First off, wet noses:
Dogs have very few sweat glands. None of these are found in the nose. To keep their noses from chapping, dogs will continually lick their noses. Breeds that have flat noses that can't be reached by the tongue have trouble with dry, cracked noses.
You may have seen a cat lick its nose, but cat tongues don't have enough saliva on them to make their noses wet.
Next, black noses:
Like humans, dogs have melanin in their skin to protect them from sunburns. Since the nose is out in the open, most dog noses have a lot of melanin, giving it a dark color. A few breeds have pink noses, while others will turn pink with age. There's also a condition called "snow nose," in which the dog's nose will turn pink during the winter when the sun's intensity is lower, then black again for the summer. This seems to be strictly seasonal, regardless of actual sun exposure.
Posted 3520 day ago