Photoperiod and parturition period in isolated or paired rats: influence of sight deprivation and social conditions

Time of delivery was studied in normal (N) or enucleated (E) rats. Isolated (1/cage) at mating (day 1 of gestation), they were all anaesthetized on day 8 and some of them were then blinded. After anaesthesia, they were kept either alone (N and E) or in homogeneous (NN and EE) and heterogeneous (NE) pairs either under a 12L-12D or a 2L-22D light-dark (LD) cycle. Activity, monitored throughout the rest of pregnancy in 2 homogeneous (NN and EE) and heterogeneous (NE) pairs of rats, kept a marked circadian rhythm under each light regimen. Birth distributions differed according to LD cycle in normal (N or NN) but not in enucleated (E and EE) rats. Under the 12L-12D regimen, parturitions of N and E rats were divided into 2 parts, the majority occurred on the afternoon of day 22 before 21:00 h, the remainder were observed after 6:00 h on day 23. Under the 2L-22D regimen, N rats gave birth over one period, mainly on day 23, whereas E rats had the same birth distribution as those subjected to the 12L-12D LD cycle. In heterogeneous pairs of rats (NE), birth times were affected by photoperiod; under the 2L-22D regimen it was intermediate between those of homogeneous pairs (NN and EE). These results indicate that the eyes were the first link of the nervous chain by which photoperiod influenced birth time. Social conditions may also modulate the photodependent mechanism in ways which remain to be determined.     

Reproductive performance of multiparous rabbit lactating does: effect of lighting programs and PMSG use

This study aims to determine if Pregnant Mare Serum Gonadotrophin (PMSG), used for oestrous synchronization in multiparous lactating does, could be replaced by one of the following lighting schedules without impairing reproductive performance: (a) 12-h L (light)/12-h D (dark) or (b) 8-h L/16-h D, until day 6 before artificial insemination (AI), when in both cases photoperiod was changed to 16-h L/8-h D and maintained until the day of AI, and in the following 4 days post Al the light hours were progressively reduced to the initial schedules. Two groups of 20 does each were respectively submitted to one of the lighting schedules specified above. All does were artificially inseminated in 6 consecutive cycles at 42 days intervals. In the first, third and fifth AIs, PMSG (20 IU/doe via sc 48 h before AI) was used in the two groups of does, whereas in the second, fourth and sixth Als no hormonal treatment was used. Degree of oestrous synchronization (also referred in text as sexual receptivity) was estimated by the colour of the vulva at AI. Reproductive performance of does was evaluated based on fertility (kindling rates), prolificity, mortality at birth, mortality at 21 days post birth, weight of the litter at 21 days post birth and number of weaned rabbits. Oestrous was better synchronized when PMSG was used with any of the two lighting programs. Without using PMSG, a photoperiod of 12-h L/12-h D until 6 days before AI resulted in a better sexual receptivity of does than 8-h L/16-h D. Fertility, prolificity, mortality of young rabbits at 21 days, the weight of the litters at 21 days and the number of weaned rabbits did not vary with the lighting program and were not affected by the PMSG treatment. Mortality at birth, however, was higher (+1 dead kit per litter) in litters housed under a light program of 12-h L/12-h D. Global productivity (number of weaned rabbits per 100 inseminated does) was better when using PMSG, for both lighting schedules. When using a photoperiod of 12-h L/12-h D until 6 days before AI, and omitting the PMSG treatment, global productivity was scarcely reduced, however, it was considerably impaired when using a photoperiod of 8-h L/16-h D until 6 days before AI and no PMSG treatment.     

Genetic parameters for early piglet weight,litter traits and number of functional teats in organic pigs

Knowledge remains limited on genetic variation and genetic correlations for traits in sows and piglets that are reared in an organic or outdoor setting. Here, we estimated genetic variance components for individual piglet weight, litter weight, litter size traits, and number of functional teats in a pig population raised under outdoor organic conditions. Data were collected from the largest organic multiplier farm in Denmark. Individual piglet weight was recorded at birth and on day 10. Number of live and dead piglets were recorded at birth, day 4, and day 11. Mean and total litter weight were calculated based on the individual weight of living piglets at birth and on day 10. The estimated heritability was highest for the number of functional teats (0.49), mean weight of a litter at birth (0.33) and on day 10 (0.25). In contrast, heritability was lowest for litter size traits (0.04–0.08) and piglet weight (0.06–0.07). Maternal heritability was much higher for individual piglet weight than direct heritability. The results showed that selection for higher mean weight results in smaller litters. Also, selection for individual birth weight of piglets results in heavier piglets at 10 days. In conclusion, this study confirmed that there is genetic variation in individual piglet weight, litter traits, and number of functional teats in organically and outdoor-reared pigs.     

