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Comparative Biochemistry and Physiology, Part B 149 (2008) 370 – 377
www.elsevier.com/locate/cbpb

Carotenoids and throat pouch coloration in the great
frigatebird (Fregata minor)
Frans A. Juola a,⁎, Kevin McGraw b , Donald C. Dearborn caDepartment of Biology, University of Miami, Coral Gables, FL, USA
School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USAc
Department of Biology and Program in Animal Behavior, Bucknell University, Lewisburg, PA, USAb

Received 4 September 2007; received in revised form 24 October 2007; accepted 24 October 2007
Available online 30 October 2007

Abstract
Carotenoid pigments are a common source of red, orange, and yellow coloration in vertebrates. Animals cannot manufacture carotenoids andtherefore must obtain them in their diet to produce carotenoid-based coloration. Male great frigatebirds (Fregata minor) display a bright red
inflated gular pouch as part of their elaborate courtship display. The basis of this coloration until now has not been investigated. Using highperformance liquid chromatography (HPLC), we investigated the types and concentrations of carotenoids that great frigatebirds circulate in theirplasma and whether male gular pouch coloration was carotenoid-based. Great frigatebird plasma collected during the breeding season contained
three carotenoid pigments in dilute concentrations—tunaxanthin, zeaxanthin, and astaxanthin—with astaxanthin accounting for nearly 85% of the
carotenoids present. Astaxanthin was the only carotenoid present in gular pouch tissue, but the concentration is the highest reported for any
carotenoid-pigmented avian tissue. Throat pouch reflectance curves were measured with a UV-VIS spectrophotometer, revealing a complex
pattern of one UV peak (approx. 360 nm), two absorption valleys (approx. 542 and 577 nm), followed by a plateau at approx 630 nm. The
reflectance curve suggests a role for additional pigments, in particular hemoglobin, in the production of color in this ornament.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Carotenoid pigments; Astaxanthin; Color signals; Sexual selection; HPLC

1. Introduction
Carotenoids are natural pigments that produce yellow, orange and red coloration in animals and are known to be an
important source of ornamental coloration in fish, lizards, andbirds (Endler, 1980; Kodric-Brown, 1989; Hill et al., 2002;
McGraw et al., 2004). Animals cannot manufacture carotenoids, which therefore must be obtained through the diet (Fox,
1979). In birds, carotenoid-based colors are common in feathers
and bare parts (McGraw, 2005), and in some species, it has been
demonstrated that color intensity is a direct product of theoverall concentration of carotenoid pigments present (Sakset al., 2003). Furthermore, experimental evidence indicates that
⁎ Corresponding author. Department of Biology, Cox Science Center, Room
73, University of Miami, Coral Gables, FL 33146, USA. Tel.: +1 305 284 2041;
fax: +1 305 284 3039.
E-mail address: frans@bio.miami.edu (F.A. Juola).
1096-4959/$ - see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.cbpb.2007.10.010

consumption of carotenoids enhances coloration (Hill et al.,
2002; Blount et al., 2003; McGraw and Ardia 2003; AlonsoAlvarez et al., 2004).
In addition to their role in color production, carotenoids
serve a variety of important physiological functions including
multiple stimulating effects on the immune system (Bendich,
1989; Olson and Owens, 1998; Hill, 1999a). Empirical evidence in birds has demonstrated an association between carotenoids and immune activity (McGraw and Ardia, 2003,
2005; Alonso-Alvarez et al., 2004). The dual functions of color
production and immunostimulation suggest that carotenoidbased coloration could convey honest information about individual quality by way of a physiological tradeoff between
expression of ornamental color and health status (Lozano,1994). Hypotheses on the costs associated with carotenoidbased coloration, however, remain controversial because solittle is known about their intake (Lozano, 1994; Grether et al.,
1999; Hill, 1999a; Hill et al., 2002; Goden and McDonough,F.A. Juola et al. / Comparative Biochemistry and Physiology, Part B 149 (2008) 370–377

