Lactation in kangaroos provides all the nutrients and energy for growth and development during most of the pouch life, this being about eight months duration in the red kangaroo, wallaroos and euros and some ten months for grey kangaroos. Joeys are not weaned until four to eight months after leaving the pouch. The benefits derived from lactation can be seen by the growth rates of pouch young. The new born of kangaroos are less than a gram in weight, but by the time of permanent emergence from the pouch they are four to five kilograms. The growth rate of the grey kangaroo is much slower than that of the red kangaroo but, because of the longer time the grey spend in the pouch, the exit weights are similar. In the period from exit until weaning the young's body more than double in weight. Again the relatively slower growth rate of the eastern grey kangaroo is compensated for by the later weaning of the young.
Rate of growth is to some degree controlled by the milk supply, since the rate of growth of the pouch young is reduced in draught. The reverse is also seen if small pouch young are fostered in the pouch of a female that is in a more advanced stage of lactation, which results in an increased growth rate. Detailed information on milk production is not available for kangaroos but the available data suggests that it follows the pattern known for the smaller tammar wallaby. In the tammar wallaby, 30 days after birth, milk production was only one millimetre per day. This increased to nine millimetres per day at 130 days, to forty-three millimetres per day at 210 days and to a peak of eighty-six millimetres per day at 250 days, the end of pouch life when the growth rate is at its highest. Not only was the quantity of the milk markedly increased but many of its characteristics were also changed. The energy content increased from 2.5 kilojoules per millimetre at day 7 to a peak of 11.2 kilojoules per millimetre at day 250. An increase in fat content accounts for much of the increase in energy content. The reduction in volume of milk towards final weaning is compensated for by the young's increasing herbage consumption.
Consumption of the milk is tailored to the requirements of the developing young. An example is the increase in sulphur-containing amino acids around the time of hair formation; hair has a high content of sulphur containing proteins. Since milk is the only source of nutrients and micronutrients, e.g. vitamins, it is presumed that their levels in the milk are adjusted to meet the differing requirements of the young as it develops. The mechanisms for these adjustments is unknown. One specific function of the milk appears to be the transfer of immunity to the newborn. At birth the young has little immunity, yet the pouch is not sterile. Around birth the mammary glands secrete a clear fluid that has free-floating cells and maternal immunoglobulins; this is similar to colostrum of placentals. Thus immunological protection provided in this early milk.
The changing characteristics of milk with the progression of pouch life brings us to another remarkable feature of kangaroo physiology: the simultaneous production of two milks of completely different characteristics at one time. This occurs when a birth takes place before the young at foot has been weaned. The obvious example of this is in the continuous breeding of red kangaroos, euros and wallaroos. It is also seen in the seasonal breeding of the two grey kangaroos, due to their relatively delayed weaning of the young at foot. In such conditions milk has to be provided for a newborn weighing about one gram and for a young at foot weighing more than four kilograms.
If the pattern of milk volume and composition is considered, the functional problem is readily appreciated. For one thing, it is obvious that the two active teats are required to act independently. How this uniquely macropodoid feature occurs is not well understood. It has been suggested that the milk let-down hormone, oxytocin, is involved via different sucking stimuli. The small, permanent attached young presumably exerts only a slight sucking pressure compared to the much larger sibling, the young at foot. Yet the overall process of lactation is very complex and so it would be expected that this fine control would also be complex.
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