Kype

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A kype is a hook-like growth that develops at the tip of the lower jaw in some male salmonids before spawning. It usually appears in the weeks leading up to migration to the spawning grounds. At the same time, a wide depression forms in the upper jaw so the kype fits into the upper jaw when the mouth is closed.

The kype is a secondary sexual characteristic. It helps establish dominance among males at the spawning grounds, and its size is thought to influence how often a male gets to spawn. It grows quickly from bony projections on the lower jaw, forming a mesh of bony needles that strengthens as it develops. Although the kype is growing, the rest of the jaw remains bone, with cartilage present in the kype tissue.

Different species show different patterns. Some salmon are semelparous (spawning once and then typically dying), while others are iteroparous (spawning multiple times). In some iteroparous species like Atlantic salmon, the kype isn’t fully reabsorbed after breeding; parts of the skeleton are remodeled, but in other fish the kype may keep growing across seasons and become a permanent feature.

Many male trout and salmon develop a kype before spawning. Females generally do not. In some species, the upper jaw becomes elongated into a “snout.” In Chinook salmon, two mating tactics exist: “hooknose” males that leave their rivers to mature and fight for mates, and “jacks” that stay in their home rivers and use sneaky fertilizations.

Kype development often goes hand in hand with other seasonal changes to the skull and body. Some scientists think it evolved through sexual selection to help males compete, while others have wondered if it serves little purpose or blocks other features. Today, the kype is viewed as a male trait used to establish dominance and potentially signal quality to rivals and mates. In some extinct relatives, enormous teeth on the upper jaw appeared without a kype, showing different mating strategies.