The endocrinology of puberty and reproductive functioning in female cotton-top tamarins (Saguinus oedipus) under varying social conditions

Sexual maturation and fertility were assessed in fourteen cotton-top tamarin (Saguinus oedipus) females under various social conditions. Six tamarin females (20-28 mo of age) showed a suppression of fertility while living with their families. Hormonal profiles demonstrated low, acyclic levels of urinary luteinizing hormone (LH) and estrone-conjugates (E1C). A rapid onset of ovarian and pituitary cyclicity occurred when four of the six females were removed from their families and paired with an unrelated male. In one female, an ovulatory LH peak occurred as early as eight days after pairing and resulted in conception and full-term pregnancy. Two of the six females were housed in total isolation for 30 days following their removal from the family and prior to pairing. Gradual increases in hormone concentrations occurred during isolation; however, there was no ovarian cyclicity until each female was paired with an unrelated male. In all six females, conception occurred before or as a result of the third ovulatory cycle. Partial isolation of a 36-mo-old female resulted in elevated LH and E1C levels, but cyclicity was not observed until the female was paired with an unrelated male. These findings indicate that removal of a female from the family alone does not initiate ovarian cycling. Sexual maturation, or puberty, occurs in female tamarins living with their families between 15 and 17 mo of age when mean LH and E1C levels began to increase. However, when a female is removed and paired at 9 mo of age with an unrelated male, elevated levels of LH and E1C may be seen by 10 and 11 mo of age. Our findings indicate that a suppression of fertility occurs in cotton-top tamarins living with their families, but that reproductive suppression does not affect the process of sexual maturation. Both removal from the family environment and stimulation by an unrelated male tamarin were necessary to induce normal reproductive activity. An acceleration of puberty occurred when a female tamarin was removed from her family early in development and paired with a male.     

Urinary excretion of oestrone conjugates and gonadotrophins during pregnancy in the Goeldi's monkey (Callimico goeldii)

Oestrone conjugate and LH/CG were measured in the urine of 4 Goeldi's monkeys during 6 pregnancies. The gestational length was a mean of 148.8 days from the post-partum LH/CG peak to parturition. CG was first detected a mean of 18.8 days after the LH/CG peak and values remained elevated for a mean of 44.8 days. Three different gonadotrophin assays were used to detect LH/CG: the mouse in-vitro interstitial cell bioassay, a mixed heterologous LH RIA, and a monkey CG RIA. The mouse in-vitro interstitial cell bioassay was useful for measuring both the LH peak which occurred post partum and the CG concentrations during pregnancy. However, both immunoassays were inconsistent in measuring LH due to poor cross-reactivity or lack of specificity; CG concentrations were measurable. Oestrone conjugates became elevated at the time of the LH/CG peak and concentrations continued to increase throughout pregnancy, reaching peak levels before parturition. The postpartum interval, pregnancy and parturition can therefore be monitored in the Goeldi's monkey by the use of urinary assays: those for bioactive LH and immunoreactive oestrone conjugates to determine the post-partum LH peak and those for immunoreactive LH/CG and immunoreactive oestrone conjugates to follow pregnancy and parturition.     

Endocrine Correlates of Reproductive Status in Breeding and Nonbreeding Wild Female Moustached Tamarins

In callitrichid primates, reproduction is usually restricted to a single female per group. Reproductive rate is high and the occurrence of a postpartum estrus can lead to simultaneous lactation and pregnancy. In contrast, nonreproductive females often show ovarian inactivity. However, most studies on callitrichid reproductive physiology have been conducted in captivity, where conditions differ considerably from those in the wild, so that reproductive conditions may be strongly modified. Using fecal estrogen and progestogen measurements to monitor female reproductive status in 2 groups of wild moustached tamarins (Saguinus mystax), we examined 1) whether reproductive females in free-ranging groups also show postpartum estrus and 2) whether nonreproductive females demonstrate signs of ovarian activity. In both reproductive females, clear changes in the excretion pattern of progestogen and estrogen metabolites over time in combination with information on parturition dates allowed us to differentiate between pregnancy, a period of postpartum ovarian inactivity lasting for 54 and 64–82 days, and a period of ovarian activity before conception. Nonreproductive females demonstrated temporal fluctuations in hormone concentrations and absolute hormone levels that were similar to ones in the breeding females during the phase of ovarian activity. The results suggest that, in contrast to most captive female tamarins, reproductive females in wild groups of moustached tamarins do not have a postpartum estrus and that nonreproductive females show ovarian activity despite the presence of a breeding female. We therefore conclude that findings from captivity should be only carefully compared to the situation in the wild.     

