Is vitamin D₃ essential for mineral metabolism in the Damara mole-rat (Cryptomys damarensis)?

The Damara mole-rat (Cryptomys damarensis) leads a strictly subterranean existence in an extensive maze of plugged burrows and eats a strictly herbivorous diet. An obvious source of vitamin D₃ (D₃), an important hormone in mineral homeostasis, therefore is lacking in this mammal. The effects of orally ingested D₃ on mineral balance were investigated. In the normal D′ status group of mole-rats, the apparent fractional intestinal absorption of calcium (91.1%), magnesium (91.2%), and inorganic phosphorous (91.2%) was high and increased slightly, (calcium, 94.5%; magnesium, 93.2%; inorganic phosphorous, 92.9%) after D₃ administration. The amount of element absorbed was positively correlated with the amount ingested (P < 0.001), suggesting that absorption occurs via a nonsaturable process. This might be an adaptation to the sparseness of natural food. The observed positive retention of calcium (94%), magnesium (85%), and inorganic phosphorous (99.9%) is attributed to their evergrowing teeth. These teeth are constantly worn down during digging and it is speculated that they represent a mineral "sink", assisting in the maintenance of mineral homeostasis. It is also hypothesized that mole-rats have adapted to an environment devoid of D₃ and can attain sufficient calcium for their needs via nonvitamin D₃-dependent processes. Although mole-rats have the "machinery" to use D₃ and D₃ can affect mineral balance, this effect may be disadvantageous if invoked.