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.     

Further hormonal suppression of eldest daughter cotton-top tamarins following birth of infants

Reproductive suppression of females is found throughout the Callitrichids. However, in many species some evidence of ovarian activity is observed in subordinate females. Subordinate cotton-top tamarin females in our colony have never been observed to ovulate in the presence of a reproductive female. However, ovarian follicular development does occur, and measurable levels of urinary estrogen and luteinizing hormone are frequently found in subordinate females. We studied 11 female tamarins living in family groups with a reproductive female. Each of the 8 eldest daughters had measurable urinary estrogen and LH levels and showed a reduction of hormonal levels when new infants were born. The 3 younger daughters showed barely detectable hormonal levels that did not change. Following the birth of infants the eldest daughters scent marked less frequently, increased time in contact with and grooming group members other than the mother, but they were more often targets of aggression than immediately prior to infant births. The eldest daughters were somewhat less involved in care of new infants than expected, although they spent much time in proximity to those carrying the infants. These results suggest that the further reduction of hormonal levels in subordinate females after the birth of infants may function to prevent these females from competing with mothers during the post-partum estrus rather than recruiting the eldest daughters as helpers for infant care. © 1993 Wiley-Liss, Inc.     

Features of female reproductive senescence in tamarins (Saguinus spp.), a New World primate.

Cyclical changes in concentration of plasma progesterone, urinary oestrone-conjugates and urinary luteinizing hormone (LH) were compared in young and older cotton-top tamarins (Saguinus oedipus) and saddle-backed tamarins (S. fuscicollis). A group of six young adult tamarin females (4-5 years of age) was sampled over eight periods of 6-8 weeks and six older (14-20 years of age) females were sampled over thirteen periods. There was hormonal evidence of ovulation in all of the sampling periods for young females; in five of thirteen periods, older females displayed no evidence of ovulation. Of the six older females, two were anovulatory in one sampling period, while one female displayed no evidence of ovulation in any of three sampling periods. Generally, females over 17 years of age either did not ovulate or displayed abnormally long periods of moderate concentrations of progesterone and oestrone conjugates. Basal concentrations of LH differed in individuals, but were not always higher in older females. In contrast to patterns of reproductive senescence in other primates, older, anovulatory tamarins displayed moderate concentrations of urinary oestrone conjugates (5-50 micrograms/mg creatinine) and plasma progesterone (8-19 ng/ml), both of which are hormones of probable luteal origin in these species. This result suggests continued production of steroids by the luteal cells of the prominent interstitial gland in reproductively senescent tamarins. This suggestion was reinforced by histological examination of the ovaries of four older, anovulatory females; few primary follicles were found. Three females had no normal antral follicles, but all females had large luteal masses. The presence of functional luteal cells in the older ovaries, which do not experience regular follicular development, may distinguish ovarian ageing in New World primates from that of Old World primates.     

Resumption of follicular activity in the early post-partum period of dairy cows

Lactating Friesian dairy cows (2nd-4th parity) which calved in spring (N = 7) or autumn (N = 15) were used. Their ovaries were examined by ultrasound scanning and blood samples were obtained daily for progesterone and oestradiol concentrations from the 5th day after calving until the first post-partum ovulation occurred. Five autumn-calving cows selected at random were bled every 15 min over a 6-h period on 1 day each week for 4 weeks after calving to assess the patterns of LH secretion. Follicular development during the post-partum anoestrous period was characterized by the growth and regression of small (less than or equal to 4 mm) and medium-sized (5-9 mm) follicles, until a dominant follicle (greater than 10 mm) was detected. The first detected dominant follicle ovulated in 14 cows, became cystic in 4 cows (all in autumn), and failed to ovulate in 1 cow. It was not possible to detect a dominant follicle in 3 cows due to scanning difficulties. The post-partum interval to detection of the first dominant follicle (mean +/- s.d.) was shorter (P less than 0.05) in autumn (6.8 +/- 1.8 days) than in spring (20 +/- 10.1 days). However, there was no significant difference between the respective intervals to first ovulation (autumn 27.4 +/- 25.9 and spring 27.3 +/- 18.9 days). Autumn-calved cows which had cysts had longer (P less than 0.001) intervals to first ovulation (58.2 +/- 23.5 days) than did normal cows (12.0 +/- 2.5 days). All cows with cysts had twin ovulations at their first post-partum ovulation. A pulsatile pattern of LH secretion was detected in the first week post-partum and LH pulse frequency was 2-3 per 6-h period in Weeks 1 and 2 post partum and increased to 5-7 pulses per 6-h period in the presence of a dominant or cystic follicle. Concentrations of progesterone in plasma during post-partum anoestrus were usually low (less than 0.2 ng/ml); oestradiol concentrations were also low (less than 5 pg/ml), but higher values (5-110 pg/ml) were observed in cows that had a dominant or a cystic follicle.     

