Variation In Litter Size Research Articles

MATERNAL AGE DRIVES SEASONAL VARIATION IN LITTER SIZE OF PEROMYSCUS LEUCOPUS

We examined the effects of maternal age and parity on litter size in 2 populations of white-footed mice (Peromyscus leucopus) in southwestern Ontario, Canada, to determine whether these factors cause seasonal variation in litter size. Litter size increased from 1st to subsequent litters among females that bred for the 1st time in their natal year, but not among females that 1st bred as overwintered adults. Thus, prior reproductive experience was not an important determinant of litter size. Maternal age accounted for approximately 70% of the variance in litter size; mean litter size was greatest among females between 150 and 250 days of age at parturition. Date of birth explained 79% of the variation in mean litter size, with litters born in summer significantly smaller than those born in spring or autumn. This effect was attributed to the fact that most summer litters were produced by primiparous young-of-the-year females. Although litter size declined among old, multiparous females, few individuals survived to the age at which reproductive senescence was apparent. Thus, reproductive senescence plays a minor role in population-wide variation in litter size.

Reproductive Traits of a High Elevation Viviparous Lizard Sceloporus bicanthalis (Lacertilia: Phrynosomatidae) from Mexico

Several species of lizards exhibit significant annual variation in reproductive traits; however, most work in this area focused on populations from temperate latitudes or low to medium elevations. We examined annual variation in litter size, neonate size, and relative litter mass in a high elevation (4200 m) population of the viviparous lizard, Sceloporus bicanthalis from the Volcano Nevado de Toluca, México. We found little evidence for annual variation in reproduction in this population. Female body size influenced litter size and litter mass. Relative litter mass in this population (0.47) was among the highest reported for any Sceloporus and may be a consequence of a nearly “annual” life cycle. Mean neonate size was not affected by female SVL or litter size, suggesting it may be optimized in this population.

Epistasis affecting litter size in mice

Litter size is an important reproductive trait as it makes a major contribution to fitness. Generally, traits closely related to fitness show low heritability perhaps because of the corrosive effects of directional natural selection on the additive genetic variance. Nonetheless, low heritability does not imply, necessarily, a complete absence of genetic variation because genetic interactions (epistasis and dominance) contribute to variation in traits displaying strong heterosis in crosses, such as litter size. In our study, we investigated the genetic architecture of litter size in 166 females from an F2 intercross of the SM/J and LG/J inbred mouse strains. Litter size had a low heritability (h2 = 12%) and a low repeatability (r = 33%). Using interval-mapping methods, we located two quantitative trait loci (QTL) affecting litter size at locations D7Mit21 + 0 cM and D12Mit6 + 8 cM, on chromosomes 7 and 12 respectively. These QTL accounted for 12.6% of the variance in litter size. In a two-way genome-wide epistasis scan we found eight QTL interacting epistatically involving chromosomes 2, 4, 5, 11, 14, 15 and 18. Taken together, the QTL and their interactions explain nearly 49% (39.5% adjusted multiple r2) of the phenotypic variation for litter size in this cross, an increase of 36% over the direct effects of the QTL. This indicates the importance of epistasis as a component of the genetic architecture of litter size and fitness in our intercross population.

Genetic Status of ESR Locus and Other Unidentified Genes Associated with Litter Size in Chinese Indigenous Tongcheng Pig Breed after a Long Time Selection