Growth rate of 21 captive-born,mother-raised cheetah cubs

This paper presents the growth rate of 21 clinically normal, mother-raised. captive cheetah cubs from birth through 45 days of age. The development of a growth curve for healthy, mother-raised cheetah cubs provides a diagnostic tool for individuals involved in cheetah propagation. Use of the curve may alert caretakers to problems early and thus help reduce the high neonatal mortality rate seen in captive-born cheetah cubs. The growth curve was constructed using 21 (11 males and 10 females) captiveborn cheetah cubs from six litters (offspring of three unrelated adult males and three unrelated adult females) born at the Columbus Zoo from September 1985 through December 1989. Each cub was weighed to the nearest gram the morning after birth and approximately the same time every consecutive morning for 45 days. The mean weight the morning after birth was 463 g (range 385–542 g). The average litter size was 3.5 (range 2–4, n = 6). The daily weight gain was 40–50 g/cub/day.     

Morning pulse of prolactin maintains seasonal quiescence in the tammar, Macropus eugenii

The pulse of prolactin, present in female tammars kept in an inhibitory photoperiod and absent 5 days after a change to a stimulatory photoperiod, was artificially maintained during stimulatory photoperiods (Exps 1A and 1B) or abolished during an inhibitory photoperiod (Exp. 2). In Exp. 1A, thyrotrophin-releasing hormone (TRH) was injected i.v. for 8 consecutive mornings and in Exp. 1B ovine prolactin was injected i.v. for 10 consecutive mornings beginning on the day the photoperiod was changed from 15L:9D to 12L:12D. In Exp. 1A, in 5 of 6 control females the prolactin pulse was present until the 3rd morning after the lighting was changed, and absent thereafter. In both experiments the early progesterone peak and the interval to birth in the treated females was significantly delayed compared to the control females; reactivation of the CL did not occur until treatment ceased. In Exp. 2, for 5 consecutive mornings, before the prolactin pulse was expected to occur, bromocriptine was injected i.m. In 2/4 treated animals the prolactin pulse was abolished and these animals showed an early progesterone peak and gave birth 28 and 29 days after treatment began. The remaining 2 animals and the controls (N = 4) did not show reactivation until after they experienced a stimulatory photoperiod. The results indicate that the prolactin pulse is the critical factor in the maintenance of the inhibition of the CL of the tammar during seasonal quiescence.     

Growth and reproduction of two species of Akodon and their hybrids

Growth and reproduction of Akodon molinae (A.m.), A. dolores (A.d.) (Rodentia: Cricetidae), and their hybrids are described. A.d. showed less fertility under animal room conditions, fewer "successful" matings, similar litter sizes, and an altered sex proportion as compared to A.m.; A.d. ovulatory index is supposed to be higher than mean litter size. X-linked lethal gene(s) and chromosome polymorphisms are possible causes of these variations. Interspecific crosses showed a marked seasonal (summer) tendency in parturitions. Intraspecific matings also produced young during winter. F1 hybrids showed a reversed tendency, while F2 matings returned to the parental pattern. A.d. male X A.m. female and F1 crosses presented larger litter sizes than the other pairings. A.m. exhibited sexual differences in weight after 60 days of age. A.d. and F1 hybrids did not show significant differences in weight of both sexes at any age. A.m. males and females were heavier than A.d. individuals except 12 months after birth. F1 hybrids born to A.m. male were heavier at birth than the A.d. female s interspecific offspring, and may be interpreted as a maternal influence of the A.m. females. Hybrids were heavier than their parents at birth and some of them were also heavier at 21 days, and probably is due to pseudoheterosis. The comparison of body measurements between sexes of both species did not give conclusive results. A.m. females were larger than females of A.d. Males of both species showed similar total lengths; A.m. males had larger body lengths and shorter tails. Body measurements were not related to weight variations. However, differences in weight and measurements between sexes appeared simultaneously at 2-6 months. This probably reflected the attainment of maturity.     