2003). Recent advances in spectrophotometric technology allow
for more objective and quantitative assessment of color whileconsidering the range of avian visual perception (Andersson andPrager, 2006).
In sexually dichromatic bird species, in which males exhibit
brighter, carotenoid-based colors, males typically circulate
higher levels of carotenoids in their blood than do females(Hill, 1995; Bortolotti et al., 1996; Figuerola and Gutierrez,
1998; Negro et al., 1998; McGraw and Ardia, 2005). Seasonalpatterns of carotenoid concentrations have been documented inboth sexes, with carotenoid concentrations in blood typically
being highest during the breeding season when the expressionof colorful traits is most pronounced (Hill, 1995; Negro et al.,1998).
A majority of studies investigating the significance of ornamental coloration in birds have focused on colorful species
from the order Passeriformes (reviewed in Hill, 1999b, and Hilland McGraw, 2006). More recent studies are beginning to shedlight on ornamental coloration in non-passerine birds (Negro
et al., 2002; Massaro et al., 2003; Blas et al., 2006; Bortolotti
et al., 2006; Velando et al., 2006; Mougeot et al., 2007). The
great frigatebird (Fregata minor) is a large marine bird from theorder Pelecaniformes that exhibits a high degree of sexual
dimorphism. Males possess conspicuous sexual ornaments, including a bright red gular pouch that is inflated during elaborate

371

courtship displays (Nelson, 1975). These courtship displays are
conducted in dense aggregations where females can assessmany potential mates at the same time (Nelson, 1975). Greatfrigatebird males exhibit dramatic seasonal variation in gularpouch appearance. At the onset of the breeding season, pouches
become bright red and extend down greater than 5 cm from thethroat (F. Juola pers. obs., Fig. 1B). Red coloration remains
intense while males actively display over the course of weeks ormonths (Fig. 1A and B). Once a male secures a mate, the gularpouch retracts into the throat (Fig. 1C) and the color begins tofade rapidly from red to orange (Fig. 1D). This decline inornamentation occurs over the course of several weeks (F. Juolapers. obs.). Throughout the non-breeding season, male gularpouches remain small and dull-orange in color. The rapid shift
in coloration after mating suggests that development and/or
maintenance of this bright red color is costly.
This striking ornament is assumed to be an example of sexual
selection by way of female choice (Nelson, 1975; Dearborn and
Ryan, 2002; Madsen et al., 2007). The goal of this study was toinvestigate the source of red coloration in male great frigatebird
gular pouches in hopes of shedding light on the possible signaling functions of this color ornament. Specifically, we tested
whether male gular pouch coloration was carotenoid-based
through identification and quantification of what, if any, carotenoids were circulating in blood and deposited in gular pouch

Fig. 1. A. Upper left: Male great frigatebird (F. minor) displaying a bright red, inflated gular pouch on Tern Island, French Frigate Shoals. B. Upper right: Between
bouts of displaying, gular pouch is extended from the throat and remains bright red. C. Lower left: After pair formation, the gular pouch is deflated and begins to retract
into the throat. C. Lower left: Within days of pair formation, pouch coloration begins to fade from red to orange.