The Tongcheng pig breed is a famous Chinese indigenous breed. The Ministry of Agriculture of China has filed it as 1 of 19 national key conservation breeds selected from more than 100 Chinese indigenous pig breeds in 2000. In order to improve the reproductive performance, it has been intensively selected to increase the litter size for about 10 years. The population randomly sampled from conservation nucleus of eight families in the Tongcheng pigs was genotyped for identification of their estrogen receptor locus polymorphisms with the PCR-RFLPs method. Only AB heterozygotes and BB homozygotes were detected, and x 2 test demonstrated that the locus was in disequilibrium at a significant level (p<0.05). In the present paper, the litter sizes in different parities were regarded as different traits. Holistic status of other unspecific and unidentified genes was estimated by using the statistical methods. Coefficients of kurtosis and skewness showed that the litter size still presented segregating characteristic in the 2nd, 5th, 7th, 8th and 9th parities. Analysis of homogeneity of variance between families confirmed the results for the 5th, 7th and 8th parities. The heritability of litter size for the 1st to 10th parities was estimated with paternal half-sib model and individual estimated breeding values (EBVs) were evaluated by a single trait animal model as well. We found that the averages of EBVs for litter size in each parity did not differ significantly between genotypes, despite the significant difference for original phenotype records in the 3rd, 4th and 5th parities (p<0.05 or p<0.01). The results may be explained by the deduction that the polymorphisms of ESR locus are no longer the important genetic base of litter size variation when the frequency of allele B accumulated in the experience of selection procedure, and further conferring that there exist special genes associated with litter size in the recent Tongcheng pigs population can be made. (Asian-Aust. J. Anim. Sci. 2004. Vol 17, No. 5 : 598-602)

Observations of Mating, Birthing and Parental Behaviour in Three Subspecies of Slender Loris (Loris tardigradus and Loris lydekkerianus ) in India and Sri Lanka

Studies of the life history parameters of slender lorises in captivity have led to conflicting results regarding gestation length, birth seasonality, interspecies variation in litter size and the degree of parental care given to offspring. During the course of field studies of Loris lydekkerianus lydekkerianus, L. l. nordicus and L. tardigradus tardigradus, data were collected on these life history variables, as well as on behaviours relating to mating. All 3 taxa displayed courtship behaviour involving the pursuit of a female by multiple males. Mating corresponded closely with captive observations, with a period of prolonged single intromissions lasting 3–11 min. One gestation period of 163 days was calculated for L. l. lydekkerianus. Births for all 3 taxa were distributed throughout the year, and males were seenmating throughout the year. All 3 taxa gave birth to singletons and twins; no subspecific pattern in litter size was evident. Females carried infants for the first 4 weeks of life and were regularly attended by males, which groomed both the mother and her offspring. After infants had been parked, female L. l. lydekkerianus and L. l. nordicus rarely returned before dawn, though males visited and played with infants. Female L. t. tardigradus maintained proximity with their infants, whilst males were not observed in proximity to infants during the night. All 3 taxa slept in social groups. High-energy milk, in combination with male care, may aid in the potentially high reproductive output of 4 infants per year.

Inheritance of litter size at birth in the Brazilian grass mouse (Akodon cursor, Sigmodontinae, Rodentia).

By means of complex segregation analysis we studied the inheritance of litter size in two large pedigrees of captive-bred colonies of the Brazilian grass mouse Akodon cursor. Genetic analysis has revealed a highly significant influence of genetic factors on the variation of litter size (heritability, h2, was estimated as 0.44). The inheritance followed the classical polygene model: neither the major-gene model nor the polygene with unequal contribution model described the data significantly better.

Interpopulational Variation in Litter Size of the Skink Mabuya agilis in Southeastern Brazil

We compared litter size (and its relationship with female SVL), minimum size of females at sexual maturity, and minimum adult size of females, among different populations [from eight coastal sea level and two midelevation (600–800 m) sites] of the live-bearing skink Mabuya agilis in the southeastern Brazilian states of Espírito Santo and Rio de Janeiro. There were no apparent differences in litter size among populations within either lowland or highland groups, but each group differed significantly from the other: highland females had larger litters (mean = 5.6, range 2–9) than those from lowland sites (mean = 3.3, range 1–6). There was a significant positive relationship between litter size and female SVL for each group, but the slope of the regression differed between lowland and highland females, with the latter tending to have larger litters for a given SVL. Highland females also reach larger body sizes, although their minimum size at sexual maturity and minimum adult size are just slightly greater than those from lowland populations. We interpret such intraspecific variation relative to possible effects of the physical environment and of genetic factors on reproduction.

