http://www.sciencemag.org/news/2018/07/got-milk-even-first-mammals-knew-how-suckle

These baby echidnas, like their platypus cousins, lick or slurp their milk from their mother’s skin.

PARIS—Mammals suck. The ability to suckle milk is a defining characteristic of the group, and it is no small feat of evolution. Nursing—as well as drinking through a straw—requires complex anatomy to seal off the airway every time we suck and swallow.