Though much has been said and thought about the female breast, there remains one aspect that has received little consideration: the actual number that a woman has, with two accepted as the immutable standard.

In fact, humans, like other mammals, can vary in their numbers of nipples and more rarely breasts, raising this little-pondered question: Why do humans, or for that matter any mammals, have the number of mammaries that they do?

The few people who have considered this question, including Aristotle and Alexander Graham Bell, have discovered that mammals follow what is now known as the ''one-half rule.'' According to the rule, the average number of young in a litter is one half the typical number of mammaries and the maximum litter size is equal to the total number of mammaries. (Humans conform perfectly to the rule with two breasts and a typical litter size of one-half that, and a standard maximum litter size, without fertility drugs, of two.)

It has remained largely a mystery, however, why mammals so slavishly follow this rule.

But now, a new study on naked mole rats, a curious breed of mammal that breaks the one-half rule, suggests that the reason for having what seem to be so many extra mammaries may be to keep sharing and fighting to a minimum in the largest litters.