Sexual communication between breeding male and female cotton-top tamarins (Saguinus oedipus), and its relationship to infant care

Male cotton-top tamarins have been shown to be responsive to female scent cues of ovulation, and are known to actively participate in infant care during the time when their mates are fertile. We measured urinary androgen levels and glucocorticoids in seven father tamarins for the first month following the birth of infants to determine 1) whether male tamarins showed an androgen response to their mate's postpartum ovulation, 2) when androgens rise relative to ovulation, 3) whether there is a glucocorticoid response, and 4) whether males alter their parenting behavior during their mate's receptive period. All of the males showed a significant increase in urinary androgens prior to the female's postpartum LH peak, which indicated ovulation. The hormonal increase, which included estradiol, occurred 3-7 days prior to the female's LH peak at a time that coincided with the female's follicular period. Corticosterone levels also peaked during that time, but did not correlate with androgen changes. Fathers did not alter their daily infant-carrying patterns relative to the androgen increase or at the time of the mate's LH peak. We conclude that male cotton-top tamarins experience an increase in androgens that coincides with their mate's postpartum ovulation, which ensures optimal fertility. However, this sexual communication does not alter father-infant interactions, which already occur at a high rate in this species.     

Nursing does affect the duration of the post-partum to ovulation interval in cotton-top tamarins (Saguinus oedipus)

Reports on callitrichid monkeys have not revealed a significant effect of nursing on interbirth intervals or on post-partum to ovulation intervals. We examined 25 post-partum intervals in cotton-top tamarin females to determine whether nursing infants would affect the length of the post-partum to ovulation interval. Urinary LH/CG and oestrone conjugates were measured in urine samples collected in the 6 weeks after birth. The post-partum to ovulation interval is the number of days between parturition and the rise of urinary LH and oestrone conjugates associated with ovulation. There was an 84% conception rate post partum. Neither mother's parity nor sex of the infants influenced the length of the post-partum to ovulation interval. The post-partum to ovulation interval for females nursing 2 infants was twice as long as for those not nursing or nursing 1 infant (P less than 0.05). The range of post-partum to ovulation interval lengths was more variable in nursing than in non-nursing females (P less than 0.01). Females spent less than 50% of observed time in contact and less than 20% of observed time nursing their infants. Neither the number of tamarins within the family nor the amount of time the mother was in contact with infants correlated with the length of the post-partum to ovulation interval. However, there was a positive correlation between the percentage time that mothers nursed 1 infant at a time and the length of the post-partum to ovulation interval (r = 0.75, P less than 0.02). The underlying mechanisms of suckling-induced delay of ovulation are present in the cotton-top tamarin as in other primate species. However, these nursing effects do not cause the substantial delay in fertility post partum that is associated with non-callitrichid primates.     

Rearing a second generation of cotton-top tamarins (Saguinus oedipus oedipus) in captivity

The average age at first parturition in captive-born female cotton-top tamarins was 31 months. Only 18% of full-term young born were successfully reared by primiparous females, but success increased with parity and 71% of babies born at 4th and 5th pregnancies were raised. 15% of litters were single births, 61% were twins and 24% were triplets. There was a seasonal distribution of births, with a clear peak in the spring months. The average interbirth interval was 294 days.     

Changes in the activity of captive cotton-top tamarins (Saguinus oedipus) over the breeding cycle

The energetic costs of reproduction are likely to be particularly high in the Callitrichidae (marmosets and tamarins), which characteristically produce twin litters. The activity of five captive breeding pairs of cotton-top tamarins (Saguinus oedipus) was investigated for a 6-month period covering the second half of pregnancy and lactation. Breeding males showed few major changes in activity over this period, but breeding females reduced their levels of locomotion during late pregnancy, and during lactation they spent up to twice as much time feeding and foraging as they did during pregnancy. These results support the hypothesis that the costs that callitrichid females incur when reproducing are likely to have been an important factor in the evolution of these species' communal rearing systems.     

