General features of monotreme reproduction

Monotremes are distinguished from other mammals (eutherians and marsupials) by the following features:

1) oviparity (ability to lay a soft shelled egg).

2) They have a true cloaca.

3) They do not have nipples or teats on their mammary glands but have hair and mammary glands.

Monotreme female reproductive system

The female reproductive tract opens into the cloaca and there are left and right reproductive tracts, with each possessing an ovary, oviduct, uterus and cervix (Temple-Smith and Grant 2001). In the platypus only one side of the reproductive tract is functional (the left), whereas both sides are functional in the short-beaked echidna. This limitation, however, this does not limit the number of eggs produced by the female platypus, in that the platypus usually produces two ova (Burrell 1927), whereas the short- beaked echidna produces only one (Griffiths 1968, 1978). Insemination may be intrauterine (Temple- Smith and Grant 2001).

Monotreme pouch

Extant female monotremes build burrows during the breeding season, for the protection of the female and her young. Before egg-laying, the female short-beaked echidna develops a pouch (marsupium or incubatorium) into which the egg is laid directly from the protruded end of the cloaca (Temple-Smith and Grant 2001). The long-beaked echidna may also develop a pouch (see figures below) (Griffiths 1978; Temple-Smith and Grant 2001). This was observed first by Owen (1865) when he dissected a female echidna. Female platypus do not develop a pouch. The function of the pouch appears to be to maintain hydration of the leathery-shelled egg. In the platypus, this hydration appears to be achieved by the female bringing moist vegetation into the burrow.

References:

Burrell H (1927) The Platypus. Angus and Robertson, Sydney.

Griffiths M (1968) Echidnas. Pergamon, London.

Griffiths M (1978) The Biology of the Monotremes. Academic Press, New York.

Owen, R. (1865). XV. On the marsupial pouches, mammary glands, and mammary fœtus of the echidna hystrix. Philosophical transactions of the Royal Society of London, (155), 671-686.

Temple-Smith PD, Grant T (2001) Uncertain breeding: a short history of reproduction in monotremes. Reproduction, Fertility and Development 13, 487–497.