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Kype
Kype
Kype
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The kype is the hook on the lower jaw which some salmonids develop before the breeding season.

A kype is a hook-like secondary sex characteristic which develops at the distal tip of the lower jaw in some male salmonids prior to the spawning season.[1][2] 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 in the upper jaw, allowing the kype to fit into the premaxilla when the mouth is closed.[3]

The kype functions as a secondary sexual characteristic and influences the formation of dominance hierarchies at the spawning grounds. The size of the kype is believed to determine male spawning frequency.

Description

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The kype grows rapidly from bony needles proliferating from the tip of the dentary (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. At the snout, the needles strengthen into Sharpey's fibres. The speed at which the kype skeleton develops results in many osteoblasts and proteoglycans appearing along the growth zone. The dentary itself is made of compact bone, but the kype tissue contains chondrocytes and cartilage.[3][4] The kype formation process has been described as "making bone as fast as possible and with as little material as possible".[4]

Some species of salmon are semelparous (they have a single reproductive bout before death) whereas others are 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.[3] 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.[5]

Occurrence

[edit]
A large male arctic char (Salvelinus alpinus) in spawning condition with a clearly visible kype

Many male trout (e.g. Brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss)) and salmon develop a kype prior to spawning periods.[5] In pre-spawning Salmo and Salvelinus males, the lower jaw elongates and the hook develops; female salmon do not develop a kype.[3] Bull trout (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.[6]

Among American species of charr, the kype reaches its maximum size in the large anadromous males, Dolly Varden trout (Salvelinus malma) and brook trout (Salvelinus fontinalis), whereas it is reportedly absent or hardly visible in large nonanadromous males, Arctic char (Salvelinus alpinus) and lake trout (Salvelinus namaycush).[7]

Similar structural changes

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In salmonids of the genus Oncorhynchus (meaning "hooked snout"), the upper jaw becomes more elongated than the lower thereby forming a "snout".[3] 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 (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.[8]

Associated seasonal changes

[edit]
Sockeye salmon (Oncorhynchus nerka)
Female (above) and male in mating condition. Note that the male has a kype, enlarged snout, humped back and deeper, more extensive colouration.

Development of the kype often occurs in association with other seasonal changes. In the Atlantic salmon (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).[9] Some salmonids may develop a predominant hump under their dorsal fin.

Function

[edit]

Charles Darwin 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.[10] 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.[3]

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,[2][3] 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.[11]

In extinct salmon

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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.[12]

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See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Kype

View on Grokipedia
from Grokipedia
A kype is a hook-like secondary sexual characteristic that develops on the distal tip of the lower jaw in mature male , such as and , during the spawning season. This structure, also known as a kype or hooked , emerges as part of the dramatic physiological changes males undergo upon returning to freshwater to , often accompanied by a reddening of the body and, in some species like pink and , the development of a pronounced dorsal hump. The kype's formation involves rapid skeletal remodeling, including the growth of specialized bone needles and Sharpey fiber bone at the dentary tip, driven by elevated levels of hormones like testosterone and growth factors. Primarily serving as a display for attracting females and a weapon in male-male to establish dominance over spawning territories, the kype enhances but imposes energetic costs, contributing to the post-spawning mortality common in semelparous species like Pacific . In iteroparous species, such as , the kype may partially resorb after spawning if the fish survives, though it can persist and affect feeding efficiency in subsequent seasons. Variation in kype size and development exists among individuals and populations, influenced by genetic, environmental, and nutritional factors, with studies indicating that larger kypes correlate with higher competitive ability during breeding.

Anatomy

Structure and Composition

The kype is defined as a hook-like elongation and curvature at the distal tip of the lower , specifically the dentary , in sexually mature salmonids. This forms a prominent secondary sexual characteristic that protrudes ventrally and anteriorly, altering the jaw's morphology for the breeding season. In terms of composition, the kype comprises a mass of , rich in fibers, that is structurally supported by a framework of skeletal needles. These needles consist of chondroid —a cartilage-like mineralized matrix with embedded osteocytes—and Sharpey-fiber reinforced , where bundles anchor the tissue to the underlying dentary's compact elements formed by osteons. Histologically, the kype surface includes odontode-like structures, represented by breeding teeth that partially embed into the skeletal tissues, contributing to its textured, tooth-bearing appearance. The tissue is highly vascularized, with blood vessels traversing the bundles and spaces to support metabolic demands. Following spawning, resorption mechanisms involve osteoclast-mediated breakdown, particularly of the apical needles, allowing partial remodeling into the original dentary bone. Variations in kype size and shape occur among individuals, with larger hooks generally corresponding to greater overall body size; for instance, kype exhibits a strong positive with fork length, ranging from approximately 7–13 mm in mature males. These differences influence the structure's prominence but maintain the core histological features across specimens.

