Sympathetic mediation of stress and aggressive competition: plasma catecholamines in free-living male tree lizards.

The sympathetic nervous system and adrenal catecholaminergic tissue act to prepare an animal for "fight or flight" by release of catecholamines into synapses and plasma. However, few studies have measured plasma catecholamines in nonmammalian vertebrates and none have measured them in free-living animals. We report plasma levels of norepinephrine (NE), epinephrine (EPI) and dopamine (DA) in free-living tree lizards (Urosaurus ornatus) bled in the field: 1) immediately after capture, 2) after 10 minutes of restraint, 3) immediately after a staged territorial encounter and 4) four minutes after the end of a staged territorial encounter. Time to capture and time to bleed after capture were also recorded for each sample. Time to capture had little effect on plasma catecholamines suggesting that plasma catecholamines do not rise during the brief pursuit by the investigator necessary to capture the lizards in the field. In contrast, plasma NE and EPI increased during blood collection. However, this response was very consistent allowing comparisons to be made using analysis of covariance to control for time to bleed. Results indicate that restraint stress caused a large increase in plasma levels of NE, E and DA. Plasma levels of NE and E, but not plasma DA, were also significantly elevated both immediately and 4 minutes after aggressive encounters, although less so than following restraint stress. These studies show that stress and aggression activate the sympatho-adrenal system. This activation could be involved in behavioral changes during aggression, meeting metabolic demands of the encounter, or both.