The Development of Social Behavior Patterns in the Mouse, in Relation To Natural Periods 1)

This paper consists of a descriptive study of the post-natal behavioral development of the house mouse, with particular emphasis on its social aspects. 2. Observations were made on ten litters, including 60 mice from four different inbred strains, up to the age of 30 days. Nine litters were raised in transparent laboratory mouse boxes, and the tenth was raised in a larger box with a more complex environment. 3. Important developmental changes which affect the nature of social relationships were found at 4 days, 12 days, and approximately 25 days. On this basis the early development of the mouse was divided into four natural periods. 4. These periods are described in detail from the viewpoint of sensory, motor, and psychological capacities, and the appearance of important patterns of social behavior. 5. During the Neonatal Period (I) the mice are naked and largely helpless, and the predominant activities are ingestive and contactual behavior. There is no reaction to sound or sight, nor an indication of learning capacity. 6. The Transition Period (II) is characterized by rapid development of sensory, motor and psychological capacities which provide the basis for future change in social behavior patterns. The period is marked by the appearance of epimeletic behavior (grooming) and an increase in investigative behavior. Other social behavior patterns remain much as before. 7. The Socialization Period (III) begins with all sense organs functional, so that the animal is for the first time in complete contact with the social environment. Motor development is rapid, and the animals give evidence of great sensivity to frightening stimuli. All the characteristic social behavior patterns of adults appear in some form during this period, and at its end one of the last neonatal patterns, nursing, disappears. The dominant behavior patterns are investigative and epimeletic (grooming). 8. During the Juvenile Period (IV) fighting behavior appears, and its end is marked by the first appearance of adult sexual behavior. 9. Certain physiological changes are correlated with overt changes in behavior. Reactions to electroshock change at 4 and 12 days. Changes in the growth curve occur with changes in ingestive behavior, which in turn is correlated with the opening of the eyes at about 12 days. 10. There is evidence that reactions to sound appear early in the Transition Period, but the startle reaction does not appear until near its close. 11. Further study is needed of the development of the capacity for simple associative learning, which on present evidence may appear early in the Transition Period. 12. When the mouse is compared with other forms it is found that the order of behavioral events varies considerably between different species, and that the order appears to be correlated with the nature of social organization typical of the species. 13. The development of behavior in the mouse is discussed with reference to possible critical periods in the development of social relationships.