Effect of photoperiod, injection of pentobarbitone sodium or lesion of the suprachiasmatic nucleus on pre-partum decrease of blood progesterone concentrations or time of birth in the rat

In Sprague-Dawley rats kept under 14L:10D (lights on 05:00-19:00 h), parturition occurred during the light phase on Day 23, and the pre-partum decrease in progesterone concentrations was observed between 07:00 and 15:00 h during the light period on Day 22. When the rats were transferred to reversed light-dark regimen (lights on 17:00-07:00 h) on Day 7, the progesterone decrease and parturition still occurred during the light period on Day 21 and 22-23, respectively. However, when rats were kept in constant darkness from Day 7, parturition occurred independently of the time of day between Day 22 and 24. A gradual decline of progesterone concentrations was randomly observed in individual rats. In Wistar rats kept under the usual light-dark regimen, parturitions were biphasic, occurring during the light periods on Day 22 and 23. The progesterone decrease occurred at the usual time even when the lighting regimen was changed only on the day of the expected progesterone decrease. However, treatment with pentobarbitone sodium at 15:00, 19:00 or 21:00 h, but not at 12:00 or 23:00 h, on Day 21 resulted in a delay of progesterone decrease and of parturition. Complete lesion of the suprachiasmatic nucleus on Day 13 or 14 led to advancement and random distribution of the time of birth. These results suggest that the time of parturition and of pre-partum progesterone decrease may be closely associated with an endogenous circadian system, and a luteolytic factor involving the nervous system may be present during a limited period before parturition.     

Reproductive patterns and birth seasonality in a South-American breeding colony of common marmosets,Callithrix jacchus

We analyzed data on captive-born and wild-caught females housed under natural conditions in a colony located in northeastern Brazil. No differences in reproductive performance were found between captive-born and wild-caught females. Twins were the most frequent litter size, followed by triplets and singletons. No parity effect was observed, with similar infant survival for nulliparous and multiparous females. No significant departures in sex ratio were detected for births and mortality of the male and female infants. The age of the females at the time of pairing showed a negative correlation with pairing-parturition length, but did not affect infant survival. The prolongation in pairing-parturition interval (PPI) and interbirth interval (IBI) was related to birth seasonality. The births were clustered in the second half of the dry season and the beginning of the wet season (November–March), and the time of pairing and the time of infant birth influenced the PPI and IBI, respectively. The use of outdoor cages, which allowed the animals to be aware of the seasonal variations in photo-period and rainfall seems to be sufficient to time the reproductive activity, even when the animals are maintained on a constant food supply.     

ON THE RELATION BETWEEN LITTER SIZE,BIRTH WEIGHT,AND RATE OF GROWTH,IN MICE

We have been concerned with the connection between size of litter and weight of litter at birth, especially in mice. The weight at birth represents, it is to be presumed (at least in mice, and for certain other cases), the weight at a particular developmental stage. The connection between number in litter (N) and weight of litter (W) has been interpreted as due to the partition of nourishment between mother and young, and on an equal basis among the several embryos of a litter. The "heterogonic" relationship which the data exhibit between N and W shows that the constant K, defined by log W = K log N + const., is independent of the species, and has an essentially constant value (0.85±) in all multiparous mammals; it is therefore regarded as a partition coefficient. In the case of power function relationships between masses of components of a single individual, the respective "drawing powers" of the several organs are diverse, and diverse magnitudes of K are encountered. With developing embryos, the intrinsic drawing powers of the tissues concerned in embryos and mothers are in each case of the same general character, at least among mammals; the constancy of K reflects this. A parallel for the case as it appears in the consideration of relative growth rates of organs in a single individual, and in which the varying magnitudes of the heterogonic growth constant K are presumed to reflect diverse drawing powers of the respective tissues, would be given by intrauterine growth of a litter containing individuals with diverse capacities for growth, —that is, individuals differing genetically with respect to the factors determining the magnitudes of w ₁. We have been dealing with the growth of litters in inbred strains. It is to be presumed that in the case of the growth of a litter containing two categories of individuals so far as concerns intrinsic drawing powers with respect to the nourishment provided by the mother, it would be possible to investigate the way in which K is open to modification. Although difficult, from the standpoint of classifying the individual young, it would appear to be distinctly worth while to make such an experiment, and we have planned it for the future. It is pointed out that for genetic purposes the ideal weight of a litter of 1 is obtainable from a series of measurements of N and W, free from disturbances affecting the apparent value of this quantity as observed in single births. This weight of an ideal litter of 1 should be employed to disentangle the effects of heterosis and fertility factors from those having to do with individual weight at birth. During the suckling period the relation ΔW/W = K (ΔN/N) is maintained for young mice, but with modifications in the case of small and large suckling litters due to (1) the time course of milk yield, and (2) the effect of litter size upon this. It is shown that a growth curve can be obtained for an ideal litter of I, under the condition of milk supply that on each day the mother is able to provide a constant fractional increase of milk for each additional young mouse in the litter. The rate of growth then adheres to the time curve of capacity for production of milk.     

