Non-invasive monitoring of ovarian function in several felid species by measurement of fecal estradiol-17β and progestins

An extraction and assay procedure to measure fecal estradiol-17β and progestin concentrations in several cat species was developed and validated for use for noninvasive monitoring of ovarian function. Fecal samples were collected over a range of 3–20 months from female tigers (three), lions (three), snow leopards (three), cheetahs (two), caracals (two), and domestic cats (five). Samples were extracted with 90% methanol, lipids removed with petroleum ether, and the estradiol and progestins in the methanol measured by radioimmunoassay (RIA). High Performance Liquid Chromatography (HPLC) fractionation and subsequent RIA of the fractions indicated that the estradiol-17β antiserum cross-reacted primarily with estradiol-17β in the feces of lions and tigers and was assumed to be specific for estradiol-17β in the feces of other species as well. However, there were several immunoreactive compounds, presumably progesterone metabolites, excreted in the feces which varied both quantitatively and qualitatively among species. The behavior of tigers, lions, cheetahs, and caracals was visually monitored during the collection period and frequency of sexual behaviors was positively correlated with increases in fecal estradiol in all species observed. The mean fecal estradiol-17β peaks were as follows: tigers, 128.0 ± 13.1; lions, 186.0 ± 14.8; snow leopards, 136.7 ± 15.9; cheetahs, 140.9 ± 9.0; caracals, 24.5 ± 4.0; and domestic cats 158.9 ± 19.3 ng/gm. Fecal progestin concentrations rose significantly (P < 0.001) only after breeding or during pregnancy and were as follows: tigers, 5.6 ± 0.6; lions, 1.9 ± 0.1; cheetahs, 8.4 ± 1.1; and caracals, 2.4 ± 0.4 μg/gm. Fecal progestins were elevated for one-half to two-thirds of the gestation length during presumed pseudopregnancy but remained elevated throughout successful pregnancies. These results suggest that ovarian function can be monitored noninvasively in the family Felidae by the measurement of fecal estradiol-17β and progestin concentrations. © 1995 Wiley-Liss, Inc.     

Fecal endocrine monitoring of reproduction in female snow leopards (Uncia uncia)

Although the snow leopard (Uncia uncia) is a common endangered felid species in zoos, little is known about the complex endocrine interactions controlling ovarian function and conception in this species. The goal of this work was to characterize ovarian activity throughout the estrous cycle, nonpregnant luteal phase (pseudopregnancy), and gestation in female snow leopards. This goal was accomplished using an enzyme immunoassay to measure fecal concentrations of estrogen metabolites (E) and progesterone metabolites (P). Fecal samples were collected from 12 female snow leopards (ages 18 months to 18 years) during one to three breeding seasons. In each breeding season, the majority of females (78%, 88%, and 100%, respectively) began to exhibit ovarian activity in December or January. The estrous cycle, defined by the first day of estrus (E ≥ 2 × basal concentration) to the first day of the subsequent estrus, was 12.7 ± 0.6 days (n = 145 cycles). Estrus lasted 4.3 ± 0.4 days with mean concentrations of fecal E during the follicular phase (1661 ± 139 ng/g feces) increasing 3.2-fold above basal concentrations (515 ± 32 ng/g feces). No spontaneous ovulations were observed in any of the cycling females. Nonpregnant luteal phases were observed in eight females that bred but did not become pregnant. The length of the nonpregnant luteal phase ranged from 11 to 72 days (45.7 ± 5.7 days; n = 10) with mean concentrations of fecal P during the luteal phase (12.46 ± 1.7 μg/g feces) increasing 6.2-fold above basal concentrations of P (2.01 ± 0.2 μg/g feces). Three of the females in the study became pregnant and gave birth after a gestation of 93 (n = 2) and 95 (n = 1) days. Fecal P concentrations during pregnancy increased to 11.64 ± 1.3 μg/g feces, or 5.8-fold above basal concentrations. The results of this study provide a comprehensive characterization of reproductive endocrinology in snow leopards, and confirm that fecal hormone monitoring is an effective way to monitor female snow leopards throughout the breeding season.     

