The red deer (Cervus elaphus) of European origin (e.g. subspecies scoticus, hispanicus, hippelaphus) is a medium sized (100-150kg mature hind weight) ruminant that exhibits highly seasonally patterns of autumn conceptions and summer births. Historic data indicate average (+/- s.d.) gestation length of 233-234 (+/- 2-4) days. Recently, however, there has been growing awareness that there is considerably greater variation in gestation length than earlier indicated and that there is a significant element of environmental, and possibly even social, control over the duration of pregnancy in this species. Imposition of variable levels of nutrition over late pregnancy of red deer hinds has been observed to influence fetal growth trajectory and gestation length, with no apparent effect on birth weight. This supports a hypothesis that under conditions of modest feed imbalance, variation in gestation length compensates for variation in fetal growth trajectory to ensure optimisation of birth weight. More recent studies on primiparous (24 month old) red deer hinds have identified surprisingly large variation in gestation length (193-263 days) compared with adult hinds (228-243 days), with earlier conceiving individuals within the primiparous cohort expressing significantly longer gestation than the later conceiving hinds, resulting in a higher level of calving synchrony than expected from known conception dates. This introduces an intriguing hypothesis of social indicative effects on parturition timing to promote within-cohort birth synchrony. Collectively, these data debunk the commonly held notion that gestation length of red deer is genetically fixed within strict limits. A review of the literature points to this as possibly a common phenomenon across a range of non-domesticated ruminant species but this conclusion is not supported by numerous conflicting studies on domestic sheep and cattle.