Development Process

The development of the kype begins with the elongation and dorsal curvature of the dentary bone in the lower of salmonids during upstream migration, forming the foundational structure for the hook-like . This initial phase is followed by the deposition of chondroid tissue, where skeletal needles composed of chondrocytes embedded in a matrix emerge, creating a spongiosa-like meshwork that provides structural support proximally. then proceeds through a combination of endochondral and intramembranous processes: endochondral involves the mineralization of near chondrocytes, while intramembranous occurs rapidly via the activity of osteoblasts that deposit distally, resulting in chondroid bone, and ventrally form Sharpey fiber bone for added stability. At the cellular level, proliferation of osteoblasts and fibroblasts drives the rapid growth of these tissues, with osteoblasts secreting and some differentiating into osteocytes or even chondrocytes to support the hybrid bone-cartilage structure. Fibroblasts contribute to the dense collagenous that anchors the forming kype, enabling its extension despite mechanical stresses. Following spawning, the kype undergoes resorption primarily through osteoclastic activity, where enzymes facilitate the breakdown of the apical skeletal needles and demineralization of the structure, reducing its length by approximately 2.8 mm and height by 1.6 mm in . This enzymatic degradation remodels the kype into compact dentary bone via the formation of osteons in the basal regions, gradually reversing the jaw morphology toward its pre-breeding state, though complete restoration may vary by individual and species. The re-initiation of kype bone growth imposes significant costs, occurring amid nutritional stress from prolonged during upstream migration, where salmonids rely on endogenous reserves and may remobilize from post-cranial or scales to fuel with minimal material for maximal structural efficiency. Hormonal triggers, such as elevated androgens, initiate these processes but are modulated by the fish's physiological state. Post-spawning resorption, in turn, recycles and minerals, providing a temporary source during severe exceeding 40% in kelts.

Occurrence

In Modern Salmonids

The kype primarily develops in male salmonids of the genera (Pacific salmon, including species such as chinook Oncorhynchus tshawytscha and coho O. kisutch), (Atlantic salmon S. salar), and (such as brook trout Salvelinus fontinalis) during the spawning season, serving as a secondary sexual characteristic at the distal tip of the lower jaw. This structure is characteristic of the family and emerges as males approach maturity, particularly in preparation for reproductive behaviors. In these species, kype development is most pronounced in anadromous males, where the lower elongates and curves to form a prominent hook, often correlating with body size and hormonal levels such as 11-ketotestosterone. For example, in Salmo salar, kype height peaks in two-sea-winter males and can be substantially reduced or absent in females, while non-anadromous resident forms exhibit diminished kype size due to differences in life history and migration. Similarly, in Oncorhynchus species, anadromous males display large kypes, with variations in prominence across species like the more elongated forms in chinook compared to coho. Population variations highlight differences between wild and domesticated strains, with wild Salmo salar exhibiting larger adjusted kype heights (e.g., means of 0.100–0.115 in Norwegian river strains) than domesticated ones (mean of -0.001), likely due to relaxed in captivity. Geographic and genetic factors also contribute to kype morphology, with phenotypic variation observed among populations. The kype's occurrence is largely restricted to the Salmonidae family, primarily within , , and , with no verified reports in related genera such as Coregonus (whitefish).

Similar Structures

In other fish taxa, analogous secondary sexual traits include nuptial tubercles observed in cyprinids such as minnows and chubs, where males develop these keratinized epidermal structures on the head, body, and fins during the breeding season to facilitate contact with females or rivals. Similarly, breeding tubercles in catostomids, or suckers like the (Catostomus commersonii), consist of tough, fleshy, keratinized knobs on the head, fins, and scales that appear seasonally in males and sometimes females, aiding in spawning interactions. These structures, while serving comparable roles in mate attraction and , differ markedly from the kype in composition, being epidermal and keratinous rather than supported by rapidly growing ossified bone. The kype's distinctive hook-shaped morphology, formed by elongation and curvature of the , provides mechanical leverage for jaw-locking during agonistic encounters, a feature not replicated in softer analogs like the contact organs in breeding male three-spined sticklebacks (Gasterosteus aculeatus), which are epidermal thickenings on fins and body used for tactile stimulation but lack bony reinforcement. In cichlids, such as those in , often manifests as elongated or protrusible oral jaws in males, adapted primarily for feeding but secondarily influencing display and combat, though these are permanent skeletal modifications rather than seasonally reversible. Within teleosts more broadly, secondary sexual traits encompass diverse modifications like opercular flaps or extensions in species such as longear sunfish ( megalotis), which function as visual ornaments during , but the kype stands out for its rapid, hormonally driven bony development and post-spawning resorption, a level of reversibility uncommon among structural traits in other fish. For non-fish parallels, the kype loosely evokes antler-like appendages in male cervids, which are seasonally regenerated bony structures employed in displays and intrasexual combat, highlighting convergent evolutionary pressures on temporary weaponry despite fundamental differences in vertebrate classes.

