The white colour of the Ostrich (Struthio camelus) egg is a trade-off between predation and overheating

Abstract

Most ground nesters lay pigmented eggs, and egg pigmentation generally matches the environment. Pigmentation of eggs has evolved as a protective device against predation, but dark-pigmented eggs can be susceptible to overheating when exposed to solar radiation. The Ostrich (Struthio camelus) lays white eggs that are unattended for the first few weeks before incubation, and are quite visible to predators. To evaluate the effect of colour on the surface and core temperatures, we painted some Ostrich eggs dark brown or white, and left some unpainted (control), and exposed all of them directly to the sun during the day. The surface and core temperatures of brown eggs were significantly higher than those of the white-painted and control eggs. In addition, the core temperature of brown eggs exceeded 37.5°C, which is the temperature at which embryo mortality starts to increase. In a second experiment, we placed eggs (brownpainted and control) in various types of vegetation to study their visibility to an observer walking towards them. The white eggs were discovered from a significantly longer distance than the brown eggs, indicating that the predation risk may be much higher for white eggs. The results thus suggest that white eggs minimise overheating and allow the Ostrich to leave its eggs unattended before incubation starts, but they are more susceptible to predation.

Effect of egg weight and amount of protoporphyrin and biliverdin in the egg shell on hatching characteristics and embryonal mortality in pheasants (Phasianus Colchicus)

“…Therefore, it was determined that crude protein was no effective on middle period embryonic mortality. However, it was reported that ambient heat absorbtion by dark eggshells was higher than by light coloured eggshells (24). Also, the avian embryos produced more radiant heat during growth in the hatching period.…”

Review: an embryo's eye view of avian eggshell pigmentation

“…Most research into the effects of eggshell pigmentation on embryonic development in wild birds has focussed on pigmentation as a means to regulate the temperature of the egg contents by reflecting sunlight (Bakken et al 1978, Bertram and Burger 1981, Westmoreland et al 2007, Lahti 2008, Magige et al 2008). As bird eggs are more tolerant to lower than to higher temperatures (reviewed by Webb 1987), the main thermoregulatory function of pigments is proposed to be the prevention of overheating the egg contents rather than protection from cooling e.g.…”

“…during incubation recesses by the participating parent. Generally, any eggshell pigmentation reduces the reflection of infra‐red wavelength light compared to the light reflection of pure white eggshells and thus increases the risk of overheating the egg by irradiation (Bakken et al 1978, Magige et al 2008). However, the protoporphyrin and biliverdin pigments employed in eggshell coloration differ from the carotenoids and melanins found in other avian tissues by providing a higher near infra‐red reflectance (Bakken et al 1978) and thus a reduced risk of overheating the egg contents.…”

Conspicuous Eggs and Colourful Hypotheses: Testing the Role of Multiple Influences on Avian Eggshell Appearance

“…Considering the central role of the egg in a bird's lifecycle and the diversity of behavioural, developmental and ecological adaptations to breeding, it would be astounding to find that a single adaptive function explains the pigmentation of any species' eggshells. While most modern studies of eggshell appearance acknowledge the fact that multiple hypotheses may be required to explain eggshell appearance, very few heed it in study design or the questions addressed (but see Bakken et al, 1978;Lahti, 2008;Magige et al, 2008). Instead, most studies choose species with seemingly clear cut selection pressures, or base their research on alternative implicit or explicit assumptions concerning the egg's environment.…”