Reproductive hormone concentrations during estrus,pregnancy, and pseudopregnancy in the Labrador bitch

Serum luteinizing hormone (LH), estrone, estradiol-17β, and progesterone were measured during the estrous cycle, pregnancy and parturition in seven adult pregnant Labrador bitches and during the estrous cycle and one gestation length equivalent in six adult pseudopregnant bitches. Although the duration of proestrus was similar in both groups, the duration of estrus was longer in the bitches that subsequently became pregnant. Mean serum LH concentrations were similar in both groups during most of the study. However, during Weeks 6 to 9 after the preovulatory LH peak, serum LH concentration was higher in both pseudopregnant and pregnant groups of animals and declined to basal levels thereafter. Mean serum estrone concentrations in the pregnant animals were higher than those of pseudopregnant animals and remained elevated throughout gestation, followed by a decline at whelping. Serum estradiol-17β levels were higher during the 4 wkimmediately following ovulation in the pseudopregnant group compared with those observed in pregnant animals. Serum progesterone concentrations generally remained higher during pseudopregnancy compared with those of the pregnant animals during gestation.In conclusion, a major difference between pregnant and pseudopregnant bitches is a pregnancy-specific elevation in estrone levels. The placenta may be a likely source of estrone during pregnancy.     

Changes in sex steroids, gonadotropins, prolactin, and inhibin in pregnant and nonpregnant Japanese black bears (Ursus thibetanus japonicus)

We examined changes in the concentrations of serum progesterone (P4), estradiol-17beta (E2), FSH, LH, prolactin (PRL), and inhibin to determine their interaction and their effect on the reproductive endocrine controls of pregnant and nonpregnant female Japanese black bears. Fourteen female bears were used in this study over a 2-yr period. In the first year, six of the bears were divided into two groups; a pseudopregnant group and a nonpregnant group. In the second year, the remaining eight bears were also divided into two groups; a pregnant group and a nonpregnant group. Pregnant and pseudopregnant bears had similar P4 trends with both groups exhibiting a significant increase in December, which is the suspected time of implantation in pregnant bears. These trends correlated with an increase in PRL levels, whereas low levels of LH were maintained throughout the year. Nonpregnant bears maintained low concentrations of P4, and compared with pregnant and pseudopregnant bears, they also exhibited a delayed elevation in PRL. Luteinizing hormone activity varied among individual animals, but regardless of reproductive status, fluctuation patterns of E2, FSH, and inhibin did not differ among bears. Our results suggest that PRL may play a luteotropic role in both pregnant and pseudopregnant bears, and is possibly responsible for inducing reactivation of the dormant corpus luteum that precedes implantation in the Japanese black bear.     

Steroid hormones and the fertilizing capacity of spermatozoa

The effects of eleven different steroid hormones on $\text{in vitro}$ development of fertilizing capacity by hamster sperm were examined. Capacitation of epididymal sperm occurred only in the presence of female genital tract secretions. Fertilizing ability of sperm was poor if estradiol-17β, cortisol, chlormadinone acetate, medroxyprogesterone acetate, or megestrol acetate were present in the incubation medium at 10^?5M, whereas similar concentrations of estradiol-17α, progesterone, norethisterone acetate, ethynodiol diacetate, or norgestrel had little effect. Testosterone was a weak inhibitor of capacitation. Capacitation activity of female uterus and oviduct washings was higher at estrus than diestrus. This activity was reduced by treating intact animals with progesterone, cortisol, or testosterone, but increased by estradiol-17β or HCG. Estradiol-17α has no effect. Activity was low in pregnant or ovariectomized hamsters. Treatment of ovariectomized animals with estradiol-17β increased capacitation activity, but estradiol-17α, HCG or progesterone treatment was ineffective.     

The reproductive status of nonbreeding group members in captive golden lion tamarin social groups

Reproductive activity is limited to only one female in many species of callitrichid primates (marmosets and tamarins): daughters and subordinate females do not produce offspring. A suppression of ovulatory cyclicity is responsible for the lack of reproductive activity in three species of callitrichids studied to date. This study evaluated the endocrine status of golden lion tamarins (Leontopithecus rosalia) housed as daughters or sons in family groups and of individuals housed in isosexual peer groups. Daughters 17 months of age and older and a subordinate female had high levels of estrogen excretion. Mean levels of estrogen excretion in these females were similar to those of nonpregnant, breeding adult females (17.14 ± 6.82 versus 11.93 ± 6.33 μg/mg creatinine, respectively). Estrogen profiles were similar to those of breeding adult females, with sinusoidal cycles in estrogen excretion. Younger daughters in family groups (10 and 12 months old) showed markedly lower levels of estrogen excretion (0.84 ± 0.58 μg/mg creatinine). Estrogen profiles lacked the sinusoidal nature of cycles in older daughters and breeding females, and elevations in estrogen excretion occurred frequently and remained elevated for 1 or 2 days. Plasma testosterone levels in males varied widely, but mean concentrations did not differ among males housed in different social conditions. These results suggest that older daughters and subordinate females may be capable of expressing normal ovarian function in the presence of a breeding adult female. This finding may account for two unusual observations in the lion tamarin: the high level of female-female aggression and the presence of groups in the wild with more than one actively breeding female.     

