Kangaroos - embryonic diapause The pattern of reproduction of the tammar wallaby is typical of the Macropodidae. The only species of macropod to differ is the musky rat kangaroo, that does not exhibit embryonic diapause, and bears twin young. With the possible exception of the pygmy possum, in all the non-macropod marsupials pregnancy length is between 1/2-2/3 the length of the oestrus cycle of an unmated female. A peak in the secretion of progesterone from the corpus luteum occurs in the middle of the oestrous cycle, the young being born as the concentration declines. During lactation, there is no embryonic diapause or post partum mating, and the activity of the ovaries is suppressed. In all Macropodidae species, the length of gestation is 80-110 % of the length of the oestrus cycle, the next oestrus cycle and mating occurring shortly before or after birth of the young. Until the young is born, the corpus luteum continues producing high levels of progesterone, production stopping abruptly after the birth. A feature of macropodid reproduction is a transient pulse of progesterone that is produced in the first week following mating, reproduction is a transient pulse of progesterone that is produced in the first week following mating, synchronising the development of the uterus and the embryo in preparation for pregnancy. After a female gives birth, and lactation begins, the corpus luteum that had formed on the ovary remains small and inactive while the young is in the pouch suckling, with the result that there is no early progesterone pulse, so the embryo remains in diapause until the older offspring leaves the pouch. If the older young dies, as when the milk production is reduced or stops, as in times of severe drought, the progesterone pulse occurs, causing the embryo in diapause to resume development and continue on to birth, and the process is repeated. that had formed on the ovary remains small and inactive while the young is in the pouch suckling, with the result that there is no early progesterone pulse, so the embryo remains in diapause until the older offspring leaves the pouch. If the older young dies, as when the milk production is reduced or stops, as in times of severe drought, the progesterone pulse occurs, causing the embryo in diapause to resume development and continue on to birth, and the process is repeated. As a result of this unusual reproductive pattern, macropod females are very flexible as to exactly when they produce young, the various breeding strategies ranging from continuous breeding, when times are good, opportunistic breeding, when times are not so good, to seasonal breeding when it is necessary. The commonest, most widespread strategy is continuous breeding, successive young being produced when the previous lactation finishes. This pattern is present in all Potoroinae species with up to 3 young being produced each year. The desert kangaroos also display this pattern in favourable times. In less favourable times they switch to the opportunistic breeding pattern, and in times like prolonged drought they they switch to the opportunistic breeding pattern, and in times like prolonged drought they become anoestrus. In the southern part of the continent, the winter rain predictably leads to nutritious food, macropods breed seasonally. They achieve this by either extending diapause until after the summer solstice, seasonal quiescence, or by seasonal anoestrus, no follicles developing in the ovaries. Discussion of particular species shows how the macropods adapt the basic pattern to suit the prevailing conditions and environment. Sources & Further reading Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.