Territorial Behavior and Cortical Brain Plasticity in Adult Male Sceloporus Occidentalis

Abstract

Territorial Behavior and Cortical Brain Plasticity in Adult Male Sceloporus occidentalis Daniel Robert PfauThe hippocampus is a brain region that can undergo tremendous plasticity in adulthood. The hippocampus is related to the formation of spatial memories in birds and mammals. In birds, plasticity in the hippocampus occurs when formation of such memories is directly relevant to survival or reproduction, such as for breeding or food caching. In reptiles, the homologues to the hippocampus are the dorsal and medial cortices (DC and MC). In several lizard, snake and turtle species, these structures have been related to spatial memory. Experimental investigations indicate that differences in DC volume are related to space use associated with differing foraging ecologies. Differences in MC volume have been associated with territory size-based mate acquisition strategies. Furthermore, territory size has previously been correlated with plasma testosterone (T) levels. Therefore, I hypothesized that neuroplasticity within the MC/DC is controlled by demands on spatial navigation and seasonal differences and that these changes may involve the action of T. During two experimental trials, male Western Fence Lizards (Sceloporus occidentalis) were placed into either large or small semi-natural enclosures and allowed to interact with a female and intruder males over the span of seven weeks. One trial was performed during the spring breeding season and the other during the summer non breeding season, to examine seasonal differences in plasticity. Blood samples were collected at initial time of capture and before sacrifice to measure plasma T. Immunostaining for doublecortin was used to determine the density of immature neurons in each region, and cresyl violet staining allowed for volume measurements of specific regions. MC cell layer neurogenesis was higher in lizards placed in large enclosures than those in small enclosures and higher in the summer than in the spring. DC volume was smaller in lizards held in large enclosures than those in small enclosures. The decreased DC volume seen lizards held in large enclosures may indicate a cost to the increased neurogenesis in the MC of lizards in the same enclosures. These results indicate a possible trade-off between DC volume and MC neurogenesis that allows for switching between the ability to solve novel spatial tasks using the DC while storing a cognitive map in the MC. During the spring, T had no relationship with MC volume, while during the summer this was negative, so effects of T on the MC may be seasonal.

Reptilian Cognition: A More Complex Picture via Integration of Neurological Mechanisms, Behavioral Constraints, and Evolutionary Context

“…[9] Similarly, when space use is increased by increasing enclosure size, the number of new cells within the MC of fence lizards increased. [93] Although this study did not dissociate neurons from other cell types, this is also consistent with another study controlling lizard space use within the context of territoriality, a spatially explicit behavior, in side-blotched lizards. LaDage et al [94] found that territorial males had increased neurogenesis rates in the MC when housed in larger enclosures relative to that seen in siblings housed in smaller ones.…”

“…Animals housed in deeper water showed increased neurogenesis rates relative to those housed in shallow water, seeming to indicate that the opportunity for increased spatial movement might have increased neurogenesis . Similarly, when space use is increased by increasing enclosure size, the number of new cells within the MC of fence lizards increased . Although this study did not dissociate neurons from other cell types, this is also consistent with another study controlling lizard space use within the context of territoriality, a spatially explicit behavior, in side‐blotched lizards.…”

Plasticity and Adult Neurogenesis in Amphibians and Reptiles: More Questions than Answers

“…In his study of the medial cortex of western fence lizards (S. occidentalis), described in Is the Production of New Cells in the Brains of Adult Amphibians and Reptiles Affected by Spatial Area Use?, Pfau [2012] found more new neurons in summer than in spring. This finding could be related to increased activity in the summer.…”