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Effect of Testosterone on the Number of Neurons and Volume of the Hippocampus in the Side-Blotched Lizard, Uta stansburiana
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It has been demonstrated in a variety of vertebrate species that there is a positive association between the hippocampus (HP) and spatial learning abilities. Spatial learning is critical to fitness and survival as it can be used to remember the location of food resources, mates, or for general navigation. Two characteristics of the hippocampus that appear to vary with differing spatial demands are neuron numbers and the overall volume; both of which have been shown to be affected by hormones. The side-blotched lizard, Uta stansburiana, exhibits three different morphs that vary in their space use patterns: orange, blue and yellow. The orange and blue morphs are territorial in nature while the yellow morph is non-territorial. Plasma testosterone (T) levels have been demonstrated to be higher in the territorial morphs compared to non-territorial morphs and as such, T might play a role in mediating spatial learning and HP attributes. We found that experimentally elevating plasma T levels induced changes to the HP, irrespective of spatial experience. More specifically, elevated T levels resulted in decreased neuron number and volume of the medial cortex in territorial lizards, but an increased medial cortex volume in non-territorial lizards. These results suggest that the effect of T on hippocampal morphology is dependent on territorial behavior associated with different morphs.