Velando, A., Beaumonte-Barrientos, R., & Torres, R. (2006). Pigment-based skin colour in the blue-footed booby: an honest signal of current condition used by females to adjust reproductive investment. Oecologia, 149, 535-542.

In this study, the authors investigated whether foot color variation in the male blue-footed booby serves as an honest signal of mate quality for female blue-footed boobies. To begin with, the authors attempted to determine whether or not foot color was structural or resulted from carotenoid ingestion, as they suspected. To do this, the authors extracted pigments from a foot skin sample and used a spectrophotometer to determine if the absorption spectra matched that of carotenoids. In addition, the authors manipulated the diets of 20 courting males kept in captivity by controlling the amounts of carotenoids ingested and then estimated foot color. The authors then wanted to determine whether foot color had any effect on female egg investment. To do this, the authors captured a courting male the day after the female’s first egg was laid and modified their foot color to a dull blue, simulating a male in poor condition. They then measured the female’s investment in a second egg. It had been previously shown that females lay heavier second eggs when ecological conditions are good, and lighter second eggs when food is limited.

Results from the first two experiments revealed that the extracted pigments matched the spectral-absorbance profile of carotenoids, and that the foot color of the captured males varied with the amount of carotenoids ingested after only 48 hours. The third experiment showed that females did in fact lay lighter second eggs when mated with males that were experimentally manipulated to have duller colored feet. Taken together, these results suggest that blue-footed booby females use the color of males’ feet as an honest signal of mate quality, and alter their investment depending on food color.

This paper pertains to our lecture and chapter in the textbook on signal honesty.

In lecture, we talked about another example in which carotenoid pigmentation was used as an honest signal: House finches use the brightness of carotenoid patches to signal a male’s foraging ability. We also talked about why senders would want to send honest signals when they could potentially cheat. Classical ethologists, such as Lorenz thought that all signals were honest because the sources of these signals were linked to the motivations of the sender, whereas in the 1970s, Dawkins & Krebs, and others, hypothesized that senders should be deceitful and therefore, an arms race occurs between signalers and discriminating receivers. However, Zahavi’s view was that receivers should only respond to signals when they are honest. One possible guarantee of honesty would be to produce a signal that requires some cost to the sender. Zahavi’s name for these costly signals is a handicap. Zahavi and Grafen contended that honest signaling evolves when the signaling is costly to males, costs to low quality males are higher than for higher quality males, and high quality males have higher probability of mating. In this paper, the authors postulated that because carotenoids are antioxidants and immunostimulants, blue-footed boobies investing carotenoids in sexual signals are doing so at the cost of lowering their immune response. In fact, when the authors manipulated the amount of carotenoids in the diets of the male blue-footed boobies, they found that those males with lower amounts of carotenoids had a lower cell-mediated immune response.