Factors Affecting Implantation Failure in Roe Deer

Reproductive performance is one of the most important life-history traits that should be routinely studied and considered in adaptive wildlife management. In the case of roe deer (Capreolus capreolus), a species with delayed implantation, which complicates studies on fetuses, corpora lutea (CL) counting is the only alternative for routine monitoring. However, because of a possible implantation failure, the reliability of this method is questionable, and factors influencing implantation success have been poorly understood so far. We analyzed 2,594 intact uteri of roe deer hunted from 2006–2015 in an Apennine population, central Italy, during winter (mid-Jan to mid-Mar). By comparing the number of CL and fetuses in the same individuals (i.e., success in blastocyst implantation), we revealed a mean implantation failure of 8.6% in a pooled sample set (regardless of the age and origin of animals), with a high inter-annual variability (range = 3.6–19.8%). Contrary to adults (x̄ ± SE = 11.1 ± 1.9%), the implantation failure in yearlings was low (4.4 ± 1.9%). Implantation success was affected by individual maternal characteristics (positive effect of body mass and negative effect of age), climatic condition in summer (positive effect of July temperature up to 23.4°C, and negative effect above this threshold), winter harshness (negative effect of snow cover duration), and altitude (negative relation with the elevation). Reproductive performance of adult female roe deer cannot be adequately measured by CL counts because of high inter-annual variability in implantation failure and important effects of female attributes and environmental factors. However, for yearlings, which also express the highest variability in the ovulation rates, CL counts provide important information on their reproductive outcome because they have low implantation failure.