Book Description

An organism’s spatial ecology allows for access to essential resources such as food, mates, and escape from predators. Home range size, or the total area an organism inhabits, varies in relation to numerous factors including seasonality. During the breeding season, home range size increases in males across taxa. In addition, males usually also have larger home range sizes than females. This implicates testosterone (T) as a possible mediator of this relationship. Indeed, T causes an increase in home range size of males in numerous species of lizards. In addition to T causing an increase in home range size, it also causes an increase in coloration, which is used as a signal to deter or elicit aggressive behaviors in lizards. Potentially, contests are less common in natural settings than in the lab due to this signaling despite increased frequency overlap of home ranges in males. The larger the home range size of males, mediated through an increase in T, the more overlap with conspecifics. With this increase in spatial demand, or home range size, there is often a corresponding increase in spatially related brain regions. In reptiles, these brain regions are the medial and dorsal cortices (MC and DC respectively). The increase in cortical brain region size due to an increase in spatial demand may be mediated by an increase in neurogenesis. Proliferation of neurons occurs along the ventricles and radiate to numerous regions in the brain including the MC. With respect to the MC, immature neurons, which express the protein doublecortin (DCX), migrate from the ventricles, through the inner plexiform layer and are integrated into the cell layer. Because DCX is only expressed in recently born, migrating neurons, it can be used to measure neurogenesis. In mammals and birds, neurogenesis and growth of certain brain regions is affected by steroid hormones, including T. Here we tested two hypotheses: (1) T affects the home range size of Sceloporus occidentalis and (2) cortical brain region volumes are related to home range size and/or T which is mediated through changes in rates of neurogenesis. We surgically castrated individuals and implanted subjects with either a T-filled implant or blank implant and then released them at their initial capture sites. In addition to these castrated individuals, subjects not subjected to castration served as unmanipulated controls. Home range size of individuals in the field was quantified using a global positioning system (GPS) unit and later delineating those GPS points using minimum convex polygons (MCPs). We predicted that (1) castrated, T-treated lizards and unmanipulated control lizards would have larger home range sizes than castrated, control lizards c and (2) MC and DC cortices would be larger in volume and contain more DCX-immunoreactive cells in the lizards with the highest circulating T levels and with the largest home range sizes. We found that increased T caused an increase in the number of blue abdominal scales. We found no differences in home range size relating to T. Likewise, T did not affect MC volumes. However, we did observe a decrease in DC volume with increasing plasma levels of T. Because T did not affect home range size, it follows that we did not find an effect of T on MC volume. However, the significant result of T causing a decrease in DC volume implies a possible trade off with regards to energetics and the maintenance of brain region volumes as prior research indicates that T in increases energy expenditure and decreases foraging efforts.