all primates (except for humans) have a bone, or bakula, in the penis. why did humans lose the bone over time? dawkins speculates:
All that females need is a dependable tool for diagnosis. Doctors don’t use an erection test in routine health check-ups–they prefer to ask you to stick out your tongue. But erection failure is a known early warning of diabetes and certain neurological diseases. Far more commonly it results from psychological factors–depression, anxiety, stress, overwork, loss of confidence and all that. (In nature, one might imagine males low in the ‘pecking order’ being afflicted in this way. Some monkeys use the erect penis as a threat signal.) It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male’s health, and the robustness of his ability to cope with stress, from the tone and bearing of his penis. But a bone would get in the way! Anybody can grow a bone in the penis; you don’t have to be particularly healthy or tough. So selection pressures from females forced males to lose the os penis, because then only genuinely healthy or strong males could present a really stiff erection and the females could make an unobstructed diagnosis.
There is a possible zone of contention here [some might say, “a bone to pick”]. How, it might be said, were the females who imposed the selection supposed to know whether the stiffness that they felt was bone or hydraulic pressure? After all, we began with the observation that a human erection can feel like bone. But I doubt if the females were really that easily fooled. They too were under selection, in their case not to lose bone but but to gain judgement. And don’t forget, the female is exposed to the very same penis when it is not erect, and the contrast is extremely striking. Bones cannot detumesce (though admittedly they can be retracted). Perhaps it is the impressive double life of the penis that guarantees the authenticity of the hydraulic advertisement.
–Richard Dawkins, The Selfish Gene, pp 307-308