There’s not a sexual bone in any man’s body—but that’s not true of many mammals. The penis of many mammal species contains a bone called the baculum, and new research shows that it evolved at least nine different times in the history of mammals, The Washington Post reports. To figure out that number, researchers looked at more than 950 species across the mammal family tree for the presence of the bone. The way it pops up again and again indicates that having a baculum probably gives some sort of an advantage—but perplexingly, the groups in which the penis bone evolved don’t seem to have anything in common, according to the study in Integrative and Comparative Biology. So even with our new understanding of its history, the purpose of the penis bone remains unknown.
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