Age as a determinant of reproductive success among captive female rhesus macaques (Macaca mulatta).

A retrospective analysis was performed on fertility outcomes among a colony of captive Indian rhesus monkeys. The analysis covered over 30 years and was based on 1443 females with a total of 11,453 pregnancies. Various determinants of fertility were assessed including birth rates, pregnancy loss, infant survival, interbirth intervals, and interval from last birth to death. Binary variables were analyzed with generalized linear models with random intercepts, while linear mixed models were used for analysis of continuous variables. Age of the dam was a significant factor in determining whether a pregnancy resulted in a birth and whether an infant survived the first 30 days with primiparous or older mothers being less likely to produce an infant surviving to that age. In contrast, sex proved to be the only significant factor in determining whether an infant lived to 1 year, with females being more likely to survive. The interval between births proved to be affected primarily by dam age, while the late death of an infant depressed the likelihood of an extended time interval between her last birth and her death. Overall, these results demonstrate that maternal age contributes significantly to a decline in fertility and older females can live relatively long periods following birth of their last infant.

Male-to-male differences in post-thaw motility of rhesus spermatozoa after cryopreservation of replicate ejaculates.

BACKGROUND: The efficiency of controlled propagation to produce rhesus monkeys of particular genotypes can be maximized by use of cryopreserved spermatozoa collected from specific males to inseminate appropriate females. But this assumes that semen from males with different genotypes can be cryopreserved with equal effectiveness. METHODS: To investigate whether spermatozoa from different Macaca mulatta males can be effectively cryopreserved when frozen under identical conditions, we collected and froze semen specimens from 13 adult, fertile males maintained at three primate research centers. RESULTS: Survival, based on post-thaw motility normalized to the pre-freeze value, was assayed within 30 minutes after thawing; it varied from 50% to 70% but declined thereafter. To examine the response of semen from individual males, we collected and froze three to six ejaculates per male from each of seven males. CONCLUSIONS: In general, semen from a given male responded reproducibly to freezing, but there were significant differences among males. The cause of these differences among M. mulatta males in post-thaw sperm survival remains unidentified.

Behavior and endocrine concentrations do not distinguish sex in monomorphic juvenile howlers (Alouatta palliata).

Behavioral observations on juvenile mantled howlers are limited by visually undifferentiated genitalia; however, animals can be sexed when they are very young or if they are captured. Behavioral data and fecal samples from juveniles during month-long field studies from 1993 to 1995 were analyzed to determine whether there are developmental differences in behavior or hormone concentrations that can be used to differentiate males from females. The subjects were juveniles of known sex and age from five different social groups on Hacienda La Pacifica, Costa Rica. Based on 749.8 hr of focal-animal sampling, there were no sex differences in daily activity patterns. There were no sex differences in proximity to mothers and other group members, and age differences reflected howler life-history patterns. There were no differences in estradiol or testosterone concentration by age or sex. Juvenile monomorphy thus extends beyond morphology to behavioral and hormonal similarity as well. Most juveniles are forced out of their natal groups and remain solitary until they join new groups by supplanting all same-sex adult group members. Monomorphy may allow them to spend more time in natal groups, and thus both reduce the solitary period and allow the juveniles to improve social skills needed for later immigration. While this strategy may benefit juvenile howlers, it remains a problem for those who wish to study juvenile sex differences from a distance.

Birth of rhesus macaque (Macaca mulatta) infants after in vitro fertilization and gestation in female rhesus or pigtailed (Macaca nemestrina) macaques.

A study was conducted to assess the possibility of using pigtailed macaques (Macaca nemestrina) as recipients for rhesus macaque (Macaca mulatta) embryos. A total of 250 oocytes were collected from 11 rhesus monkeys during 12 follicular aspirations. We performed 15 embryo transfers with two embryos each into rhesus recipients, which resulted in eight pregnancies, of which two were lost during the second trimester. Among the remaining six pregnant rhesus macaques, two were carrying twins, resulting in the birth of eight infants. Twelve transfers of rhesus embryos into pigtailed macaques resulted in one pregnancy and the birth of one infant. Fetal growth and development were monitored by monthly ultrasound examinations, during which biparietal measurements were taken and compared with those derived from 22 pregnant control monkeys. In vitro fertilization-derived singletons tended to develop faster than did twins and naturally conceived control singletons during the initial months of pregnancy and weighed more at birth than did twins. There were pronounced morphologic changes in the placenta of the rhesus that developed in the female pigtailed macaque. These included an irregular shape, elevated placenta-to-birth-weight ratio, and an abnormal length and diameter of the umbilical cord. Histologic analyses of the rhesus-pigtailed placenta showed evidence of maternal-placental floor infarction and thrombosis of the spiral artery with resulting infarction of the villi. These results demonstrate that pigtailed macaques can carry rhesus fetuses to term, but further studies are necessary to determine the cause of the decreased pregnancy rates and observed placental abnormalities.

Four-year study of controlled timed breeding of rhesus monkeys (Macaca mulatta).

As part of the timed breeding colony at Tulane National Primate Research Center, exogenous progesterone administration (5 mg/day for 10 days) has been used to select conception dates by inducing artificial luteal phases in female rhesus monkeys. A retrospective analysis of data obtained during four breeding seasons (1998-2001) revealed that conceptions occurred an average of 18 days after the last administration of progesterone. The age of the female to be bred, previous pregnancy history, and timing of breeding during the breeding season were determined to be critical factors in the success of the procedure. The benefit of this method of timed breeding is that it does not require tracking of menstrual cycles, which can be labor-intensive and requires that animals be monitored several months in advance of breeding to determine each female's individual cycle length. It also provided an efficient use of breeding-age males.

Behavioral, immunological, and hormonal responses associated with social change in rhesus monkeys (Macaca mulatta).

Twelve animals to be socially reorganized in a group of 49 animals were compared with ten animals in a socially stable group (n = 51) for reproductive hormones, cortisol concentrations, immune function, and behavioral patterns for a 6 week baseline and 16 week postreorganization period. Cortisol values, mitogen-induced lymphocyte blastogenesis, and rates of behavior reflected social events. Increased cortisols, increased noncontact aggression, and decreased mitogen response were associated with animal introductions. Increased cortisol concentration and decreased mitogen response correlated significantly only in animals removed from the group for trauma, and females successful at group membership exhibited lower cortisol concentrations. High-ranking animals in small stable groups exhibited significantly higher lymphocyte proliferation than middle- or low-ranking animals. While relative ranks established in small groups were maintained following reorganization, baseline cortisol values, immune measures, or male testosterone values did not predict success at group membership. © 1996 Wiley-Liss, Inc.