Variations in the concentrations of androstenedione in peripheral plasma of women the day and during the menstrual cycle

The target tissues (e.g., hypothalamus, pituitary, uterus and vagina) of mature female ovariectomized rats show selective uptake of radioactivity in one hour after the injection of 6,7, ³H-estradiol-17β in a dose of 0.1 μg per 100 g body weight. Injection of 100 μg norethindrone or norgestrel per 100 g body weight 15 min before or 15 min after the administration of tritiated estradiol reduced the radioactivity in most target tissues, and also in the non-target tissues to a lesser extent. The uptake of radioactivity in the pituitary and uterus is reduced more by norethindrone than by norgestrel treatment when these Steroids were injected 15 min after estradiol-17β injection. It appears that there exists a competitive inhibition of estradiol-17β by these contraceptive Steroids in the rat. It is speculated that such competition with estradiol-17β may be an inherent property of the 17-substituted 19-nortestosterone group of Steroids.     

Impacts of the season and reproductive status on fecal reproductive and adrenocortical steroid metabolites in zoo Cuban crocodiles (Crocodylus rhombifer)

Conservation strategies for crocodilians often include captive breeding to create stable assurance populations. Evaluating adrenal and gonadal hormone patterns can provide animal managers with data to more effectively monitor animal welfare and reproductive status. This study evaluated the effects of season (breeding, nesting, or off), sex (male and female), and reproductive status of females (egg-laying/housed with a male or non-laying/housed solo) on concentrations of fecal glucocorticoid metabolite (FGM), fecal androgen metabolite (FAM), and fecal progestogen metabolite (FPM) in seven Cuban crocodiles, Crocodylus rhombifer, at the Smithsonian's National Zoological Park. Overall, seasonal changes in FGM and FPM concentrations were only observed in egg-laying females; FGM and FPM concentrations were both higher during the nesting season compared to the breeding and off seasons. Seasonal changes in FAM concentrations were only observed in males; males had higher FAM concentrations during the breeding and nesting seasons compared to the off season. Future studies investigating the use of fecal hormone metabolites in crocodilians are necessary to understand differences between individuals and species, to further elucidate the interactions between hormones and environmental factors, such as social housing, and to develop long-term datasets for the management of this species.     

皮质醇的周年变化与雌性虹鳟性腺成熟的关系

利用放射免疫分析法对饲养于恒定水温和自然光照下的雌性虹鳟血浆中皮质醇和性激素含量的周年变化进行了测定．结果表明：１）根据性腺结构指数和性激素分泌量判断,三龄时,雌性虹鳟达到性成熟;２）在血浆中不仅性激素而且皮质醇的变化水平与性腺结构指数的变化高度相关．排卵前性激素的水平都较高,伴随着排卵的进行性激素水平下降．而且在产卵季节虹鳟血浆中皮质醇水平也较高,三龄时皮质醇水平与性腺结构指数的相关系数为０．８６．这些结果提示,皮质醇在虹鳟的繁殖过程中可能发挥某种作用．     

Estrous cyclicity and reproductive success are unaffected by translocation for the formation of new reproductive pairs in captive red wolves (Canis rufus)

