Why humans lost their penis bone
Study traces evolutionary history of the baculum.
By David Shultz.
This study has been the first to demonstrate that baculum presence has correlated with intromission duration over the course of primate evolution.
https://www.science.org/content/article/why-humans-lost-their-penis-bone
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.
Postcopulatory sexual selection influences baculum evolution in primates and carnivores
Abstract
The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection (particularly, high levels of postcopulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ancestral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intromission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.
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Baculum length could not be predicted from testes mass in either primates (n = 46, p = 0.139, R2 = 0.03) or carnivores (n= 32, p= 0.231, R2= 0.37) (electronic supplementary material, table S2).
We found positive evidence for correlated evolution between baculum presence and intromission duration in primates (n = 299, log BF = 4.78; table 2). Ancestral state reconstructions and model rates indicate that baculum presence and short intromission durations (mean probability = 0.73) preceded a shift to prolonged intromission (figure 2). After long intromission durations had evolved, the baculum was rarely lost. However, the baculum was often lost if intromission duration remained short. Long intromission durations rarely became short again when a baculum was present. By contrast, when a baculum was absent, intromission duration switched frequently between being long and short.
The hypothesis that postcopulatory sexual selection influences baculum length was tested through a series of phylogenetic t-tests (table 3). In line with our predictions, we find that species in both the primate and carnivore orders in which intromission is prolonged have significantly longer bacula than species in which intromission is short (n = 53, p = 0.000 and n = 41, p = 0.018, respectively). Primates in polygamous mating systems were found to have significantly longer bacula than those in other mating systems (n = 65, p = 0.032). Finally, seasonally breeding primates have significantly longer bacula than primates that do not breed seasonally (n = 63, p = 0.045).
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This study has been the first to demonstrate that baculum presence has correlated with intromission duration over the course of primate evolution. The result highlights the interplay between morphological and behavioural phenotypes over evolutionary time. The baculum physically supports and protects the male’s penis [12,14], and assists the transfer of semen towards a female’s cervix [12,15]. However, it also plays an important role in facilitating prolonged intromission, which itself may be a sexually selected behaviour, aimed at increasing reproductive success by delaying females from re-mating [12].
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Polygamous mating systems and limited breeding seasons create high levels of postcopulatory sexual competition. In this environment, prolonging intromission could delay a female from re-mating, thus increasing a male’s chance of successfully fertilizing her under competitive conditions. Ensuring that the urethra is unrestricted and there is as little distance as possible for sperm to travel is a way of increasing the amount of sperm transported to the cervical canal. The baculum serves as a supportive structure during prolonged intromission, both protecting the urethra and preventing it from being constricted