Delayed Implantation in Marsupials
All bar one species of the family Macropodidae (kangaroos) may undergo embryonic diapause. Embryonic diapause is a strategy whereby an animal can delay the development and birth of her offspring. This reproductive strategy is utilised by around 100 different mammals. The embryo or blastocyst remains a ball of hollow cells and does not implant in the uterus. It may remain dormant for up to a year.
Embryonic diapause is also known as delayed implantation. Macropods are not the only animals to show this trait. Rodents, bears, and mustelids (such as the badger) undergo embryonic diapause. The order Artiodactyla has only one species that undergoes embryonic diapause and that is the roe deer.
Although much is still unknown about embryonic diapause, it would seem that the suckling stimulus may postpone the initiation of the pre-oestrous phase. Generally, pregnancy does not prevent ovulation or fertilisation but the corpus luteum (essential for maintaining pregnancy) is held dormant by the suckling stimulus.
In most macropod species, the female mates soon after the birth of a joey. The embryo enters a state of suspended animation. The hormone prolactin is produced in response to the sucking stimulus and blocks further development of the blastocyst. As the joey begins to leave the pouch, eating other food and lessening its milk intake, growth of the embryo is reinitiated.
In macropods, there is a release of progesterone in the first week following mating. This release synchronises the development of both the uterus and the embryo. However, while the older offspring is still in the pouch and suckling, the embryo remains passive. Should the older offspring die through accident or because of severe hardship as in drought conditions, the embryo will recommence its development.
Female macropods seem to be very flexible as to when they will produce young. Some, including the Potoroinae species, breed continuously, with new offspring appearing as the previous lactation nears its end. Under favourable conditions, desert kangaroos also display this continuous cycle. When conditions are generally not so good, they bear young from time to time to coincide with less 'bad' conditions. During prolonged drought, they may not bear young at all.
Kangaroos in the southern part of the continent are more inclined to breed seasonally, waiting until winter rains produce an abundance of green feed.
Embryonic diapause is a great advantage to a mother. The birth of the offspring can be timed to occur during favourable times such as when food is available, when the weather is kind and/or when previous offspring have been weaned. The mother is able to give herself and her offspring every chance of a successful outcome.
There is now a distinction between two types of embryonic diapause. Facultative diapause or facultative arrest is associated with metabolic stress. This metabolic stress is generally lactation. However in the case of the brine shrimp, encysted embryos may be produced in the autumn when temperatures are low and there is high salinity, both features not conducive to high survival rates of nauplii (live shrimp offspring). The cysts can survive for over a year with no oxygen but, when favourable conditions return, they will hatch and continue their development.
Obligate diapause (obligate arrest) allows flexibility in the timing of births to coincide with favourable environmental conditions. Thus roe deer may mate in July or August and not give birth until the following May or June. Not only that but the offspring is usually twins of opposite sexes! Pinnipeds (seals and sea lions), mustelids (weasels and otters), ursids (bears), armadillos and one species of fruit bat are other mammals which undergo embryonic diapause.
Embryonic diapause is just one of Nature's many wonderful innovations designed to help her subjects survive in the wild.