Abstract
Solar radiation is an important driver of animal coloration, not only because of the effects of coloration on body temperature but also because coloration may protect from the deleterious effects of UV radiation. Indeed, dark coloration may protect from UV, but may increase the risk of overheating. In addition, the effect of coloration on thermoregulation should change with egg size, as smaller eggs have higher surface‐volume ratios and greater convective coefficients than larger eggs, so that small eggs can dissipate heat quickly. We tested whether the reflectance of eggshells, egg spottiness, and egg size of the ground‐nesting Kentish plover Charadrius alexandrinus is affected by maximum ambient temperature and solar radiation at breeding sites. We measured reflectance, both in the UV and human visible spectrum, spottiness, and egg size in photographs from a museum collection of plover eggshells. Eggshells of lower reflectance (darker) were found at higher latitudes. However, in southern localities where solar radiation is very high, eggshells are also of dark coloration. Eggshell coloration had no significant relationship with ambient temperature. Spotiness was site‐specific. Small eggs tended to be light‐colored. Thermal constraints may drive the observed spatial variation in eggshell coloration, which may be lighter in lower latitudes to diminish the risk of overheating as a result of higher levels of solar radiation. However, in southern localities with very high levels of UV radiation, eggshells are of dark coloration likely to protect embryos from more intense UV radiation. Egg size exhibited variation in relation to coloration, likely through the effect of surface area‐to‐volume ratios on overheating and cooling rates of eggs. Therefore, differential effects of solar radiation on functions of coloration and size of eggshells may shape latitudinal variations in egg appearance in the Kentish plover.
Highlights
One of the most thoroughly studied topics of animal coloration is that of melanism, the occurrence of individuals that are darker in pigmentation
Consistent with this hypothesis, a geographical variation in the degree of melanism has been found in various taxa of ectotherms, with darker individuals being found at higher latitudes and lighter ones at lower latitudes (Alho et al, 2010; Brakefield, 1984; Moreno Azócar et al, 2015; Rapoport, 1969)
The effect of coloration on thermoregulation should change with egg size, so that there would be a positive relationship between egg size and dark coloration (Gates, 1980), a prediction supported by theoretical models and empirical evidence (Clusella-Trullas et al, 2008; Schweiger & Beierkuhnlein, 2016)
Summary
One of the most thoroughly studied topics of animal coloration is that of melanism, the occurrence of individuals that are darker in pigmentation. Because of this there are some biophysical mechanisms with which birds may counteract the negative effects of high temperature on embryos’ overheating One of such mechanisms is eggshell color, as lighter eggs overheat less quickly than darker eggs when exposed to direct solar radiation (Gómez et al, 2016; Lahti & Ardia, 2016; Montevecchi, 1976). The effect of coloration on thermoregulation should change with egg size, so that there would be a positive relationship between egg size and dark coloration (Gates, 1980), a prediction supported by theoretical models and empirical evidence (Clusella-Trullas et al, 2008; Schweiger & Beierkuhnlein, 2016) Another biophysical mechanism with which birds may counteract the negative effects of high temperature on embryos’ overheating is egg size, as smaller eggs have higher surface-volume ratios and greater convective coefficients than larger eggs, so that small eggs can dissipate heat quickly We tested whether there are variations in egg size in relation to eggshell color, so that lighter eggs should be smaller, given the thermal advantages of such eggs in hotter environments
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