雌性川金丝猴尿中生殖激素变化的研究

用人促黄体生成素／绒毛膜促性腺激素诊断药盒及雌激素诊断药盒,定期测定４只雌性川金丝猴尿中黄体生成素和绒毛膜促性腺激素的活性及其与雌激素含量的关系。实验证明,两种人用诊断药盒可以测出金丝猴尿中的促黄体生成素（ＬＨ）／绒毛膜促性腺激素（ＣＧ）及雌激素（Ｅ）的变化情况。基本上反映了金丝猴的卵泡形成、排卵和黄体生成的时间以及妊娠的情况。ＬＨ／ＣＧ及Ｅ,每月有一个大的峰值,其后１－４天内有月经或尿潜血出现。妊娠初期ＬＨ／ＣＧ急剧上升,持续一个月达到最高峰后即急剧下降。Ｅ在ＬＨ／ＣＧ下降后开始升高,持续３－４个月达到最高峰,高峰下降后半个月左右分娩,分娩后重新升高,但低于原水平。金丝猴的繁殖行为,主要表现为邀配和交配,９－１２月频率最高。根据ＬＨ／ＣＧ曲线的变化可以推断受精的大约日期。     

Preparental hormone levels and parenting experience in male cotton-top tamarins, Saguinus oedipus

Male cotton-top tamarins, Saguinus oedipus, display hormonal changes associated with parenting prior to the birth of their infants. Here we examined the hormonal changes that occurred in experienced and inexperienced fathers during the postconception period, prior to the birth of infants. Noninvasive techniques were used to collect urine from 10 male cotton-top tamarins (5 experienced and 5 inexperienced breeders) three times weekly during the 6-month gestation period. Samples were analyzed for prolactin, testosterone, dihydrotestosterone (DHT), and cortisol, averaged by gestational month. Experienced males showed elevated prolactin during the mate's 3rd gestational month, and the elevation correlated with the number of infants surviving from the previous birth (0, 1, and 2) but not with outcome of the current pregnancy. However, an experienced male with no infants present still showed elevated prolactin and some inexperienced males showed elevated prolactin just before parturition, suggesting noninfant cues are also important. While prolactin levels were influenced by the male's prior infant experience, testosterone levels did not differ between experienced and inexperienced males. Testosterone levels were significantly elevated for all males during the 3rd, 4th and 5th months but had no relationship with number of infants present or with outcome of current pregnancy. DHT decreased during the second half of pregnancy compared with testosterone but this finding was not consistent for every male. No significant changes occurred in cortisol levels. These results suggest that infant-rearing experience affected the hormonal responsitivty of the male to his mate's current pregnancy.     

Prolactin levels during the periparturitional period in the biparental cotton-top tamarin (Saguinus oedipus): interactions with gender, androgen levels, and parenting

Unlike biparental bird and rodent species, mating and parenting occur simultaneously in cotton-top tamarins, Saguinus oedipus, providing a unique model for examining endocrine interactions. This study was designed to determine the relationship of prolactin to testicular androgens during parenting and mating. Specifically we examined (1) the patterns of postpartum prolactin excretion in male and female tamarins with and without infant survival; (2) the relationship between androgen and prolactin levels during the periparturitional period in male tamarins; and (3) male hormonal response to the postpartum mating that occurs simultaneously with paternal care. All females showed an elevation in prolactin during the first week postpartum and when infants died, female prolactin levels decreased significantly. Infant survival during the first 15 days did not influence male levels of prolactin, cortisol, or the testicular androgens testosterone and dihydrotestosterone (DHT). Prolactin did not show an inverse relationship with any of the androgens (testosterone, DHT) studied during the postpartum period. No significant differences in hormones were found between prepartum levels and the first, second, and third 5-day blocks postpartum for all 10 males. Males who had infant-care experience showed an increase in testosterone during the first 5 days postpartum and prolactin levels correlated with the number of previous births a male had experienced. However, the most significant changes in testosterone occurred in males whose mates ovulated during the first 15 days following birth in contrast to males whose mates ovulated later than 15 days. These results indicate that unlike females, male tamarins are not showing hormonal changes in response to infants. Urinary androgens did not show an inverse relationship to prolactin in male tamarins, but were elevated concurrent with the female's fertile period.     

