Hormonal control of embryonic diapause and reactivation in the tammar wallaby.

Suckling is the most important and best-known influence that the marsupia offspring exerts on its mother but recent evidence suggests that this is not the only way it proclaims its presence. The pregnant female differs from the non-pregnant in several features, which appear to be due to the presence of the conceptus in the uterus. The converse influence of the female on the embryo is best known in kangaroos and wallabies because the facultative and obligate embryonic diapause they exhibit has provided excellent opportunities for experimental investigation of the environmental and hormonal control of early pregnancy. Results from the tammar wallaby, which exhibits both types of diapause, show that the initiation of diapause occurs on Day 8 p.c. The sequence of events that lead to blastocyst reactivation, after removing the suckling stimulus (facultative diapause), takes seven days and involves a reduction in prolactin secretion from the anterior pituitary, reactivation of the corpus luteum, increase in endometrial secretion and reactivation of the blastocyst. The ultimate control of obligate diapause involves alteration of photoperiod but the role of prolactin is still uncertain.