colour mutations such as albinos and white spots throughout the pelage occur. The summer fur is somewhat lighter, and dirty in tone, with more reddish highlights. Differences from American mink The European mink is similar to the American mink, but with several important differences. The tail is longer in the American species, almost reaching half its body length. The winter fur of the American mink is denser, longer and more closely fitting than that of the European mink. Unlike the European mink, which has white patches on both upper and lower lips, the American mink almost never has white marks on the upper lip. The European mink's skull is much less specialised than the American species' in the direction of carnivory, bearing more infantile features, such as a weaker dentition and less strongly developed projections. The European mink is reportedly less efficient than the American species underwater. Behaviour Territorial and denning behaviours The European mink does not form large territories, possibly due to the abundance of food on the banks of small water bodies. The size of each territory varies according to the availability of food; in areas with water meadows with little food, the home range is , though it is more usual for territories to be . Summer territories are smaller than winter territories. Along shorelines, the length of a home range varies from , with a width of . The European mink has both a permanent burrow and temporary shelters. The former is used all year except during floods, and is located no more than from the water's edge. The construction of the burrow is not complex, often consisting of one or two passages in diameter and in length, leading to a nest chamber measuring . Nesting chambers are lined with straw, moss, mouse wool and bird feathers. It is more sedentary than the American mink, and will confine itself for long periods in its burrow in very cold weather. Reproduction and development During the mating season, the sexual organs of the female enlarge greatly and become pinkish-lilac in colour, which is in contrast with the female American mink, whose organs do not change. In the Moscow Zoo, estrus was observed on 22–26 April, with copulation lasting from 15 minutes to an hour. The average litter consists of three to seven kits. At birth, kits weigh about , and grow rapidly, trebling their weight 10 days after