Energy requirements of captive cotton-top tamarins (Saguinus oedipus oedipus)

Food intake of captive cotton-top tamarins was measured at various stages of the life cycle and energy intake was calculated from the results. The intake of adults for maintenance was found to vary around 500 kJ X kg-3/4 X day-1 and to decrease with age. During the last 8 weeks of pregnancy, females increased in weight by about 2 g X day-1 but energy intake did not show a marked rise above the maintenance range. During lactation, their energy intake appeared to double. Measurements of intake at various stages of growth were compared with a model of the energy requirements for growth which predicted intake quite closely. Compared with non-primate mammals of similar size, the growth rate of this species is slow, and because of this, the energy intake during growth and lactation is also comparatively low.     

Effect of days postpartum and exogenous GnRH on reproductive hormone and ovarian changes in postpartum suckled beef cows

Two experiments were conducted to determine the effect of days postpartum and exogenous gonadotropin releasing hormone (GnRH) on reproductive hormone and ovarian changes in postpartum suckled beef cows. In experiment 1, eight suckled cows were bled at .5 hour intervals for 4 hours on days 7, 14, 21 and 28 postpartum. Although mean concentrations of plasma luteinizing hormone (LH) were positively correlated with days postpartum, mean concentrations did not differ. The mean maximum change and the variance of plasma LH were low on days 7, 14, 21 and 28 postpartum. Although the number of cows with an ovarian follicle and follicular size increased with days postpartum, mean concentrations of estradiol-17beta did not change. The interval from parturition to the first detected ovarian follicle and the first postpartum estrus was 17.5 +/- 2.6 days and 36.0 +/- 2.2 days, respectively. An elevation in plasma progesterone was detected about one week prior to the first postpartum estrus in 6 of the eight cows in the absence of corpora lutea. In experiment 2, gonadotropin releasing hormone (GnRH) induced ovulation in 4 of the 8 cows treated on day 27, 28 or 29 postpartum whereas none of the 8 saline treated cows ovulated to treatment. The interval from parturition to first estrus and conception were similar for both groups (P >.10).     

Excretion of estrone,estradiol, and progesterone in the urine and feces of the female cotton-top tamarin (Saguinus oedipus oedipus)

The excretion of three gonadal steroids was studied in the urine and feces of female cotton-top tamarins (Saguinus oedipus oedipus). Each steroid, ¹⁴C-estrone, ¹⁴C-estradiol, and ¹⁴C-progesterone, was injected into a separate female cotton-top tamarin. Urine and feces were collected at 8 hr intervals for 5 days on the three tamarins. Samples were analyzed to determine the proportion of free and conjugated steroids. Steroid excretion patterns were determined by sequential ether extraction, enzyme hydrolysis, and chromatography. Labeled estrone was excreted in a slow and continuous manner into the urine (57%) and feces (43%) with 90% of the steroid conjugated. The nonconjugated form had an elution profile identical to ³H estrone, but the conjugated portion was not completely hydrolyzed by enzyme. Labeled estradiol was excreted primarily in the urine (87%) and was released rapidly. Over 90% of the injected ¹⁴C-estradiol was excreted in urine as a conjugate, of which 41% was converted to an estrone conjugate and the remaining 59% was excreted as a polar estradiol conjugate. Labeled progesterone was excreted primarily in the feces (95%), 61% of which was free steroid. Four to six individual peaks of radioactivity were found when using celite chromatography and high performance liquid chromatography (HPLC), indicating that progesterone is metabolized into several urinary and fecal metabolites. One of these peaks matched ³H-progesterone and others may be pregnanediols, pregnanetriols, and 17-hydroxyprogesterone. These steroidal excretion patterns help explain the atypical hormonal patterns seen during the tamarin ovarian cycle.     

Profiles of LH, FSH and progesterone in postpartum dairy cows and their relationship to the commencement of cyclic functions

In 25, 3-to-13 year old, dairy cows (Braunvieh and Hoehenfleckyieh) FSH, LH and progesterone plasma profiles were determined by RIA. Blood was sampled at 6-hour intervals from parturition to 40–78 days postpartum, and the results correlated with the commencement of cyclic functions. For FSH, generally basic values were recorded, without characteristic features associating any values with the onset of cyclic ovarian activities or the occurrence of the first heat. LH profiles varied greatly between individuals with regard to the onset of elevations, regularity of patterns and peak values. The first preovulatory LH peak was recorded 17.3±9.8 days (range 4–46) postpartum. The first heat occurred on day 28.4±16 (range 6–55) postpartum, indicating that 13/23 cows ovulated without behavioral estrus, as reproductive cycles were re-established. Peak LH values increased with progressive cycles (1st peak 5.7±4.8 ng/ml; 2nd peak 11.8±8.7 ng/ml; 3rd peak 13.5±9.9 ng/ml plasma). Progesterone values also showed great variations in the profile of their first postpartum elevation. In 13/25 cows the first cycle was shortened (13.1±2.9 days), prolonged in 3 animals (34±4 days) and normal in 7 cows (20.4±1.9 days). Heat, preovulatory LH peak and progesterone profile were normal in all animals on subsequent cycles. Two animals did not start cycling.     

