Your Penis Used to Have a Bone (Most Mammals Still Do)

The Baculum: A Real Bone in Real Penises

The baculum (plural: bacula) — also called the os penis or penis bone — is a bone found in the penis of many placental mammals. It aids erection mechanically: the bone provides rigidity, and blood flow provides the rest. In species with a baculum, erections are partially structural and partially hydraulic.

The walrus has the largest baculum of any mammal — up to 63 cm (about 25 inches). Bears, dogs, cats, bats, rodents, and most primates all have one too. Our close evolutionary cousins, chimpanzees and gorillas, have small bacula (about 10–18 mm). Bonobos have them. Orangutans have them.

Humans do not. Neither do spider monkeys, horses, elephants, or whales. But among the great apes, we're the odd ones out.

Why Humans Lost It

The prevailing theory connects baculum loss to mating strategy. Species that have prolonged intromission (extended mating duration) tend to have larger bacula — the bone helps maintain rigidity over long periods. Species with shorter mating durations and monogamous or pair-bonding tendencies tend to have smaller or no bacula.

When early human ancestors shifted toward pair bonding and reduced sperm competition (compared to, say, chimpanzees), the evolutionary pressure to maintain a baculum decreased. At the same time, sexual selection may have favored a fully hydraulic system — because a blood-based erection is actually a more honest signal of health.

The Honest Signal Theory

Here's the compelling part: a bone-based erection tells a potential mate nothing about the male's health. A sick animal with a baculum gets just as rigid as a healthy one. But a blood-based erection — which requires healthy cardiovascular function, proper hormone levels, intact neural pathways, and good overall circulation — is a real-time readout of biological fitness.

In other words: your erection is a health report card, delivered in real time, that cannot be faked. A partner selecting for a strong, reliable erection was selecting for overall health. Evolution kept the honest signal and ditched the cheat code.

The Remnant: The "Os Analog"

Interestingly, humans haven't completely lost all trace of the baculum. A structure called the os analog — a distal ligament that buttresses the glans penis — plays a supporting role in the penile fibroskeleton. Researchers describe it as a remnant of the ancestral baculum, repurposed into connective tissue. The ghost of a bone, doing a different job.

Fun Comparative Facts

• Walrus: Baculum up to 63 cm (25 inches). The largest of any mammal.
• Gorilla: Baculum only ~18 mm despite massive body size. Also relatively small penis. Low sperm competition (one dominant male mates with the group).
• Chimpanzee: Small baculum, but relatively large testicles — high sperm competition (multiple males mate with the same female).
• Humans: No baculum. Largest penis of any great ape relative to body size. Medium-sized testicles. Our anatomy suggests a mating strategy somewhere between gorilla and chimp.