Urinary gonadotropin and estrogen excretion during the postpartum estrus,conception, and pregnancy in the cotton-top tamarin (Saguinus oedipus oedipus)

Hormonal profiles during postpartum estrus, time of conception, and pregnancy were determined in urine samples from six cotton-top tamarins (Saguinus oedipus oedipus). Noninvasive collection techniques permitted daily sampling throughout lactation and pregnancy. Urinary estrone (E1), estradiol (E2), and both bioactive and immunoreactive luteinizing hormone/chorionic gonadotropin (LH/CG) measures revealed an interval of 19 ± 2.07 (S E M) days between parturition and the postpartum ovulatory LH peak. An increase in both E1 and E2 was seen prior to the LH peak; however, E1 and E2 continued to increase to their highest concentrations after the LH peak. Since postpartum ovulations resulted in pregnancy, neither postpartum estrus nor conception was suppressed by lactation. The length of gestation (measured from the LH peak to parturition) was 183.7 ± 1.14 (S E M) days, which is at least 30 days longer than that previously reported for other callitrichid species. Both E1 and E2 reached their maximum levels during midpregnancy but showed a rapid decline at parturition. Gestational levels of CG were first detectable approximately 20 days after the LH peak and continued to be elevated for approximately 80 days. The Sub-Human Primate Tube Test (SHPTT) for pregnancy did not detect the LH Peak and was less sensitive than other methods in detecting CG. Two RIA methods and a bioassay technique could not distinguish between LH and CG. We concluded that monitoring both estrogen and LH concentration was needed to determine when ovulation occurs in the cotton-top tamarin, since peak values of estrogen are seen after the ovulatory LH peak. Also, these tamarins were pregnant the majority of the time, indicating an unusually high fertility rate in contrast to most noncallitrichid primate species.     

Reproduction of the emperor tamarin (Saguinus imperator) in captivity,with comparisons to cotton-top and golden lion tamarins

Little information has been published on the reproductive biology and behavior of the emperor tamarin (Saguinus imperator). We analyzed twelve years of data on emperor tamarins at the Los Angeles Zoo and made comparisons with data on cotton-top tamarins (Saguinus o. oedipus) and golden lion tamarins (Leontopithecus rosalia) from the same collection. Secondary sex ratios did not differ significantly from 50:50. Births were not strictly seasonal for any species. The number of infants reared had a significant effect on interbirth interval for all species, with shorter intervals when only one or no infants were reared, but females did sometimes conceive early in lactation. In emperor tamarin families, all fathers and most older siblings carried new infants, usually beginning within a few days after a birth. Previous exposure to younger siblings did not appear to be critical to the development of competent parental behavior by zooborn emperor tamarin females.     

The costs of infant carrying in captive cotton-top tamarins

Marmosets and tamarins have a communal rearing system in which all group members help to care for the twin infants characteristic of this family of primates. Helpers are likely to incur time and energy costs by contributing to infant care. Predictions that cotton-top tamarin (Saguinus oedipus) helpers would change their behavior when carrying infants because of reduced mobility and/or a need for increased vigilance were tested in a captive colony. Tamarins carrying an infant spent significantly less time feeding, foraging, moving, or engaging in social activities such as grooming than they did when not carrying. Frequencies of scratching, autogrooming, and scent marking were significantly reduced in carriers, suggesting that their mobility was reduced. However, carriers were significantly less likely to be vigilant (measured by direction of gaze) than when not carrying. Further observations showed that carriers spent more time in concealed areas than they did when not carrying and were probably therefore adopting a cryptic strategy to reduce predation risks to themselves and to infants. These results demonstrate that tamarin helpers pay costs by carrying infants. Some possible compensating benefits are indicated.     

