To the best of my knowledge, there is no animal which consumes milk outside
of infancy--except man.

To the best of my knowledge, there is also no animal which consumes the
milk of -other species- in any significant quantity--except man.

Moreover, it would be wrong to say that man regularly consumes milk,
because most humans do not.  It is only in the West (and some limited parts
of Africa) that people eat dairy on a regular basis.  In most of the world,
regular consumption of the milk of animals would be considered either
disgusting or simply a bizarre aberration of white Westerners.

Both the carbohydrates (especially lactose) and the proteins (such as
casein and lactoglubulin) are quite different in character than the
proteins or carbohydrates humans normally encounter in their other foods.

It's not clear to me when humans started the practice of herding animals in
order to drink their milk.  Perhaps some more knowledgeable member of the
list can share with us some thoughts on that?