Effect of age on reproductive traits
The minimum and maximum values, the least squared mean and its standard error of female’s age (AF), litter size at birth (LSB), kit’s individual weight at birth (IWB) and statistical comparisons between nulliparous and multiparous female rabbits are presented in Table 1. We found that the nulliparous female rabbits (young females) had their first delivery around 6 month of age and the multiparous female rabbits (older females) at more than one year old at their fifth delivery (P<0.001). Besides, litter size was greater in young females than older ones (+1.7 kits, P<0.05). However, kits of young females weighed lesser at birth compared to that of those multiparous female rabbits (-11.7 g, P<0.001). This difference disappeared when the litter size was included in the model as covariate (-0.5 g, P>0.10). Therefore, larger litters in young female are related to a lower weight of their kits (P<0.001).
In commercial rabbit breeding, females are normally inseminated at 10-12 days post-delivery; thus, in multiparous females, there is an overlap between lactation of their current litter and gestation of their future litter. The high body energy deficit in multiparous females and the negative interactions between lactation and fertility are well known
(Fortun-Lamothe et al., 1999; Theau-Clement and Roustan, 1992; Xiccato et al., 2004), which would explain a lower litter size in multiparous females than nulliparous ones. In this sense, literature in rabbits shows that reproductive performance is age dependent and the litter size at birth is increased as parity increased reaching the peak at the 3
rd parity
(Das and Yadav, 2007; Xiccato et al., 2004) then decreased gradually thereafter
(Mahmoud, 2013). As weight of kits at birth in nulliparous females is concerned, it is well established that female’s body growth has not finished during the first pregnancy because of the feed intake is lower than thereafter
(Rommers et al., 1999) and that a greater embryonic uterine overcrowding decreases available uterine surface area for the development of each placenta and nutrient supply to embryo
(Argente et al., 2006 and 2008), which would explain kits born from nulliparous females are lighter than those multiparous ones.
Effect of age on haematological parameters
Even though rabbit is widely used as experimental animal model for monitoring evolution in infectious and non-infectious diseases
(Fan et al., 2014; Peng et al., 2015), there is scarce information about evolution in haematological reference values with age in this species. Using appropriate reference values are vital in these studies. Table 2 shows the minimum and maximum values, the least square mean and its standard error of red blood cells (RBC), white blood cells (WBC) and differential count for lymphocytes, neutrophils, monocytes, eosinophils and basophils both in nulliparous and multiparous female rabbits,
i.e. young and older animals. Nulliparous and multiparous females had similar RBC count (P>0.10). This finding showed that the values of RBC do not change with age and they are consistent with reference data of RBC in other populations
(Hewitt et al., 1989; Jain 1993; Jeklova et al., 2009). In contrast to our observation for RBC, WBC and lymphocytes decrease substantial with age (10.50 109/L in nulliparous female vs 7.01 109/L in multiparous females, P≤0.05,
e.g. -33% for WBC; 6.99 109/L in nulliparous females vs 2.78 109/L in multiparous females, P≤0.001,
e.g. -60% for lymphocytes). Accordingly, the percentage of lymphocytes to WBC count was lesser in multiparous females than nulliparous ones (-26% for LWR, P≤0.001) and the percentage of neutrophils to WBC ratio was higher in multiparous females than in nulliparous ones (+26% for NWR, P≤0.001). A reduction in number of lymphocytes may related to higher ratios of both neutrophils to lymphocytes (NLR) and monocytes to lymphocytes (MLR) in multiparous females than in nulliparous ones (+95% for NLR and +5% for MLR, P≤0.01). For the rest of haematological parameters, there were no differences between young and older females.
This strong decrease in WBC and lymphocytes would be related to impairment of the adaptive immune function primarily because of the decline in production of naïve lymphocytes in the bone marrow and thymus as well as the expansion of incompetent memory lymphocytes
(Weng, 2006). Monocyte to lymphocyte ratio (MLR) has been recently proposed to a simple biomarker of immune system; moderate and low values are related to a good combative power against disease in the host, by contrary high value is associated to impair immune system
(Zelmer et al., 2018). On the other hand, neutrophil to lymphocyte ratio (NLR) has been proposed as an indirect biomarker of stress
(Jain, 1993). Stress stimulates release of glucocorticoids from the adrenal glands that induces neutrophilia or lymphopenia, or both, increasing NLR
(Davis et al., 2008). We stress that, in healthy rabbits, NLR increases from 1:2 at 2 months of age until 1:1 at 1 year of age
(Washington and Van Hoosier, 2012), as a consequence of maturation of the hypothalamic-pituitary-adrenal axis over time
(Hillmann et al., 2008). Chronic stress is associated with immune dysfunction. In this regard, the highest values in NLR and MLR of multiparous females would agree with a higher adrenal activity and in consequence an increase in the susceptibility to infections in older animals.
Effect of physiological status on haematological parameters
Table 3 shows the minimum and maximum values, the least square mean and its standard error of red blood cells (RBC), white blood cells (WBC) and differential count for lymphocytes, neutrophils, monocytes, eosinophils and basophils at mating and at delivery. There was difference only in monocytes count. In this sense, monocytes count doubled at delivery compared to mating (0.28 109/L at mating vs. 0.59 109/L at delivery, P≤0.01). Accordingly, its percentages increased in WBC count (+3%, P≤0.01) and number of lymphocytes (+3% for MWR and +5% for MLR, P≤0.01).
