Barnacles are sessile (stuck in place) so they can't move to find a mate. Evolution's solution: grow the longest penis-to-body ratio in the animal kingdom. A barnacle the size of your thumbnail can have a penis several inches long that reaches out blindly through the water to find a neighbor. Scaled to human proportions, that's roughly 300 feet — longer than a football field.
Even more remarkable: barnacles can adjust their penis shape based on water conditions. In rough waters, they grow shorter, thicker penises to resist current. In calm waters, they grow longer, thinner ones for maximum reach. Adaptive penis engineering.
The Argentine lake duck holds the record for longest penis relative to body size among vertebrates. Its penis is corkscrew-shaped, can be as long as its entire body, and deploys from a coiled resting state in less than half a second. The female reproductive tract corkscrews in the opposite direction — an evolutionary arms race between the sexes that's been going on for millions of years.
The blue whale has the largest penis of any living animal — roughly 8-10 feet long and about 1 foot in diameter. But relative to its 80-100 foot body length, that's actually only about 10% of body length — proportionally smaller than many much smaller animals. Size anxiety is relative, even in the ocean.
The echidna (a spiny egg-laying mammal from Australia, because of course it's from Australia) has a four-headed penis. During mating, it uses two heads at a time while the other two shut down, alternating between pairs. Scientists believe this evolved to match the female echidna's two-branched reproductive tract. Evolution doesn't optimize for our comfort level.
Flatworms are hermaphrodites — every individual has both male and female reproductive organs. When two flatworms meet, they engage in "penis fencing" — literally stabbing at each other with their penises. The loser (whoever gets pierced first) becomes the mother and has to carry the eggs. Reproduction as combat.
Most mammals — dogs, bears, walruses, primates (except humans) — have a baculum: an actual bone inside the penis. A walrus baculum can be 2 feet long. Humans are among the few mammals that rely entirely on blood pressure (hydraulics) for erections, with no skeletal support. This is why cardiovascular health directly affects erectile function in humans but not in most other mammals.
This is where it gets interesting for human ego:
Why Humans Are Biggest: Evolutionary biologists believe human penis size evolved through a combination of sperm competition (early human mating was likely semi-promiscuous, like bonobos), female mate choice (visible genital size may have been a selection factor in bipedal species where genitals are prominently displayed), and possibly as a "sperm displacement" mechanism during multi-partner scenarios. Whatever the reason, evolution gave humans the biggest primate penis by a wide margin.
The animal kingdom teaches three lessons about penis size:
Next time you're worried about your 5 inches, remember: a 400-pound gorilla is working with 1.5. And he's doing fine.
You've already beaten every other primate. Now see where you fall in the human data.
Get Your PercentileNeufeld CJ, Palmer AR. (2008). "Precisely Proportioned: Intertidal Barnacles Alter Penis Form to Suit Coastal Wave Action." Proceedings of the Royal Society B, 275(1638):1081-1087.
McCracken KG, et al. (2001). "Are Ducks Impressed by Drakes' Display?" Nature, 413:128.
Dixson AF. (2009). Sexual Selection and the Origins of Human Mating Systems. Oxford University Press.
Veale D, et al. (2015). "Am I Normal?" BJU International, 115(6):978-986.