Nipples serve a purpose on a female cat; she uses them to nurse her young. Male kittens don't produce milk, but they still have nipples. Most male mammals share the trait, which can make it hard to tell males and females apart when they're babies.
Feeling Them Out
Male kittens have tiny nipple buds that form two rows on their bellies. A male kitten has nipples in the same configuration as females: four parallel sets for a total of eight nipples. In kittens, the buds are so small you can't see them through their soft belly fur. You can feel them, though. Run your fingers along his tummy, and you'll touch the little, hard bumps.
How Nipples Work
Nipples are essential in the survival of mammals. Most mammal females nurse their young. A female's mammary glands produce milk, which the babies access through their mother's nipples. Males have a small amount of similar mammary tissue around the nipples, but it's non-functional. In rare occasions, this tissue in males can have the same problems females sometimes experience, such as breast cancer.
Male Mammal Nipples
Since male nipples on kittens and other mammals don't work, it seems logical the males wouldn't develop nipples at all. In a few cases, such as rats and horses, that's true -- the males of those species don't have nipples. But in other mammals, including people, males and females start out developing the same as fetuses. As male DNA kicks in, the male hormones such as testosterone stop the development of some traits -- like full sets of mammary glands -- and start the development of others, including reproductive organs. Because it doesn't hurt males to have nipples, most mammals haven't evolved away from them.
Because male and female kittens have nipples, it can be a bit complicated to determine the genders. Gender becomes more evident on mature cats, but kittens aren't fully developed. Your vet can help you if you have questions. Both genders have two openings on their rear ends. In females, the openings are close together, almost touching. In males, the openings are spaced farther apart with hair filling in between. It's often easier to tell when you look at two kittens side by side -- the space difference is more obvious when you can compare a female with a male. It's also easier as the kittens age. At around 6 weeks old, you can begin to see the small bump of the scrotum in the males as well.