Genetic and Non-genetic Causes of Variation in Gestation Length, Litter Size and Litter Weight in Goats

This study was made with 631 does of 3 genetic groups and 1,112 of their kids allocated into 3 different locations to examine gestation length, litter size and litter weight in goats. Attributes studied were genetic group, parity, age of dam at kidding, weight of dam at breeding, season and location. Genetic group of dam affected significantly (p<0.01) size of litter but not gestation length and weight of litter. Selected Black Bengal (SBB) genetic group performed better in litter size and litter weight than random bred Black Bengal (RBB) and its crossbreds with Jamunapari ♂ (JBB). Litter size and weight were significantly affected by age of dam at kidding (p<0.01), weight of dam at service (p<0.001) and parity (p<0001). Size and weight of litter were found highest in SBB does, or does having 35-40 months of age at kidding, or does with 19-20 kg live weight at service, or at 4th parity. Season, location and parity also affected significantly (p<0.05) gestation length. Shortest gestation length was found in 5th parity or in summer season, whereas the longest was in 2nd parity or in winter season. (Asian-Aust. J. Anim. Sci. 2002. Vol 15, No. 6 : 772-776)

Energetics and litter size variation in domestic dog Canis familiaris breeds of two sizes

We measured resting metabolic rate (RMR), daily energy expenditure (DEE) and metabolisable energy intake (MEI) in two breeds of dog during peak lactation to test whether litter size differences were a likely consequence of allometric variation in energetics. RMR of Labrador retrievers (30 kg, n=12) and miniature Schnauzers (6 kg, n=4) averaged 3437 and 1062 kJ/day, respectively. DEE of Labradors ( n=6) and Schnauzers ( n=4) averaged 9808 and 2619 kJ/day, respectively. MEI of Labradors ( n=12) was 22448 kJ/day and of Schnauzers ( n=7) was 5382 kJ/day. DEE of Labrador pups (2.13 kg, n=19) was 974 kJ/day and Schnauzers (0.89 kg, n=7) were 490 kJ/day. Although Labradors had higher MEIs than Schnauzers during peak lactation, there was no difference in mass-specific energy expenditure between the two breeds. Hence, it is unlikely that litter size variation is a likely consequence of differences in maternal energy expenditure. Individual offspring were relatively more costly for mothers of the smaller breed to produce. Therefore, litter size variations were consistent with the expectation that smaller offspring should be more costly for mothers, but not that smaller mothers should per se invest more resources in reproduction.

Genetic components of litter size variability in sheep

Genetic components of litter size variability in sheep

Selection on purebred and crossbred Performance for litter size in pigs*

Abstract. The present study deals with estimation of genetic parameter for purebred and crossbred Performance of live born piglets, in order to choose the optimal selection method. Data sets of two pure breeds, line L03 and L04, with 5,422 sows, a two line crossbred, L303, with 3,553 sows and a three line crossbred, L350, with 3,609 sows of a North-German breeding Company were recorded. Estimated genetic correlation between purebred and crossbred Performance were rg = 0.59 and 0.40 for reciprocal crosses L03xL04 and L04xL03, respectively. Further investigations showed that the genetic correlation is influenced by genotype-environment interactions between a nucleus farm and a farm on production level. Full-sib effects showed a proportion of FS = 0.06 on the phenotypic variance of litter size. They were confounded with additive genetic variance and permanent environment variance, when full-sib effects were neglected. The percentage of equal selected purebred sires of line L03 were 80% when 30% of the sires selected on purebred or crossbred breeding values. Accuracy of estimated breeding values of purebred sires increased when crossbred Information were considered additionally from 0.32 to 0.38 for line L03 and 0.46 to 0.47 for line L04. Genetic correlation between purebred and crossbred Performance, the genetic connectedness between nucleus and production and the presence of genotype-environment interactions were analysed to have high influence on the value of additionally considered crossbred Performance.

