One difference between humans and other primates is that human females do not show external signs of their fertile period. A variety of explanations for this have been proposed. The purpose of this note is to suggest an explanation which, so far as I am aware, has not appeared in the literature.

Consider a primitive group of humans in which reproduction is primarily by short term mating. Males induce females to bear and rear their children by spot payments, exchanging food for sexual access. For expository simplicity, assume each non-pregnant female is fertile one day in thirty and copulation with a fertile female always results in conception.

There is some equilibrium price for sexual access; suppose it corresponds to a month's food. This raises a problem. Food is not storable for very long, money does not exist. How is a male to offer a female more than one or two day's food for sexual access?

One solution is some form of long term contract. The male feeds the female for a month, at the end of which she copulates with him. While this is a possible solution, it depends on mechanisms to prevent cheating on social rules sufficient to enforce the contract--to keep the female from accepting food for twenty-nine days and then refusing copulation, thus keeping her reproductive options free for a second payment from another male.

Concealed ovulation provides an alternative solution that does not depend on such mechanisms. Now the female can trade a one in thirty probability of conception for one day's food. More generally, concealed ovulation makes it possible for the female to sell sexual access in smaller units, hence makes possible a higher price in a situation where the price is constrained by the availability of suitable forms in which to pay it.