They’re not exactly pleasant behaviours
Has it ever happened to you that when you wanted to fry an egg and you cracked it open you found two egg yolks in it? If you had that experience you would probably have wondered whether two chicks could have developed from such an egg. Fact is that although two embryos may initially grow, one soon overtakes the other and causes its death, so that in the end only a single chick actually hatches. Can embryos really be so murderous?
There is a medical phenomenon termed “Vanishing Twin” in humans and it means that originally there were two foetuses in the uterus, but during the pregnancy one got absorbed by (or incorporated into) the other foetus, who then contained part of its twin in its body. A classmate of mine had a jaw once removed from inside his back and that jaw stemmed from an absorbed twin. The chicken story above is therefore not at all unique.
Once hatched, the elimination of a sibling that shares the same nest is quite a common occurrence in many species of birds of prey. In the eagle Aquila pomarina, usually two eggs are laid, but it is the second egg that is always a little smaller than the first. The first hatchling has, thus, a double advantage. Being bigger and stronger it uses that advantage to gradually overpower and kill its little brother or sister through suffocation by climbing up and sitting on it, by stealing its food share or by physically injuring it. In Antarctic skua gulls as well as some owls the strongest chick has frequently been observed to swallow its weaker nest mate, so that one wonders whether the second egg laid and the weaker sibling isn’t actually a kind of food provisioning carried out and wisely planned by the parent bird in case it might later not be able to obtain sufficient nutrition for its young.
However, not only siblings can kill each other (as already the Bible tells us, hence the term Cainism for this kind of behaviour), but parents, too, may kill their own young, perhaps in order to solve the problem of overcrowding in the nest or to reduce the risk of being discovered by a predator or to eliminate the chance of a sick individual spreading a disease. And that’s infanticide. A gosling, for example, that behaves oddly or has sustained a handicapping injury cannot count on any sympathy from “Mother Goose”, but on the contrary is in danger of being chased and finished off by her (just think of Hans Christian Andersen’s tale “The Ugly Duckling”). To be different in looks and behaviour can be life threatening in animals (and not just them: think of human albinos in some societies).
In caged or domesticated mammals the elimination of the afterbirth through consumption by the mother can initiate the eating-up of the newborn and zoo veterinarians keep a watchful eye on this, although such behaviour should be seen as pathological. The opposite, namely the killing and use of a parent by the offspring as a source of food, also occurs, but it is rarer still and, as far as I know, a regular and normal occurrence only among some small gall midges, the ibis fly, the wood borer beetle Micromalthus debilis (and possibly a few other arthropod species as well). In these species the male larvae hollow out and consume their mother. That I’d call a sacrifice and human parents, who proudly talk about all the sacrifices they make for their children, had better take a note of this.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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