Behavimtral Bra#~ Research, 60 (1994) 43-50 © 1994 Elsevier Science B.V. All rights reserved. 0166-4328/94/$07.00 43 BBR 1562 Developmental changes in spatial learning in the Morris water-maze in young meadow voles, Microtus pennsylvanicus L.A.M. Galea*, K.-P. Ossenkopp, M. Kavaliers NeuroscieHce Program, Univerxitv (?f We,~tern Ontario. Loluton, Olllario, N6A 5C2 Catlada (Received 22 July i993) (Revised version received 29 September 1993) (Accepted 30 September 1993) Key won£v Developmental change; Spatial learning; Sex difference; Water-maze: Swimming ability'; Ontogeny' of learning; Meadow vole Spatial learning in pre- and postweaning meadow voles, (Microtus pemTsyh,anicus) was examined in a Morris water-maze task. The learning performance of 10-day-old (preweaning) and 15-, 20- and 25-day-old (postweaning) male and female voles was assessed by measuring the latency to reach a hidden platform by each animal twice a day for 5 days. Voles of all age groups were able to learn the spatial task with Day 10 and Day 15 voles acquiring the task more slowly than did Day 20 and Day 25 voles. There were no significant sex differences in task acquisition in any of the four age groups. In addition, although swimming speed was related to age, with older animals swimming faster than younger ones, differences in swim speed did not account for the faster acquisition by the older animals. These results show that both preweaning and postweaning voles can successfully learn a spatial task. This is in contrast to preweaning laboratory rats which cannot successfully acquire a similar spatial task. These findings indicate that there are species differences in the ontogeny of spatial learning, which are likely related to the ecological and behavioural developmental characteristics of the species. Furthermore, in contrast to the sex difference in water-maze performance obtained in adult, breeding meadow voles who demonstrate a sex difference, there were no significant sex differences in the spatial performance of the juvenile w)les. This suggests that sex differences in spatial learning in the meadow vole do not appear until voles reach reproductive adulthood. INTRODUCTION Investigations of the ontogeny of spatial learning in rodents have been almost exclusively restricted to labo- ratory rats ~°'24-'5"27->. Results of initial investigations with food-reinforced tasks yielded some evidence of spatial learning in early development l°'2s. These findings were, however, potentially confounded by the alterations in the developmental growth that can occur under food restriction 31. More recently, the Morris water-maze spatial task, whereby individuals have to acquire and retain the spatial location of a submerged hidden platform ~7, has been used to investigate the ontogeny of spatial learn- ing 24"25'27">). Results of these studies have suggested that preweaning laboratory rats (Day 17 after birth) cannot learn the spatial location of a fixed hidden plat- form, while postweaning rats do learn to spatially navi- gate to a hidden platform :425. It has been further indicated that, although preweaning rats have the visual *Corresponding author. Fax: (1) (519) 661-3961. Email: Galea (-I\'a x r. s sc L/ivvo. c a navigational skills to acquire the task and learn to navi- gate to a cued visual platform, preweaning rats appar- ently lack the spatial navigational skills to find a hidden platform 25. This apparent lack of spatial navigational skill in preweaning rats is reinforced by the finding that even when preweaning rats learned to find a visible platform they did not retain the task 24. Whether or not this developmental pattern in spatial learning is similar among all rodents or unique to rats remains to be determined. None of the previous studies of the ontogeny of learning have examined the effects of developmental changes in swim speed or swimming ability on spatial learning in preweaning animals. Although preweaning rats do learn to swim to a visual platform, none of the previous studies have statistically-controlled for pos- sible developmental differences in swim speed 24"25. As the rate of acquisition in the visible platform task in the prior study of Rudy et al. 25 was slow, there may have been some motoric immaturity in the preweaning ani- mals that was responsible for the longer response latencies in both the visible and hidden platform tasks. Thus, an analysis of swim speed might provide SSDI: OlO6-4328(93)EOI53-R