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tissue. In addition, we tested whether differences exist in circulating carotenoid levels between males and females of three differing physiological states: courting, incubating, and non-breeding.
Integumentary coloration is predicted by levels of carotenoids inplasma for at least six bird species (reviewed in McGraw, 2006). In
dichromatic bird species that exhibit carotenoid-based coloration,colorful males commonly circulate higher levels of carotenoids
than do females (Bortolotti et al., 1996; McGraw et al., 2003;
McGraw and Ardia, 2005). In addition, females may circulate
higher levels of carotenoids during the physiological state prior to
egg-laying, in order to divert carotenoid pigments into egg yolk
(Blount et al., 2000).
Finally, we describe the spectral curve of the male gularpouch and investigated whether any spectral properties of thiscurve might provide further insight into the basis of this colorful
ornament. Pigments absorb light according to characteristic
absorption functions, the patterns of which can provide insight
into the types of pigments present in animal tissues (Andersson
and Prager, 2006). In particular, carotenoids absorb light in theblue–green wavelengths (430–500 nm) and consequently produce distinct reflection peaks in the yellow, orange, and red
wavelengths (McGraw et al., 2007).
2. Materials and methods
We collected blood and tissue samples for this study on Tern
Island, French Frigate Shoals, in the northwestern Hawaiian
Islands. There are roughly 2000 nesting attempts by great frigatebirds (Fregata minor) on this 14-ha island each year
(Dearborn and Anders, 2006). One hundred and twelve adultgreat frigatebirds of known breeding status (61 displaying
males, 10 incubating males, 11 non-breeding males, 10 searching females, 10 incubating females, 10 non-breeding females)
were captured, either by hand or with a hand net, between 0900
and 1400 h from February–March 2006. Breeding status wasdetermined by behavior and plumage. Displaying males exhibit
bright red inflated gular pouches and freshly molted black
plumage, while non-breeding males exhibit retracted, dull-orange pouches and an overall faded brownish-black plumage
color. Females seeking mates fly low around the breedingcolony and appear to be visually inspecting males with inflated
throat pouches. These females also exhibit freshly molted black
plumage; plumage of non-breeding females is also faded
brownish-black in color. At the time of capture, approximately
1 mL of blood was collected from the brachial vein of each bird.
Plasma was separated from blood by centrifugation of blood
samples at 12,000 g for 5 min. Centrifugation was completed
within 4 h of blood collection, and all plasma samples werethen stored at − 20 °C until carotenoid analysis approximately
4 months later. Eight gular pouch tissue samples were collectedfrom adult male great frigatebirds found dead (within ∼24 h)in the breeding colony from 1999–2006. Pouch samples werestored at −20 °C.
The types and amounts of carotenoids in plasma and gularpouch tissue were analyzed using high-performance liquid
chromatography (HPLC). Plasma analysis followed previously
published methods (McGraw et al., 2002, 2006), with the ex-

ception of using 50 μL of plasma from each sample for carotenoid extractions (because pigment concentrations were solow). Briefly, reverse-phase chromatography was run with a
Waters Alliance 2695 autosampler instrument (Waters Corporation, Milford, MA, USA) equipped with a Waters YMC Carotenoid C-30 5 um column (4.6 × 250 mm) that was heated to
30 °C with a built-in column heater. We used an isocratic
solvent system (42:42:16, methanol:acetonitrile:dichloromethane, v/v/v), at a flow rate of 1.2 mL/min for 12 min, toanalyze polar carotenoids after initial tests revealed that no nonpolar carotenoids (i.e. cryptoxanthins or carotenes) were present. The detection limits of our system were 0.01 μg/mL for
plasma analyses and 0.01 μg/g for pouch analyses.
For each gular pouch, a 0.5 g cube of flesh was sectioned andprocessed as follows: The gular pouch tissue was ground inpresence of 2 mL hexane:TBME (1:1) for 5 min. The solvent
was transferred to a new tube, while the grinding jar was rinsed
with 1 mL hexane: MTBE to recover any residual pigment. The
solvent was then centrifuged at ca. 500 g for 5 min. and transferred to a new tube where it was evaporated to dryness under
nitrogen, then resuspended in 1 mL 0.02 M KOH in order tosaponify (attempts to analyze carotenoids from unsaponified
material were unsuccessful). It was then placed in darkness atroom temperature for 4 h, after which 1 mL saturated NaCl wasadded. The solution was vortexed, 2 mL ddH2O was added, andthe solution was again vortexed. Next, 2 mL MTBE was added
and the solution was shaken vigorously for 1 min. This solution
was then centrifuged at ca. 500 g for 5 min, and finally evaporated to dryness. Standard HPLC methods (McGraw et al.,2006) were then followed exactly as for the plasma samples.
Reported carotenoid concentrations in pouch tissues refer to dry
weight of the starting material.
During the mating season of 2007, reflectance curves wereobtained from 10 displaying male and 10 incubating male great
frigatebirds using a UV-Vis spectrophotometer (USB4000, configured range 250–850 nm, Ocean Optics Inc.) with a pulsed
xenon lamp (PX-2) as a light source. Using a black rubber
sheath that excluded ambient light, we held a bifurcated micron
fiber-optic probe 3 mm from and perpendicular to the pouch
tissue. All measurements were conducted indoors, in a dark

Fig. 2. Representative two-dimensional HPLC chromatogram depicting carotenoids detected in the plasma of frigatebirds.

