RESULTS

Litter Size, Sex Ratio, and Infant Mortality

One hundred seventynine births were recorded at the CRCP from October, 1979 to June, 1989 of which 94.4% (169) were live births (Tables I and II). With the exception of one twin birth, all of them were singletons (99.4%). Most of the births took place at night or dawn. At this time, 154 infants of 179 births have been sexed, yielding a sex ratio of 1:1.1 (male:female). The remaining infants were not handled until after the age of 6 months. Twelve (3 males and 9 females) of 169 live births died the first week, a neonatal mortality rate at 6 days of 7.1%. Of these 12, 11 infants were rejected by their parents and one had locomotor paralysis. After the first week and during the first 3 months after birth, six infants (four males and two females) died. After the first 3 months and by the end of 1 year after birth another six offspring (two males and four females) died. The survival rate of live offspring at the first year of age was therefore 85.8%. The newborns were not weighed in order to avoid disturbing the mother and infants.

Sexual Maturity (Youngest Age of Conception)

The mean age at first birth in a group of nine females born in captivity was 40.56 _ 7.82 (S.D.) months. However, one female gave birth at the age of 25 months. Female age at first pairing varied with male availability. The mean age in a group of 12 males born in the colony when their female mates produced offspring was 42.17 ± 10.73 (S.D.) months. As in the females, most males were removed from their parents and paired with females at different ages ranging from 1.5 to 3 years old. At the present time the colony has 85 potential breeding pairs of which 63 (74.1%) are reproductively active (9 are captive born). Of the remaining 22 potential breeding pairs, 9 captive born breeding pairs were introduced to the colony in 1988 (Table D and have been paired for less than 1 year; thus offspring were not expected.

Interbirth Interval and Reproductive Success

Seventyfive interbirth intervals were estimated ranging from 5 to 29 months (X= 12.72 ± 5.72 S.D.). Fortyfive (60%) of these interbirth intervals were between 5 and 12 months. Thirtytwo (28%) from 113 pairs with more than I year in the colony failed to reproduce. Table III shows the breeding performance among different groups of owl monkeys introduced into the colony between 1979 and 1988. The highest average number of births per year of pairing was 1.0 and the lowest 0.4; the mean for the 10 years was 0.6.

TABLE II. Offspring Produced by Aotus nancymai From the CRCP Between February, 1979 and June, 1989*

*M, male; F, female; N, newborn; T, twins (male and female); A, abortion; 1, January to April; 2, May to August; 3, September to December; , start of pairing; *, end of pairing.
^a The "T" before the female number means it was captive born.

Seasonality

Despite the fact that there were births all year round, a consistent birth peak was observed from October to January in 1986, 1987, 1988, and 1989 (Fig. 1). Sixtysix (52.3%) of the 126 births recorded at the CRCP between July, 1986 and June, 1989 occurred in the months of October, through January.

DISCUSSION

Litter Size and Infant Mortality

We agree with Dixson [1983] that owl monkeys rarely produce twins. Our survival rate of live offspring was lower than that reported by Rieckmann et al.

TABLE III. Summary of the Breeding Experience With Various Groups of Aotus nancymai Introduced Into the Colony.

Year	No. of pairs addeda	Pairing period (months)	No. of offspring	No. of abortions	Average pairing period (months)	Average no. oflive births during each 12 months of pairing
1979	11	985	49	7	89.5	0.6
1980	3	248	10	2	82.7	0.5
1981	2	101	3	0	50.5	0.4
1982	1	48	2	0	48	0.5
1983	2	146	13	0	73	1
1984	6	355	15	4	59.2	0.5
1985	11	463	26	12	42.1	0.7
1986	16	559	23	3	34.9	0.5
1987	20	556	31	2	27.8	0.7
1988	7	104	8	0	14.8	0.9
Total	79	3619	180	30	45.8	0.6

Between February, 1979 and June, 1988

Only successful breeding pairs are considered. [19801. Our figure of 92.9% remains close to Rieckmann's rate of 95.7%, although Rieckmann did not specify his method for calculating survival rate.

Sexual Maturity

.The youngest age of conception in a female was calculated to be 20.5 months. This captive born animal gave birth at 25 months, and the conception date was calculated by subtracting a 133 day gestation period [Hunter et al., 1979]. The youngest age at conception of a paired male at the time of his mate's parturition was 28 months. It was estimated that his first fertile copula occurred at 23.5 months by subtracting a gestation period of 133 days. These results agree with Dixson [1983] who reports that it is probable that owl monkeys are reproductively mature by 1824 months.

Interbirth Interval and Reproductive Success

Dixson [19831 found an interbirth interval for the owl monkey ranging from 166 to 419 days. The mean interbirth interval was 253 days (8 months), shorter than that observed in our colony (381 days, 12.7 months). An analysis of 48 interbirth intervals for A. trivirgatus obtained from a laboratory colony and from other sources showed a mean value of 271 days or 9 months [Hunter et al., 1979]. Estimates of the gestation period in Aotus monkeys vary considerably from 126 days [Hall, 1979], 133 days [Hunter et al., 1979], 146 days [Rieckmann et al., 1980], to 148159 days [Elliot et al., 1976; Merritt, 1976]. Although the estimate of 133 days by Hunter et al. [1979] refers to one female, it is based on a single timed mating and we consider it to be most accurate. One female who had an abortion had a live birth after 141 days.

Fig. 1 Distribution by months starting the annual cycle from August 1986 to June 1989 of Aotus noncymai births at the CRCP, Iquitos, Peru.

Seventynine breeding pairs have produced 180 offspring in a period of 3,619 months (Table III). This represents an average of about one offspring for every 20 months that a pair was housed together. Rieckmann et al. [1980] reports a higher breeding performance of about one live offspring for each 12 months that a pair was housed together, with owl monkeys already adapted to captivity. Our results may be skewed towards longer intervals because newly captured individuals were constantly admitted to the colony, requiring adaptation periods before conception could occur.

Seasonality

According to Dixson [1983], Rieckmann et al. [1980], Hunter et al. [1979], Cicmanec & Campbell [1977], and Elliot et al. [1976], under conditions of controlled lighting and heating, owl monkeys show no apparent annual birth peak nor birth season. On the other hand, we have found that under natural photoperiod, temperature, and humidity conditions, Aotus nancymai shows an annual birth peak (Fig. 1). Also it seems that A. nancymai shows birth seasonality in the wild [Wright, 1981]. This birth peak also may be the reason why our mean interbirth interval is around 12 months and not shorter as previously reported [Dixson, 1983; Hunter et al., 1979]. Apparently, birth seasonality coincides with the rainy season and availability of food resources in the wild.