Physiological Changes

Seasonal Development

The development of the kype in male salmonids aligns closely with the reproductive cycle, beginning in late summer or early autumn during the upstream migration to natal spawning grounds. In (Salmo salar), kype formation initiates as males enter freshwater, typically around October, and progresses over 2–3 months to reach its peak size and prominence just prior to or during spawning in late autumn to early winter. This timeline ensures the structure is fully formed when males establish spawning territories, enhancing their competitive interactions. Environmental cues primarily trigger kype development, with shortening photoperiods acting as a key seasonal signal to synchronize maturation with the reproductive period. Decreasing water temperatures during migration further promote these changes by influencing metabolic rates and allocation toward secondary sexual traits. The physiological stress from the arduous upstream migration, including prolonged and high energetic costs, also accelerates the onset of final maturation stages, including kype growth. Kype formation integrates into the salmonid life cycle across spawning strategies, occurring in first-time spawners such as grilse (after one sea winter in ) as well as in larger, multi-sea-winter males. In iteroparous species like , the kype remodels post-spawning in survivors to support potential future reproduction. Conversely, in semelparous Pacific salmon ( spp.), the kype achieves full expression at spawning but regresses amid rapid senescence, culminating in death shortly after egg deposition. Observational field studies in wild populations demonstrate that kype size positively correlates with spawning readiness, as males with larger kypes exhibit greater dominance in agonistic encounters and higher mating success on spawning grounds. For instance, in , kype height relative to body size predicts hierarchical rank among competing males, signaling peak physiological preparedness for .

Hormonal Regulation

The development of the kype in male salmonids is primarily regulated by the hypothalamic-pituitary-gonadal (HPG) axis, which activates during sexual maturation to drive secondary . (GnRH) from the stimulates the to release (FSH) and (LH), which in turn promote gonadal production of androgens such as testosterone and 11-ketotestosterone (11-KT). These hormones elevate significantly during the upstream migration and spawning period, correlating with kype . Elevated levels of testosterone and 11-KT are the primary drivers of kype formation, acting directly on osteoblast-like cells to stimulate deposition and elongation. Serum 11-KT concentrations show a strong positive with lower length in maturing males, reaching peaks that coincide with rapid kype growth. These androgens bind to β (ARβ) expressed in kype tissues, including chondrocytes in the upper jaw and osteoblasts in the lower jaw's spongiosa-like , promoting cellular proliferation and production for the hook's curvature. (GH) and insulin-like growth factor-1 (IGF-1) provide supportive roles in tissue deposition, facilitating overall skeletal remodeling despite declining GH levels during ; however, androgens override GH to sustain kype-specific growth. The HPG axis activation is synchronized with environmental cues like photoperiod and temperature changes in late summer to autumn, ensuring kype development aligns with spawning readiness. Post-spawning, loops involving reduced release and declining levels inhibit further growth, leading to kype resorption through osteoclast-mediated remodeling as prioritize survival over reproduction. Experimental studies confirm the endocrine control of kype . Intraperitoneal injections of 11-KT in maturing male ( nerka) induced precocious hooknose formation, demonstrating direct androgenic effects independent of full gonadal maturation. In ( gorbuscha), serum 11-KT supplementation in juveniles enhanced jaw tissue elongation, with ARβ expression upregulated in response, supporting accelerated development. experiments in maturing (Salmo salar) reduced secondary sexual trait expression, including diminished kype size, underscoring the necessity of gonadal androgens via the HPG axis.

Function

In Mating and Competition

In male salmonids, the kype serves as a key structure in intra-sexual , functioning as a during aggressive encounters with rival males. During spawning, males engage in physical combats to defend territories and access to females, where the elongated, hooked lower acts like a for fighting rivals. This behavior establishes hierarchies, with dominant males using the kype to deter subordinates, thereby securing priority in opportunities. The kype also plays a role in inter-sexual selection as a visual cue during displays. Larger kypes signal , genetic quality, and competitive ability to females, influencing and increasing the likelihood of acceptance during spawning approaches. In (Salmo salar), larger kypes correlate with dominance and higher mating success, potentially signaling status to females. Observational studies in wild populations confirm that kype size correlates with higher success. Studies indicate that larger body size and secondary sexual traits like the kype correlate with higher success in male through competitive advantages.