EFFECTS OF LITTER SIZE ON THE ENERGETICS OF REPRODUCTION IN A HIGHLY PRECOCIAL RODENT, THE GUINEA PIG

Litter size strongly affects reproductive output and is therefore of central interest for the understanding of life-history evolution. I investigated the effect of variation in litter size (natural and manipulated) on the energetics of reproduction in the highly precocial guinea pig (Cavia porcellus). Unlike the situation in altricial species, increasing litter size in this precocial rodent influenced maternal performance (increase in litter mass and decrease in mass of individual offspring) as strongly during gestation (68 days) as during lactation (20 days). To cover the energy cost of offspring production, mothers increased their daily food intake. Their daily increase in metabolizable energy intake varied significantly with litter size during gestation but not during lactation. Instead, offspring in large litters consumed more solid food during lactation than did offspring in small litters. The efficiency of energy conversion into offspring production was not affected by litter size, neither during gestation nor during lactation. Furthermore, manipulation of litter size did not influence a mother's total energy cost of lactation. As a consequence, offspring in enlarged litters grew significantly slower than offspring in reduced litters despite similar body masses at birth and significantly greater consumption of solid food by nursing offspring in large litters. Results suggest that the effect of litter size on the energetics of reproduction in the precocial guinea pig differs from that of altricial rodents. Consequently, ecological implications of variation in litter size in the precocial guinea pig might also be very different to those in altricial species.

Factors affecting maternal care in an income breeder, the European roe deer

1. Maternal care is a major component of demographic tactics in mammals. In ungulates most work has been done on capital breeders (e.g. bighorn sheep), which rely heavily on body reserves to raise their young. Roe deer, in contrast, are close to the income breeder end of the capital–income breeder continuum, and show high levels of maternal care.2. The aim of this study was to explore the factors determining the level of maternal care in roe deer, in particular the effects of maternal body weight, mother's parity, litter size and year of birth on the amount of prenatal care (i.e. the average mass of an offspring multiplied by the number of offspring) and postnatal care (i.e. the average growth rate of the offspring multiplied by their number). The study was carried out on a captive population of roe deer fedad libitum, and in a wild population.3. In both populations prenatal care increased with increasing maternal body weight. In the population fedad libitumthis effect was found in light females only (&lt; 22 kg); in the wild population the positive relationship between maternal body weight and prenatal care was entirely accounted for by variation in litter size (i.e. heaviest females produced larger litters) and density (i.e. females were lightest in years with high population density). Parity did not affect prenatal care.4. In 14 females fedad libitumthere was no relationship between postnatal care and maternal body weight. Multiparous females had higher levels of postnatal care. In contrast, 20 wild females showed a positive relationship between postnatal care and maternal body weight, and only litter size affected the level of postnatal care.5. Even after accounting for the confounding effects of maternal body weight, parity, litter size and population density, we found no trade‐off between pre‐ and postnatal care in any of the two populations. The conditions under which the pattern of maternal care could impose trade‐offs that affect the individual offspring are discussed.

Uterine blood flow in sows: effects of pregnancy stage and litter size.

Female pigs were assigned to three groups at 94 days of age: a control group (CTR), a group undergoing the ligation and severing of the left oviduct (LIG), and a group undergoing right hysteroovariectomy (HHO). They were inseminated at 307 days of age. At 35 days of pregnancy, an ultrasonic transit time flow probe was implanted around the middle artery of one uterine horn in 33 sows and uterine blood flow was measured during thirteen 24-h periods between 44 and 111 days. Despite large differences in ovulation rate per uterine horn (4.8, 8.3 and 16.9 in the LIG, CTR and HHO groups, respectively), variation of litter size was considerably reduced with advancement of pregnancy (3.0, 6.6 and 10.8 foetuses per uterine horn at 35 days, and 3.0, 5.8 and 4.9 at 112 days (slaughter), respectively). Uterine blood flow increased linearly during pregnancy. It was lower in the LIG sows (0.82 to 1.74 L x min(-1) x horn(-1) from 44 to 111 days) than in the CTR and HHO sows (1.22 to 2.84 and 1.09 to 2.63 L x min(-1) x horn(1), respectively). It was more closely related to litter weight than to litter size and amounted to 0.42 L x min(-1) x kg foetus(-1) at 111 days. Uterine blood flow per foetus decreased when litter size increased. It increased from 0.31 to 0.72, 0.26 to 0.60 and 0.20 to 0.43 L x min(-1) x foetus(-1) from 44 to 111 days when there were 2 to 3, 4 to 5, and 6 to 8 foetuses in the uterine horn, respectively. This explains why piglets from large litters are lighter at birth.

