thumb|Young roe deer
Capreolus is a genus of deer, the roe deer. It contains two species native to Eurasia, and fossil evidence also supports their former presence in North America.
Etymology
English roe is from Old English ra or rá, from raha, from Proto-Germanic , cognate to Old Norse ra, Old Saxon reho, Middle Dutch and Dutch ree, Old High German reh, German Reh. It is perhaps ultimately derived from a PIE root *rei-, meaning "streaked, spotted or striped".
The word is attested on the 5th-century Caistor-by-Norwich astragalus -a roe deer talus bone, written in Elder Futhark as , transliterated as raïhan.
In the English language this animal was originally simply called a 'roe', but over time the word 'roe' has become a qualifier and the creature is now usually called a 'roe deer'.
The Koiné Greek name πύγαργος, transliterated 'pygargos', mentioned in the Septuagint and the works of various writers such as Hesychius, Herodotus and later Pliny, was originally thought to refer to this species (in many European translations of the Bible), although it is now more often believed to refer to the addax. It is derived from the words wikt:πυγή| 'buttocks' and wikt:ἀργός| 'white'.
The taxonomic name Capreolus is derived from capra or caprea, meaning 'billy goat', with the diminutive suffix -olus. The meaning of this word in Latin is not entirely clear: it may have meant 'ibex' or 'chamois'. The roe deer was also known as capraginus or capruginus in Latin.
Systematics
Roe deer are most closely related to the water deer Hydropotes, and, counterintuitively, the three species in this group, called the Capreolini, are most closely related to elk and moose Alces.
Although roe deer were once classified as belonging to the subfamily Cervinae, they are now classified as part of the Capreolinae, which includes the deer from the New World.
Palaeontology
Roe deer are thought to have evolved from a species in the Eurasian genus Procapreolus, with some 10 species occurring from the Late Miocene to the Early Pleistocene, which moved from the east to Central Europe over the millennia, where Procapreolus cusanus occurred, and the western species first appeared in Europe 600 thousand years ago.
The Siberian roe deer had split into two subspecies, C. pygargus pygargus and C. pygargus tianschanicus in the interval between 229 and 462.3 thousand years ago.
It was previously assumed that Capreolus only ever inhabited Eurasia, the likely ancestral home for the Capreolinae. However, a 2014 paleontological study recovered fossil remains of the species Capreolus constantini, previously known only from Russia, from the Pliocene of Hidalgo, Mexico. This suggests the existence of a previously unknown dispersal event of Capreolus from Eurasia to North America across Beringia during the Pliocene, where the genus is no longer found in the present day. In line with Haldane's rule, female hybrids of the two taxa are fertile while male hybrids are not. Hybrids are much larger than normal and a cesarean section was sometimes needed to birth the fawns, becoming larger than their mothers at the age of 4–5 months. F1 hybrid males may be sterile, but backcrosses with the females is possible. In northeastern Poland there is also evidence of introgression with the Siberian deer, which was likely introduced. In some cases, such as around Moscow, former introductions of European stock is likely responsible.