This study investigated possible female-related causes for inconsistent success among reproductive pairs in the zoo-based red wolf (Canis rufus) population. Females (n = 13) at seven institutions were assessed for evidence of ovulation and normal reproductive cycles through the measurement of estradiol and progesterone metabolite excretion in feces. Fecal cortisol metabolites (FCM) were also measured. Factors potentially affecting FCM and/or estrous cyclicity were recorded, including exhibit status (on vs. off), reproductive history (proven vs. unproven), copulatory behaviors (ties observed: yes or no), pregnancy/parturition (pups or no pups produced), and translocation before the observed breeding season (yes or no). No differences were observed in baseline FCM between females housed on versus off-exhibit (p = .46) or between females producing pups and those who did not (p = .19). Baseline FCM were significantly lower among females observed in copulatory ties compared to females never observed in a tie (p = .04), and tended to be higher in females translocated before the breeding season compared to females in existing reproductive pairs (p = .11), and among historically unproven females compared to proven females (p = .11). All females evaluated had an endocrine profile indicative of ovulation and among the four females translocated to be paired with a new male before the breeding season, two had successful pregnancies, producing litters. Therefore, despite observed differences in baseline FCM among factors, estrous cyclicity and reproductive success are unaffected by translocation for the formation of new reproductive pairs in the zoo-based red wolf population.     

Influences of retrochiasmatic surgical transection on mating and serum ovarian hormone levels in the androgenized rat

In previous studies female rats were shown to increase mating after retrochiasmatic surgical transections prior to ovariectomy and after ovariectomy with and without hormone replacement. This study was designed to determine if retrochiasmatic surgical transections (FC) or sham-FC would produce similar increased mating in androgenized rats. Four of thirty-one (13%) testosterone propionate (TP)-treated rats showed minimal mating without surgery, and 27 (87%) failed to mate. Mating occurred up to 30 days following FC, whereas temporary mating resulting from sham-FC had declined to presurgical levels by 30 days after surgery. Mating after ovariectomy was not facilitated by daily injections of 0.2μg of estradiol cypionate, but was facilitated after daily injections of 1.6 μg. TP-treated rats with FC showed more mating after 1.6 μg estradiol cypionate injections than the sham-FC and non-TP-treated controls. Serum estradiol-17β and progesterone levels at autopsy did not differ between TP-treated rats mating without surgery and those failing to mate. TP-treated rats with sham-FC surgery showed levels of estradiol-17β and progesterone similar to those of nonsurgical TP-treated rats and were combined into a single control group for comparison to the FC group. Serum levels of estradiol-17β did not differ between the control and FC group, whereas serum progesterone levels were decreased in the FC group. Serum levels of estradiol-17β and progesterone in TP-treated rats without FC were similar to baseline levels observed in this colony during the 4-day estrous cycle, but were less than peak levels observed at similar times on proestrus and diestrus Day 2, respectively. One possible effect of early androgen treatment would be to initiate a sustained inhibitory input to the mediobasal hypothalamus from the septal-preoptic area regions, thus causing deficits in mating. The lordosis responding in the androgenized rat after surgical interruption of the preoptic area-anterior hypothalamic continuum suggests that an inhibitory input was disrupted. It was further suggested that the higher mating observed in the FC group compared to the sham-FC group was not the result of higher ovarian hormone levels.     

Seasonal variations in circulating levels of progesterone and estradiol in unmated adult female woodchucks (Marmota monax) in captivity.

The annual profile of serum levels of progesterone (P) and estradiol-17 beta (E2) was characterized in a seasonally breeding rodent, the woodchuck (Marmota monax). Hormonal levels were determined in serum samples taken at weekly or biweekly intervals from unmated female woodchucks maintained all year indoors under controlled conditions of photoperiod and temperature. Annual fluctuations included a rise of E2 levels during late January through February, followed by a modest increase in plasma P concentrations by late March, the latter attaining peak values during April and May. A temporal dissociation of peak values of circulating levels of P and E2 during the annual reproductive cycle was also detected. The timing of changes in serum levels of P and E2 in these captive woodchucks corresponded to reproductive events during the normal breeding season of the woodchuck in the southern part of its range.     

Use of behavioral measures to assess reproductive status in maned wolves (Chrysocyon brachyurus)