Latitudinal variation in litter size of polar bears: ecology or methodology?

I used data reported in the scientific literature to examine latitudinal variation in litter size of polar bears in 18 different populations. No relationship was found between litter size and latitude using non-weighted regression. Regression weighted by sample size indicated a negative relationship with latitude. However, sampling biases caused by latitudinal differences in the timing of sampling and cub mortality after den emergence could produce a latitudinal cline. Stratifying analyses by sampling area, at dens or away from dens, revealed no latitudinal trend in litter size. Observed differences in litter size among populations may result from variation in demography and ecosystem productivity that is not simply related to latitude. I conclude that available data do not provide support for a biologically significant latitudinal cline in polar bear litter size.

Ontogeny of intestinal safety factors: lactase capacities and lactose loads.

We measured intestinal safety factors (ratio of a physiological capacity to the load on it) for lactose digestion in developing rat pups. Specifically, we assessed the quantitative relationships between lactose load and the series capacities of lactase and the Na+-glucose cotransporter (SGLT-1). Both capacities increased significantly with age in suckling pups as a result of increasing intestinal mass and maintenance of mass-specific activities. The youngest pups examined (5 days) had surprisingly high safety factors of 8-13 for both lactase and SGLT-1, possibly because milk contains lactase substrates other than lactose; it also, however, suggests that their intestinal capacities were being prepared to meet future demands rather than just current ones. By day 10 (and also at day 15), increased lactose loads resulted in lower safety factors of 4-6, values more typical of adult intestines. The safety factor of SGLT-1 in day 30 (weanling) and day 100 (adult) rats was only approximately 1.0. This was initially unexpected, because most adult intestines maintain a modest reserve capacity beyond nutrient load values, but postweaning rats appear to use hindgut fermentation, assessed by gut morphology and hydrogen production assays, as a built-in reserve capacity. The series capacities of lactase and SGLT-1 varied in concert with each other over ontogeny and as lactose load was manipulated by experimental variation in litter size.

Genetic determination of individual birth weight and its association with sow productivity traits using Bayesian analyses.

Genetic association between individual birth weight (IBW) and litter birth weight (LBW) was analyzed on records of 14,950 individual pigs born alive between 1988 and 1994 at the pig breeding farm of the University of Kiel. Dams were from three purebred lines (German Landrace, German Edelschwein, and Large White) and their crosses. Phenotypically, preweaning mortality of pigs decreased substantially from 40% for pigs with < or = 1 kg weight to less than 7% for pigs with > 1.6 kg. For these low to high birth weight categories, preweaning growth (d 21 of age) and early postweaning growth (weaning to 25 kg) increased by more than 28 and 8% per day, respectively. Bayesian analysis was performed based on direct-maternal effects models for IBW and multiple-trait direct effects models for number of pigs born in total (NOBT) and alive (NOBA) and LBW. Bayesian posterior means for direct and maternal heritability and litter proportion of variance in IBW were .09, .26, and .18, respectively. After adjustment for NOBT, these changed to .08, .22, and .09, respectively. Adjustment for NOBT reduced the direct and maternal genetic correlation from -.41 to -.22. For these direct-maternal correlations, the 95% highest posterior density intervals were -.75 to -.07, and -.58 to .17 before and after adjustment for NOBT. Adjustment for NOBT was found to be necessary to obtain unbiased estimates of genetic effects for IBW. The relationship between IBW and NOBT, and thus the adjustment, was linear with a decrease in IBW of 44 g per additionally born pig. For litter traits, direct heritabilities were .10, .08, and .08 for NOBT, NOBA, and LBW, respectively. After adjustment of LBW for NOBA the heritability changed to .43. Expected variance components for LBW derived from estimates of IBW revealed that genetic and environmental covariances between full-sibs and variation in litter size resulted in the large deviation of maternal heritability for IBW and its equivalent estimate for LBW. These covariances among full-sibs could not be estimated if only LBW were recorded. Therefore, selection for increased IBW is recommended, with the opportunity to improve both direct and maternal genetic effects of birth weight of pigs and, thus, their vitality and pre- and postnatal growth.

