Embryonic diapause, or delayed implantation as it is sometimes known, is sa
id to occur when the conceptus enters a state of suspended animation at the
blastocyst stage of development. Blastocysts may either cease cell divisio
n so that their size and cell numbers remain constant, or undergo a period
of very slow growth with minimal cell division and expansion. Diapause has
independently evolved on many occasions. There are almost 100 mammals in se
ven different mammalian orders that undergo diapause. In some groups, such
as rodents, kangaroos, and mustelids, it is widespread, whereas others such
as the Artiodactyla have only a single representative (the roe deer). In e
ach family the characteristics of diapause differ, and the specific control
s vary widely from lactational to seasonal, from estrogen to progesterone,
or from photoperiod to nutritional. Prolactin is a key hormone controlling
the endocrine milieu of diapause in many species, but paradoxically it may
act either to stimulate or inhibit growth and activity of the corpus luteum
. Whatever the species-specific mechanisms, the ecological result of diapau
se is one of synchronization: It effectively lengthens the active gestation
period, which allows mating to occur and young to be born at times of the
year optimal for that species.