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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

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Summary 1.Modelling thermal environments at high resolution becomes simpler when using operative temperature, which condenses microclimate and morphology into an index of thermal stress. Operative temperature can be mapped using large numbers of 'operative temperature thermometers', hollow models that duplicate external properties of the animal. 2. As climatologists predict that air will warm by 2-4°C by 2100, biologists must be able to distinguish climate change from systematic errors in operative temperature of the same magnitude. 3. A systematic error in operative temperature of 2°C or a similar amount of climate warming can change predicted surface activity and indices of habitat quality, thermoregulatory precision and predation risk by 5-12%, and in some cases more than 30%. 4. As construction details of operative temperature thermometers can affect their accuracy by 2°C or more, biologists should use detailed physical models calibrated against living animals over potential ranges of postures, orientations and microclimates. 5. Water-filled models do not measure operative temperature correctly, fail to capture thermal extremes and are an unnecessary complication as one can easily compute the body temperature of moving or stationary animals from body mass and the spatio-temporal distribution of operative temperatures.

Studies of the spectral reflectance of the eggs of 25 species of birds from nine families disclosed uniformly high reflectance (often above 90 percent) in the near infrared. This property is associated with the presence of the eggshell pigments protoporphyrin and the bilins. These pigments allow coloration for cryptic or other purposes with minimum solar heating, a combination not possible with the melanin pigments typical of vertebrates.

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