Family MUSTELIDAE 105 



at base: about 3] _> inches (90 mm.)- Weight: approximately 20 

 pounds. 



The skull is large, especially the braincase, and is about 105 

 mm. (4 inches) long and 70 mm. {2^\ inches) wide. It is not 

 heavily ridged. The auditory bullae are flattened, and the teeth 

 are massive. Dental formula: I 3/3, C 1/1, Pm 4/3, M 1/2. 



Life History. — The river otter lives along streams and lakes. 

 Only infrequently does it wander far from them on excursions 

 from one body of water to another. Its large den, usually with 

 an entrance diameter of about 10 inches, is never more than a 

 few hundred yards from water. Usually it is in a stream bank 

 or on a lake shore, either above or below water, and protected 

 by roots of large trees or by overhanging banks. For its den, 

 the otter may use a natural cavity or a burrow, such as that 

 of the woodchuck. Breeding occurs in winter, and about 1 1 

 months later a litter, usually of three young, is born. The male 

 usually stays in the general vicinity of the den, but the female 

 does not let him join the group until the young are old enough 

 to travel. During their third or fourth month, the cubs are 

 forced to learn to swim. A family group may hunt and fish over 

 a waterway of 10 or more miles during the season. The river 

 otter feeds on crayfish, frogs, turtles, earthworms, aquatic in- 

 sects, and fish. 



Signs. — Most conspicuous of all otter signs are the slides — 

 the places where the animals gather to enjoy a slide down clay 

 or snow banks into water. These slides are a foot or more wide 

 and may be a dozen to many feet long. Otters remove every 

 stick and stone from the slides, and, as the water dripping from 

 their pelts makes the clay or snow very slippery, the descent 

 is fast. 



The surest place to see otter tracks, fig. 23, is on the shores 

 and islands of large rivers. When much snow is on the ground, 

 the otter sometimes propels itself by pushing itself along, mak- 

 ing a deep groove in the snow; even when it walks in shallow 

 snow its short legs allow its body to drag, leaving a continuous 

 trail. When traveling at considerable speed through deep snow. 

 the otter bounds in such a way that it leaves a well-marked, 

 full-length impression of its long body on the trail. 



Distribution. — The river otter was once fairly common along 

 the large streams of Illinois, but apparently by the early 1800 s 

 it was scarce in most parts of the state. Since 1900. it has been 