Adaptive Significance

The kype confers significant adaptive benefits to male salmonids by enhancing through male-male and female mate choice. In species such as (Salmo salar), larger kype height is positively correlated with dominance rank in hierarchies, enabling dominant males to approach ripe females more frequently, secure more s, and effectively guard nests or females against rivals. The kype facilitates physical interactions that determine access to breeding opportunities. However, kype development entails notable costs, primarily energetic, as resources are allocated to rapid skeletal remodeling during upstream migration when cease feeding. Empirical studies in S. salar demonstrate that fork length-adjusted kype height is significantly reduced in domesticated strains compared to wild ones (e.g., mean adjusted height of -0.001 in domesticated vs. 0.100–0.115 in wild strains), reflecting a where relaxed in captivity diminishes investment in this trait. In natural populations, this reduction could lower spawning success if escaped farmed males interbreed with wild . Overall, the net fitness impact of the kype balances these benefits and costs, with selection favoring larger sizes in competitive streams to maximize lifetime reproductive output, as evidenced by correlations between kype dimensions and mating frequency in S. salar. Quantitative genetic analyses further reveal heritable components, with quantitative trait loci explaining 5–7% of variance in kype size, underscoring its role as a target of . Similar functions are observed in other salmonids, such as Pacific species, where the kype aids in and display, though specific variations may exist due to life history differences.

Evolutionary History

In Extinct Salmonids

Paleontological evidence for kype-like structures in extinct salmonids is sparse, primarily due to the seasonal and soft-tissue nature of the kype, which rarely preserves in the fossil record. Direct fossilization of the cartilaginous and components is uncommon, but ossified elements, such as modified dentaries, can be inferred from imprints, variations, and in skeletal remains. These challenges limit insights into prehistoric secondary sexual characteristics, with most evidence derived from well-preserved cranial s in sedimentary deposits. Fossil records indicate the presence of kype-like jaw modifications in early salmonids dating back to the Eocene. By the , more pronounced structures appear in Salminae, such as the exaggerated dentaries in , a giant Pacific salmon from the Miocene-Pliocene of the . This species displays in the lower jaw, with male dentaries showing irregular bone density mesial to the tooth row, interpreted as evidence of kype development for spawning competition. In contrast, pre-Cretaceous fossils lack any such structures, aligning with the absence of salmonids before this period. These findings imply that kype-like traits evolved after the , coinciding with the diversification of salmonids in the and the adoption of anadromous lifestyles in the , which heightened male-male competition during riverine spawning.

Origins and Evolution

The family is estimated to have diverged from other salmoniform fishes around 50–60 million years ago, during the period. This timeline aligns with the Salmonidae's colonization of freshwater environments in , marking a shift from marine or estuarine ancestors to riverine habitats that favored anadromous and potamodromous life histories. The kype likely evolved later within the family, with direct fossil evidence of kype-like structures appearing by the in species such as . The evolution of semelparity—reproducing once before death—further coincided with this period, intensifying reproductive investment and promoting traits like the kype as part of spawning adaptations. Selective pressures driving kype diversification included through male-male combat and display, where larger kypes enhanced competitive success during mate guarding. also favored the kype for nest defense in fast-flowing riverine spawning grounds, where the hook-like structure aided in holding position and deterring rivals amid egg burial behaviors. These pressures contributed to the trait's elaboration across Salmoninae and Thymallinae subfamilies, though variability emerged in response to and life-history differences. Genetically, the kype is a polygenic trait influenced by multiple quantitative trait loci (QTL), with notable associations on linkage groups SSA1 (explaining ~6.6% of variance) and SSA23 (explaining ~5.0% of variance adjusted for body ). in has led to reduced kype expression due to relaxed natural and , as artificial breeding removes competitive pressures, resulting in smaller hooks in farmed lines compared to wild populations. Phylogenetically, the kype is absent in basal teleosts outside Salmoniformes, representing a derived innovation within the family. It shows convergence with jaw ornaments in distantly related clades, such as elongated oral jaws in cichlids adapted for trophic niches. Potential losses occur in certain lineages, like reduced or absent kypes in some Coregoninae (whitefishes) and select chars, possibly due to shifts toward lacustrine habitats with less intense spawning competition.

References

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