A newborn bear cub is actually smaller than a newborn porcupine. The off' 

 spring of a two hundred and fifty pound black bear mother may weigh only 

 nine or ten ounces at birth. According to Seton the bear commonly suckles 

 her young throughout the summer and therefore usually breeds every other 

 year. The smaller mammals usually mate early in the year, their short gesta- 

 tion periods enabling their young, like those of the larger mammals, to be bom 

 when living conditions are near the optimum. Thus foxes, with a gestation 

 period of fifty days, usually mate in February, when one may often find their 

 tracks making a network in the snow along woodland paths. The ground 

 squirrels emerge from hibernation about the time that succulent vegetation 

 first appears and mate even before making up for their winter's fast. It has 

 been suggested that the development of their sexual glands may be the 

 stimulus that arouses them from hibernation. A few of the small mammals, 

 such as mice and rabbits, may have tvv'o or three broods a year. The opossum, 

 the only North American marsupial, may also have two broods a year. Its 

 young are born in a very immature condition and complete their development 

 in a pouch covering the mammary glands. The armadillo, the only native 

 edentate, normally has four identical young at one time, all derived from the 

 same fertilized egg. Many of the mammals seem to be monogamous, a few, 

 like the deer, practicing polygamy. As to whether the same pair mates in 

 successive years or whether new mates are chosen each season, very little 

 is yet known. In many cases the male continues his association with his mate 

 after the breeding season and aids in the care and feeding of the young. 

 Sometimes the association persists over a year, and family groups of the two 

 parents and several almost grown young may be seen, as with the coyotes. 

 Some animals, like the timber wolves, rejoin in a pack as soon as the young 

 are big enough to play their part in the hunt. Some mammals, although not 

 especially sociable, tend to form groups, possibly because the young merely 

 move over to the edge of their parents' domain when they set up their own 

 homes. Striped ground squirrels may be quite numerous in certain old pas- 

 tures or cemeteries in the middle west, and on the great plains prairie dog 

 "tovv'ns" of great extent have been described. One continuous colony two 

 hundred and fifty miles long and one hundred miles wide is recorded in south- 

 western Texas, and seven thousand, two hundred burrows covering about one 

 square mile have been counted in eastern Arizona. In general, young carnivores 

 are likely to scatter far more widely, when the family breaks up, than are young 

 herbivores. Probably the degree of social life is quite dependent upon the 

 amount of competition for food. 



Several little known branches of mammal study are of interest. Exam- 

 inations of individual hairs under the microscope show surprising differences, 

 especially after they are cleared by several hours immersion in xylol or other 

 clearing fluids. A few hairs found at the mouth of a burrow or entrance 



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