I once read the novel “Kappa” by the acclaimed Japanese novelist Akutagawa and was reminded of it when I was contemplating writing this blog about an embryonic diapause, a situation in which an embryo stays and waits in the womb to develop and to come out not until conditions are optimal for its emergence. In Akutagawa’s novel the offspring decides when and if they want to be born. Something a little similar to that actually exists amongst several species of mammals, although it seems that here the maternal organisms control and decide the best time for the young to be born.
Diapause is a common strategy for invertebrates like, for example, insects, to survive harsh and inhospitable conditions. Insects may shut down their metabolism, cease to move around and become unresponsive to stimuli until environmental conditions improve and ‘revive’ those insects or their eggs or developmental stages so that they may continue what had been interrupted by the diapause. In some way it is similar to the hibernation that many species, including some mammals, undergo to get through the winter without having to search for food and to keep the body warm in the cold season.
In the embryonic diapause, it’s not the adult individual (as with hibernation) that reduces or interrupts its own development or activity; it only retards the embryo’s development and postpones the birth of the offspring. The most widely quoted and famous “practitioners” of this strategy are marsupials like kangaroos. A kangaroo can have three joeys of different degree of development at the same time. The important feature of an embryonic diapause, often referred to also as dormancy, is the delayed implantation of a fertilised egg cell, an embryo-in-waiting. While the latest baby kangaroo may be firmly attached to its mother’s nipple in the pouch (where in case of the larger species it may stay for at least 200 days), an older sibling, able to climb in and out the pouch, still gets its milk of a different content from a different set of nipples. The third and youngest (and no more than a fertilised egg at that stage) can be implanted in one of the marsupial’s two uteruses, where (in case of a larger kangaroo species) it’ll stay for 30-40 days, before being born as a bean-sized, naked and pink baby with strong arms to climb into the pouch to become attached to the life-saving and life-sustaining nipple. Therefore, it’s the mother that controls when the fertilised egg is allowed to continue its development in the womb.
Implantation delays such as the ones just outlined for marsupials are also known from other mammals, where they occur in bears, some deer, seals and fruit bats in a somewhat less spectacular fashion than that known to occur in case of the kangaroos. Here the objective is to let the birth happen during the most favourable environmental and seasonal conditions, but implantation delays can also occur in response to a female’s situation with regard to lactation – although it is more common that actual fertilisation and egg cell availability are suppressed by a lactating female.
Closer to the “Kappas” of Akutagawa’s novel come the embryos of altricial birds like chicks, ducks, etc., for the eggs in these birds are laid over a period of many days and yet they all hatch at more or less the same time. However, here again the mother bird has a decisive role to play, for a fertilised bird egg starts to develop in earnest only when incubated at the right temperature and that happens to be when egg laying has come to an end and sitting on the eggs has commenced. For many species of birds that period of the offspring’s development also involves the fathers, who take turn warming the eggs when the female has to find food for herself.
The rare condition of two uteruses in humans and implantation of fertilised egg cells in both (at different times) can cause a woman, like recently reported in a Bangladeshi lady, to have two separate births separated by a whole month. This, however, does not qualify as an embryonic diapause -but I don’t think the Bangladeshi lady would mind one little bit whether or not it qualifies.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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