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Physiological (rapid) change of color in horned lizards

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Rapid color-change in three congeneric desert lizards of diverse size was studied and compared. Using in vitro skins, the highest % skin-darkening values (15-19%) of dose-response curves for α-melanophore stimulating hormone (α-MSH) were determined for each species. In the smallest species, Phrynosoma modestum, melanophores responded to α-MSH over the broadest range, 2.5 x 10-10- 6.5 10-8 M. Isoproterenol, a β-adrenoceptor agonist, was less effective than α-MSH at darkening skins, 5% in P. cornutum. Norepinephrine (NE), an α-adrenoceptor agonist, dramatically lightened α-MSH darkened skins of P solare and P modestum. Catecholamine responses were further investigated with the α-adrenoceptor blocker Dibenamine and the β-adrenoceptor blocker oxprenolol. Phrynosoma cornutum did not respond to NE. The effects of temperature on α-MSH activity were studied in vitro and in vivo. Skins in vitro darkened most rapidly in response to α-MSH at 41°, to an intermediate degree at 22°, and least at 6°C. Lizards in vivo exhibited the darkest color at 22°, lightest at 41°, and intermediate color at 6°C. A potent α-MSH receptor agonist, [Nle4, D-Phe7]-α-MSH, was used to further study the significance of temperature on in vivo α-MSH regulation of color. Both P. cornutum and P. modestum placed on black-and-white backgrounds for twenty-four hours failed to show background matching. Diel color changes associated with thermoregulatory needs are apparently consistent with crypsis, an antipredator strategy; cool lizards darken (to increase solar thermal gain) in early morning and late afternoon when background shadows are prominent, and warm lizards lighten (to reduce solar thermal gain) at midday when the albedo of the environment is high.

Affiliations: 1: Southwestern Research Station, The American Museum of Natural History, Portal, Arizona 85632, USA, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, USA


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