Physical condition at weaning affects exploratory behaviour and stereotypy development in laboratory mice.

Laboratory mice of the ICR strain develop two morphologically distinct forms of stereotypy in the post-weaning period when kept under standard housing conditions. Wire-gnawing develops from single bites at the cage-lid during exploratory climbing whereas jumping originates from exploratory rearing at the cage wall. In a recent study nude mutants of ICR that were physically retarded around the age of stereotypy development performed more rearing but less climbing just after weaning and subsequently developed more jumping but less wire-gnawing than ICR mice (Würbel, H., Stauffacher, M. and von Holst, D. 1996. Ethology 102: 371-385). The two source behaviour patterns, climbing and rearing, could represent alternative strategies serving the same goal (e.g. exploration of external stimuli, escape). Since the ability to climb may depend on physical strength, preference for either of these two patterns may be determined by physical condition to some extent. As a consequence, the prefered pattern may determine the later form of stereotypy. To test these two hypotheses, 48 male ICR mice were assigned to three groups of 16 each. One group was prematurely weaned at day 17 to mimic physical retardation, the others were weaned at the standard weaning age of 20 days and divided into two groups of high and low weaning weight. Premature weaning was associated with a shift in the relative proportions of climbing and rearing towards rearing. Although heavy males showed most climbing and least rearing, weaning weight did not significantly affect performance of these two patterns. Subsequently, all mice developed stereotypic wire-gnawing but none of them showed stereotypic jumping. Although weaning age affected the performance of the two source behaviour patterns in the predicted way, the differences may not have been large enough to cause different forms of stereotypy. However, both, premature weaning and low weaning weight resulted in higher stereotypy performance when adult.