Controls over reproductive phenology among ungulates: allometry and tropical‐temperate contrasts

Ungulates inhabiting high latitudes schedule the timing of conceptions so that offspring are born during the most favourable nutritional conditions for reproductive success. The optimal period for births is less reliably predictable in tropical and subtropical savanna environments where plant growth is governed by rainfall, suggesting that reproductive phenology could be influenced more proximately by resources affecting the body condition of females around the time of conceptions. To assess how these controls operate, we compared the timing of births and conceptions among tropical and subtropical savanna ungulates with the patterns shown by ungulates in northern temperate or subarctic latitudes. The association between the timing of births and the onset of plant growth early in the growing season is less consistent among tropical savanna ungulates than among ungulates inhabiting northern temperate environments, and apparently subject to other influences affecting vegetation phenology. Nevertheless, birth peaks seem to coincide with the time of the year when forage quality is expected to be best for offspring survival and growth for most tropical or subtropical ungulates with gestation periods shorter than a year. When gestation time exceeds one year, proximal effects of nutritional conditions around the time of conceptions apparently become overriding and birth synchrony with early season plant growth is no longer effective. Proximate nutritional influences on conceptions may also govern the somewhat diffuse spread of births shown by ungulate populations in equatorial latitudes where photoperiod cues controlling oestrus and mating cannot be used to schedule the later timing of births.     

Influence of month and climatic factors on the period of birth,litter size,birth weight and preweaning mortality of sheep in Ghana

Relationships between month and clinatic factors-rainfall, ambient temperature, relative humidity (RH) and temperature-humidity index (THI)-with time of parturition, litter size, birth weight and preweaning mortality were studied in Djallonke and Dja lonke x Sahelian sheep. Births (4142) occurred all year round although there were significant differences in the number of births per month. Maximum litter size did not coincide with the maximum births. Peak conceptions occurred in the early parts of the major rainy season. There were also significant differences in birth weights and mortalities (%) per month. Maximum and minimum values (of birth weighs and mortalities) occurred in the latter part of the rainy seasons and dry periods respectively. There were significant positive correlations between litter size and cumulative rainfall in the 4th–6th months periods prior to lambing. Birth weights of both males and females were positively and significantly associated with minimum temperature and cumulative rain-fall in the 2- and 3-months period prior to lambing respectively. There were significant positive correlations between mortality and minimum and maximum RH and cumulative rainfall in the 3 months after lambing.     

Photomanipulation of sexual maturation and breeding cycle of the steppe polecat (Mustela eversmanni) and other techniques for more rapid propagation of the species

Twenty steppe polecats were divided into 2 groups, each consisting of 4 males and 6 females, and subjected to either a natural photoperiod (controls) or alternating periods of short (8 h light/16 hr dark for 8-9 weeks) and long days (16 h light/8 h dark for 16-20 weeks). The experimental photoperiod significantly accelerated sexual maturation in both sexes, with males developing maximal testis size within 57 days and females breeding after an average of 52 days exposure to 16L/8D. Males in the experimental group completed 2 1/2 testicular cycles and participated in mating during 3 successive breeding seasons during the 18 month period whereas males in the control group completed a single testicular cycle and only had an opportunity to mate during a single breeding season. Females in the experimental group produced 3 litters whereas females in the control group only gave birth to a single litter. Litter size averaged 6.9 +/- 2.0 (n = 23) and did not significantly differ with age, parity, or treatment. Pseudopregnant females returned to estrus within 12 days after the expected date of parturition, were bred, and gave birth to kits. Polecats which were subjected to the experimental photoperiods completed more molting cycles and underwent more photoperiod-induced changes in body weight than those in the control group. Death or removal of kits within 8 days after birth resulted in 12/12 females returning to estrus within 6-26 days. Eleven of these females were remated and gave birth to kits. Eight domestic ferrets readily accepted neonatal polecat kits and 5 successfully reared kits, although kit survival was quite poor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic neuropeptide Y infusion during lactation suppresses pup growth and reduces the length of lactational infertility in rats

