Decline In Litter Size Research Articles

Effect of rearing cross-fostered piglets in litters of differing size relative to sow functional teat number on preweaning growth and mortality.

Litter sizes of commercial sows have increased considerably over recent decades, and often exceed the number of functional teats on the sow. The objective of this study was to evaluate the effect of litter size after cross-fostering relative to sow functional teat number on piglet preweaning growth and mortality. A total of 39 litters (561 piglets) were used in a randomized complete block design; blocking factors were farrowing day and sow parity, body condition score, and functional teat number. Three Litter Size treatments were compared (relative to sow functional teat number): Decreased (two piglets less); Control (same number of piglets); Increased (two piglets more). Piglets were randomly allotted to treatment at 24 h after birth to form litters of the appropriate size, with similar mean and CV of birth weight within block. Weaning weights (WW) were collected at 19.5 ± 0.50 d of age; preweaning mortality (PWM) was recorded. Litter sizes were between 11 and 17 piglets, depending on block and treatment. The Decreased treatment had lower (P ≤ 0.05) PWM than the Increased (7.7% and 17.9%, respectively); the Control was intermediate (11.5%) and not different (P > 0.05) from the other treatments. The rate of decline in litter size from birth to weaning was greater (P ≤ 0.05) for the Increased than the Decreased treatment (−0.16 vs. −0.05 piglets per day), with the Control (−0.09 piglets per day) being intermediate and different (P ≤ 0.05) to the other two treatments. Litter sizes at weaning were greater (P ≤ 0.05) for the Increased than the Decreased treatment (13.3 and 11.3, respectively); the Control treatment was intermediate (12.6) and not different (P > 0.05) to the other treatments. The log odds of PWM increased with the decreasing birth weight, at a similar rate (P > 0.05) for all Litter Size treatments. However, the intercept was greater (P ≤ 0.05) for the Increased compared with the Decreased treatment; the Control was intermediate and different (P > 0.05) to the other two treatments. Mean WW tended (P = 0.07) to be greater for the Decreased (6.17 kg) compared to the Control and Increased treatments (5.86 and 5.84 kg, respectively). In conclusion, increasing litter size after cross-fostering relative to the number of functional teats of the sow increased piglet PWM, and tended to decrease WW.

Reproduction, ageing, and longevity in two species of laboratory rodents with different life histories.

Social subterranean rodents of the Bathyergidae family are known to have extended longevity and some signs of negligible senescence, although the manifestation of these traits depends on the reproductive status of individuals. Such enormous life history peculiarities are usually explained by the specificity of a subterranean way of life. If so, all subterranean rodents, regardless of their taxonomic position, are expected to have higher maximum lifespans and shorter senescence periods than the related above-ground species. In this study we compared the mortality rates and age-related reproductive activity and physical conditions in two sympatric rodents of the Cricetidae family: subterranean mole voles and above-ground dwarf hamsters. Mole voles have a maximum lifespan that is as high as two times the maximum lifespan of the dwarf hamsters; however, only a few mole voles outlasted the maximum lifespan of dwarf hamsters. Dwarf hamsters were generally more fecund than mole voles, which manifests both in a higher number of litters and larger litter sizes. Neither species demonstrated a significant age-related decline in litter size or muscle strength, although there were negative trends for dwarf hamsters. We conclude that some evidence of extended longevity and slow ageing do occur in mole voles, but due to the relatively short "subterranean" phylogenetic history of the species, this evidence is not as pronounced as in the social species of Bathyergidae family.

PohnB6F1: A Cross of Wild and Domestic Mice That Is a New Model of Extended Female Reproductive Life Span

In the search for novel genetic diversity that affects the timing of life history traits, we investigated a wild-derived stock of mice (Pohn). Early generations showed extended reproductive life span; however, this phenotype diminished with propagation of the stock. Out-crossing latter generation Pohn mice to C57BL/6J (B6) mice produced PohnB6F1 hybrids with remarkably extended reproductive life spans-mean age at last litter of 647 +/- 32 days-longer than for the parental strains (70% longer than Pohn, 88% longer than B6) and longer than for highly heterogeneous crosses of laboratory mice. Litter size among young PohnB6F1 adults was similar to parental stocks, but their age-related decline in litter size was delayed by 150-200 days, resembling the earlier Pohn generations. Possibly, out-crossing Pohn mice unmasked cryptic alleles that promote extended female reproduction. This work establishes the PohnB6F1 hybrid as a new model for delayed reproductive aging.