Fluctuating Resources and the Evolution of Litter Size in the Arctic Fox

Fluctuations in essential resources cause a strong selection pressure on the ability to adjust parental investment accordingly. In the dog family, Canidae, variance in female prebirth investment is adjusted by litter size. The arctic fox, Alopex lagopus, is a small canid living on the northern tundras of the world. It has the largest known litter size in the order Carnivora, up to 18 young, and litter size is highly variable. We have analysed data from arctic fox populations throughout the species circumpolar range. In some areas, arctic foxes feed on strongly fluctuating populations of small rodents. In contrast, they have more stable food resources at bird cliffs and along coast lines. Food availability determines arctic fox litter and population sizes. A comparison between fluctuating and stable arctic fox populations showed that fluctuations are associated with large litter sizes. There were significant differences in litter size means, maxima and variances, as well as in placental scar count means. We have discussed five hypotheses on the determination of variation in litter size: one energetic, one genetic (based on density variation), one diet-determined, one based on reproductive allocation and one based on differences in reaction norms. Our findings suggest that litter size in the arctic fox is determined by the combined effect of immediate resource levels and the degree of resource predictability. We describe reaction norms that suggest how litter sizes result from adaptive plasticity within each of two genetic strategies where, according to the jackpot hypothesis, populations with unpredictable food resources generally have larger litter sizes. Within each genetic strategy, or reaction norm, litter sizes are adjusted through a number of plastic traits. These traits are influenced by nutritional limitations and include reduced ovulation rates, prenatal losses, and litter size reduction during the lactation period.

State-Dependent Optimization of Litter Size

Four inter-related theories demonstrate that the quality or state of the parent influences its optimum litter size. A review of each model highlights their similar predictions, sets the stage for an expanded theory and demonstrates that three are really special cases of state-dependent optimization. The state-dependent theory helps to explain three paradoxes in life history. l. Why the most commonly observed litter size is often smaller than the one that yields the maximum number of recruits. 2. Why litter size may increase or decrease in high-quality habitat. 3. Why there may be no reproductive cost associated with increased litter size. Tests of state-dependent optimization are likely to be complicated because the essential state variables are often left undefined, and because many state variables may interact with one another to determine a parent's quality. Composite state-variables, such as female body size, may overcome some of the limitations that otherwise arise from interacting variables. In white-footed mice, significant body-size effects interact with the environment and with a variety of life-history characters. The resulting networks of interactions, complicated as they are, may be caused by a relatively small number of underlying processes. Density-dependent habitat selection appears, at least for white-footed mice, to represent a fundamental process accounting for much of the environmental state-dependent variation in litter size.

Numbers of Teats and Pre- and Post-Natal Litter Sizes in Small Diprotodont Marsupials

Because many species of diprotodont marsupials (including all large ones) have litters of a single young, it seems to be assumed tacitly that there is little variation in litter size within and between species in this group. In contrast, polyprotodont marsupials have litters that are typically larger; variation in litter size between species is related to body size, diet, and ecology; and intraspecific variation in litter size also occurs. The largest litters in diprotodont marsupials are found in the small possums (eight species from the families Burramyidae, Acrobatidae, and Tarsipedidae), but there also is variation in litter size beyond that which can be explained by body size, reflecting a fast-slow continuum among small possums. There also are geographical differences in litter size within some species. Number of teats varies among species and also varies within three of the species. Production of supernumerary young occurs in some species such that number of teats effectively limits litter size. In other species, number of young at birth is smaller than number of teats, so litter size is limited by number of eggs shed at ovulation and embryos carried in utero. The largest mean litter sizes are found in species that produce supernumerary young, and these are the fast species among small possums.