Penis bones are found in at least nine orders of mammals, including primates, carnivores, rodents, and bats. Called a baculum, this bone sits inside the tip of the penis above the urethra and develops from connective tissue rather than being part of the main skeleton. It varies wildly across species, from a tiny sliver in a mouse to a 75-centimeter (roughly 30-inch) club in a walrus.
Which Mammal Groups Have a Baculum
The baculum has been documented in nine mammalian orders: Primates, Carnivora (meat-eaters like dogs, bears, and seals), Rodentia (rats, squirrels, beavers), Chiroptera (bats), Afrosoricida (tenrecs and golden moles), Eulipotyphla (hedgehogs and shrews), Dermoptera (colugos, or “flying lemurs”), Soricomorpha (a broader group of shrews and moles), and Lagomorpha (the American pika, a small relative of rabbits). That last group was only recently confirmed, meaning the bone keeps turning up in lineages where scientists didn’t expect it.
Not every species within these groups has one. Dogs, cats, bears, raccoons, otters, seals, and walruses all have well-developed penis bones. Among primates, many monkeys and all great apes except humans possess one. Among rodents, squirrels and chipmunks are particularly well-represented, with at least 51 species in the squirrel family alone confirmed to have the bone. And among bats, some species have it while others don’t, with no clean dividing line.
The Walrus Holds the Record
The walrus has the longest baculum of any living mammal: up to 75 centimeters, about the length of a human thigh bone. Walrus penis bones, called “oosik” in Yupik, have long been collected as curiosities and carved into tools or decorative objects in Arctic Indigenous cultures. Among carnivores more broadly, bears and seals also have large, robust penis bones, while smaller species like weasels and minks have proportionally elaborate ones with hooks and curved tips.
Notable Animals That Lack One
Humans do not have a baculum, and neither did Neanderthals or any other known extinct human species. Whales, dolphins, marsupials (kangaroos, koalas), hyenas, and hoofed animals like horses, cows, and elephants all lack the bone entirely. These groups achieve erections through blood pressure alone, without any skeletal support.
The absence in humans is particularly interesting because our closest relatives, chimpanzees and gorillas, do have one (though it’s small, only about 6 to 8 millimeters in chimps). Evolutionary analysis suggests humans lost the baculum with 100% confidence in modeling studies. Some primates in South America have also lost it independently, and researchers have found that even species that lost the bone as adults still begin developing one in the embryo before it degenerates, suggesting the genetic blueprint remains in place.
Why the Bone Exists
The leading explanation is called the prolonged intromission hypothesis. In species where males face intense competition from other males, mating for a longer time can delay a female from mating again, improving the first male’s chances of fathering offspring. The baculum acts as a supportive rod, keeping the penis rigid during these extended mating sessions and protecting the urethra from collapse.
The evidence is strong. Species with longer mating durations have significantly longer penis bones, in both primates and carnivores. Primates that mate with multiple partners (polygamous species) and those that breed only during a specific season, both situations where male competition is fierce, have significantly longer bacula than species with less competitive mating systems. Seasonal breeders face especially intense pressure because all males are competing within a narrow window.
Shape matters too, not just length. Research on weasels, otters, and their relatives found that species with longer mating durations and larger testes (another marker of sperm competition) tend to have more complex baculum tips, featuring hooks, bends, and forked ends. These elaborate shapes likely increase friction or improve the fit during copulation, helping males stay coupled longer.
Shape Varies Enormously Across Species
The baculum is one of the most morphologically diverse bones in the entire mammalian skeleton. Between species, it ranges from a smooth, simple rod to a deeply grooved, hooked, or scooped structure. Features like urethral grooves running along the shaft, abrupt bends near the tip, and forked or bifurcated ends are common. Taxonomists have historically used baculum shape to distinguish closely related species of rodents and bats that look nearly identical on the outside.
This diversity is not random. It reflects millions of years of sexual selection pushing each species toward a shape that serves its particular mating strategy. In the weasel superfamily alone, researchers identified specific tip features (hooks, bends, cross-section shape) that statistically predicted mating duration and relative testes size. The bone isn’t gained or lost just once in evolutionary history either. It has been independently gained and lost multiple times across the mammal family tree, suggesting it evolves rapidly in response to species-specific reproductive pressures.
Females Have a Counterpart
Many female mammals have a corresponding bone in the clitoris called a baubellum (or os clitoridis). It tends to be smaller and less consistent than the baculum. Of 145 species studied that had a well-developed penis bone, 12 lacked any trace of a clitoral bone, and another 17 had it in some individuals but not others. The baubellum appears more evolutionarily unstable, meaning it’s gained and lost more easily than the baculum. Dogs, cats, bears, raccoons, seals, some squirrels, and several primate species are among those where a baubellum has been documented, though its function remains far less studied.