Explaining the seasonal decline in litter size in European ground squirrels

In European ground squirrels Spermophilus citellus as in many ground squirrel species, late born litters are composed of fewer young than early born litters. Two alternative though not mutually exclusive hypotheses may explain this seasonal pattern of change in litter size. On the one hand, the production of few large young late in the season may be an adaptation to time limitations on the offspring, that have to complete growth and fattening prior to hibernation. Then one would expect a trade‐off between offspring number and size as the breeding season progresses. At its extreme, this hypothesis would predict that total maternal effort should be equal independent of litter size. Alternatively, litter size may be determined by physiological limitations on the mother, in that highly constrained mothers breed later and produce smaller litters. Then one would expect reduced overall maternal effort in highly constrained mothers of smaller litters. In this case, a trade‐off between litter size and offspring size would not be expected. We found that total maternal effort in terms of gestation length and the duration of lactation increased with increasing litter size, thus supporting the second hypothesis. Lactation was not terminated at natal emergence. It extended a relatively long period of time beyond the time of first litter emergence depending on litter size. During prolonged lactation, individual young of large litters made up body mass to young of small litters. As a consequence, juvenile weaning body mass was unaffected by litter size although offspring body mass at natal emergence was inversely related to litter size. This additional weight gain in young of large litters compensated for initial survival disadvantages and presumably affected fecundity at yearling age.

Peri-implantation Embryonic Loss and Its Related Uterine Factors in Aged Hamsters.

The embryonic development and changes of the uterine tissue during pregnancy were examined morphologically to reveal causes of the age-related decline in litter size in the golden hamster. Uteri were obtained from young (9-12-week old, n=48) and aged hamsters (43-53-week old, n=47) at estrus and on Days 2 to 16 of pregnancy. Viability of conceptuses were examined in half of implants from Day 6 onwards by a dissecting microscope. The remaining implants were evaluated histologically. The number of resorbing implants was significantly larger in the aged hamsters than in the young females on Day 6 (4.4 vs. 1.2, p<0.05), although no difference was found in the overall mean number of ova ovulated (14.8 vs. 14.2, respectively). Delays in the embryonic development and uterine response were observed in all aged hamsters throughout gestation. Re- sorbing implants noted in the aged hamsters were accompanied with stunted decidual cells having pale cytoplasm (incomplete decidualization) and heavy infiltration of endometrial granulocytes. It is suggested that the embryonic wastage may occur much crucially during the peri-implantaion period rather than late gestational period, due to the retarded development of embryos and less- developed decidualization, and that endometrial granulocytes may function in resorption of greatly retarded embryos in the aged hamsters.

An abnormal pattern of embryonic development during early pregnancy in aging rats.

There is recent evidence that a decline in fertility and litter size precedes the cessation of regular estrous cyclicity in middle-aged female rats. This decline in litter size is related to a decrease in the number of normal blastocysts that are present on Day 5 of gestation, immediately prior to implantation. Thus, the pattern of embryonic development during the first 5 days of pregnancy may be altered in middle-aged rats, resulting in fewer implanting embryos and smaller litter sizes. The present study examined the ovulation rates, fertilization rates, and the patterns of embryonic development in regularly cyclic, young and middle-aged females during the first 5 days of pregnancy. Examination of the numbers of ovulated ova revealed that the ovulation rate was significantly reduced in 12- to 14-mo-old females (13 mo; 9.0 +/- 1.0/rat), but not in 9- to 11-mo-old females (10 mo; 12.2 +/- 0.8/rat), as compared to that in young animals (12.8 +/- 1.0/rat). However, there was no decrease in fertilization rate in either the 10-mo or 13-mo group. While the total numbers of embryos present on Days 2-5 were similar among all 3 groups, embryos from 10-mo females displayed a delayed pattern of development and an increased incidence of morphological abnormalities. These changes in embryo development were even more pronounced in the 13-mo group. By Day 5 of pregnancy there was a significant reduction in normal blastocysts in 10-mo (7.3 +/- 1.2/rat) and 13-mo (6.0 +/- 1.6/rat) rats, as compared to young females (10.6 +/- 0.9/rat).(ABSTRACT TRUNCATED AT 250 WORDS)