Effects of reducing the remating interval after parturition on the fertility and plasma concentrations of luteinizing hormone, prolactin, oestradiol-17 beta and progesterone in lactating domestic rabbits

Primiparous crossbred does were remated on Day 1 (n = 15) or 14 (n = 25) post partum and killed on Day 10 post coitum to assess their fertility. Blood samples were taken during the pre- (0-12 h post coitum) and post- (1-10 days post coitum) ovulatory periods and plasma was assayed for luteinizing hormone (LH), prolactin, oestradiol-17 beta and progesterone. Ovulation response was significantly greater (P less than 0.01) and ovulation rate significantly lower (P less than 0.001) in does mated on Day 1 than in those mated on Day 14 post partum. Does failing to ovulate on Day 14 post partum exhibited no preovulatory LH surge and had significantly lower (P less than 0.05) premating concentrations of oestradiol-17 beta and prolactin than those ovulating at this time. No significant differences in hormone concentrations were observed during the preovulatory period between does ovulating on Days 1 and 14 post partum, with the exception of oestradiol-17 beta. Concentrations of this hormone were significantly lower (P less than 0.01) in does mated on Day 1, at 1 h post coitum. We conclude that (i) fertility was affected by the remating interval after parturition, (ii) ovulation failure was associated with an absence of the preovulatory LH surge and a reduction in premating concentrations of oestradiol-17 beta and prolactin and (iii) the lower ovulation rate in early lactation was apparently caused by a reduction in ovarian competence to respond to the gonadotrophic stimulus.     

Influence of undernutrition in the lactating cow on ovulation induction by serum gonadotropins (PMSG) during the post-partum period

Two experiments were carried out to assess the influence of undernutrition on postpartum ovulation in nursing Charolais cows after PMSG injection. In each experiment, the nursing cows were divided into 2 groups: one fed at a low nutritional level and the other at a normal nutritional level. In the first experiment, 19 animals were injected on post-partum days 15 and 30 with 600 IU of PMSG; in the second experiment, 34 received the same injection on post-partum day 54 after 9 days of priming with a Norgestomet implant. On post-partum day 15, only one cow in each group ovulated. At post-partum day 30, 1 out of 8 cows at the low nutritional plane ovulated vs 5 out of 9 at the normal nutritional plane (P less than 0.05). Likewise, on post-partum day 54, 5 out of 14 cows at the low nutritional plane ovulated vs 17 out of 17 at the normal nutritional level (P less than 0.05). Therefore, there is a time during the post-partum period when the nursing cow ovary does not respond to PMSG by ovulation. The length of this time is increased by undernutrition.     

Cross-generic Food Sharing in Tamarins

Adult-infant food-sharing behavior is a major component of the infant care strategies of callitrichids (marmosets and tamarins). It is particularly well-developed in cotton-top tamarins (Saguinus Oedipus) and lion tamarins (Leontopithecus spp), which show frequent adult-initiated food offering, as well as sharing of food in response to begging by infants. This report documents a case of cross-generic food sharing, in which a male golden-headed lion tamarin (Leontopithecus chrysomelas) shared food with an infant cotton-top tamarin. The lion tamarin provided more food to the infant than its mother did. This emphasizes the importance of this behavior in the reproductive strategies of the communally-rearing Callitrichidae and raises questions about mechanisms that underly it.     

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.     

