Social Organization and Behavior Edit
Owl monkeys or night monkeys live in closely-knit social groups of two to five individuals made up of one adult of each sex and young of various ages (Wright 1978; 1989; 1994; Fernandez-Duque et al. 2001). While researchers long assumed that owl monkeys lived in family groups, because there is always a single reproductive female and a single reproductive adult male that cares for the offspring, research from at least one species, A. azarae, reveals that the social structure may be more flexible than a monogamous family group. Frequently, resident adults are displaced by other same-sexed adults which then assume the breeding and care position in the social group. Males are equally as likely as females to be replaced (Fernandez-Duque in press). This process of replacement is characterized by high levels of physical aggression between the new adult attempting to displace the resident adult male or female and can lead to permanent disfigurement or even death of the resident owl monkey (Fernandez-Duque 2004).
Aotus groups move together cohesively, with individuals never more than ten meters (32.8 ft) from one another. Aggressive or agonistic behavior between group members is exceptionally rare (Wright 1981; 1994). Owl monkeys exhibit some inter-group aggression at their home-range boundaries and meetings at fruit trees on the perimeter of their territories include loud vocalizations, stiff-legged jumping displays, chasing, wrestling, and piloerection. These encounters usually only last about ten minutes, after which neither group "wins" but rather retreats into its own territory (Wright 1994). Peaks of social behavior, including inter-group aggression, occur most often when the moon is bright (Wright 1989). Other social behaviors seen in owl monkeys include grooming and play. Grooming is not often seen (less than one grooming bout per month occurs between adults within a group), but play is an important social behavior that is especially prevalent on bright nights. The adult male and his offspring play by wrestling and chasing, and often exhibit patterns of "pounce and retreat" and "rough and tumble" play four to five times per day in bouts of five to 20 minutes. Adult females rarely play with their offspring (Wright 1981).
Because the primary social unit is the adult pair, both males and females disperse from their natal groups when they reach sexual maturity to find mating opportunities and start their own family groups. Both males and females leave the group between two and three years of age and range solitarily until they can find mates and establish a new territory (Fernandez-Duque & Huntington 2002; Fernandez-Duque in press). While establishment of new groups is not well-studied, subadult group members usually lag behind other group members during the month prior to their dispersal and spend more time away from the group, vocalizing and even sleeping in different trees, indicating they are preparing to leave the group and find a mate (Wright 1994; Fernandez-Duque & Huntington 2002). The timing of dispersal is also fairly uniform across groups, between August and January, and could either be related to the presence of new infants within the group or an attempt to find a mate before the mating season begins in May (Fernandez-Duque & Huntington 2002).
Almost all of the information about owl monkey reproductive characteristics comes from captive studies of the animal and new research on wild owl monkeys is beginning to reveal a very different picture. In captivity, Aotus has a monogamous mating system and the long-term bond between the adult pair probably lasts for life (Kinzey 1997). In the wild, the bonds between adult owl monkeys are dynamic and pairs separate regularly (Fernandez-Duque 2004). Male owl monkeys enter puberty at an early age, about one year old, when they begin to adopt physical characteristics of adult males including testicular growth and activation of the scent glands located under the base of the tail (Dixson 1994). As adults, male owl monkeys have extremely low sperm counts, probably as an adaptation to their monogamous mating system; only reproducing with one female, one time per year means it is unnecessary and energetically wasteful to produce high levels of sperm. Females are capable of reproducing as young as two years old, but normally do not give birth until about three years old (Gozalo & Montoya 1990). Physical examination of wild-caught A. a. azarae reveal that individuals do not reach adult body mass or have fully adult characteristics until about four years of age and age at first reproduction is at about five years (Fernandez-Duque in press). The ovarian cycle lasts about 16 days and menstruation does not occur in female owl monkeys. Mating occurs very infrequently both in the wild and in captivity and seems to be timed with ovulation (Dixson 1994; Fernandez-Duque et al. 2002). Once pregnant, gestation lasts 133 days and Aotus births are almost always singletons although twins are occasionally seen (Dixson 1994). In captivity, births occur year-round, but there is consistent peak in births from October to January. Among wild owl monkeys in the Argentinean Chaco, births coincide with the beginning of the rainy season, between late September and late November, and may be related to temperature and light conditions. Females that mate in April or May have a wide resource base available to support them during their energetically expensive pregnancy while infants born at the beginning of the rainy season will be mature enough to survive the extreme temperature drops seen in the dry season in May (Fernandez-Duque et al. 2002). Changes in photoperiodare probably the cue for mating to begin which is why similar peaks in births are seen both in the wild and in captivity (Fernandez-Duque et al. 2002). The interbirth interval is about 12 months for both wild and captive owl monkeys (Dixson 1994; Fernandez-Duque et al. 2002).