The Seasonal Decline in the Litter Size of Meadow Voles

Variation in litter size was examined in meadow voles ( Microtus pennsylvanicus ) during a 14-month period in 1979–1980 near Ann Arbor, Michigan. Mean litter size declined sharply each autumn, at the end of the breeding season. This pattern of autumnal decline in litter size is widespread in the genus Microtus . We examined four hypotheses that might might explain the seasonal decline in litter size: the decline reflects possibly adaptive changes in the birth mass of offspring; the decline reflects differential investment in offspring of one sex; the decline reflects a decrease in the mass of mothers; and the decline reflects a change in the average age or parity of mothers. To evaluate these hypotheses, we examined changes in litter size, masses of young and mothers, the sex ratio of litters, and associations among these reproductive variables by bringing live-caught pregnant females into the laboratory and observing their litters. Mean neonatal mass, total mass of litters, and mother's mass varied significantly during the breeding season, but preliminary estimates of age and parity of mothers did not. The number of young in a litter was correlated strongly and positively with the postpartum mass of the mother and correlated weakly and negatively with the mass of neonates. No single hypothesis adequately accounted for seasonal changes in litter size. At best, the first and third hypotheses together could account for about half of the seasonal variation in litter size.

Chronological changes in fertility, fecundity and steroid hormone secretion during consecutive pregnancies in aging rats.

During aging in female rats the incidence of regular estrous cyclicity decreases progressively. Concomitant with this decrease are declines in both fertility and fecundity in aged females. The causes for these age-related losses in reproductive function are currently unknown. To reveal the biology and endocrinology of pregnancy in aging females, we performed a longitudinal study in a group of multiparous rats bred once every 2 mo. Beginning at 4 mo and continuing through 12 mo of age in the same individual females, we determined the chronological changes in estrous cyclicity and fertility; examined the number of implantation scars; estimated the fetal resorption rate; and assessed the patterns of progestin, androgen, and estrogen secretion during 5 consecutive gestations. During each pregnancy, blood samples were obtained from the mothers on Days 2, 5, 8, 11, 14, 16, and 19 for measurement of steroid hormone concentrations. Laparotomies were performed after parturition to count the number of implantation scars. To estimate the number of fetal resorptions, the difference between the number of implantation scars and the total number of pups delivered was calculated for each animal. Fertility during aging followed a similar but more dramatic pattern of decline than did the incidence of regular cyclicity. While almost all females became infertile when their regular ovulatory cycles ceased, an increasing proportion of middle-aged (10-12 mo of age) regularly cyclic rats also failed to reproduce successfully. These observations indicate that the loss of fertility in aging female rats is due largely, but not exclusively, to the cessation of regular ovulatory cycles. Beginning at 8 mo and continuing to 12 mo of age, a significant (p less than 0.01) decline in litter size was observed in these multiparous rats. This decline was associated with a similar decrease in the number of implantation scars. Neither the estimated number of fetal resorptions nor the number of dead pups delivered per pregnant rat changed significantly during 5 consecutive gestations. There were, however, age-related gradual increases in the percentage of fetal resorptions/implantation scars and of dead pups/total number of pups delivered. These data indicate that the age-related decline in fecundity is due mainly to reproductive failures early in gestation. The patterns of decreases in both fertility and fecundity found in repeatedly sampled and laparotomized females were similar to those seen in untreated multiparous rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Lifetime breeding studies in fully fed and dietary restricted female CFY Sprague-Dawley rats. 1. Effect of age, housing conditions and diet on fecundity

