Blue-footed Booby - Foot Pigmentation

Foot Pigmentation

The blue color of the webbed feet in Blue-footed Boobies comes from carotenoid pigments obtained from the diet. The pigments are antioxidants and stimulants for the immune system, so there is a trade-off between immune function and signaling, since any incorporation of the pigments into the feet is a direct detraction from those that could be used for immunity and detoxification. The blue feet are sexual signals that reliably indicate the condition of the male boobies, and coloration is favored through sexual selection. The brightness of the feet decreases with age, so females tend to choose males with brighter feet (younger males). Females prefer younger males because they have higher fertility and increased ability to provide paternal care than older males. In a cross-fostering experiment, it was shown that foot color really does reflect paternal contribution to raising chicks, because chicks raised by foster fathers with brighter feet grew faster than chicks raised by foster males with duller feet. Blue feet also indicate the current condition of males. Those that were experimentally food-deprived for forty-eight hours experienced a decrease in foot brightness. This is most likely because the deprivation of food reduces the amount of lipids and lipoproteins in the body which are used to absorb and transport carotenoids. Thus, the feet are rapid and honest indicators of a booby’s current level of nourishment.

Researchers found that females continuously evaluate their partners’ condition based on foot color. In an experiment, males whose partners had laid a first egg in the nest had their feet dulled by make-up. Consequently, the females laid smaller second eggs a few days later. Since duller feet usually indicate a decrease in health and possibly genetic quality, it is adaptive for females to decrease their investment in the second egg. The smaller second eggs contained not only less yolk concentration, which could in turn influence embryo development, hatching success, and subsequent chick growth and survival, but also contained less yolk androgens. Androgen plays an important role in chick survival, so this also shows that female Blue-footed Boobies use the attractiveness of their mates to determine how much resources they should allocate to their eggs. This possibly supports the "Differential Allocation Theory", which predicts that parents would invest more in the care of their offspring when paired with attractive mates.

Females are not the only ones assessing their partner’s reproductive value. Males, too, evaluate females and adjust their own investment in the brood according to their mate’s condition. Like males, females also have a trade-off between signaling ornamentations and fecundity, because paler females actually produce more eggs in a brood. Females that lay larger and brighter eggs are in better condition and have greater reproductive value. Therefore, males tend to display higher attentiveness and parental care to larger eggs, since those eggs were produced by a female with apparent good genetic quality. Smaller, duller eggs garnered less paternal care. Female foot color was also an indication of female condition. In an experiment where the color of the eggs was muted by researchers, it was found that males were willing to exercise similar care for both large eggs and small eggs if his mate had brightly colored feet, whereas males paired with dull-footed females only incubated larger eggs. Interestingly, researchers also found that males did not increase their care when females exhibited both bright feet and high-quality offspring.