The structures and physiology of the mammary glands of the lactating Red kangaroo are the most complex to be found in the class Mammalia. The female Red kangaroo can exhibit, simultaneously, two glands regressing from the tubular condition found at birth, one gland growing and differentiating from the tubular to an alveolar condition supporting a small pouch young, and a fourth in a state of full lactation supplying a young at heel. As lactation proceeds the two regressing glands become quiescent and then later re-differentiate to the oestrus or birth condition independently of the lactating glands.
As well as differing anatomically the two lactating glands are very different biochemically; the milk for the sustenance of the pouch young contains a large amount of palmitic and relatively little oleic acid whereas the gland supplying the young at heel secretes a milk with little palmitic and much oleic acid.
The amount of crude lipid in the milk of the Red kangaroo sampled from day 1–4 day to day 360 of lactation tends to increase. The fatty acid complement of the milk fat taken day 1–4 to the 72nd day differed greatly from that taken at 100–360 days. The early milk triglycerides contained on the average 44·2% palmitic acid and 24·1% oleic, whereas in mature milks the proportions were reversed. Short chain fatty acids (C4-C12) were virtually absent which was surprising to find in the milk fat of a herbivore. The Red kangaroo can elaborate simultaneously the two kinds of milk fat just described in two different glands supplying a small pouch young and a large young at heel respectively. It is suggested that early milks containing far more palmitic than oleic acid may only be found in mammals that have very special young such as the echidna, platypus and marsupials.
The structure of the mammary gland from the condition found in the very young virgin doe to that of a doe suckling a 312-day young is described. The Red kangaroo has four mammary glands one of which, suckling a small pouch young, differentiates and grows along with growth of the pouch young and produces milk with a high palmitic acid content, another, suckling a young at heel, has large fully differentiated and thin-walled alveoli, while the other two (non-suckled) show progressive regression from the condition at birth, followed by redifferentiation at c. 128th day of lactation, both activities being independent of the activities of the lactating glands.