Paw Morphology

The arrangement of palmar pads on the forepaws is sexually dimorphic.[^1] Males have three large pads, with one pad at the base of each thumb, and another directly under the knuckles. Females have the same two lower pads, but the single upper pad is replaced by several smaller pads in order to make room for a lactation patch. The lactation patch appears as an undifferentiated patch of grayish-black skin, the same as the rest of the furless portion of the paw. When exposed to saliva, the patch begins oozing bluish-white milk.

Vulpithecine ink and milk evolved out of similar excretory structures, which is why they are both located on the forepaws, and why the milk is bluish. The milk has potent antimicrobial properties to account for the fact it's being excreted from a surface in constant contact with the ground. Lactation is not linked to the reproductive cycle, and may occur at any time after reaching maturity. The on demand nature of lactation evolved in order to make climbing easier. If lactation happened automatically it would make for slippery paws at inopportune times.

[^1]: (Doylist explanation: If dogs sweat through their paws, and monotremes sweat milk, then monotreme dogs should sweat milk through their paws. At least I think that's why I did this. I honestly can't remember lol.)