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373

Table 1Comparison of plasma carotenoid concentrations and percent composition ofeach carotenoid present in male and female great frigatebirds
Males (n = 82)

Females (n = 30)

Concentration (μg/mL)
Astaxanthin0.97 ± 0.05
0.59 ± 0.07
Tunaxanthin0.18 ± 0.03
0.13 ± 0.02
Zeaxanthin0.04 ± 0.004
0.03 ± 0.01
Total carotenoids
1.18 ± 0.07
0.75 ± 0.49
Percent composition (of total plasma carotenoids)
Astaxanthin
82.00 ± 0.01
78.88 ± 0.02
Tunaxanthin
14.94 ± 0.01
16.71 ± 0.02
Zeaxanthin
3.06 ± 0.003
4.41 ± 0.01

ZP

− 4.69− 0.83− 0.53− 3.86b0.0010.41
0.60
b0.001− 2.10− 1.70− 0.86

0.04
0.90
0.39

Mann–Whitney U tests were used to compare differences between males andfemales (Z-statistic). Mean ± SE are given for each group.

room. Each curve was generated relative to a white standard(WS-1, Ocean Optics). Using SpectraSuite software (Ocean
Optics), we averaged 20 sequential spectra for each of fivearbitrarily selected locations on each pouch sample. To obtain an
average spectral curve for displaying males and for incubating
males, the median percent reflectance for each 10 nm increment
was averaged for the 10 individuals from each category, acrossthe wavelengths relevant to the avian visual spectrum (300–700 nm).
2.1. Statistical analysis
We used SYSTAT 11 (Systat Software, Inc.) for all statisticalanalyses. We used Mann–Whitney U tests to compare differences in the carotenoid concentrations and percent composition of each carotenoid present in the plasma of male and female

Fig. 4. Representative two-dimensional HPLC chromatogram depicting
carotenoids detected in the pouch tissue of frigatebirds.

great frigatebirds. We used a two-way ANOVA to compare
differences between sex and three time periods. We conducted
post-hoc analyses of differences between individual groups
using Fisher's least significant difference test.
3. Results
Great frigatebird blood plasma contained three carotenoid
pigments: astaxanthin, tunaxanthin, and zeaxanthin (total =1.06 ± 0.06 μg/mL; mean ± SE). Astaxanthin accounted for84.5 ± 0.010% of the total plasma carotenoids (0.87 ± 0.04 μg/mL).

Fig. 3. Great frigatebird plasma carotenoid concentrations by sex and breeding
status. Sample sizes were courting males (61), courting female 10), incubating
males (10), incubating females (10), non-breeding males (11), non-breeding
females (10). Error bars represent standard errors of the means.

Fig. 5. Mean spectral reflectance curve of the throat pouch of displaying malegreat frigatebirds (n = 10). Measurements were averaged from 5 readings takenfrom the non-inflated portion of skin under the bill. Solid line represents themean reflectance curve. Dotted lines represent the upper and lower 95% confidence limits.

