The roe deer, Capreolus capreolus, is the only artiodactyle known to exhibit delayed implantation (Short & Hay, 1966). In Great Britain, mating normally occurs between late July and mid-August and after fertilization development proceeds to the blastocyst stage before growth is arrested (Prell, 1938). Blastocysts remain free in the uterine lumen for about 5 months from August until early January before growth is resumed, implantation occurs and development proceeds normally. The majority of kids are born between mid-May and mid-June, and twins are common.
In the roe, the long period of delayed implantation ends in early January very soon after the winter solstice (22nd December) when the days are just beginning to lengthen (Short & Hay, 1966). The change in photoperiod may, therefore, act as an environmental cue, causing changes in the reproductive system of the doe and resulting in the resumption of embryonic development. This seems probable
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