In lactating rats, food restriction potentiates the already high levels of hypothalamic neuropeptide Y (NPY). To investigate the role that high levels of NPY might play in the prolongation of lactational infertility that typically accompanies a food restricted lactation we investigated the effects of chronic central infusions of NPY in ad libitum-fed lactating females. First, we compared the effects of intracerebroventricular (icv) infusion of NPY from Days 12-19 postpartum at a dose of 14.4 microg/day with a similar treatment in nonlactating females. In subsequent experiments we examined the effects of NPY infusions into the lateral ventricle at doses of 6 or 20 mug/day or unilaterally into the medial preoptic area at a dose of 1 microg/day from either Days 12-19 or 7-21 postpartum. Effects on food intake; female body weight; and, where appropriate, litter weight and length of lactational diestrus were compared between NPY and vehicle-treated females. As expected NPY infusion produced a robust increase in body weight and food intake in nonlactating females that was accompanied by a suppression of cyclicity. By contrast NPY treatment in lactating rats resulted in a marked decrease in litter growth and an earlier termination of lactational diestrus.     

Maternal age does not affect tetrahydrocannabinol's in utero actions in rats

The potential contribution of maternal age to tetrahydrocannabinol's (THC) in utero effects in rats was studied. Pregnant animals were intubated with 25, 10 or 0 mg/kg of THC from gestation day six to parturition. Animals in the 10 and 0 mg/kg groups were pair fed to those given the 25 mg/kg dose. Each series of doses was administered to females three, four or six months of age. THC lowered maternal weight gain and weights of offspring at birth and at 21 days of age, but did not affect litter size, spontaneous alternation or passive avoidance learning in offspring. Increased maternal age was associated with smaller litter size and lower birth weight and weight at 21 days, but did not interact significantly with THC.     

Mid-arm and chest circumferences at birth as predictors of low birth weight and neonatal mortality in the community.

In developing countries, where about three quarters of births occur at home or in the community, logistic problems prevent the weighing of every newborn child. A study was performed to see whether other simpler measurements could be substituted for weight to identify neonates of low birth weight and those at risk. A study of 520 hospital births showed a strong correlation (p less than 0.001) between other anthropometric variables and birth weight, but the correlation was maximum for chest circumference (r = 0.8696) and mid-arm circumference (r = 0.8110). A mid-arm circumference of less than or equal to 8.7 cm and a chest circumference of less than or equal to 30 cm had the best sensitivity and specificity for identifying neonates with a birth weight of 2500 g or less. Measurements on 501 consecutive live births in the community were recorded and the infants followed up at specified ages. Mid-arm circumference was again significantly correlated to birth weight (r = 0.6918). Neonatal mortality showed an inverse relation but postneonatal mortality an inconsistent relation with mid-arm circumference. A mid-arm circumference of less than or equal to 8.7 cm and a birth weight of less than or equal to 2500 g were equally useful in predicting neonatal outcome. Mid-arm and chest circumferences are simple, practicable, quick, and reliable indicators for predicting low birth weight and neonatal outcome in the community and can be easily measured by paramedical workers in developing nations.     

Age specific fecundity,litter size,and sex ratio in the chacoan peccary (Catagonus wagneri)

Ten years of data on the reproductive biology of the Chacoan peccary (Catagonus wagneri) were analyzed to determine average litter size, sex ratio, timing of births, and individual and age-specific fecundity. Data were obtained from a captive herd of chacoan peccaries located in the western Paraguayan Chaco. Births peak in the austral spring months of September, October, and November, with fewer litters born during the dry season months of June, July, and August. The average litter size was 2.4 with a sex ratio of 56:44 (M:F). There was no significant difference in litter size among individual females or among females of different ages. Finally, there was some individual variation in the age at which sows produced their first litter, but no discernible variation among sows in the average time between litters. Zoo Biol 16:301–307, 1997. © 1997 Wiley-Liss, Inc.     

