SEXUALLY DIMORPHIC SPATIAL-LEARNING VARIES SEASONALLY IN 2 POPULATIONS OF DEER MICE

Spatial learning in photoperiodically induced breeding (reproductive) and non-breeding (non-reproductive) adult male and female deer mice (P eromyscus maniculatus) was examined in a Morris water-maze task. Sexua lly mature, adult male and female deer mice that were derived from eit her a mainland population (P. m. aretmisae) or an island population (P . m. angustus) were required to learn the spatial position of a hidden , submerged platform in a water maze. Deer mice were tested either dur ing the breeding season (summer; long day photoperiod) or during the n on-breeding season (winter; short day photoperiod) with a total of six blocks of four trials conducted in a single day. Retention was tested with two probe trials which occurred one and three days after acquisi tion. During the breeding season male spatial task acquisition was sup erior to female spatial task acquisition for both populations. In cont rast, during the non-breeding season there were no significant sex dif ferences in spatial acquisition for either population. This change in sexually dimorphic spatial learning was due to female spatial-performa nce decreasing from non-breeding season to the breeding season and mal e spatial-performance increasing over the same period. Both population s displayed similar seasonal variations in sexually dimorphic water-ma ze task performance. There were, however, overall population differenc es in water-maze task performance that were related to the ecology of the mice, with the insular mice displaying shorter latencies to reach the hidden platform than did the mainland deer mice. It is suggested t hat changes in the levels of sex hormones which vary across the season in deer mice may account for the seasonal fluctuations in spatial acq uisition in both populations and that these changes differentially inf luence the spatial learning of male and female mice.