The capacity of female CFY Sprague-Dawley rats to bear multiple litters when breeding was commenced at increasing ages was assessed under the following conditions: continuous housing with male (GC), intermittent breeding of group (GI) and individually housed (I) fully fed animals, restriction of food intake (DR), refeeding of dietary restricted rats (DRA). The percentage of females pregnant, number of litters produced and total litter size were assessed. The number of litters or percent pregnant was unaffected by individual housing but there was a significantly greater decline in total litter size with increasing age of mother when compared to (GI) rats. The slope of the regression line in (I) was identical to that for (GC) females and may reflect the burden of continuous breeding or the stress of individual housing. Both litter size and overall fertility were decreased by dietary restriction but these females did retain their capacity to breed as late as 937 days in contrast to 720 days for any group of fully fed animals. There was no decline in litter size with increasing age in (DR). Returning (DR) rats to full feeding resulted in a rate of decline in litter size identical to that for (GI). All groups of (DRA) produced a maximum of three litters, commencement of breeding at 750 days resulting in production of a litter as late as 952 days.

Reproductive senescence in the Chinese hamster (Cricetulus griseus).

The female Chinese (C.) hamster (Cricetulus griseus) exhibits a progressive decline in litter size after the third litter. The mean number of litters produced was 4.9 +/- .5 for 25 C. hamsters, and the mean age of the C. hamsters at the time of delivering the last litter was 16.4 +/- .5 months. Most aged C. hamsters (17 to 20 months old) retained a relatively normal estrous cycle, although fewer ovulated, either spontaneously or after mating, when compared with younger hamsters (5 to 6 months old). Progesterone injections given to aged (16 to 21 months old), otherwise nonproducing, C. hamsters after mating permitted 61% to become pregnant once again and 34% to deliver viable young. Early infant mortality in these litters was greatest, however, and growth of the young was retarded in comparison with earlier litters. It appears as if age-related changes in the ovary and uterus are primarily responsible for reproductive senescence in the female C. hamster.

Genetic and maternal environmental factors influencing litter size and reproductive efficiency in mice.

Lines of mice which have been genetically differentiated in litter size at birth and 6-week body weight by single-trait or index selection were randomly assigned to be reared in postnatal litter sizes (PLS) of N8, N12 or N16. The lines were selected as follows: large litter size (L+), large 6-week body weight (W+), selection index to decrease litter size and increase 6-week body weight (L-W+), selection index to increase litter size and decrease 6-week weight (L+W-) and random selection (K). Line and PLS differences in female body weight at weaning and 6 weeks were significant (P less than .05), with W+ greater than L+ greater than L-W+ greater than K greater than L+W- and N8 greater than N12 greaater than N16. Although there was a significant interaction between line and PLS, ranking of the lines was the same at each PLS. Females of each line by PLS group were exposed to males of the same group. Line and PLS differences (P less than .05) in female body weight at copulatory plug, adjusted for age, exhibited the same rankings as the earlier weights. Litter size at birth (dead plus alive), number born alive and number born alive per female exposed each revealed similar differences (P less than .05) in the ranking of lines and PLS. No significant interactions were found. Means for litter size were as follows for each line: L+ (17.4) greater than [W+ (14.1), L+W- (14.3)] greater than K (12.0) greater than L-W+ (10.8), and for each PLS: [N8 (14.0), N12 (13.9)] greater than N16 (13.2). The decline in litter size as PLS increased did not differ significantly from linearity. Adjusting litter size for body weight at copulatory plug eliminated the significant effect of PLS [N8 (13.9), N12 (13.7), N16 (13.4)]. The established genetic differences in litter size among lines due to selection were considerably larger than the maternal environmental effect due to postnatal litter size. Because of the negligible interaction between line and PLS, valid inferences concerning genetic differences among lines in litter size may be made at any postnatal litter size between eight and 16.

Maternal age related pre- and post-implantation fetal mortality: A strain survey

Pre and post-implantation mortality were examined as a function of maternal aging in six inbred mouse strains. These strains were chosen to evaluate the effects of altered immunity (A/J and NZB/J) and premature reproductive aging (CBA). The decline in litter size observed with maternal aging does not appear to be the result of a decrease in the number of ova produced. The frequency of both pre- and post-implantation mortality increases with maternal age in all strains examined. Although the highest levels of pre- and post-implantation mortality were found in the NZB/J strain, which features altered immune responses, results from the A/J females, which also display immune dysfunction were at the other end of the spectrum. Thus, no consistant relationship was seen between altered immune function and fetal mortality. The CBA strain which is characterized by premature reproductive decline and marked maternal age-related fetal aneuploidy also demonstrated high levels of pre- and post-implantation mortality, even in the young maternal age range.