The variability in the interval between estrus and ovulation in cattle and its determinants

Fertility of Holstein cows has been decreasing for years and, to a lesser extent, the fertility of heifers too but more recently. A hypothesis to explain this phenomenon may be that the chronology of events leading to ovulation is different for those animals bred nowadays when compared to what was reported previously; this would result in an inappropriate time of insemination. Therefore, two experiments were designed to investigate the relationships among estrus behavior, follicular growth, hormonal events and time of ovulation in Holstein cows and heifers. In the first experiment, the onset of estrus, follicular growth, patterns of estradiol-17beta, progesterone and LH, and the time of ovulation were studied in 12 cyclic Holstein heifers that had their estrus synchronized using the Crestar method; this was done twice, 3 weeks apart. The intervals between estrus and ovulation, estrus and the LH peak, and between the LH peak and ovulation were, respectively, 38.5 h +/-3.0, 9.1 +/- 2.0 and 29.4 h +/-1.5 (mean+/- S.E.M). The variation in the interval between estrus and the LH peak explained 80.6% of the variation in the interval between estrus and ovulation. The intervals between estrus and the LH peak, and estrus and ovulation were correlated with estradiol-17beta peak value (r=-0.423, P <0.04 and r=-0.467, P<0.02, respectively). Positive correlation coefficients for the number of follicle larger than 5 mm, and negative correlation coefficients for the size of the preovulatory follicle with the intervals between estrus and LH peak, LH peak and ovulation, and estrus and ovulation suggest an ovarian control of these intervals. In respect to its role to explain the variation in the interval between estrus and ovulation, the variation in the interval between estrus and the LH peak was evaluated further in a second set of experiments utilizing 12 pubertal Holstein heifers and 35 Holstein cows. The duration of the interval between the beginning of estrus and the LH peak was longer in heifers than in cows (4.15 h versus -1.0 h; P <0.002); the variation for this interval was higher in cows than in heifers (S.E.M.= 1.2 h versus 0.8 h; P=0.01). According to the results of these studies it can be proposed that estradiol and other product(s) of ovarian origin regulate not only the duration of intervals between the onset of estrus and the LH surge but also between the LH surge and ovulation. From the results obtained in the first experiment, it may be postulated that differences observed between cows and heifers for the duration of the interval between onset of estrus and the LH surge as well as for the variation of this interval would be observed also for the interval between the onset of estrus and ovulation. Therefore, on a practical point of view, the long interval between the onset of estrus and ovulation and the high variation of this interval, especially in cows, may be a source of low fertility and should be considered when analysing reproductive disorders.     

Birth spacing in langur monkeys (Presbytis entellus)

This paper presents 10 years of reproductive data on birth interval length and 5 years of data on reproductive behavior postpartum from a captive colony of gray langur monkeys (Presbytis entellus)housed in Berkeley, California. Birth intervals of females following different pregnancy and nursing schedules are compared. Females whose infants survive to the age of 9 months have a median birth interval of 15.4 months. The experimental separation of mothers from infants for a period of 2 weeks, 6 to 9 months postpartum, had no significant effect on the median birth interval length. Females experiencing a pregnancy failure or the loss of a neonate had median birth intervals of 9.6 and 10.7 months, respectively. These intervals were significantly shorter than the birth intervals of females whose infants survived to 9 months, showing that the presence of a nursing infant delays the female’s time to next conception by approximately 5 to 6 months. Females experienced a median of three estrous periods (two estrous cycles) before conceiving postpartum, regardless of pregnancy outcome or length of infant survival, and females rarely conceived during their first estrous period postpartum. Weaning did not occur until after the mother’s next conception. These data indicate that, in populations of langurs characterized by average birth intervals of 15 to 16 months, the loss of an infant after the age of 5 to 6 months will not accelerate a female’s ability to conceive or shorten the birth interval length. The available data on birth spacing from populations of free-ranging langurs are reviewed. It could not be demonstrated that non-Himalayan populations are characterized by birth intervals which are as long as 20 to 24 months. Rather, it is suggested that female langurs inhabiting seasonally arid sites, such as Jodhpur, Abu, and Dharwar, may be capable of producing infants on the average of every 15 to 16 months. Flexibility in the timing of births and the lack of well-defined birth seasons at these sites may be explained by this species’ dietary and digestive adaptations. Additionally, data on birth spacing and the age of missing infants from the above field sites, where it has been suggested that infanticide following changes in male leadership occurs habitually, do not lend support to the sexual selection hypothesis of infanticide as proposed by S. Hrdy (1974, 1977).     

