Why do dogs nipples turn black?

Dogs' nipples can turn black due to several reasons, including hormonal changes, age, or skin conditions. As female dogs mature or go through heat cycles, their nipples may darken due to increased blood flow and hormonal fluctuations. In some cases, darkening can also be a sign of underlying health issues, such as infections or tumors, so it's important for pet owners to monitor any changes and consult a veterinarian if concerned.