MOLE

in zoology, any of numerous small, often blind, burrowing mammalsespecially the 22 species constituting the family Talpidae (order Insectivora), the true moles. For ordinal relatives of this name, see golden mole; for analogues in Australia, see marsupial mole. Talpids occur throughout temperate Eurasia and North America. Most are quite small, with round bodies, short legs and tail, flat, pointed head, and small or vestigial eyes; most lack external ears. Typically, the forelimbs are set almost on opposite sides of the shoulders and are shortened and rotated. The five toes of the forefoot are broad clawed, making a shovel for digging or a paddle for swimming. The fur is velvety and brushes in any direction without resistance. Most moles have one to seven young (usually about four) once or twice a year, usually in a grassy nest underground. Sexual maturity is reached in 6 to 12 months, and the life span is brief. Moles are active day and night in rapid cycles of work and rest. Most spend their lives making galleries in the soil. Tunnels near the surface are made in search of earthworms, grubs, and other invertebrates: a mole may consume more than its weight in food daily. Deeper burrows, to about 3 metres (10 feet), are for occupancy. The surface-burrow ridges on the ground and the deep-burrow vents (molehills) annoy gardeners; furthermore, moles sometimes damage the roots of plants. On the other hand, moles stir and aerate the soil and kill noxious organisms. A few birds and mammals prey on moles despite their strong odour. Typical moles are the four species of the Old World genus Talpa, including the 15-centimetre (6-inch) common mole (T. europaea) of Britain to Japan. They have stubby tails. The American shrew mole (Neurotrichus gibbsii) is a long-snouted species of wet coastal forests of northwestern North America; at 10.5 cm, including its 3-cm tail (minimum measurements), it is the smallest American mole. It swims and climbs. In the shrew moles (Uropsilus, Nasillus) of eastern Asia, the forefoot is virtually unmodified: these are primarily surface animals. Perhaps the tiniest talpid is the long-tailed shrew mole (Scaptonyx fusicaudus) of western China and northern Burma: its head and body may be only six cm long, but it has a three-cm tail. The North American common, or eastern, mole (Scalopus aquaticus), of the U.S. east of the Rockies, may be 20 cm long and is gray in the north, coppery brown in the south and west. The star-nosed mole (Condylura cristata) of northeastern North America has 22 pink, tentacle-like touch organs set radially on its nose. It often leaves its burrow, and it swims well. Semi-aquatic talpids of the Old World include the desman (q.v.). also spelled Mol, in chemistry, a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles. The mole designates an extremely large number of units, 6.0221367 1023, which is the number of atoms determined experimentally to be found in 12 grams of carbon-12. Carbon-12 was chosen arbitrarily to serve as the reference standard of the mole unit for the International System of Units (SI). The number of units in a mole also bears the name Avogadro's number, or Avogadro's constant, in honour of the Italian physicist Amedeo Avogadro (17761856). Avogadro proposed that equal volumes of gases under the same conditions contain the same number of molecules, a hypothesis that proved useful in determining atomic and molecular weights and which led to the concept of the mole. (See Avogadro's law.) The number of atoms or other particles in a mole is the same for all substances. The mole is related to the atomic weight, or mass, of an element in the following way: one mole of carbon-12 atoms has 6.022137 1023 atoms and an atomic weight of 12 grams. In comparison, one mole of oxygen consists, by definition, of the same number of atoms as carbon-12, but it has an atomic weight of 16 grams. Oxygen, therefore, has a greater mass than carbon. This reasoning also can be applied to molecular or formula weights. The concept of the mole helps to put quantitative information about what happens in a chemical equation on a macroscopic level. The mole can be used to determine the simplest formula of a compound and to calculate the quantities involved in chemical reactions. When dealing with reactions that take place in solutions, the related concept of molarity is useful. Molarity (M) is defined as the number of moles of a solute in a litre of solution. in dermatology, pigmented, flat or fleshy skin lesion, composed for the most part of an aggregation of melanocytes, the cells of the skin that synthesize the pigment melanin. In thicker moles, nerve elements and connective tissue are also present. Moles vary in colour from light to dark brown or black; when deposition of melanin occurs in the dermis, the deeper layer of the skin located underneath the epidermis, the lesion has a bluish cast. Moles may be present at birth; more frequently, they appear and evolve in character during childhood. A new mole is usually flat and of the junctional type (junction nevus), so called because it is located between the dermis and the epidermis. It sometimes remains there, in which case the possibility of malignant development is increased. In most instances, however, the original mole evolves into a slightly raised lesion located in the dermis (intradermal nevus). Examination of the tissue of an actively changing mole in a child may show transformations resembling cancer, but actually such lesions are benign; malignant melanoma is almost never seen until after puberty. The following developments are indicative that a mole may be undergoing cancerous changes, giving rise to malignant melanoma: (1) development of a flat pigment zone around the base of the mole, (2) progressive enlargement of an existing mole in adults, (3) increase in pigmentation, or darkening, of a mole and, more frequently, a loss of evenness in pigmentation, with variations from very light to very dark (probably the single most significant sign of developing malignant melanoma), (4) loss of hair from a mole (hairy moles rarely undergo cancerous changes), and (5) advanced obvious symptoms, such as ulceration and bleeding. It should be noted that melanomas do not derive from pigmented moles only; approximately 25 percent of these tumours arise in normal skin. During pregnancy, existing moles may enlarge and new ones may appear. Moles will sometimes disappear with age. See also nevus.