Velando, A., R. Beamonte-Barrientos, and R. Torres. 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.

This study was conducted on the blue footed booby (Sula nebouxii), an iteroparous species that forms monogamous pairs. Female blue footed boobies lay two to three eggs a year and the young are cared for by both parents for a relatively extended period of time. The article discusses that iteroparous organisms are predicted to adjust their investment in offspring based on the potential negative impacts on future reproduction through continuous evaluation. Female blue footed boobies select males based features including the color of the feet; a dull blue is undesirable and a bright green color resulting in a female permitting increased copulation. Researchers theorized that carotenoids at least partially control foot color and that this attribute is a rapid, honest indicator of potential male investment in offspring and/or genetic quality. The researchers theorized that females would continue to evaluate their investment in reproduction based on assessment of their mate’s feet color and that females would adjust their investment in breeding downward with a perceived decrease in male quality.

Researchers first investigated the source of variation in male foot color and the speed with which that color changed. Researchers experimentally allocated captive birds into a group which was not fed for 48 hours, a group that was fed a carotenoid-poor diet, and a group that was fed a carotenoid-rich diet. Foot color varied with nutritional condition and carotenoid consumption, foot color was affected by the end of the 48 hour period. Food deprived males’ foot color reverted to a nearly-original color within 24 hours of restoration of diet.

Subsequently, researchers took males from breeding pairs after the first egg was laid and either experimentally dulled the color of the feet or conducted a sham procedure. The weight of the first and second egg was then measured. The second eggs produced by females mated to a modified male were significantly smaller than those produced by females that were mated to a control male. Smaller eggs have a lower hatching success rate and offspring have a lower survival rate. Smaller eggs also require less initial investment by females to produce. Researchers concluded that the foot color of male blue footed boobies was indeed a rapid and honest indicator of male quality that is evaluated by females not only during courtship but continually in order to adjust female investment in offspring based on perceived direct or indirect benefits likely to be proffered by the male.

We discussed in class that because of anisogamy, which is different gamete sizes between males (smaller) and females (larger), the greater cost of reproduction to females should cause them to more carefully regulate their reproductive activity. This article indicates that the constant evaluation of mate quality is one such regulation as current investment in egg production and offspring-rearing appears to have a greater impact on future reproduction by females. Based on this article, mating in the blue footed booby has all the hallmarks of intersexual selection as discussed in class and the book: females have control over mating and males use elaborate mating displays and ornamental structures (blue feet) to attract females. As this article discusses, female control in mating appears to be the case, as one would expect in an intersexual selection system. Here the researchers theorized that either direct benefits (improved assistance with raising offspring) or indirect benefits (higher quality genes) were the reason females would continually evaluate and adjust their investment in reproduction. To me it seems much more likely that direct benefits play a role here, these individuals are socially monogamous and the female is reducing the egg size and therefore likely decreasing the number of offspring that must be cared for by with the help of a male that has presumably at least temporarily become less fit. It seems to me that the adjustment a female would make if compensating for mating with a male with low-quality genes would be to increase extra-pair copulations, thereby obtaining genetic material from a higher-quality male.