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Complex penis bones capable of removing a rival’s semen could be a way for males to ensure their paternity with a long-term mate.

The baculum bone, found in the penis of most male mammals, determines the shape of the penis tip itself and varies widely in form across species, says Charlotte Brassey at Manchester Metropolitan University, UK. Humans are one of a few primate species to have evolved this bone away, while many carnivores have well-developed bones with complex curves and hooks.

Brassey’s team used 3D X-ray imaging to compare the bacula of 82 carnivore species, including dogs, wolves, lions, bears, otters, walruses and minks, and concluded that penis bone shape might have a role in what they call “post-copulatory sexual competition”.

When females mate with more than one male during the same fertility cycle, the sperm of those males compete to fertilise the few available eggs. The design of the penis tip could displace sperm already present in the reproductive tract, clearing the way for the new male, though it isn’t possible to know without imaging the penis inside the female’s reproductive tract – something the team hopes to do in future.

The honey badger’s baculum even looks like an ice cream scoop, says Brassey. “It really seems to just be designed to scoop out other sperm and then cup the cervix.”

The team found that the most complex bacula in the study were those of animals generally considered to be socially monogamous, meaning they usually live as a pair for an extended period. Simpler structures – smoother and straighter, like rods – were more commonplace in multiple-mate species which don’t form long-term pairs, like some seals.

While it might seem counter-intuitive, many socially monogamous females actually mate outside of their “couple”, meaning the complex penis bone shapes may have evolved as a way for males to ensure their paternity, says Brassey.