Parental Care Edit
Among A. azarae studied in the Argentinean Chaco, rates of infant survival to six months are extremely high, around 96%, while in captivity, survival rates through the first year are about 85.8% (Gozalo & Montoya 1990; Fernandez-Duque et al. 2002). These high survival rates may be attributed to low incidences of predation and almost constant parental care. Owl monkey males are extremely involved in the socialization and care for dependent offspring. Researchers have long assumed that this pattern of parental care is seen because males are certain of their paternity because of their mating patterns and because of the potential benefits to the infant (Wright 1981). Genetic tests have not been conducted to confirm that male caregivers are fathers and until this is done, other explanations for this behavior should also be explored (Fernandez-Duque pers. comm.). Male owl monkeys that constantly carry dependent offspring may be better equipped to flee from predators or danger than a lactating female of equal size. Because lactation is nutritionally expensive and energetically taxing, females may lack the energy necessary to run with infants that are relatively large compared to their body size (Wright 1984). The infant, therefore, gains an advantage if being carried by an adult male compared to its mother.
Newborn owl monkeys weigh, on average, 96.5 g (3.40 oz) and instinctively ventrally cling to the mother. The infant will exhibit this pattern of attachment until about three to four weeks of age, when it will begin to cling dorsally to its carrier (Dixson 1994). Developmental stages have been observed in captivity. Starting on the first day of life, the father begins to carry the infant, and though he will carry the infant almost exclusively for the first 2 months of life, the mother carries and nurses the neonate in the first week of life (Wright 1984; Dixson 1994). After the first week, the mother only carries the infant when it nurses, approximately 10 to 20% of the time, the remaining 80 to 90% of its life is spent on the father (Wright 1986). Beginning as early as 22 days or as late as 46 days, infants get off the carrier and begin to eat solid food as a supplement to nursing (Dixson 1994). As they age, infants are carried increasingly less such that by five months of age, they are totally independent. As they gain this independence though, there are some conflicts between the father and infant starting at about four months; conflicts over carrying include the infant holding on to the father tightly while he attempts to bite or push the infant off. During this time, there is very little carrying by the father, but the infant still rides on his back as the group travels between trees that are between three and four meters apart (9.84 and 13.1 ft) (Wright 1984). Even though infants travel independently by five months of age, they are not weaned completely until about seven months and in the instance of any disturbance or stressor, they will return to the parents for support (Dixson 1994). Older siblings that are still in the group when an infant is born take turns carrying it, but they are limited to carrying during the first four weeks of life, after which the infant is too big for them to support (Wright 1984).
Owl monkeys communicate through chemical signaling and scent marking, vocalizing, and by using some visual cues (Wright 1994; Bolen & Green 1997). Specialized scent glands under the base of their tails secrete chemical signals that convey information to other individuals including sexual identification and route-marking from sleeping sites to food resources (Wright 1989; 1994). They also exhibit a behavior known as "urine washing" in which they urinate on their feet and hands and as they walk over branches, leaving a scented path (Wright 1981). Their honed sense of smell may also help them distinguish ripe from unripe fruits in an environment in which color is dulled (Wright 1989).
Unlike their highly developed sense of smell, the auditory system among owl monkeys is unspecialized compared to other primates. However, they do have a specialized throat pouch which inflates and increases the volume and power of calls (Wright 1981). They have not been observed giving a territorial call like other monogamous primates such as gibbons (Hylobates species), but their vocal repertoires may serve to reinforce bonding between individuals of a group as well as inter-group interactions at home range boundaries (Wright 1981). The aggressive call or "war whoop" of owl monkeys is a call consisting of a series of a dozen low notes or grunts which increase in volume and power into hoots; this call is exhibited by both sexes and heard only during direct conflict with another group. Subadult males and females use the same type of call in a shorter series of three to five hoots as a long-distance contact call when they are looking for a mate (Wright 1981; Kinzey 1997). The generalized alarm call of the owl monkeys is a series of "sneeze-grunts" while softer calls that sound like purrs are used to reinforce bonds between group members or alert them to desirable food sources. When infants express desire for food or contact to their parents, they squeak loudly (Wright 1981).