The relationship between female urinary estrogen excretion and mating behavior in cotton-topped tamarins,Saguinus oedipus oedipus

Daily urinary estrogen excretion in six singly housed adult female cottontopped tamarins was measured by radioimmunoassay. Each female was paired with a male for 1 hr every day, during which behavioral observations were conducted on the pair from behind a oneway glass screen. During each observation period the duration or frequency of occurrence of various behaviors was recorded. In this paper, the daily variations of (1) total frequency of male mounting, (2) total frequency of male mounting with thrusting, and (3) latency to first male mount are presented in relation to the measured pattern of female estrogen excretion. The results reveal a high degree of behavioral variation between pairs, although a relationship between rhythmic changes in sexual activity and female estrogen excretion in individual pairs is apparent. Mating occurs throughout the female cycle and also during at least the first 2–3 weeks of pregnancy. A gestation period of 170–172 days is estimated in respect of one female, in agreement with a previous observation of one gestation of at least 166 days. The high frequency and intensity of mating recorded in this controlledintroduction study give support to the proposition that mating is important in pairbonding in this monogamous species. It is concluded that under the experimental procedure followed, mating behavior did not reliably reflect the female’s reproductive hormonal state.     

Serum profiles of luteinizing hormone,progesterone and total estrogens during the canine estrous cycle

Serum levels of LH, total estrogen and progesterone were measured daily by radioimmunoassay during proestrus, estrus and early diestrus in five beagle bitches. Occurrence of the LH peak relative to the onset of estrus was quite variable ranging from 3 days before to 7 days after the onset of estrus. Serum LH levels were elevated for 3 days with a peak value of 25 ± 2 ng/ml reached 2.4 days after the start of estrus. LH levels were ≤ 2 ng/ml when measured at other times during the estrous cycle. Estrogen titers ranged from 84 ± 39 pg/ml at 9 days before the LH peak to 175 ± 15 pg/ml coincident with the LH peak. A broad estrogen peak was evident beginning 5 days before and continuing for 5 days after the LH peak. An estrogen surge was seen in 4 of 5 dogs immediately preceding or coincident with the LH peak suggesting that LH release in the bitch is triggered by a sharp elevation in estrogen levels. Serum progesterone levels rose from ≤ 5 ng/ml before the LH peak to 46 ± 6 ng/ml 6 days afterwards.     

Fertility status,dominance, and scent marking behavior of family-housed female cotton-top tamarins (Saguinus oedipus) in absence of their mothers

The effect of the mother's absence on the fertility status and expression of scent marking behavior was examined in 12 sexually mature female cotton-top tamarins (Saguinus oedipus) living in four motherless families. Behavioral data and urine samples were collected on an average of five times per week and levels of estrone-glucuronide (E1G) were quantified directly by radioimmunoassay to establish whether the females showed ovarian cyclicity. Of the 12 females investigated only the dominant female in each motherless group exhibited regular E1G cycles. By contrast, low and acyclic hormonal profiles were found for subordinate sisters. In addition to differences in the patterns and levels of E1G, scent gland morphology and rates of scent marking differed between females: dominant, cycling females had elaborate glands and significantly higher rates of scent marking than subordinates. These results indicate that the presence of the mother (breeding female) is one important factor regulating ovarian function in cotton-top tamarin daughters. The findings also show that dominance status is a vital determinant of each female's subsequent physiological reproductive competence, with only the dominant female obtaining complete fertility and probably inhibiting ovulatory activity in her subordinate sisters. Scent marking behavior seems to be involved in the regulation of this phenomenon of intrasexual reproductive competition.     