Captive breeding efforts of the threatened maned wolf (Chrysocyon brachyurus) have been plagued by a lack of reliable reproduction and a high rate of neonatal mortality. A particular problem for animal managers has been the ability to detect pregnancy in a noninvasive manner. Pseudopregnancies are common, and many staff hours are expended preparing for a birth that may not occur. The objectives of our study were to document changes in behavior during the breeding season in captive maned wolves in order to determine if behaviors other than sexual (i.e., copulation) could be used to distinguish nonbreeding from breeding pairs and, further, whether successful breeding pairs (young born) could be distinguished behaviorally from breeding pairs that did not produce young (pseudopregnant). Between 1988 and 1994, behavioral data were collected during the annual reproductive season from 52 maned wolf pairings (27 males, 26 females) housed at 17 North American institutions. Breeding animals showed significant increases in rates of affiliative behaviors (approach, friendly) and the amount of time spent in close association (social) during the estrous period, compared to pre- and post-estrous periods. In contrast, the behavior of nonbreeding pairs varied little throughout the breeding season. Discriminant function analysis (DFA) showed that several behavioral measurements during certain reproductive periods were useful in discriminating between nonbreeding and breeding animals (rates of friendly and social behavior for males, marking and social behavior for females). Unsuccessful (pseudopregnant) and successful breeding pairs could be further distinguished on the basis of agonistic and approach behavior frequencies (successful females showed higher rates of agonistic and lower rates of approach behavior during certain periods; successful males had higher rates of both behaviors). Discriminant equations for key behavioral measurements and examples of their practical application are presented. DFA provides animal managers with an effective, noninvasive technique for assessing the reproductive status of maned wolf pairs. © 1996 Wiley-Liss, Inc.     

Longitudinal fecal hormone analysis for monitoring reproductive activity in the female polar bear (Ursus maritimus)

The objective was to identify suitable enzyme immunoassays to monitor gonadal and placental function in the female polar bear. Immunoreactive progesterone, progesterone metabolite (PdG), estrogen, and androgen metabolite (T) concentrations were measured in fecal samples collected over 24 mo from captive female bears (N = 20). Whereas fecal extracts produced displacement curves parallel to the standard curve for each respective steroid, T and PdG more accurately reflected reproductive events. Concentrations of fecal T increased (P < 0.05) during the breeding season, and brief spikes were associated with estrus and mating. A postovulatory increase in PdG was not always detected, but sustained baseline T after mating appeared consistent with ovulation. Parturient bears excreted higher PdG concentrations (P < 0.05) during expected time of embryo implantation in Fall, and a late gestational rise in fecal T occurred 30 days prepartum. Many nonparturient bears also had a PdG rise in the Fall, suggesting they experienced either pregnancy loss or a pseudopregnancy. Differentiating pregnant and pseudopregnant states was not achieved using fecal PdG alone, but when combined with fecal T, comprehensive diagnoses could be made. Nonparturient bears demonstrated elevated (P < 0.05) fecal T during summer months, whereas parturient bears did not. In summary, noninvasive hormone monitoring techniques were established for the female polar bear. Although this study was directed at facilitating management and breeding efforts of captive polar bears, the methods could be applied to studies of reproductive function in wild populations.     

Frustrated appetitive foraging behavior, stereotypic pacing, and fecal glucocorticoid levels in snow leopards (Uncia uncia) in the Zurich Zoo

This study hypothesized that permanently frustrated, appetitive-foraging behavior caused the stereotypic pacing regularly observed in captive carnivores. Using 2 adult female snow leopards (Uncia uncia), solitarily housed in the Zurich Zoo, the study tested this hypothesis experimentally with a novel feeding method: electronically controlled, time-regulated feeding boxes. The expected result of employing this active foraging device as a successful coping strategy was reduced behavioral and physiological measures of stress, compared with a control-feeding regime without feeding boxes. The study assessed this through behavioral observations and by evaluating glucocorticoid levels noninvasively from feces. Results indicated that the 2 snow leopards did not perform successful coping behavior through exercising active foraging behavior or through displaying the stereotypic pacing. The data support a possible explanation: The box-feeding method did not provide the 2 snow leopards with the external stimuli to satisfy their appetitive behavioral needs. Moreover, numerous other factors not necessarily or exclusively related to appetitive behavior could have caused and influenced the stereotypic pacing.     

Femme Fatale — the Case of the Threespine Stickleback

In addition to courting females, male threespine sticklebacks must guard their nests from attacks on the eggs by female cannibals. We hypothesized that the presence of females in high densities in tide pools adversely affects a male's health and reproductive success. In the laboratory males housed with conspecific females during the breeding season were in poorer physical condition and died sooner than solitary males and males housed with conspecific males or with heterospecific females.     

