Osteophagy is the practice of eating bones. It occurs among both carnivorous and herbiviorous animals. Among carnivores, it is an ecological niche where a species can extract more nutrition from the same food resources than competitors can, by being able to crack apart and digest bones. These adaptions may be dental, as in spotted hyenas and wolverines, or physiological and behavioral, as in bearded vultures. Among non-carnivores, the word refers to a behavior where individuals of a species consume bone, usually for supplemental minerals. Some species of herbivore, such as the North American porcupine, are especially attracted to chewing mineral-rich materials, but many mammals that live on vegetation may do so. Most vegetation around the world lacks sufficient amounts of phosphate to support bone growth. Phosphorus is an essential mineral for all animals, as it plays a major role in the formation of the skeletal system, and is necessary for many biological processes including: energy metabolism, protein synthesis, cell signaling, and lactation. Phosphate deficiencies can cause physiological side effects, especially pertaining to the reproductive system, as well as side effects of delayed growth and failure to regenerate new bone. The importance of having sufficient amounts of phosphorus further resides in the physiological importance of maintaining a proper phosphorus to calcium ratio. Having a Ca:P ratio of 2:1 is important for the absorption of these minerals, as deviations from this optimal ratio can inhibit their absorption. Dietary calcium and phosphorus ratio, along with vitamin D, regulates bone mineralization and turnover by affecting calcium and phosphorus transport and absorption in the intestine. It has been suggested that osteophagy is an innate behavior that allows animals to supplement their phosphorus and calcium uptake in order to avoid the costly effects of deficiencies in these minerals. Osteophagic behavior has been observed in pastoral and wild animals, most notably ungulates and other herbivores, for over two hundred years. Osteophagy has been inferred from archaeological studies of dental wear in Pleistocene fossils dating back 780,000 years. It has been seen in domestic animals, as well as red deer, camels, giraffes, wildebeest, antelopes, tortoises, and grizzly bears. Due to differences in tooth structure, herbivores tend to chew old dry bones that are easier to break, while carnivores prefer to chew softer fresh bones. Variations of the behavior have also been observed in humans. While osteophagy has been regarded as a beneficial behavior to combat mineral deficiencies in animals, osteophagic practices have also been observed to be detrimental to the dentition of herbivores. It has been observed that the pattern of wear on the cheek teeth of herbivores is congruous to the manner in which herbivores hold and chew bones. A major cost of osteophagy is therefore significant wear on teeth and dental breakage in herbivores, whose teeth did not evolve to enable the regular consumption of hard materials but rather for the grinding of vegetal fibers.
Animals
Wolverine Wolverines are observed finding large bones invisible in deep snow and are specialists at scavenging bones specifically to cache. Wolverine upper molars are rotated 90 degrees inward, which is the identifying dentition characteristic of the family Mustelidae (weasel family), of which the wolverine has the most mass, so they can crack the bones and eat the frozen marrow of large animals. This structural feature helps the wolverine be successful as a scavenger and adapt to a frozen habitat.
Porcupine Porcupine species including the largest, African porcupine and North American porcupine, are nocturnal bone collectors of thousands of bones, stored inside their den and in open piles in their vicinity. The bones do not satisfy seasonal nutritional deficiency, they prevent overgrown teeth but the shavings are ingested as the bulk of their diet.
Tortoise Osteophagy in desert tortoises has largely been observed in captivity, and more rarely in the wild where osteophagy observed above ground is quick and seldom, usually lasting only a few minutes. Desert plants are a major food source for desert tortoises (Gopherus agassizii), as they have a mainly herbivorous diet. In addition to desert plants, desert tortoises also consume vulture feces (which contain bones), soil (layers contain calcium), mammal hairs, feathers, arthropods, stones, bones of conspecifics, as well as snake and lizard skin castings. Desert tortoises have been observed to exhibit mounting behavior, aggressive biting, and repeated striking of carcasses when practicing osteophagy. Osteophagy in herbivores has been viewed to serve as a source for supplemental minerals. Desert plants grow in mineral-deficient soil, and may be a cause of mineral deficiency in desert tortoise diets, resulting in the intake of this supplemental material. An observational study of tortoises near St. George, Utah, found that the tortoises exclusively consume the Mojave Desert's white stones, which are composed of calcite (mostly calcium carbonate), as opposed to the brown, grey, or other colored stones. The ingestion of these white stones is attributed to the deliberate intake of additional calcium. Furthermore, it is thought that these additional sources of food are sources of not only calcium, but also other nutrients including phosphorus, sodium, iron, copper, and selenium. It has also been hypothesized that osteophagy is a practice necessary for the maintenance of desert tortoise shells. This parallels the phenomenon of osteophagy in birds, in which snail shells are ingested by egg-laying females to supplement the increased calcium needed for eggshell formation. Therefore, it would be expected that the increased physiological needs of juvenile and gravid female tortoises would also increase mineral demands and promote ingestion of bones, stones, and soil. Alternatively, the need to consume supplemental minerals may serve the purpose of detoxifying plant compounds, or may serve other purposes not related to nutrition such as to dislodge gut parasites.
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