Summary. Lactation was initiated and confined to the mammary gland to which the neonate attached at birth, and continued for about 360 days. The lactating gland continued to grow for over 200 days, increasing 7- to 10-fold in size between Days 50 and 250. The young at these times weighed approximately 35 and 2500 g respectively. The young left the pouch permanently at 200–220 days, although they continued to suck intermittently for a further 160 days. Twenty-six days after evacuation of the pouch a second young was born and this attached to one of the three unused nipples. Thereby a second lactation developed in parallel to the previous one, but over 200 days in arrears.
The nipples of the non-lactating mammary glands were about 6 mm long and 2 mm in diameter: these increased to >50 mm long and 4 mm in diameter at Day 100 of lactation. Each nipple contained 18–33 separate galactophores. Recordings were made of intramammary pressure in cannulated galactophores of wallabies under barbiturate anaesthesia. Bolus injections of 2–30 mU oxytocin caused increases in intramammary pressure after a latency of 23–60 sec, and pressures peaked at 20–60 mmHg after 50–100 sec. Infusions of oxytocin produced multiple increases in intramammary pressure with peaks at intervals of about 4 min.
The threshold dose of oxytocin for the induction of a mammary contraction increased from approximately 2 to 15 mU oxytocin between Days 50 and 250 of lactation. Likewise, there was an increase in the latency to the onset of contraction and a decline in the peak pressure generated between early and late lactation. Injections of small doses of oxytocin induced contraction only of mammary glands in early lactation, whereas larger doses caused the contraction of both 'young' and 'old' glands.
The decline in oxytocin sensitivity during lactation permits milk ejection in response to small releases of oxytocin to be confined to the mammary gland to which the neonate is continuously attached. Conversely, the release of a large quantity of oxytocin, in response to the sucking of the juvenile at foot, would cause milk ejection in both lactating mammary glands.