Visible sympathetic activity as a social signal in Anolis carolinensis: changes in aggression and plasma catecholamines.

Darkening of postorbital skin in Anolis carolinensis occurs during stressful situations and is stimulated by sympathetic activation of beta(2)-adrenergic receptors via adrenal catecholamines. This eyespot forms more rapidly in dominant males during social interaction. Eyespot darkening (green to black) appears to function as a social signal communicating sympathetic activation and limiting aggressive interaction. To assess the value of the eyespot as a social signal, males were painted postorbitally with green, black, or red paint. Each male was exposed to a mirror following acclimation to the cage. The total number of aggressive displays toward the mirror image was greatest when eyespots were masked by green paint. In contrast, black or red artificial eyespots, regardless of size, inhibited biting behavior toward the mirror image. The most aggressive males, those who saw a reflected opponent with no eyespot (hidden with green paint), had significantly higher levels of all plasma catecholamines. These results suggest that A. carolinensis use information from the eyespot to assess their opponent's readiness to fight and thereby determine whether to be aggressive. Darkened eyespots are capable of inhibiting aggression, whereas aggressive displays from an opponent in the mirror without darkened eyespots do not. Darkened eyespots reflect rapid changes in plasma NE, DA, and Epi that may signal dominant social status.