Reorientation ability in redtail splitfin (Xenotoca eiseni): role of environmental shape, rearing in group and exposure time

When passively disoriented in an enclosed space, animals use the geometry of the environment (angular cues and metrically distinct surfaces) to find a position. Wheth- er the ability to deal with geometry is a mechanism available at birth, with little influ- ence of previous experience with the same kind of information, is still debated. We reared fish (Xenotoca eiseni) in tanks of different shape (circular or rectangular) either singly or in group and tested at different ages (at one week or one, five or ten months). Fish were trained to reorient in an enclosure with a distinctive geometry (a rectangu- lar arena) and a blue wall providing non-geometric, featural information. Then, they were tested after an affine transformation that created conflict between geometric and non-geometric information as learned during training. We found that all fish, since one-week old, use significantly more the geometry of the enclosure for reorientation independently from the experience in circular or rectangular tanks. At one month of age, we observed a modulatory effect of rearing experience during learning with an advantage of individuals reared singly in rectangular cages, but no difference was evident at test. Furthermore, such effect on learning propensity disappeared later in development, i.e., when fish were trained at five or ten months of age. These results confirm that the use of geometric information provided by the shape of an enclosure is spontaneous and inborn, and that a modulatory effect of experience can appear brief- ly during ontogeny, but experience is not essentially needed to deal with geometry.