Zur frage einer kombinierten wirkung eines chemischen mutagens und strahlensterilisierter Nahrung im mutagenitäts- und reproduktionstest bei der Maus

The possible intensification of the mutagenic effect of cyclophosphamide (Endoxan) by the feeding of a radiation-sterilized diet (dose, 4·5 Mrad) was studied in 2000 NMRI/Han mice. In a dominant lethal test, males were pretreated with 100 mg Endoxan/kg body weight. The greatest sensitivity towards Endoxan was observed during the late-spermatid stage. No significant differences were detected between the control group (Endoxan plus non-irradiated diet) and the experimental group (Endoxan plus radiation-sterilized diet). In this test, radiation-sterilized feed showed no co-mutagenic effect when combined with Endoxan treatment. In a reproduction study of 7 months duration (continuous mating without lactation periods), the females were treated every 2 wk with 20 mg Endoxan/kg body weight. The decline in litter size with increasing number of litters (i.e. with advancing age of the females) was more pronounced after treatment with the chemical mutagen than in the untreated group. Increases in the frequency of abortions and in premature sterility resulted from Endoxan treatment. During the entire observation period, no effects from the intake of radiation-sterilized food were detected.

Microtus Population Biology; III. Reproductive Changes in Fluctuating Populations of M. ochrogaster and M. pennsylvanicus in Southern Indiana, 1965‐67

About 1,200 Microtus pennsylvanicus and 1,000 M. ochrogaster were collected over a 3—year period by sequential trapping of snap—trap lines in 15 grassland areas in southern Indiana. Peak densities were reached in 1966 for all but two of the populations studied. Reproductive data were obtained to determine whether weight at sexual maturity, pregnancy rate, living embryo production, prenatal mortality, and body weight varied in relation to fluctuations in numbers. The rate of winter breeding was high in both species of vole between the cyclic increase and cyclic peak years. Breeding was continued at a reduced level during the fall of the cyclic peak and was greatly reduced during the winter between the cyclic peak and decline. In both species sexual maturity occurred at higher weights in periods of peak population density and lower weights in periods of low density. Only heavy individuals remained fecund during the winter months of November—February. Sexual maturity was reached at the same weight in males and in females. Litter size was correlated with body size. In M. pennsylvanicus litter size was not significantly different in multiparous and primiparous females, and litter size remained constant in the summer breeding seasons of the increase, peak, and decline years. Difference between seasons was significant, the fall—winter—spring periods having a 14% lower litter size than the summer period. In M. ochrogaster living embryo counts were 25% lower during the period of cyclic peak in multiparous females than they were during the years of cyclic increase or decline. This observation was confirmed independently in a population that was out of synchrony. A decline in litter size was not associated with the year of cyclic decline in either species. Corpora lutea counts were correlated with body size. The pattern established for living embryo counts was nearly identical to the ovulation pattern. Prenatal mortality did not change significantly, either seasonally or yearly, and thus was not higher in the decline phase for either species. Testes weight and body weight were correlated. The testes weight of non—fertile Microtus pennsylvanicus differed little among the summers of population increase, peak, or decline. Testes development was inhibited during fall and winter. Fertile males taken during the summer of population peak and the summer of decline had heavier testes than males taken during the summer of increase. A midsummer drop in testes weight was observed during three breeding seasons. Non—fertile M. ochrogaster had only slightly reduced testes weight during the winter between the population increase and peak, but testes weight declined significantly during the winter following the population peak. Testes weights of fertile males changed little during the summer of population increase, but they increased steadily during the summer of population peak. A significant decline in testicular weight occurred during the winter following the population peak. High body weights were associated with peak populations in both species of Microtus. The pattern of change in weight distributions was similar to that described for other cyclic voles and lemmings.

