Devenport, J. A., Luna, L. D., & Devenport, L. D. (2000). Placement, retrieval, and memory of caches by thirteen-lined ground squirrels. Ethology, 106(2), 171-183.

Mechanisms governing placement and retrieval of scatter hoards were investigated in 13-lined ground squirrels (Spermophilus tridecemlineatus). The squirrels used several placement tactics, including relatively deep burial of seeds, avoidance of prominent objects, and cryptic placement, which would reduce chance discovery by competitors but that might also increase the difficulty of retrieval for the hoarder. Nevertheless, scatter hoards were unfailingly retrieved within a day or two after placement, despite experimental elimination or displacement of local sensory cues emanating from the sites. Artificial caches placed close to true caches were not discovered, indicating that recovery attempts were quite precise. These results imply that 13-lined ground squirrels rely heavily on spatial memory for retrieval and are the first experimental demonstration of the importance of memory for cache recovery in a natural population of mammals.

Jacobs, L. F., & Shiflett, M. W. (1999). Spatial orientation on a vertical maze in free-ranging fox squirrels (Sciurus niger). Journal of Comparative Psychology, 113(2), 116-127.

To determine how squirrels return to remembered locations in an arboreal environment, wild fox squirrels (Sciurus niger) were trained on an outdoor vertical maze. Squirrels were trained on 1 route and tested with all routes accessible. Possible mechanisms of spatial orientation were distinguished with manipulations such as rotations, shifts, and blocked routes. Squirrels consistently used an extra- maze, allothetic frame of reference to orient and appeared to organize their memory of the route hierarchically. This study demonstrates that a laboratory technique, the maze, can be successfully brought into the field to measure mechanisms of spatial orientation under natural conditions in free-ranging wild rodents. Such studies will allow researchers to determine what bind of spatial information is acquired by wild animals under natural conditions and how this information is used.

Kleim, J. A., Barbay, S., Cooper, N. R., Hogg, T. M., Reidel, C. N., Remple, M. S., & Nudo, R. J. (2002). Motor learning- dependent synaptogenesis is localized to functionally reorganized motor cortex. Neurobiology of Learning and Memory, 77(1), 63-77.

The regional specificity and functional significance of learning- dependent synapto-genesis within physiologically defined regions of the adult motor cortex are described. In comparison to rats in a motor activity control group, rats trained on a skilled reaching task exhibited an areal expansion of wrist and digit movement representations within the motor cortex. No expansion of hindlimb representations was seen. This functional reorganization was restricted to the caudal forelimb area, as no differences in the topography of movement representations were observed within the rostral forelimb area. Paralleling the physiological changes, trained animals also had significantly more synapses per neuron than controls within layer V of the caudal forelimb area. No differences in the number of synapses per neuron were found in either the rostral forelimb or hindlimb areas. This is the first demonstration of the co- occurrence of functional and structural plasticity within the same cortical regions and provides strong evidence that synapse formation may play a role in supporting learning- dependent changes in cortical function. (C) 2001 Elsevier Science.

Lavenex, P., Steele, M. A., & Jacobs, L. F. (2000). Sex differences, but no seasonal variations in the hippocampus of food- caching squirrels: A stereological study. Journal of Comparative Neurology, 425(1), 152-166.

Recent studies have described sex differences in the relative size of the hippocampus that are associated with sex differences in space use in birds and short-lived mammals. A correlation between spatial learning and increased hippocampal volume has also been demonstrated in food-caching animals. Such results suggest that sexually dimorphic spatial learning (sex differences in space use during the breeding season) and seasonal variations in food-caching behavior (spatial memory for cache locations) might correlate with morphological changes in the hippocampus of adult long-lived mammals. We used modern stereological techniques to examine the volume and neuron number of the structures forming the hippocampal complex identate gyrus, CA3, and CA1) of wild adult eastern gray squirrels (Sciurus carolinensis) throughout the year. We observed differences in brain size between samples collected at different times of the year (October, January, and June). Our analysis showed sex differences, but no seasonal variations, in the volume of CA1 stratum oriens and stratum radiatum. There were no sex differences or seasonal variations in the relative volume or the number of neurons of any other layer of the structures forming the hippocampal complex. These results confirm the existence of sex differences in the structure of the hippocampus; however, this sexual dimorphism does not vary seasonally in adulthood and is likely to result from developmental processes. These results do not support the hypothesis that seasonal variations in food-caching behavior might correlate with morphological changes, such as variations in volume or neuron number, in the hippocampal complex of adult long-lived mammals. J. Comp. Neurol. 425:152-166, 2000. (C) 2000 Wiley-Liss, Inc.

Millesi, E., Prossinger, H., Dittami, J. P., & Fieder, M. (2001). Hibernation effects on memory in European ground squirrels (Spermophilus citellus). Journal of Biological Rhythms, 16(3), 264-271.

Effects of hibernation on memory were tested in European ground squirrels (Spermophilus citellus). The animals were trained in summer to successfully accomplish two tasks: a spatial memory task in a maze and an operant task on a feeding machine. One group hibernated normally, and the other was prevented from hibernation by maintaining ambient temperature at 22 degreesC. In spring, the same tasks were repeated for both groups and their individual performances compared to the initial training phase. The experimental groups differed significantly in both tasks. The nonhibernating animals had higher levels of retention and needed significantly fewer trials to relearn the tasks than the group that had hibernated. In addition to testing the retention of conditioned tasks, social memory was also studied. The ground squirrels were given a social recognition test in spring with one familiar and one unfamiliar conspecific. In contrast to the conditioned tasks, social memory did not seem to be affected by hibernation. The results indicate negative effects of hibernation on the retention of conditioned tasks, which could produce important constraints on animals. A potential explanation for this memory loss might be changes in neuronal activity which occur during hibernation.

Schwagmeyer, P. L., Parker, G. A., & Mock, D. W. (1998). Information asymmetries among males: implications for fertilization success in the thirteen-lined ground squirrel. Proceedings of the Royal Society of London Series B-Biological Sciences, 265(1408), 1861-1865.

The classical view of mammalian mating competition focuses on combat and threat. By contrast, field studies have revealed that male mating success in some species is more strongly determined by mate location ability than by physical dominance. In thirteen-lined ground squirrels, competition in locating oestrous females is exacerbated by sperm competition that favours the first male to mate with a female. We used a female- removal experiment to identify the distinguishing characteristics of males that were the first at finding and mating with females. Each focal female was observed the day before she entered oestrus; the identities of males that made contact with her and the locations of their interactions were recorded. The following morning the females were temporarily removed, and we monitored male search behaviour in their absence. Males that arrived first were those that had spent more time with the female the previous day relative to their rivals. Time invested the day before, in turn, was highly correlated with male search persistence and familiarity with the female's likely whereabouts. These results demonstrate that differential fertilization success can arise from information asymmetries among males: the advantaged individuals are those that have greater a priori knowledge of the reproductive state and spatial locations of prospective mates than rivals.