Though it has been argued that the black-tailed deer is a species, virtually all recent authorities maintain it as a subspecies of the mule deer (Odocoileus hemionus). Strictly speaking, the black-tailed deer group consists of two subspecies, as it also includes O. h. sitkensis (the Sitka deer). The black-tailed deer group and the mule deer group (sensu stricto) hybridize, and mule deer appear to have evolved from the black-tailed deer group. Despite this, the DNA of the white-tailed deer and mule deer are similar, but differ from that of the black-tailed deer.
The gestation period for does is six to seven months, with fawns being born in late May and into June. Twins are the rule, although young does often have only single fawns. Triplets can also occur.