A comparison of the in vitro metabolism of estrone,estradiol-17β and estrone-3-sulfate by the livers of young virgin female,pregnant and female fetal guinea-pigs

The metabolism, $\text{in vitro}$, of [³H]-estrone, [³H]-estradiol-17β and [³h]-estrone-3-sulfate by the livers of pregnant, young virgin female and female fetus guinea-pigs has been compared using 900 g supernatants and microsomes. The ability of the guinea-pig livers to synthesize polyhydroxylated estrogens has been found to be small. The major metabolites isolated were unconjugated estrone and estradiol-17β or their glucuronides. The percentage of sulfates was lower after incubations with [³H]-estrone than with [³H]-estradiol-17β. A kinetic study with microsomes has shown a direct conversion of estrone-sulfate to estradiol sulfate. Fetal microsomes have been found to exhibit a more active hydrogenation of estrone to estradiol-17β than microsomes from young female or pregnant animals.     

Behavior in adult pairs of captive snow leopards (Panthera uncia)

Eight adult pairs of snow leopards (Panthera uncia) were observed for one to three years in the months December through March to determine the species' social and reproductive characteristics in captivity. To statistically examine the occurrence of behaviors as a function of estrus, the observation weeks were divided into three time blocks: before estrus, estrus, and after estrus. Using percentage of scan samples as an estimate of time spent in various behaviors, 16 behaviors and combined behavior categories were examined for (1) behaviors that differentiated successfully from unsuccessfully breeding pairs, (2) sex differences in behavior, (3) significant correlations between pair members, and (4) behaviors that showed time block effects. The rationale for identifying a behavioral profile of successful breeders in snow leopards was to aid zoos in their captive management programs by increasing their knowledge of the social behavior of this species. By finding correlates to breeding success, informed decisions on whether to change partners after a certain period of time, how to group the cats, and the optimum strategy for a survival plan can be made.     

Effects of capture stress on free-ranging, reproductively active male Weddell seals

Physiological stress responses to capture may be an indicator of welfare challenges induced by animal handling. Simultaneously, blood chemistry changes induced by stress responses may confound experimental design by interacting with the biological parameters being measured. Cortisol elevation is a common indicator of stress responses in mammals and reproductive condition can profoundly influence endocrine response. We measured changes in blood cortisol and testosterone induced by handling reproductively active male Weddell seals (Leptonychotes weddellii) early and late in the breeding season. Weddell seals have the highest resting cortisol levels of all mammals yet showed a clear, prolonged elevation in cortisol in response to capture. Responses were similar when first caught and when caught a second time, later in the breeding season. Baseline testosterone levels declined over the breeding season but were not altered by capture. Administering a light dose of diazepam significantly ameliorated the cortisol response of handled animals without affecting testosterone levels. This may be an effective way of reducing acute capture stress responses. Male breeding success in years males were handled was no different to the years they were not, despite the acute capture response, suggesting no long-term impact of handling on male reproductive output.     

Lactation‐associated infertility in Japanese monkeys (Macaca fuscata) during the breeding season

To investigate the endocrine factors in Japanese monkeys (Macaca fuscata) responsible for the suppression of the estrous cycle during the first reproductive season after delivery (150–360 days postpartum), peripheral blood was taken to measure plasma concentrations of follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone, estradiol‐17β, immunoreactive (ir)‐inhibin, and cortisol. The results demonstrated that during the breeding season of lactating Japanese monkeys, circulating concentrations of FSH (1.7–2.7 ng/ml), LH (308.5–461.0 pg/ml), estradiol‐17β (<62.6 pg/ml), and progesterone (145.0–453.0 pg/ml) remained low and were similar to the nadir levels observed during both the normal menstrual cycles and the nonbreeding season. Concentrations of ir‐inhibin, which is secreted from both follicles and corpus luteum in female Japanese monkeys, were also low (300.5–585.0 pg/ml). This strongly suggests that no follicular development occurs during lactation. Serum concentrations of cortisol (261.0–519 ng/ml) were higher during lactation than during the nonbreeding season. Since babies were often seen suckling their mothers during the study, the results indicate that the increased cortisol levels were associated with suckling‐induced secretion of corticotrophin‐releasing hormone (CRH) and adrenocorticotropic hormone (ACTH). The results of this study indicate that a long period of postpartum infertility in lactating Japanese monkeys, with apparent inhibition of follicle growth and anovulation, is due to weak gonadotropin stimulation, which may occur as the result of a suckling stimulus. Zoo Biol 22:65–76, 2003. © 2003 Wiley‐Liss, Inc.