out. During the juvenile period, another set of infants is usually born and carrying and play with infants also characterizes this period of development (Yamamato 1993). Between nine and 14 months, the sub-adult stage begins, characterized by the full repertoire of adult behaviors as well as puberty. By 15 months, common marmosets have reached their adult weight and are capable of reproduction but do not reproduce until social conditions are adequate (Yamamato 1993).
COMMUNICATIONLike all primates, vocal and visual communication is important to common marmosets. Facial expressions and vocalizations convey information about social status, emotional state, and intent to other individuals (Stevenson & Rylands 1988). Because of their small size and the natural habitats they are found in, visual signals are important in close-range communication while vocal communication is more important over longer distances (Jones 1997). Some expressive facial and postural positions include the "partial open mouth stare," "frown," and "slit-stare" which are used to signify alarm, aggression, and submission. When common marmosets flatten their ear-tufts close to their heads in "tuft-flatten" position, this signifies submission, fear, and sometimes curiosity of new objects (Stevenson & Rylands 1988).
Photo: Ludwig Miller
Unlike the apes and Old World monkeys, smell is very important to New World monkeys. They have a specialized organ in their nasal cavity called the vomeronasal organ that allows them to process chemical signals in a focused manner and discern information about other animals (Evans 2003). Because of the presence of this "second nose," scents are very important tools of communication in New World monkeys, and common marmosets convey information by marking objects with secretions from specialized scent glands on their chests and around their anus and genitals (Lazaro-Perea et al. 1999). The main information conveyed by scent-marking includes demarcating home range and resources within that range, signifying social status, and advertising reproductive status (Stevenson & Rylands 1988; Lazaro-Perea et al. 1999; Ziegler et al. 2005).