REGULATION OF PHYSIOLOGICAL COLOR-CHANGE IN DORSAL SKIN OF MALE TREE LIZARDS, UROSAURUS-ORNATUS

Urosaurus ornatus is a small desert lizard that utilizes rapid changes of dorsal skin color for thermoregulation and social signaling. WO pe rformed an investigation on the hormonal control of color change utili zing skins, in vitro, of U. ornatus (1) during the species' reproducti ve season (July) and (2) when not sexually active (September). Alpha-m elanocyte stimulating hormone (alpha-MSH) was a potent skin darkening agent, the ED50 being 10(-10) M in July. The sensitivity of the skins to alpha-MSH decreased. by about nine-fold (ED50 = 9 x 10(-10) M) in S eptember. The beta-adrenoceptor agonist isoproterenol (in the presence of the alpha-adrenoceptor blocker, Dibenamine) was only a partial mel anophore agonist, because the maximal darkening elicited in its presen ce was approximately 50% of the darkening promoted by alpha-MSH. The r esponses to isoproterenol were also significantly higher in July (17%) than in September (11%). In the presence of both alpha-MSH and isopro terenol applied simultaneously to the skins, tile darkening responses were not significantly higher than in the presence of alpha-MSH alone, either in July or September. Neither melatonin (10(-9) to 10(-6) M) n or norepinephrine (10(-8) to 10(-5) M, in the presence of the beta-adr enoceptor blocker, propranolol) were effective lightening agents. Thes e results suggest that alpha-MSH probably is the physiological melanot ropin regulating color change for thermoregulation in U. ornatus and t hat there is no synergistic action between alpha-MSH and catecholamine s that might be responsible for the intensity of darkening by socially dominant males. The seasonal differences in sensitivity of sexually a ctive and non-sexually active animals indicate that gonadal hormones m ay have a facilitatory role for the responsiveness of the skins of U. ornatus to melanotropic hormones.