A kype is a hook-like

secondary sex characteristic A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during pubert ...

which develops at the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

tip of the lower jaw in some male salmonids prior to the

spawning Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...

season. The structure usually develops in the weeks prior to, and during, migration to the spawning grounds. In addition to the development of the kype, a large depression forms in the two halves of the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

in the upper jaw, allowing the kype to fit into the premaxilla when the mouth is closed. The kype functions as a

secondary sexual characteristic A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during puberty ...

and influences the formation of

dominance hierarchies In the zoological field of ethology, a dominance hierarchy (formerly and colloquially called a pecking order) is a type of social hierarchy that arises when members of animal social groups interact, creating a ranking system. Different types of ...

at the spawning grounds. The size of the kype is believed to determine male spawning frequency.

Description

The kype grows rapidly from bony needles proliferating from the tip of the

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

(the anterior and largest of the bones making up the lower jaw). The needles form a mesh, but do not interfere with the connective tissues used by

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

. At the snout, the needles strengthen into

Sharpey's fibres Sharpey's fibres (bone fibres, or perforating fibres) are a matrix of connective tissue consisting of bundles of strong predominantly type I collagen fibres connecting periosteum to bone. They are part of the outer fibrous layer of periosteum ...

. The speed at which the kype skeleton develops results in many

osteoblasts Osteoblasts (from the Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts functi ...

and

proteoglycan Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to w ...

s appearing along the growth zone. The dentary itself is made of

compact bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...

, but the kype tissue contains

chondrocyte Chondrocytes (, ) are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although the word '' chondroblast'' is commonly used to describe an immatu ...

s and

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

. The kype formation process has been described as "making bone as fast as possible and with as little material as possible". Some species of salmon are

semelparous Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered ''semelparous'' if it is characterized by a single reproduction, reproductive episode before death, and ''iteroparous ...

(they have a single reproductive bout before death) whereas others are

iteroparous Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered ''semelparous'' if it is characterized by a single reproduction, reproductive episode before death, and ''iteroparous ...

(they spawn multiple times after maturation). In iteroparous cases, at least in Atlantic salmon, the kype is not fully resorbed after the breeding season, although basal parts of the kype skeleton are re-modelled into regular dentary bone. Some fish never lose their kype. Rather, as they re-enter subsequent spawning seasons, their kypes continue to grow. This fast growing skeletal tissue fuses with the dense dentary, becoming a permanent, growing kype.

Occurrence

Many male

trout Trout (: trout) is a generic common name for numerous species of carnivorous freshwater ray-finned fishes belonging to the genera '' Oncorhynchus'', ''Salmo'' and ''Salvelinus'', all of which are members of the subfamily Salmoninae in the ...

(e.g.

Brown trout The brown trout (''Salmo trutta'') is a species of salmonid ray-finned fish and the most widely distributed species of the genus ''Salmo'', endemic to most of Europe, West Asia and parts of North Africa, and has been widely introduced globally ...

(''Salmo trutta'') and

rainbow trout The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributary, tributaries of the Pacific Ocean in North America and Asia. The steelhead (sometimes called steelhead trout) is an Fish migration#Classification, ...

(''Oncorhynchus mykiss'')) and

salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...

develop a kype prior to spawning periods. In pre-spawning ''

Salmo ''Salmo'' is a genus of ray-finned fish from the subfamily Salmoninae of family (biology), family Salmonidae, and is part of the tribe (biology), tribe Salmonini along with the sister genera ''Salvelinus'' and ''Salvethymus''. Almost all ''Salmo ...

'' and ''

Salvelinus ''Salvelinus'' is a genus of Salmonidae, salmonid fish often called char or charr; some species are called "trout". ''Salvelinus'' is a member of the subfamily Salmoninae within the family Salmonidae. The genus has a northern circumpolar distrib ...

'' males, the lower jaw elongates and the hook develops; female salmon do not develop a kype.

Bull trout The bull trout (''Salvelinus confluentus'') is a char of the family Salmonidae native to northwestern North America. Historically, ''S. confluentus'' has been known as the " Dolly Varden" (''S. malma''), but was reclassified as a separate speci ...

