All male primates orgasm, but not all female primates, such as gibbons.

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The Great Female Orgasm Debate: Adaptive Theory

To evolutionary biologists, the male orgasmic function makes perfect sense. In decidedly unromantic terms, it ensures the survival of the human species by offering a physiological motivation to pursue and inseminate sexual partners. The speed and consistency with which heterosexual men achieve orgasm via vaginal intercourse also supports its classification as a beneficial adaptation [source: Keim].

Not so for the female orgasm. Partly because of the previously discussed gender gap in orgasm frequency and disparities between clitoral and vaginally stimulated climaxes in women, evolutionary biologists have yet to conclusively explain why women also experience the Big O. But that doesn't mean they're lacking in theories, which collectively split into an academic tug-of-war between two major camps: adaptation and byproduct.

Adaptation proponents spin the female orgasm as an evolutionarily favorable trait like the male orgasm. To them, natural selection rooted for the pleasurable physiological response because it enhanced female's reproductive fitness. Orgasms not only sexually satisfy females, bonding them to a partner, but also provide a rubric for evaluating suitable mates by revealing who could excite them toward ecstasy [source: Keim].

Another contribution to the adaptation school of thought is the "sperm up-suck theory," which contends that uterine contractions along with oxytocin release during orgasm suction potent semen upward into the vaginal canal [source: Lloyd]. According to zoologist Robin Baker, who popularized the long-held theory with his 1996 book "Sperm Wars," orgasms lend women a helping hand by innately spotting -- or at least up-sucking -- the most virile men. If the up-suck theory, which was originally tested on female pigs, was true, that would endow female orgasms with a reproductive advantage by increasing a woman's fertility.

But Baker's thesis didn't go unchallenged, and a June 2011 reanalysis of the related scientific literature poked holes in the up-suck strategy for a number of reasons. For instance, Baker's research included only a single subject, and additional studies found no conclusive evidence of orgasm-induced sperm retention in sexually aroused women [source: Levin]. Hence, the great female orgasm debate continues.