About Kypes - Kype.net

What is a Kype?

A kype is a hook-like secondary sex characteristic that develops at the distal tip of the lower jaw in some male salmonids prior to the spawning season. This distinctive feature only appears in males and is absent in females, making it a clear example of sexual dimorphism in fish.

The structure usually develops in the weeks prior to, and during, migration to the spawning grounds. As the kype forms, a large depression simultaneously develops in the two halves of the premaxilla in the upper jaw, allowing the kype to fit when the mouth is closed (though often the kype is so large that the mouth cannot fully close).

Kypes are most commonly observed in various species of salmon and trout, particularly during their breeding season. The size and shape of the kype can vary significantly between species and even between individuals of the same species.

Kype Formation

The kype forms through a remarkable biological process. It grows rapidly as bony needles proliferate from the tip of the dentary (the anterior and largest of the bones making up the lower jaw). These needles form a mesh-like structure but do not interfere with the connective tissues used by bone marrow.

This skeletal transformation is triggered by hormonal changes as the fish prepares for spawning. The growth of the kype coincides with other physical changes that occur during this period, including changes in body coloration and, in some species, the development of a hump on the back.

Interestingly, in some species that spawn multiple times (iteroparous species), the kype is partially resorbed after the spawning season ends. However, the basal parts of the kype skeleton are often remodeled into regular dentary bone, potentially leading to larger kypes in subsequent spawning seasons.

Key Features of Kypes

Understanding the structure and function of these remarkable adaptations

Definition & Structure

A kype is a hook-like secondary sex characteristic that develops at the distal tip of the lower jaw in some male salmonids prior to the spawning season. The structure usually forms in the weeks before migration to spawning grounds, creating a distinctive curved hook.

Formation Process

The kype grows rapidly from bony needles proliferating from the tip of the dentary (the anterior bone in the lower jaw). These needles form a mesh but don't interfere with connective tissues used by bone marrow. The growth is triggered by hormonal changes preparing the fish for reproduction.

Evolutionary Function

The kype functions as a secondary sexual characteristic and influences the formation of dominance hierarchies at spawning grounds. Males with larger kypes are typically dominant over those with smaller kypes. The feature may also play a role in female mate choice, with females potentially preferring males with well-developed kypes.