Mother kangaroos are able to rear multiple joeys (young kangaroos) at different stages of development at the same time. According to Wikipedia, "the mother is able to produce two different kinds of milk simultaneously for the newborn and the older joey still in the pouch." What biological mechanisms are able to ensure that the correct joey gets the correct kind of milk? Do different teats produce different types of milk and somehow the joeys know which teat to suck from; or is each teat able to deliver both types of milk and the teat is able to tell from the mouth conformation (or something else) which type of milk to deliver?
The younger Joey remains attached to the teat from which it receives its milk for the period of nine months until it leaves the pouch. So that would be teat from which milk suitable for the newborn is available (reference 1 and reference 2). The older Joeys also have an incredible sense of smell by which it can detect the teat from which it gets the milk it requires but it stands to reason that they cant get milk from the teat to which the younger joey is attached. (reference)