The effect of drought on reproduction
If there is a drought at this time, as happens often in central Australia the fertilised egg in the uterus still remains dormant. But if there has been rain and there is good pasture, then the egg resumes its development. Thirty three days later, another bean sized neonate will emerge from the mother's cloaca. The female will then immediately mate again. But the first-born does not give up its milk supply so easily. It returns regularly to feed from its own teat. The female kangaroo in effect has three young dependents on her, each at a different stage of development. One active young at foot which grazes but comes back to suckle, a second, the tiny neonate, sucking at her teat in the pouch; and a third the fertilised egg waiting further development.
It is a commonly held notion that the marsupials are backward creatures, scarcely much of an improvement on those primitive egg layers, the platypus and echidna. That is a long way from the truth. The marsupial method of reproduction must certainly have appeared very early in mammal history, but the kangaroos have refined it marvellously. No other creature anywhere can compare with the female kangaroo who, for much of her adult life, supports a family of three in varying stages of development.