Aotus: The Owl Monkey's Natural History
- Aotus azarae: Azara's owl monkey
- Aotus brumbacki: Brumback's owl monkey
- Aotus hershkovitzi: Hershkovitz's owl monkey
- Aotus infulatus: Feline owl monkey
- Aotus lemurinus griseimembra: Lemurine owl monkey
- Aotus miconax: Andean owl monkey
- Aotus nancymai: Ma's owl monkey
- Aotus nigriceps: Black-headed owl monkey
- Aotus trivirgatus: Northern owl monkey
- Aotus vociferans: Noisy owl monkey
The northern owl monkey is found in southern Venezuela and north central Brazil (Nowak, 1999). This species lives in forests where the trees are evenly dispersed. The southern owl monkey is found in the countries of Bolivia, Paraguay and northeastern Argentina (Nowak, 1999). This species lives in low vine forest and the tall forest, and is predominately found in the middle stratum of the forest (Wallace et al., 1998). In Argentina this species is found in both low and tall forests that have dense canopies (Rathbun and Gache, 1980).
The owl monkey is a frugivorous species that also eats leaves insects and small ripe fruit (Kinzey, 1997). An advantage of foraging at night is decreased competition from diurnal frugivores and the presence of larger nocturnal insects (Wright, 1989).This species spends all of its life up in the trees (Kinzey, 1997). Owl monkeys become more active when the moon is bright (Wright, 1984), and they travel the same route during the night, so they may memorize routes when they travel when the moon is brighter (Wright, 1989). Southern species are known to eat the seeds of the species Brosimum alicastrum, which differs from other members of the genus Aotus (Wright, 1985). In the cold season in Paraguay this species eats more leaves than other members of the genus Aotus, because during this time of the year in Paraguay, fruits are very scarce (Wright, 1989). In Argentina the predominate food for this species was found to be fruits and flowers (Rathbun and Gache, 1980).
This species feeds more frequently during the first hours of activity (Garcia and Braza, 1987). The southern owl monkey sleeps in groups in trees which are located in the middle stratum of the forest, and the trees have a mean height of 12.8 meters (Garcia and Braza, 1993). The sleeping sites have a mean height of 10.8 meters (Garcia and Braza, 1993). In the trees the sleeping locations are either thickets, dense foliage or lianas (Garcia and Braza, 1993). Garcia and Braza (1993) found the use and distribution of the trees used as sleeping sites is related to food availability and structural characteristics of the habitat. The mean group size for the southern owl monkey is three individuals, with a range from two to five (Wallace et al., 1998). In Argentina this species has been shown to be active during the day (Rathbun and Gache, 1980). This species can also be active during the day in Paraguay, where they were seen to forage for a couple of hours during the day (Wright, 1989). The mean group size for this species for populations found in Argentina is 2.9 individuals (Rathbun and Gache, 1980).
The basic group of owl monkeys is composed of a breeding pair and their offspring. This species has a monogamous mating system (Kinzey, 1997). The monogamous mating system is correlated with having males present to help raise offspring and having food resources in predictable, uniform patches (Wright, 1986). Both males and females disperse in this species (Kinzey, 1997). The young stay with their birth group until between 2.5 and 3.5 years old, then they disperse (Kinzey, 1997). Social grooming is not common for members of this genus (Fleagle, 1988). In this species the father becomes the main carrier of the infant and only gives the infant to the mother to suckle (Jantschke et al., 1998). If the father dies when the infant is still young, other siblings will assume the caregiver role, but not the mother, and the infant does become independent sooner than if the father cared for it (Jantschke et al., 1998). Jantschke et al. (1998) found that an infant reared without a father became independent at 12 weeks, versus 33 weeks for an infant with a father.
Aotus is usually reported as a highly monogamous primate. Groups are often composed of an adult male and female and up to three infants and juveniles. However, reliable observations of up to five multi-adult groups moving through the canopy together have been reported (L. T. Rosengreen, pers. comm.) and multiple-adult groups nesting together (Hernandez-Camacho and Cooper, 1976). Other reports of larger groups of up to 30 individuals found in fruiting trees are probably aggregations of smaller groups, a point supported by a high degree of agonistic behavior between individuals in the larger groupings (Wright, 1981).
Owl monkeys have a relatively short interbirth interval of about one year, which is possible due to high levels of paternal care which alleviates much of the energetic costs to the mother (Garber and Leigh, 1997). Male owl monkeys carry the young, defend them from predators, play with them and instruct them (Wright, 1985). The social system benefits the male, usually limited by access to females, by providing high parental certainty (Garber and Leigh, 1997), low infanticide rates and short interbirth intervals. Owl monkeys are unique in that a female will actively refuse to carry the young if the male is unavailable, going so far as to violently pull the infant off of her back. Two birth peaks occur, one at the end of the dry season and the other in the middle of the wet season (Wright, 1985). Gestation is about 133 days (Hunter et al., 1979). Young disperse at two to three years (Wright, 1985) and probably pass into a nomadic, "vagabond" stage before pair-bonding with a female (Charles-Dominique, 1977).
Male-male aggression is common and is a factor in keeping groups apart (Moynihan, 1964). Agonistic encounters involve back arching, stiff-legged jumping, pilo-erection, urination and defecation, as well as giving clicking/grunting alarm calls. Identical agonistic displays occur between conspecifics and other species (Wright, 1978).
Aotus is highly territorial. Territories are extremely small for a primate of its size, usually no greater than 10 ha (Wright, 1985). Although little work has been done on the dynamics of territory size and location over time, it has been suggested, based on the behaviors of nocturnal prosimians, that Aotus is relatively sedentary (Charles-Dominique, 1977). This is supported by Isbell's (1994) conclusion that use of unfamiliar areas by primates may increase the rates of predation. Relatively mobile, daily path lengths are reported to be approximately three-quarters of a kilometer. Territories between neighboring groups overlap extensively (Wright, 1978). Aotus has been reported to come out of trees and cross open savanna to move between forest patches (Rathbun and Gache, 1977). Population densities in Peru are reported to be around 25 to 50 individuals per km2 (Moynihan, 1976).
Aotus is a relatively noisy monkey, uttering loud contact and locomotory notes (Moynihan, 1976). Olfaction is an important component of communication and Aotus marks substrates by rubbing a gland at the base of its tail and exuding a brown, oily substance (Wright, 1981).
Breeding and Reproduction
The northern owl monkey most often gives birth to a single offspring, although twins have been recorded. The interbirth interval for this species is one year. Copulation attempts tend to be brief and rapid, with the male and female first quietly approaching each other (Moynihan, 1964). Then the male performs social sniffing, and the female may social sniff while the male is doing the same to her (Moynihan, 1964). Mating is done dorso-ventrally, with the male inserting his penis under the female's tail, with the female moving the tail slightly (Moynihan, 1964). The male usually makes three to four pelvic thrusts, with ejaculation occurring on the last thrust (Moynihan, 1964).