(''Salvelinus confluentus'') are adfluvial (adults spawn in streams but subadults and adults migrate to lakes for feeding) and sometimes develop a kype, however, although this may occur in some populations, it remains absent in others. Among American species of charr, the kype reaches its maximum size in the large

anadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...

males, Dolly Varden trout (''Salvelinus malma'') and

brook trout The brook trout (''Salvelinus fontinalis'') is a species of freshwater fish in the char genus ''Salvelinus'' of the salmon family Salmonidae native to Eastern North America in the United States and Canada. Two ecological forms of brook trout h ...

(''Salvelinus fontinalis''), whereas it is reportedly absent or hardly visible in large nonanadromous males,

Arctic char The Arctic char or Arctic charr (''Salvelinus alpinus'') is a cold-water fish in the family Salmonidae, native to alpine lakes, as well as Arctic and subarctic coastal waters in the Holarctic realm, Holarctic. Distribution and habitat It Spaw ...

(''Salvelinus alpinus'') and

lake trout The lake trout (''Salvelinus namaycush'') is a freshwater Salvelinus, char living mainly in lakes in Northern North America. Other names for it include mackinaw, namaycush, lake char (or charr), touladi, togue, laker, and grey trout. In Lake Sup ...

(''Salvelinus namaycush'').

Similar structural changes

In salmonids of the genus ''

Oncorhynchus ''Oncorhynchus'', from Ancient Greek ὄγκος (''ónkos''), meaning "bend", and ῥύγχος (''rhúnkhos''), meaning "snout", is a genus of ray-finned fish in the subfamily Salmoninae of the family (biology), family Salmonidae, native to col ...

'' (meaning "hooked snout"), the upper jaw becomes more elongated than the lower thereby forming a "snout". In some species, the development of the "kype" (in this study defined as the distance from the middle of eye to the tip of the snout) is used as an indicator of a difference in behavioural mating strategies.

Chinook salmon The Chinook salmon (''Oncorhynchus tshawytscha'') is the largest and most valuable species of Oncorhynchus, Pacific salmon. Its common name is derived from the Chinookan peoples. Other vernacular names for the species include king salmon, quinn ...

(''Oncorhynchus tshawytscha'') express one of two fixed alternative reproductive tactics. Individuals expressing these are referred to as "hooknose" or "jack". Hooknose males leave their natal rivers at the end of their first year of life, but then return after maturing for 3 to 5 years on average. Once returned, they fight for position in a dominance hierarchy to gain closer access to spawning females. Alternatively, jacks are presumably resident in their natal rivers their entire lives, reach sexual maturity precociously (after 2 years), and use a sneaking tactic, by darting from nearby refuges to steal fertilisations from hooknose males.

Associated seasonal changes

Development of the kype often occurs in association with other seasonal changes. In the

Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Hucho taimen, Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlan ...

(''Salmo salar''), kype development is accompanied by a morphogenesis of bones and cartilages in the ethmoidal zone (the anterior region of the skull) changing the appearance of both jaws, the appearance of "breeding teeth" and resorption of scales (more so in males than females). Some salmonids may develop a predominant hump under their

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

Function

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

considered the kype to be a product of sexual selection and as a tool for fighting among males. For example, male salmon have been seen in the wild using their kype to firmly grasp an opponent's tail.Groot C and Margolis L (1991
''Pacific salmon life histories''
UBC Press. p. 144. . Others have suggested it has no function, and observed the kype seems to prevent the use of the breeding teeth which sometimes develop alongside the kype. One suggestion was that the kype is merely the result of a surplus of chemicals, not used for the production of sex products. Today, the kype is regarded as a secondary sex characteristic displayed by males at the spawning grounds. Therefore, its function is considered to be helping the fish establish a hierarchy among other males where those with larger kypes are dominant over animals with smaller kypes, and/or characteristics that could be of importance in inter- and intra-sexual evaluations of individual quality. The size of the kype is believed to determine male spawning frequency.

In extinct salmon

The extinct sabertooth salmon, '' Oncorhynchus rastrosus'', was first named for its prominent premaxillary dentition. It possessed an enormous conical tooth on each premaxilla. There is no visible kype on the dentary, implying a different strategy for forming mate dominance.

Gallery

File:PSM V04 D040 Head of a kelt.jpg File:Annual report of the Conservation Commission (1912) (19175729410).jpg

References

{{reflist, 30em Ethology Fish anatomy