Changes in circulating inhibin levels during pregnancy and early lactation in the Japanese monkey

Changes in immunoreactive (ir-) inhibin concentrations in serum throughout pregnancy and early lactation up to one month after parturition were characterized in 6 Japanese monkeys (Macaca fuscata fuscata) by a heterologous radioimmunoassay (RIA) based on a bovine RIA. Serum levels of FSH, LH/monkey chorionic gonadotropin (mCG), estradiol-17 beta, and progesterone were also monitored for the entire period. Ir-inhibin levels in the serum were low (under 0.5 ng/ml) before conception. Three marked increases in serum ir-inhibin levels were found during pregnancy. The first increase was noted during early pregnancy, with a peak (2.2 +/- 0.2 ng/ml) at Day 22 of pregnancy (Day 0 = day of LH surge). The second increase was noted after Day 38 until Day 72 of pregnancy, when a peak value was noted (19.0 +/- 1.4 pg/ml). Plateau levels were maintained until late pregnancy, and a final rise was evident near the term with a peak (36.7 +/- 3.8 ng/ml) at Day 158 of pregnancy, 5 days before parturition. After parturition, ir-inhibin levels in the serum plummeted to nonpregnant levels within one day, and were maintained during early lactation. The first rise in serum inhibin during pregnancy was parallel to the rise of mCG and estradiol-17 beta, and the second and third rise were well correlated with serum estradiol-17 beta. Serum FSH was maintained at low levels throughout pregnancy, followed by a slight increase after parturition when serum inhibin decreased abruptly. Both bioactivity and immunoreactivity of inhibin were detected in the placental homogenates obtained at 120 days of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endocrine control of estrous cycle in mithun (Bos frontalis)

The objective of the present study was to establish the profiles of luteinising hormone (LH), follicle stimulating hormone (FSH), estradiol 17beta (E2) and progesterone (P4) secretion and their interrelationships during the natural estrous cycle of mithun (Bos frontalis). Daily blood samples were collected from second or third postpartum estrous cycles for determination of plasma concentrations of LH, FSH, E2 and P4. Concentration of P4 was found to be lowest on the day of estrus. It increased following estrus, attained the highest concentration on day 11 and decreased thereafter. Concentrations of LH and FSH varied significantly (p<0.01) during the first and last 6 days of the cycle and their variations were found to be synchronised. Both LH and FSH attained a biphasic peak during the estrous cycle. This biphasic peak lasted on from day -5 to day 3 of the cycle. The variations in maximum LH and FSH concentrations of both the phases did not differ significantly. During the entire estrous cycle, the E2 concentrations attained either one peak or two peaks. The first peak, approximately on day 4 before estrus was common in all animals. One additional peak was found on the day of estrus in 45% animals. A significant (p<0.01) negative relationship was found between P4 and, LH and FSH during the first and last 6 days of cycle. But a significant (p相似文献   

The role of the young in the control of the hormonal events during lactation and behavioral weaning in the golden hamster

This study was designed to determine the relationship between the behavioral and physiological changes occurring in the hamster over the course of lactation. Experimental (E) females were given litters of six 3-day-old pups on Day 13 of lactation and allowed to raise them. Control (C) females raised their own litters of six pups. Groups of E and C females were decapitated at 1000 and 1700 hr when their pups were either 8, 18, or 28 days old. Nursing was observed for 1 hr on the 3 days prior to autopsy. Nursing behavior disappeared by Day 28 in C females. E females exhibited nursing behavior at levels equal to those observed in C females until E pups were 28 days old and 38 days had elapsed since parturition. Despite the fact that E mothers continued to nurse pups on day 18 postpartum when pups were 8 days old, E pups showed lower growth rates than did C pups. Prolactin (PRL) levels remained elevated when E pups were 8 days old even though E pups did not grow normally. PRL levels decreased over time in both C and E females and reached baseline by Day 28 although nursing behavior was still elevated in E females. Thus, nursing behavior did not stimulate PRL release during late lactation. Estradiol (E₂) levels in C females remained at baseline until Day 28 when levels increased. LH, FSH, and Progesterone (P) levels in C females showed a dramatic diurnal pattern which disappeared by Day 28, when levels dropped. Levels of E₂, P, LH, and FSH in E females closely paralleled those of C females only when groups were compared with respect to pup age. Thus, behavioral weaning and the return to estrous cyclicity appear to be dependent upon the age of the pups rather than the time elapsed since parturition. However, milk production and PRL release appear to be more closely tied to the number of days postpartum and can be dissociated from the amount of nursing behavior observed.