FSH and LH variations in beef cows during the postpartum period

To study the plasma gonadotrophin profiles of 9 cows after parturition, blood samples were obtained every 20 min for 12 hrs on three occasions between 5 and 50 days postpartum and analysed by RIA techniques. The time of the first ovulation, as judged by plasma progesterone levels, varied from 30 to more than 60 days postpartum. Variations in mean levels of FSH and LH were not significantly correlated with the postpartum interval. However, the mean levels of plasma FSH and number of LH pulses were lower in females which had not ovulated than in those which had. The cows could be classified into four groups: group 1 with less than 4 LH pulses in 12 hrs and a mean plasma FSH level less than 138 ng/ml; group 2 with more than 4 LH pulses in 12 hrs and varying plasma FSH levels; group 3 with less than 4 LH pulses in 12 hrs and a mean plasma FSH level greater than 138 ng/ml; group 4 which had ovulated. This classification indicated that the LH and FSH levels progressed significantly (2.46 to 3.56 ng/ml, P less than 0.05; 120 to 159 ng/ml, P less than 0.01, respectively) from groups 1 to 3, and that they decreased in the females which had ovulated (group 4). Since the time of the first ovulation after parturition varied, it was not possible to demonstrate any relationship between that interval and the mean plasma gonadotrophin profiles. However, when ovulation was considered as time zero there was a clear increase in plasma gonadotrophin before ovulation.     

Cyclic excretion of urinary oestrogens in female tamarins (Saguinus oedipus)

Urine was collected from 6 female cotton-top tamarins (Saguinus o. oedipus) and urinary oestrone and oestradiol concentrations were measured by radioimmunoassay. Oestrone was excreted at 50-fold higher concentrations than oestradiol. Five females showed patterns of regular oestrone cyclicity, with a mean peak-to-peak oestrone cycle of 23.6 +/- 1.2 days. Levels of oestradiol tended to vary with levels of oestrone excretion, but peaks were less pronounced and more variable. The sixth female, diagnosed as having 'wasting marmoset syndrome', had very low levels of excreted oestrogens, suggesting infertility. We suggest that urinary oestrone provides a good index to ovarian cyclicity in female cotton-top tamarins.     

Pregnancy weight gain: marmoset and tamarin dads show it too

Paternal behaviour is critical for the survival of offspring in many monogamous species. Common marmoset (Callithrix jacchus) and cotton-top tamarin (Saguinus oedipus) fathers spend as much or more time caring for infants than mothers. Expectant males of both species showed significant increases in weight across the pregnancy whereas control males did not (five consecutive months for marmoset males and six months for cotton-top tamarin males). Expectant fathers might be preparing for the energetic cost of fatherhood by gaining weight during their mate's pregnancy.     

Time of ovulations in dairy goats induced to superovulate with porcine follicle stimulating hormone during and out of the breeding season

Alpine dairy goats were induced to superovulate at the end of a progestagen treatment with porcine follicle stimulating hormone (pFSH) during the breeding season (n = 10 goats) and out of the breeding season (n = 10 goats). Occurrence of estrus and of the luteinizing hormone (LH) peak were checked every 4 h. Ovulations were determined every 6 h by ovarian laparoscopic examination. Among the parameters studied, the mean interval from sponge removal to the onset of estrus did not differ whatever the season of treatment, but the variability was higher for females treated out of the breeding season. Ovulations began during the laparoscopic control period for nine of ten goats during the breeding season vs seven of ten goats out of the breeding season. For these 16 females, on which the LH peak and beginning of ovulation were known, the season did not affect the intervals between the onset of estrus and the LH peak and between the LH peak and the beginning of ovulation. When ovulations are observed by laparoscopy every 6 h, for any given goat 54.9% of total ovulations (counted 7 d after estrus) occurs in less than 6 h, and 87.1% in less than 12 h. Although the interval between the LH peak and the ovulation is quite constant, the additive variabilities of the intervals between the sponge removal and the onset of estrus and between the onset of estrus and the LH peak precluded the determination of an optimal time for artificial insemination (AI) by timing sponge removal or onset of estrus.     

