Nature’s strangest love story: Why banana slugs chew off their mates’ penises

The penis of a banana slug is, to put it mildly, impressive. Banana slugs typically grow to be 6 to 8 inches long, and remarkably, their erect penis can match this length. One species, Ariolimax dolichophallus, even carries a scientific name that literally translates to “long penis.” Adding to their anatomical curiosity, a banana slug’s penis emerges from its genital pore, which is located on its head.

Heath theorised that perhaps these penises had been “cast off” and that the stumpy remainders were early stages of regeneration. Determined to solve this mystery, he collected approximately 200 slugs for laboratory study. When laboratory observations proved insufficient, Heath ventured into the field to witness slug mating firsthand.

 

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The mating dance

The mating ritual of banana slugs begins with what Heath described as “vicious love nips.” The slugs then curl around each other to form what resembles a bright yellow yin-yang symbol. As both slugs possess male and female reproductive organs, they proceed to insert their penises into each other. According to Nature World News, “One slug provides the sperm, while the other slug is at the receiving end. The partner exchanges sperm as well.” This intimate coupling can last for many hours.

It’s what happens next that caught Heath’s attention. He observed that as the slugs were withdrawing their penises, “one of the animals turned its head and commenced to gnaw upon the walls of the organ.” This biting was described as “unusually vigorous,” and “within a very few minutes the penis was entirely severed.”

Scientists now refer to this phenomenon as “apophallation” — the practice of slugs chewing off one another’s penises. Studies have shown that when apophallation occurs, both slugs often end up penisless. And contrary to Heath’s initial belief, banana slug penises do not regenerate.

The evolutionary puzzle

Why would an animal engage in such seemingly self-destructive behaviour? In his 1916 paper, Heath speculated that the amputated penis might serve as a “plug” to retain sperm. He also suggested that the presence of observers or predators might prompt this behaviour as a means for the slugs to separate quickly.

Modern researchers have proposed alternative theories. One possibility, as Nature World News explains, is that “Much like wolves, trying to chew off their foot to escape a trap, slugs may also chew up their partner’s penis to get away.” However, Brooke L.W. Miller, who studied this behaviour as a graduate student, disputes this idea. She favours a different explanation: sperm competition. From an evolutionary perspective, organisms aim to produce as many offspring as possible. For hermaphrodites like banana slugs, apophallation may be a strategic move to outcompete rivals.

The evolutionary strategy isn’t foolproof, however. The Last Word On Nothing report points out that “the decision to chew another slug’s penis off isn’t as clearcut as it might seem. When one slug begins chomping, the other typically responds in kind. And then they both lose in this evolutionary penis-chewing arms race.”

Despite extensive research since Heath’s initial observations more than a century ago, the exact reason for this remarkable behaviour remains something of a mystery. What’s certain is that the sex lives of banana slugs offer a fascinating window into the sometimes bizarre adaptations that evolution can produce.