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How some carnivorous mammals manage to keep their claws sharps.

Canada HyperNova Offline
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( This post was last modified: 05-02-2017, 09:19 AM by HyperNova )

Some carnivorous mammal manage to keep their claws sharps. That's especially true when we talk about cats in general. It seems to be a common assumption that cats manage to keep their claw sharp because of their retractability. Alought claw retractability might help keeping your claws sharps, that's probably not the primary reason why their claws stay sharp. Domestic cats, lions, tigers, bobcats and probably the vast majority of cats are equipped with a "claw-shedding mechanism". This mechanism is also present on other carnivorous mammal, the following explain how this mechanism work :

''The morphology of cornified structures is notoriously difficult to analyse because of the extreme range of hardness of their component tissues. Hence, a correlative approach using light microscopy, scanning electron microscopy, three-dimensional reconstructions based on x-ray computed tomography data, and graphic modeling was applied to study the morphology of the cornified claw sheath of the domesticated cat as a model for cornified digital end organs. The highly complex architecture of the cornified claw sheath is generated by the living epidermis that is supported by the dermis and distal phalanx. The latter is characterized by an ossified unguicular hood, which overhangs the bony articular base and unguicular process of the distal phalanx and creates an unguicular recess. The dermis covers the complex surface of the bony distal phalanx but also creates special structures, such as a dorsal dermal papilla that points distally and a curved ledge on the medial and lateral sides of the unguicular process. The hard-cornified external coronary horn and proximal cone horn form the root of the cornified claw sheath within the unguicular recess, which is deeper on the dorsal side than on the medial and lateral sides. As a consequence, their rate of horn production is greater dorsally, which contributes to the overall palmo-apical curvature of the cornified claw sheath. The external coronary and proximal cone horn is worn down through normal use as it is pushed apically. The hard-cornified apical cone horn is generated by the living epidermis enveloping the base and free part of the dorsal dermal papilla. It forms nested horn cones that eventually form the core of the hardened tip of the cornified claw. The sides of the cornified claw sheath are formed by the newly described hard-cornified blade horn, which originates from the living epidermis located on the slanted face of the curved ledge. As the blade horn is moved apically, it entrains and integrates the hard-cornified parietal horn on its internal side. It is covered by the external coronary and proximal cone horn on its external side. The soft-cornified terminal horn extends distally from the parietal horn and covers the dermal claw bed at the tip of the uniguicular process, thereby filling the space created by the converging apical cone and blade horn. The soft-cornified sole horn fills the space between the cutting edges of blade horn on the palmar side of the cornified claw sheath. The superficial soft-cornified perioplic horn is produced on the internal side of the unguicular pleat, which surrounds the root of the cornified claw sheath. The shedding of apical horn caps is made possible by the appearance of microcracks in the superficial layers of the external coronary and proximal cone horn in the course of deformations of the cornified claw sheath, which is subjected to tensile forces during climbing or prey catching. These microcracks propagate tangentially through the coronary horn and do not injure the underlying living epidermal and dermal tissues. This built-in shedding mechanism maintains sharp claw tips and ensures the freeing of the claws from the substrate.''

''That a superficial layer of the cornified claw sheath is shed when domesticated cats appear to sharpen their claws by scratching rough surfaces, such as tree bark or rough textiles, is well known among pet owners and veterinarians, and was mentioned earlier by De Weerdt (1927). Siedamgrotzky (1871), though, described a continuous abrasion as the mechanism for keeping claws sharp and seems to have been unaware that cats shed horn caps off their claws. Interestingly, in Dyce et al. (2002), a shed horn cap of a cornified claw sheath is shown as an example of an actual cat claw. However, the internal architecture of the cornified material that allows such a controlled shedding of its superficial layer without cracks propagating into the underlying living tissues has not been analysed and cannot be inferred from the currently available information on the morphology of claws. Not only domesticated cats but also large cats, such as lions, tigers and bobcats (staff at the Baton Rouge Zoo, personal communication), small dogs (Barbara Luikhart, Galvez Veterinary Clinic, Prairieville, Louisiana, personal communication), and even horses (K.-D. Budras, Institute of Veterinary Anatomy, Free University Berlin, unpublished observation of a Przewalski horse) can shed their claws or hooves. Hence, the phenomenon of shedding superficial parts of cornified digital end organs is of a more general interest and warrants a better understanding of the morphology of cornified end organs to explain the mechanisms that are involved.''

Scanning electron microscopy picture of shed horn caps off the tips of cornified claw sheaths of an adult female cat. (a) Proximal view (specimen SC no. 3). (b) Side view (specimen SC no. 5).

*This image is copyright of its original author

''The microcracks expose edges of horn layers to water loss. In combination with aging and continuing internal strain throughout the cornified claw sheath, these processes promote the formation of intercellular cracks that propagate from the peripheral microcracks through the horn preferentially along the interfaces between the cornified cell layers. In this manner, the separation of superficial layers can proceed tangentially to the interface between the living tissues and the cornified epidermis without creating perpendicular cracks that would damage the underlying living tissues. Hence, the ongoing simultaneous horn production and formation of microcracks result in superficial layers of coronary horn being separated from underlying layers of coronary horn through tangential cracks that propagate through the horn material starting from the superficial microcracks. As can be seen from the surface structure of shed horn caps, several microcracks may appear before a particular microcrack spreads between two layers of horn and separates a horn cap from the main part of the cornified claw sheath in preparation for the shedding of the horn cap. The actual removal of this horn cap is probably facilitated by the characteristic scratching behavior of cats on rough surfaces of bark or textile.''

"The selective regime for the evolution of the claw-shedding mechanism in cats is likely to be connected with the need to maintain sharp cutting edges and a sharply pointed tip, as well as to be able to free the claws from struggling prey or other substrates without the risk of pulling the entire claw sheath off the vascularized and innervated dermal claw bed. At this point in time, it is less clear why small dogs and even horses have evolved the capacity for shedding their claws or hooves, respectively. It may be hypothesized that the shedding of superficial horn layers of claws and hooves is more widespread among terrestrial vertebrates than hitherto assumed and that, therefore, the shedding mechanism of cats is only more pronounced and effective because it has evolved under the selective regime for climbing and prey catching. Additional studies will be needed to clarify these and other questions concerning the evolutionary history of cornified digital end organs.''

All of the above paragraphs and picture are quoted from this scientific article : The structure of the cornified claw sheath in the domesticated cat (Felis catus): implications for the claw-shedding mechanism and the evolution of cornified digital end organs

If you have any informations about similar mechanisms present on other animals, please share it in this thread.
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