Matching the green wave: growing season length determines embryonic diapause in roe deer

The roe deer (Capreolus capreolus) is Europe's most widespread ungulate, notable for its unique trait of embryonic diapause (delayed blastocyst implantation after mating) and an ongoing debate regarding how climate change affects its parturition timing. Given the relatively constant timing of the rut, roe deer could cope with advancing greening by adjusting its diapause end. Here, we bridge the gap on factors influencing roe deer's diapause by analysing 390 uteri from legally hunted roe deer females in Germany (2017-2020), which we macroscopically examined for the presence of visible embryonic tissue to retrospectively identify the diapause end date. By employing a marginal Cox proportional hazard model, we tested associations between female phenotypic attributes, environmental conditions and the probability of ending embryonic diapause prematurely. Our results confirmed that high-quality, well-conditioned and prime-aged females tend to terminate embryonic diapause earlier. We also demonstrated for the first time that on a population-averaged level, the growing season length in the year of conception significantly influences the diapause timing, even explaining the much-debated shifts in parturition dates in roe deer over the last seven decades. Increased knowledge of mechanisms involved in embryonic diapause may also help decipher embryo-maternal interactions in general, including in vitro fertilization.