Effect of maternal age on reproductive capacity

THE MENOPAUSE is such a striking event in thle life of women that it tends to overshadow all other aspects of the period of life associated with reproductive decline. In reality this event is only a step in a process which typically begins several years before and continues for a variable period of time after the last menses. Furthermore, this term has only limited application in a comparative study of reproductive decline, since it should only be applied to the human and those subhuman primates in which the superficial portion of the endometrium periodically undergoes destruction as a result of progesterone withdrawal or, at times, estrogen withdrawal throughout reproductive life. Although bleeding also occurs in dogs, cows, and a few other species, this event takes place at a different time in the cycle and is more in the nature of seepage unaccompanied by tissue destruction. Consequently other criteria for comparing ,reproductive decline in different species have had to be utilized. One criterion which has been widely used in the study of laboratory rodents is the decline in litter size which is associated with increasing maternal age. Unfortunately this guideline also has limited application, since other polytolcous species have not been studied sufficiently to provide us with comparable information.

The effects of inbreeding on the components of litter size in mice.

Twenty-four lines were bred from a base population of outbred Q mice by continued full-sib mating. Inbreeding depression in litter size at birth was observed. This decline in litter size was analysed in terms of ovulation rate, the incidence of preimplantation mortality and the incidence of postimplantation mortality. Pregnant females were dissected at 17½ days' gestation and the numbers of corpora lutea, or eggs, and of live and dead embryos were counted. Matings were arranged so that separate estimates of the effects of inbreeding in the mother and in the litter on the components of litter size could be obtained.In the first generation of inbreeding when the inbreeding coefficient of the litter was raised from 0 to 25% decline in litter size was attributable to an increased incidence of preimplantation mortality.In the second and fourth generations decline in litter size was attributable to (1) a reduction in the number of eggs ovulated by the inbred mothers, (2) an increased incidence of preimplantation mortality which resulted from inbreeding in the mother. No evidence of significant effects on mortality of inbreeding in the litter was obtained in the later generation of inbreeding.These findings are discussed in the context of previous work on the effects of inbreeding and crossing on litter size and its components in mice and pigs.

Effect of maternal age on viability of ova and uterine support of pregnancy in mice.

The technique of ova transplantation was used to study the causes of the decline in litter size in ageing mice. Only 14% of morulae and blastocysts obtained from the uteri ofyoung donors survived to term when transplanted into the uteri of old hosts (13 to 24 months of age), whereas 48% survived in young hosts (2 to 7 months of age). A slightly higher percentage of grossly abnormal ova was recovered on the 4th day of pregnancy from the uteri of old mice than from young mice (13% compared with 5%) However, normal-appearing morulae and blastulae from old mice survived as well as those from young mice when transferred into uteri of young hosts. It was concluded that the initial decline in litter size in aged mice is probably due to an unfavourable uterine environment.

LONG-TERM REPRODUCTIVE PERFORMANCE OF FEMALE MICE

Summary. The relation of litter size to litter order was studied in eighteen normal mice (two-ovary females) and twenty-two mice in which one ovary had been removed before sexual maturity (one-ovary females). Both groups showed an initial increase in litter size, followed by a plateau period, and finally a period of linear decline in which the mean number of young born decreased by one in each successive litter. A survey of other published data on long-term reproductive performance in polytocous mammals shows that this pattern is the usual one among mice and rats, but that in pigs the decline in litter size in later life is much less marked. The mean number of young in the first litter was not significantly different in the two groups. After the first litter, number of young was consistently lower in the one-ovary than in the two-ovary group, and the linear decline of litter size started sooner. The number of corpora lutea in the ovaries at autopsy, 16 weeks after the last litter had been born, was high, and the number in the remaining ovary of the one-ovary animals did not differ significantly from the number in both ovaries together in the two-ovary group. We conclude that the lower mean litter size in the one-ovary group during the plateau period is an effect of prenatal mortality due to the presence of double the normal number of embryos in a single uterine horn. In both groups, the decline in litter size in the second half of reproductive life, which limits the total reproductive output of the female, must be due to a factor causing increased embryonic loss rather than a diminished supply of eggs. Ageing of the uterus itself is probably involved, leading to increased embryonic mortality both at and after implantation. The earlier onset of the decline in one-ovary females would then be due to overloading and premature ageing of the single functional uterine horn. In the pig, the regulation of total reproductive output is probably not primarily uterine.