Luteinizing hormone in blood plasma of post-partum buffaloes (Bubalus bubalis)

The changes in luteinizing hormone concentration were measured by heterologous double-antibody radioimmunoassay in blood plasma of 28 Murrah buffaloes. The LH concentration fluctuated between 0.22 to 0.78 ng/ml during first 21 days post-partum. The level increased significantly (P < 0.001) at estrus. The basal LH concentration of second and third week post-partum was inversely related to the first post-partum ovulation interval.     

The benefits of helpers: Effects of group and litter size on infant care in tamarins (Saguinus oedipus)

The Callitrichidae (marmosets and tamarins) typically give birth to twins, and infant care is shared by all group members. The potential benefits to callitrichids of having helpers were investigated in a study of 21 captive cotton-top tamarin (Saguinus oedipus) infants, living in groups with two to 12 older members. Time carried and suckled and amount of food received from other family members were recorded for each infant during the first 12 weeks of life. The results showed that infants in larger groups were carried more and received more food than those in smaller groups. Twin infants in larger families were less likely to be on the same carrier. Singleton infants were carried more than twins but did not receive more food. On average, individual caretakers in larger groups carried less and shared less food with infants than those in smaller groups. When parental contributions to care were analyzed, no effects of group size were found on mothers' contributions to carrying or food sharing, but fathers in larger groups both carried infants less and shared less food with them. There appear to be several benefits of a communal rearing system to cotton-top tamarins. 1) Infants may receive more care in larger families, thus increasing their chances of survival. 2) The burden of care is spread over several animals, reducing the costs to a given individual. 3) The parents, particularly fathers, may benefit most from reducing the costs of investing in the present litter and increasing their ability to invest in future litters.     

Pattern of follicular growth and resumption of ovarian activity in post-partum beef suckler cows

The ovaries of 18 post-partum beef suckler cows were examined daily, using ultrasound, from Day 5 post partum until a normal oestrous cycle was completed. Periods of growth and regression of medium-sized (5-9 mm) follicles were identified before one medium follicle became dominant (single large follicle greater than or equal to 10 mm). The mean (+/- s.e.m.) number of days from parturition to detection of the first post-partum dominant follicle was 10.2 +/- 0.5. The first post-partum dominant follicle ovulated in 2/18 (11%) cows. The interval from calving to first ovulation (mean +/- s.e.m. = 35.9 +/- 3.3 days) was characterized by the growth and regression of a variable number (mean = 3.2 +/- 0.2; range 1-6) of dominant follicles. The maximum diameter of the dominant follicle increased as the cows approached first ovulation (P less than 0.05). Behavioural oestrus was not detected in 16/18 (89%) cows at first ovulation. Following first ovulation, the length of the subsequent cycle was short (mean = 9.7 +/- 0.5 days; range 8-15 days) in 14/18 (78%) cows and was characterized by the development and ovulation of a single dominant follicle. During oestrous cycles of normal length (mean = 20.6 +/- 0.5 days; range 18-23 days) one (N = 2), two (N = 7) or three (N = 8) dominant follicles were identified. The growth rate, maximum diameter or persistence of non-ovulatory dominant follicles before first ovulation or during oestrous cycles were not different (P greater than 0.05). These data show that, in beef suckler cows, follicular development and formation of a dominant follicle occur early after parturition and the incidence of ovulation of the first dominant follicle is low. The number of dominant follicles that develop before first ovulation is variable; first ovulation is rarely associated with oestrus and short cycles are common after first ovulation. It is concluded that prolonged anoestrus in post-partum beef suckler cows is due to lack of ovulation of a dominant follicle rather than delayed development of dominant follicles.