Mammary glands are basically modified sweat glands that develop from the germinal epithelium. Sounds sexy, no? While humans typically only develop one pair, dogs typically have 5 to 6 pair. During fetal development, when the basic genitalia and sex organs are forming, they are undifferentiated in males and females until a certain time period. Eventually, early sex hormones kick in and shape boys parts into something special but not without leaving behind a trace of that early sexless, what’s-it-gonna-be stage, i.e., nipples. The male nipples are just vestigial though, and without female hormones, they are non-functioning and lack the “secretory” ability to release or produce milk. In a spayed female, nipples are still present but are smaller in size. Intact females have more prominent mammary glands compared to spayed females. Obviously, the hormonal ones have got the biggest breasts, a fact which I’m sure the male of our own species are highly aware.
Material from It's a Dog's Life... but It's Your Carpet, available at Amazon.com. Copyright © 2008 Justine Lee Veterinary Consulting, LLC.