BackgroundIn recent years, there has been an increasing interest in the genetic determination of environmental variance. In the case of litter size, environmental variance can be related to the capacity of animals to adapt to new environmental conditions, which can improve animal welfare.ResultsWe developed a ten-generation divergent selection experiment on environmental variance. We selected one line of rabbits for litter size homogeneity and one line for litter size heterogeneity by measuring intra-doe phenotypic variance. We proved that environmental variance of litter size is genetically determined and can be modified by selection. Response to selection was 4.5% of the original environmental variance per generation. Litter size was consistently higher in the Low line than in the High line during the entire experiment.ConclusionsWe conclude that environmental variance of litter size is genetically determined based on the results of our divergent selection experiment. This has implications for animal welfare, since animals that cope better with their environment have better welfare than more sensitive animals. We also conclude that selection for reduced environmental variance of litter size does not depress litter size.

Resilience is the ability of an animal to return soon to its initial productivity after facing diverse environmental challenges. This trait is directly related to animal welfare and it plays a key role in fluctuations of livestock productivity. A divergent selection experiment for environmental variance of litter size has been performed successfully in rabbits over ten generations. The objective of this study was to analyse resilience indicators of stress and disease in the divergent lines of this experiment. The high line showed a lower survival rate at birth than the low line (−4.1%). After correcting by litter size, the difference was −3.2%. Involuntary culling rate was higher in the high than in the low line (+12.4%). Before vaccination against viral haemorrhagic disease or myxomatosis, concentration of lymphocytes, C-reactive protein (CRP), complement C3, serum bilirubin, triglycerides and cholesterol were higher in the high line than in the low line (difference between lines +4.5%, +5.6 µg/ml, +4.6 mg/ml, +7.9 mmol/l, +0.3 mmol/l and +0.4 mmol/l). Immunological and biochemical responses to the two vaccines were similar. After vaccination, the percentage of lymphocytes and CRP concentration were higher in the low line than in the high one (difference between lines +4.0% and +13.1 µg/ml). The low line also showed a higher increment in bilirubin and triglycerides than the high line (+14.2 v. +8.7 mmol/l for bilirubin and +0.11 v. +0.01 mmol/l for triglycerides); these results would agree with the protective role of bilirubin and triglycerides against the larger inflammatory response found in this line. In relation to stress, the high line had higher basal concentration of cortisol than the low line (+0.2ng/ml); the difference between lines increased more than threefold after the injection of ACTH 1 to 24, the increase being greater in the high line (+0.9 ng/ml) than in the low line (+0.4 ng/ml). Selection for divergent environmental variability of litter size leads to dams with different culling rate for reproductive causes and different kits’ neonatal survival. These associations suggest that the observed fitness differences are related to differences in the inflammatory response and the corticotrope response to stress, which are two important components of physiological adaptation to environmental aggressions.

The aim of this study was to investigate whether uterine capacity (UC) in rabbits was related to uterine horn length and weight and whether these uterine traits and vascular supply were related to fetal development and survival. Data from 48 unilaterally ovariectomized (ULO) does of the High and 52 ULO does of the Low UC lines of a divergent selection experiment on UC were used. Does were slaughtered on d 25 of fifth gestation. The High line showed higher ovarian weight (0.08 g, P < 0.05) linked to a higher ovulation rate (1 ovum, P < 0.05) and greater length of the empty uterine horn. There were no differences between lines in the remaining doe traits. The number of implanted embryos and live fetuses, fetal survival, and uterine weight and length were positively associated and explained most of the observed variation. Average weights of the live fetuses and their fetal and maternal placentae were not related to uterine weight and length. The linear regression coefficient of full uterine horn length on the number of live fetuses was 2.43 +/- 0.21. The weight of the full uterine horn showed a small quadratic relationship (P < 0.05) with the number of live fetuses. Full uterine horn length, after adjusting for the number of embryos, was negatively associated (P < 0.001) with the number of dead fetuses. The linear regression coefficient of average fetal placental weight of the live fetuses on number of implanted embryos was higher (P < 0.10) in the Low line (-0.23 +/- 0.04 vs. -0.12 +/- 0.04). The linear regression coefficient of average weight of the live fetuses on the average weight of their fetal placentae was higher (P < 0.10) in the High line (2.56 +/- 0.47 vs. 1.27 +/- 0.57). The High line was more efficient, most likely because an increase in intrauterine crowding has a lesser effect on the development of fetal placentae and fetuses. The fetal position within the uterus did not affect the proportion of dead embryos. Fetuses with placentae receiving a single blood vessel had a higher probability of death (P < 0.001) and the lowest weight. There was no difference between lines for individual weight of the live fetuses, but the High line showed higher individual weights of fetal (P < 0.01) and maternal placentae (P < 0.10). Live fetuses in the midportion of the uterus were lighter in weight (P < 0.05) than in the oviductal and cervical regions (20.3 vs. 21.6 and 21.7g). Increasing uterine capacity increases uterine length and decreases weights of fetus and fetal placenta in rabbits.

A study was performed to evaluate the use of unilateral ovariectomy for the measurement of uterine capacity in rabbits through a comparison of the relationships between ovulation rate, number of implanted embryos, and litter size in unilaterally ovariectomized (ULO) and intact does. Data from 211 ULO and 323 intact does were analyzed. The animals were derived from a synthetic line previously selected on litter size. Laparoscopy was performed on all does during their second gestation 12 d after mating and the number of corpora lutea and implantation sites were recorded. Intact and ULO does had the same ovulation rate, confirming the presence of compensatory ovarian hypertrophy in the remaining ovary of the ULO does. The number of implantation sites in the ULO group (11.3) approached the number found in the control group (12.6). Embryonic survival (until implantation) was lower (P < .01) in ULO does (.77) than in intact does (.88), but fetal survival (after implantation) was the same in both groups. The ULO females produced litters 77% of the size of those of the normal control females. Pre- and postimplantation survivals were not related in intact does but seemed to be related in ULO does through an effect on the number of implantation sites. The coefficient of the regression of number of implantation sites on ovulation rate was positive in control does (.62 +/- .06) and was also positive in ULO does (.31 +/- .07), showing that a higher ovulation rate would have resulted in a higher number of embryos being implanted in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

To examine the genetic and phenotypic parameters of uterine capacity and its components in rabbits, we performed a divergent selection experiment for uterine capacity. Rabbit does were unilaterally ovariectomized, and a laparoscopy was performed at midgestation to count the number of corpora lutea and implanted embryos. Selection was performed on litter size in all parities. Genetic parameters were estimated by residual maximum likelihood. For most traits, phenotypic and genetic correlations were in the same range. In unilaterally ovariectomized rabbit does, litter size had a low genetic (.34) and phenotypic (-.08) correlation with ovulation rate, a high genetic (.71) and phenotypic (.60) correlation with the number of implanted embryos, and a high genetic correlation (.89) and moderate phenotypic correlation (.51) with fetal survival. Embryonal survival had a low phenotypic (-.26) or genetic (.12) correlation with fetal survival. A component of uterine capacity, fetal survival in one overcrowded uterine horn, had a low heritability (.05), and consequently limited possibilities for improvement by selection. Uterine capacity, estimated as litter size in one overcrowded uterine horn, had a moderately low heritability (.16). Divergent selection on uterine capacity was effective and a correlated response was found in implanted embryos. No correlated response was found in fetal survival and number of dead fetuses between implantation and birth.