Reception of photoperiodic information by fetal Siberian hamsters: role of the mother's pineal gland

The rate of reproductive development in juvenile male Siberian hamsters is strongly influenced by daylength (photoperiod). Recent studies indicate that reception of photoperiodic cues begins during fetal life. The present experiments yielded a further demonstration that developing male Siberian hamsters receive information about the photoperiod to which their mother is exposed during pregnancy. The possibility that photoperiodic information is transmitted from mother to young after birth was investigated by cross-fostering young gestated on 12L and 16L to mothers from the other photoperiod. Litters were cross-fostered on the day of birth and then were transferred, along with their foster mothers, to 14L. We found no influence of the mother after birth, indicating that transmission of photoperiodic information from mother to young must occur during gestation. To determine if the pineal gland of the mother is required for this response, adult females were pinealectomized or sham-operated and paired with intact males in 12L, 14L, or 16L. After parturition parents and offspring were exposed to 14L. The influence of prenatal photoperiod on postnatal testicular development in 14L was blocked by pinealectomy of the mother. Postnatal testicular development was retarded in offspring that experienced a photoperiod transfer from either 15L to 14L or 8L to 12L at birth. In contrast, the inhibitory effect of a transfer from 16L to 14L at birth was abolished when juvenile males were exposed to a single long photoperiod (16.3 h light) at age 17-21 days and then were returned to 14L.     

Time of parturition in rats after melatonin administration or change of photoperiod

Two experiments were carried out with rats isolated at mating (Day 1 of gestation) and kept in a standard light regimen of 14 h light (14L:10D). All treatments started on Day 8 of gestation; periodicities of 23:45 h, 24:00 h and 24:15 h were applied to the light phase (14L) in Exp. I and to daily treatment with melatonin (0.3 mg/rat) or its vehicle in Exp. II. In Exp. II, the animals were placed in a continuous dim light regimen and injections were given at a time corresponding to lights off. In all groups, rats delivered on the afternoon of Day 22 and on the morning of Day 23 after a cessation of parturitions. The rates of births during these two times depended on the periodicities of the light phase or those of melatonin administration. With a periodicity of 24:15 h, 85.7% of rats in Exp. I and 85.7% of rats in Exp. II delivered on Day 22. With a periodicity of 23:45 h, 83.0% of births occurred on Day 23 in Exp. I and 57.7% in Exp. II with melatonin instead of 25.9% in the corresponding vehicle controls. These results suggest that melatonin secretion may be a mechanism whereby photoperiod regulates the time of parturition in the rat.     

Relation of parity and estrous cyclicity to the biology of pregnancy in aging female rats

In the female rat, the incidence of regular estrous cyclicity and fertility decreases progressively during aging, and the causes for these are unknown. To reveal the biology of pregnancy in aging rats, we performed a longitudinal study in a colony of multiparous rats bred every 2 mo. Beginning at 4 mo and continuing to 12 mo of age in these same individual females, we determined the chronological changes in estrous cyclicity, examined the relationship between the estrous cycle pattern and fertility, and recorded the numbers of live and dead pups delivered at term. In separate groups of 4- to 12-mo-old multiparous rats, we counted the number of ova present in the oviducts (ovulation rate) one day after mating and the number of grossly normal blastocysts found in the uteri on Day 5 of pregnancy. Similar studies were also performed in primiparous rats of 8, 10, and 12 mo of age. The cessation of regular cyclicity during aging occurred significantly (p less than 0.01) earlier in virgin than multiparous rats. Fertility followed a similar but more dramatic pattern of decline than did the incidence of regular cyclicity in both the multiparous and virgin females. Few irregularly cyclic and persistent-estrous females had fertile gestations after mating, and increasing proportions of regularly cyclic females also failed to reproduce successfully at middle age (8-12 mo). Thus, regular ovulatory cycles were essential but not sufficient for fertile gestations in aging rats. Beginning at 6 mo of age, the litter sizes of multiparous rats decreased progressively, and these decreases were associated with a similar decline in the number of live but not dead pups delivered. Also, the percentage of dead pups/total number of pups delivered increased steadily during aging in multiparous (from 14% to 69%) but not primiparous females. The litter sizes of 8- to 10-mo-old primiparous females were not different from those of multiparous rats. However, the litter sizes of irregularly cyclic rats were consistently smaller than those of regularly cyclic females. Thus, parity had little effect on fecundity in aging females, whereas the cessation of regular ovulatory cycles during aging greatly decreased both the incidence of fertility and the litter size.(ABSTRACT TRUNCATED AT 400 WORDS)