A feature of the gestation period of marsupials is its short duration. The foetal marsupial may spend as little as twelve to thirteen days in the reproductive tract. Why is the gestation period so short and what happens in this time?
In female bird and mammals the reproductive tract has two primary functions. The first is the reception and transport of spermatozoa to the egg so that fertilisation can occur. The second function is to provide the egg with coats and 'shells' and to provide nourishment for the developing embryo. The complex sequence of events which make this possible is called the oestrus cycle. There are two phases of the cycle: the pro-oestrus or follicular phase and the luteal or secretory phase. These names refer to stages which occur in the activity of the ovary during the cycle, but are reflected in the whole reproductive tract.
The broad sequence of changes in the ovaries and the genital tract are similar in most marsupials. The breeding cycle starts with the pro-oestrus phase, during which the ovaries enlarge and egg containing follicles grow and mature. Cell division and growth of the uterine epithelium and secretory glands occur at this time. The vaginal regions also increase in size and secretory activity to enable the reception and transport of seminal fluid. These initial changes are controlled by increased levels of oestrogen produced in the ovary by the cells of the growing follicles. These oestrogen induced changes reach their peak about the time of oestrus or 'heat', which is when the female is receptive to males and copulation may occur. Ovulation, the bursting of the ovarian follicles and the shedding of eggs, occurs one to two days after oestrus. Growth of the uterine glands continues at this time and for a further short period. The epithelium of the vaginal area regresses, its job of transporting and storing spermatozoa being finished.
The brief post-oestrus proliferative phase in the uterus is followed by a secretory or luteal phase, which is under the control of the hormone progesterone, produced by the corpus luteum. The corpus luteum is a gland that forms the wall of the ruptured ovarian follicle. During the luteal phase the uteri become highly secretory and swollen and vascular. Abundant secretory material is poured into the uterine lumen to provide nutrients for the growth of the embryo and its associated membranes. The luteal phase varies in length in different marsupials and is followed by a regressive phase during which there is a reduction in size complexity and secretory activity of the uterine glands. Anoestrus, an overall shutdown of the system, follows in those species which have only one oestrus per breeding season. However in continuos breeders or species that have more than one oestrus a regressive or post-luteal phase grades into the next pro-oestrus phase.
Insemination and a resulting pregnancy do not interrupt the oestrus cycle of marsupials as they do in placentals. This is a significant feature of marsupial reproduction. Pregnancy does not inhibit the continuation of the oestrus cycle, but lactation does, the ovarian inhibition being mediated by the suckling stimulus. If continued lactation is prevented by the removal of the young soon after birth the next oestrus occurs when expected. In many marsupials gestation does not last past the luteal phase, but the situation is variable, notable in kangaroos.
The reproductive cycle of kangaroos differs from the basic marsupial pattern in that gestation is extended from about half the oestrus cycle to occupy almost all of it. The embryo in the kangaroo is maintained in the uterus well into the next pro-oestrus phase, with birth occurring only a day or two before the next oestrus and mating (Table 1).
The relatively longer gestation period of kangaroos is associated with a lengthening of the secretory phase in the uteri via the corpus luteum. The aim of the longer gestation is to produce a young that is larger and more advanced. This may seem surprising, given its very small size at birth, but the newborn kangaroo is relatively bigger and more developed than non-macropodoid marsupials. This increased development has been show to have significance in the journey of the newborn to the pouch and its attachment to the teat. The more advanced state, particularly in neurological organisation, means that the investment in the single young is less likely to go astray.
An unusual feature of reproduction in kangaroos is embryonic diapause. During diapause a viable embryo is carried in the uterus for long periods (many months) with its development stunted at the stage of a blastocyst, which consists of some seventy to a hundred cells and is only about a quarter of a millimetre in diameter. Because of the extension of the gestation period into the pro-oestrus phase of the next ovulation, mating and ovulation can occur a day or two after birth. (The Grey kangaroos are an exception to this pattern, with the Western grey's not showing diapause at all.) The embryo produced following this post-partum mating will not successfully develop past the blastocyst stage until a specific signal is given. If a young has not taken up residence in the pouch and started to nurse there is a specific pulse of progesterone secretion about six days after birth. The pulse is necessary if the blastocyst is to continue development. If there is a young suckling in the pouch then lactation inhibits the corpus luteum via the hormone prolactin. The corpus luteum then becomes quiescent and no pulse of progesterone occurs and the blastocyst enters diapause. It will only recommence development when the lactational inhibition is removed, either by loss of the pouch young or by the reduced lactation toward the end of the pouch life.
The general timing of events in the reproductive cycles of kangaroos is basically similar for all species (Table 1), but Grey kangaroos do differ in some details from the Red kangaroos and Wallaroos. There are also difference in timing among the subspecies of the Wallaroos and limited data also suggests that the Antilopine kangaroo may have a longer cycle than the Red kangaroo.
With Red kangaroos, Euros and Wallaroos breeding is continuous under good conditions. The oestrus cycles are about thirty-five days, one to three longer than the gestation or pregnancy period. There is a post birth oestrus with mating one to three days after the birth. The embryo resulting from this mating enters embryonic diapause if the recently born young has succeeded in reaching the pouch and attaching to a teat. The length of pregnancy after loss of a pouch young and reactivation of the dormant blastocyst is thirty to forty days, the longer periods being associated with a greater intensity of suckling from a young at foot. If the female is not carrying a dormant blastocyst the time from loss of the pouch young to the next oestrus will be about thirty-two days. In the usual course of events the young from the blastocyst is born is born within a day of the permanent pouch exit of the previous young.
The Grey kangaroos can breed at any time of the year but they are usually seasonal breeders. Oestrus and births generally occur September to March with the peak with the peak activity occurring in late spring and early summer. The Eastern grey has a more variable breeding pattern than the Western grey, especially in the northern parts of its range. Grey kangaroos differ from Red kangaroos and the Wallaroo group in that gestation takes less of the oestrus cycle, especially in the Eastern grey where the pregnancy period may be some ten days shorter than the oestrus cycle.
Diapausing blastocysts are rare in Eastern grey kangaroos and function differently from the pattern seen in Red kangaroos and the Wallaroo group. Immediate post-birth oestrus does not occur. Under favourable conditions a female will mate when the pouch young is about six months old, with the resulting embryo quiescent due to lactational inhibition. Again the stage of arrested development is that of the seventy to a hundred cell unilaminar blastocyst. The embryo recommences development to be born immediately after final pouch exit by previous young, at ten months of age, or if the pouch young is prematurely lost. In Western grey kangaroos diapausing blastocysts do not occur.
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