Osteophagy

Osteophagy is the practice in which animals, usually herbivores, consume bones. Most vegetation around the world lacks sufficient amounts of phosphate. 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. Osteophagic behavior has been observed in pastoral and wild animals, most notably ungulates and other herbivores, for over two hundred years. It has been seen in domestic animals, as well as red deer, camels, giraffes, wildebeest, antelopes, tortoises, and grizzly bears.

Animals

Turtles

Desert plants are a major food source for desert tortoises (Gopherus agassizii), as they have a mainly herbivorous diet. However, while desert plants are high in calcium content, desert tortoises have been observed to consume other substances which are also rich in calcium. 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. 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. Desert plants grow in mineral-deficient soil, and may be a cause of mineral deficiency in desert tortoise diets, resulting in the intake of supplemental material. It has also been hypothesized that osteophagy is a practice necessary for the maintenance of desert tortoise shells. 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.

Cattle

In the late 1800s, a then relatively unknown disease called botulism was seen in very high levels in South African cattle, especially those that grazed in pastures with low phosphorus levels. Researchers found that feeding the cattle sterile bonemeal, or corn with unnaturally high levels of phosphorus, nearly eliminated botulism. The simplest conclusion for this was that the botulism symptoms were caused by a lack of phosphorus.

In the early 1900s, Sir Thomas Thieler revisited the issue, and began following herds of cattle to observe their behavior. Incredibly, he found that the phosphorus-deficient cattle would eat the decomposing bones of dead cattle and other animals, and that this activity was highly correlated to botulism. Over the next several years, he was able to show that a bacterial strain living in the decomposing carcasses, Clostridium botulinum, was the true cause of the disease. The cattle would eat the carcasses to replenish their phosphorus deficiency, and would contract the disease.

Bears

Grizzly bears (Ursus arctos), after emerging from hibernation, may be experiencing a skewed phosphorus-to-calcium ratio due to the lack of consumption of animal resources during the period of hibernation. In winter conditions, while grizzly bears may be able to maintain calcium intake and absorption due to ingestion of plants and vitamin D absorption from solar radiation, low protein availability results in phosphorus deficiency. This low protein availability is often the case prior to green-up in ecosystems. Therefore, bones can serve as a valuable source of minerals at times where animal protein availability is low.

Giraffes

Giraffes rely solely on browsing to maintain their diet, which consists primarily of leafy material. However, they are commonly observed supplementing their diet with bones. Although the exact purpose of this behavior is unknown, it is hypothesized that the ingestion of bones serves as an additional source of calcium and phosphorus. While leaves usually serve as a sufficient source of these nutrients, calcium and phosphorus concentrations in the leaves vary seasonally with rainfall; the giraffes' osteophagic behavior has been observed to parallel this variance in mineral concentration.

The benefits of this behavior remain unclear. Researchers have found that it is actually unlikely that the giraffes can sufficiently digest the bones to extract the calcium or phosphorus. There is also evidence to suggest that osteophagy is associated with the development of kidney stones in giraffes.

Domestic Dogs

While the media often portrays domestic dogs chewing bones, this is slightly misleading. Dogs chew bones only to eat any residual meat left on them, so it is not truly a form of osteophagy. Most modern toy "bones" for dogs are actually rawhide, which is simply dried animal skin.

Humans

Pica

Pica is the craving and consumption of non-nutrient substances that can cause health risks. Osteophagy in humans would be considered a form of pica. Unlike phosphorus in most animals, pica is associated with iron deficiencies in humans. Geophagy, the eating of clay, is another form of pica that is more commonly observed.

Religious Practice

The Yanomami tribe live as nomads in the Brazilian and Venezuelan Amazon. When a tribe member dies, it is a custom for their family to “set their spirit free” in a religious ritual. During this ritual, the tribe grinds/pulverizes their bones to a fine ashen powder and mixes the powder into a plantain soup, which is eaten by the family of the deceased. It is possible that this ritual originated as a way to increase phosphorus and other minerals in the tribe’s diet, though it may just be a religious ritual without any other purpose.

See also

• Geophagy