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Understanding how quickly physiological traits evolve is a topic of great interest, particularly in the context of how organisms can adapt in response to climate warming. Adjustment to novel thermal habitats may occur either through behavioural adjustments, physiological adaptation or both. Here, we test whether rates of evolution differ among physiological traits in the cybotoids, a clade of tropical Anolis lizards distributed in markedly different thermal environments on the Caribbean island of Hispaniola. We find that cold tolerance evolves considerably faster than heat tolerance, a difference that results because behavioural thermoregulation more effectively shields these organisms from selection on upper than lower temperature tolerances. Specifically, because lizards in very different environments behaviourally thermoregulate during the day to similar body temperatures, divergent selection on body temperature and heat tolerance is precluded, whereas night-time temperatures can only be partially buffered by behaviour, thereby exposing organisms to selection on cold tolerance. We discuss how exposure to selection on physiology influences divergence among tropical organisms and its implications for adaptive evolutionary response to climate warming.

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Total surface of the animal [equation (Z)]. Effective radiating area of the animal [equation (2)]. Proportionality constant between K. and Kz [equation (27)]. Heat capacitance of the animal [equation (711. Diameter of the bodv of the animal. excluding appendages (Table 2). Total heat lost by evaporation [equation (1411. Evaporative heat loss from the respiratory system [equation (7)]. Evaporative heat loss from the outer surface [equation (5X. Evaporative heat loss from the skin surface of a furred animal [equation (6)]. Efficiency of evaporation at the outer surface of a furred animal [equation (23% Efficiency of evaporation at the outer surface of a naked animal [equation (24)]. Efficiency of evaporation at the skin surface of a furred animal [equation (22)]. Heat transfer coefficient to the ground [equation (711. Convective conductance, free or forced [section 2(B); equation (511. Free convection conductance [section Z(B)]. Forced convection conductance [section 2(8)]. Turbulence intensity in the direction of air flow [equation (25)]. Thermal conductance [equation (l)]. Thermal conductance of fur or leather layer [equation (5)]. Newton's Law dry thermal conductance [equation (1411. Overall thermal conductance [equation (lo)]. Overall thermal conductance under standard conditions [equation (19% Thermal conductance of skin layer [equation (611. Thermal conductance of skin and fur/feather layers [equation (ll)]. Conductivity of fur or feather layer [Table 21. Conductivity of skin layer [Table Z]. Metabolic heat production [equation (7)]. Oxygen consumed by metabolic processes [section 3(A)]. Effective net metabolic heat production [equation (13)]. Effective net metabolic heat production under standard conditions [equation (19)]. Superscript used to note that a parameter applies to a taxidermic model [section 4(~)1. Numerical correction factor used in linearizations [equation (3)]. Time period [equation (26)]. Rate of heat flow [equation (l)]. Absorbed thermal and solar radiation [equation (4)]. Emitted thermal radiation [equation (211. Net rate of heat transfer bv all radiative nrocesses when T, = F,:, if N = 0 [equation A HEAT TRANSFER ANALYSIS OF ANIMALS 341