Men notice some things when a woman ovulates

Ovulating Lap Dancers Make More Money
[Via Big Think]

I love the Bonobos, those crazy little apes. You know what I like best about them? Just like us, those little guys like to have sex all the time. I am not kidding; living in the primal horde is like having been 20 years old in the 1970s. Unlike other ape species, female bonobos engage in sex even when they are not fertile. They do it just for fun. They even like girl-ape on girl-ape action from time to time.

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Female strippers made almost $90 more a shift when they were ovulating than the week after and $170 more than when menstruating. And strippers on the pill, who do not ovulate, made almost $700 less a month than those not on the pill.

So there appears to be some sort of non-verbal cueing that makes men pay more money when they sense an ovulating woman. Wonder what it is?