Juola FA, McGraw K, and Dearborn DC. 2008. Carotenoids and throat pouch coloration in the great frigatebird (Fregata minor). Comparative Biochemistry and Physiology: Part B 149: 370-377.

In the February 24 lecture on light signal reception, we covered a list of pigments found in many phyla and examples of animals that utilize each pigment type for communication. Porphyrins, characterized by their coordination complexes of metallic ions and neutral/negatively charged organic groups, were represented by the great frigatebird (Fregata minor). Males of this large pelagic species attract potential mates by inflating gular pouches that appear bright red during the breeding season. This coloration was discussed as being due to increased blood flow in the pouch during the breeding season. However, I recalled that when visiting Central America and seeing similar birds, I was told the red color may also have been due to an influx of carotenoids, which also reflect red, orange, and yellow light.

To solve the controversy over great frigatebird gular pouch coloration, I consulted the Juola et al article, which sought to quantify how much of the male frigatebird’s skin coloration was due in part by carotenoids. These pigments are not biologically manufactured by the birds, but instead must be derived from the diet. The authors noted that they were unsure how exactly frigatebirds would acquire these carotenoids, but that they could assume their diets to be particularly rich in squid and crustaceans, both of which have high concentrations of a carotenoid example we saw in class: astaxanthin. Thus, frigatebird advertisement of carotenoid enhanced coloration could constitute an “honest” signal of reproductive fitness, presumably broadcasting a male’s ability to catch food and perhaps, his future suitability for feeding chicks.

Juola et al conducted a series of high pressure liquid chromatography (HPLC) and spectrophotometric tests utilizing blood and tissue samples from male and female great frigatebirds sampling on Tern Island in northwestern Hawaii. Blood was from birds identified as either non-breeding (with dull plumage, males not displaying) or breeding (with freshly molted plumage, males either displaying or incubating eggs and females actively searching for mates). Gular pouch tissue samples were acquired from recently dead males found on the island and preserved at -20ºC.

HPLC results of blood samples did indeed recover three separate carotenoids: astaxanthin, tunaxanthin, and zeaxanthin, which were identified by reflectance spectra. Of these carotenoids, astaxanthin pigment was most common in the plasma of the birds, at an average of 0.87 micrograms per milliliter or about 85 percent of the total carotenoid volume. This figure tended to be higher for males courting, incubating eggs, or simply not taking part in courtship displays at all, but higher for females only when courting. Carotenoids are known components of egg yolk, and seem to function in immune response, so it may make sense that courting females would have slightly higher circulating levels of carotenoids in the blood, as they are preparing to lay eggs.

In the gular pouch tissue of males, only astaxanthin was found, but the concentration was very high—with a mean of 1269 micrograms per gram of tissue, way higher that average circulatory levels of the same chemical. Some outliers had concentrations exceeding 3000 micrograms per gram tissue. Although the red color is indeed augmented in part by blood circulation to the pouch, to the knowledge of the authors, this is actually the highest carotenoid concentration found in unfeathered bird tissue ever. Juola et al made some postulations as to why the level of circulating carotenoids seemed so low in male courting birds—perhaps they were sequestering dietary carotenoids in the pouch during courting, and these were slowly released back into the blood stream after acquiring a mate.

My only criticism with this study was that all tissue pouch samples were from males found dead on the Tern Island colony, and there seemed to be no attempt to quantify age or time of dead with each sample. I would expect only recently dead individuals that were displaying prior to death to approximate the carotenoid content of living male frigatebirds. If the carotenoids are such an “honest signal” why would males with such high astaxanthin content be dead? I would find the results of a live bird tissue analysis, hopefully using samples acquired with minimal invasion (some sort of thin film/swab and culture technique?) to be much more convincing of the true levels of carotenoids in the live male frigatebird gular pouch.