These are caused by skin gland secretions and disappear in the first two to three weeks of life. (womenandinfants.org)
Stimulation is both palpable and chemical, through skin gland secretions. (eol.org)
The mammals are the class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species and specialized skin glands in monotremes that seep or ooze milk; the presence of hair or fur; specialized teeth; three minute bones within the ear; the presence of a neocortex region in the brain; and endothermic or «warm - blooded» bodies. (sciencedaily.com)