The underground ecotope is dark. Apart from the consequences for sensory orientation and communication, absence of light also influences some physiological functions. One of them is mineral metabolism. On the one hand, subterranean rodents have an especially high requirement for calcium because their large teeth are constantly worn down during digging. In the African mole-rat, Cryptomys, the visible part of the incisors regenerates completely every week. Also, calcium may be excreted in high concentrations as calcium carbonate through urine, a mechanism to deplete tissues of carbon dioxide. On the other hand, it is common knowledge that vitamin D (and principally D3, cholecalciferol), which is needed for effective absorption of calcium from the gut (its deficiency causes rickets), is synthesized in the skin by the action of sunlight. Rochelle Buffenstein and colleagues have demonstrated that several species of African mole-rats, although in the perpetual state of vitamin D3 deficiency due to their lightless environment, have evolved other physiological mechanisms to absorb calcium effectively (indeed, up to 91% of minerals can be extracted) from their diet.