Despite slang terms
that imply otherwise, the human penis contains no bones. The same cannot be
said for many of our closest evolutionary relatives: Chimpanzees and bonobos
both have penis bones (a macaque one is pictured), also known as bacula. To
find out why some primates have the feature whereas others don’t, researchers
traced the bone’s evolutionary history through time. The baculum first evolved
between 145 million and 95 million years ago, the team reports online today in
the Proceedings of the Royal Society B. That means it was present in the most
recent common ancestor of all primates and carnivores.
Why some descendants,
like humans, lost their bacula appears to be due to differences in mating practices:
In primates, the presence of a penis bone was most tightly correlated to
increased intromission duration, i.e. how long the penis penetrates the vagina
during intercourse. Longer intromission times often occur in species with
polygamous mating practices—where multiple males mate with multiple females—as
seen in bonobos and chimps, but not humans. This system creates an intense
competition for fertilization, and one way for males to reduce a female’s
access to additional mates is to spend more time having sex with her
themselves. The penis bone facilitates this by supporting the penis during sex
and keeping the urethra open.
http://www.sciencemag.org/news/2016/12/why-humans-lost-their-penis-bone
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