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Zeaxanthin and tunaxanthin accounted for 2.8 ± 0.003% and12.7 ± 0.010% of the total plasma carotenoids, respectively(0.04 ± 0.004 μg/mL; 0.16 ± 0.02 μg/mL). A representative 2dimentional HPLC chromatogram of plasma is presented inFig. 2. Males had significantly higher astaxanthin concentrations in their plasma than did females, when individuals fromall breeding stages were combined (Table 1). The relative
proportion of astaxanthin was significantly higher in males
compared to females, while the relative proportions of tunaxanthin and zeaxanthin did not differ between males andfemales (Table 1). Total carotenoid concentrations in plasma
differed significantly between the sexes (two-way ANOVA,
sex: F1,106 = 36.08, P b 0.001; Fig.3, but not in relation tobreeding category (two-way ANOVA, F2,106 = 0.77, P = 0.47;
Fig. 3). However, there was a significant interaction between
sex and breeding category (two-way ANOVA, sex ⁎ categoryinteraction: F2,106 = 18.976, P b 0.001; Fig. 3). Post-hoc comparisons showed that non-breeding males circulated significantly higher carotenoids compared with courting males(Fisher's least-significant difference, P b 0.001), courting females (P = 0.002), incubating females (P b 0.001), and nonbreeding females (P b 0.001). In addition, incubating males
circulated significantly higher levels compared with courting
males (P = 0.004), incubating females (P b 0.001), and nonbreeding females (P b 0.001). Courting males circulated higher
levels than incubating females (P = 0.007) and non-breeding
females (P = 0.006). Finally, courting females circulated higher
levels than incubating females (P = 0.003) and non-breeding
females (P = 0.003). Astaxanthin was the only carotenoid present
in gular pouch tissue. Pouch tissue concentrations (1268.8 ±282.5 μg/g; mean ± SE) were extremely high compared tototal plasma concentrations. One gular pouch had an astaxanthin concentration exceeding 3000 μg/g (3668.0). A representative 2-dimentional HPLC chromatogram for pouch
tissue is presented in Fig. 4.
The average reflectance curve for gular pouches of displaying males (n = 10; 5 spectra averaged for each) showed aFig. 6. Mean spectral reflectance curve of the throat pouch of incubating malegreat frigatebirds (n = 10). Measurements were averaged from 5 readings fromthe skin patch under the bill. Solid line represents the mean reflectance curve.
Dotted lines represent the upper and lower 95% confidence limits.

distinctive pattern containing an initial peak in the UV range atapproximately 360 nm. In addition, small absorption valleys
occurred at approximately 542 nm and 577 nm, followed by aplateau at approximately 630 nm (Fig. 5). The average reflectance curve for gular pouches of incubating males (n = 10, 5
spectra averaged for each) was similar to that of displaying
males. There was an initial peak in the UV range at approximately 360 nm, and the final plateau again appears at approximately 630 nm. However, the absorption valleys present in
the displaying male pouch spectra at 542 nm and 577 nm are notpresent in the incubating male pouch (Fig. 6).
4. Discussion
Carotenoids are a known source of ornamental coloration ina variety of bird tissues, including feathers (Stradi, 1999;
McGraw et al., 2001; McGraw and Hardy, 2006), bills (Bennett
et al., 1996; McGraw and Ardia, 2005), and legs (Bortolotti
et al., 1996; Casagrande et al., 2006). Far less attention has been
given to colorful fleshy parts in birds (e.g. wattles, combs, eyerings, pouches) and the potential role that carotenoids play in theexpression of these colors (but see Negro et al., 2006; Mougeot
et al., 2007). Because the concentration of astaxanthin, a red
carotenoid, in male great frigatebird throat pouch tissue was sohigh, it is likely that astaxanthin contributes substantially to the redcolor of these ornaments. This has implications for the information
content of this display signal, as astaxanthin is a known stimulantof the immune system and a powerful antioxidant (Hussein et al.,2006; McGraw, 2006). Previous work on this population demonstrated variation in displaying male pouch color (Dearborn
and Ryan, 2002). Males capable of displaying astaxanthin-richpouch tissue may be advertising superior fitness if only excess
carotenoids beyond what are needed for basic physiological functions is available for diversion into throat pouch tissue. If females
select mates based on coloration of their throat pouches, selectionof males with the most astaxanthin-rich pouches could potentiallyresult in direct benefits to females. Perhaps the most intensely
colored males are in better condition and are more efficient foragers, able to provide superior parental care. In addition, females
could gain indirect benefits if the ability of males to acquire or
assimilate carotenoids is a genetic trait that could be inherited bytheir offspring.
High concentrations of carotenoids have been reported inother avian tissues, such as in the feathers of American goldfinches (McGraw and Gregory, 2004). One throat pouch tissue
analyzed in our study exceeded 3000 μg/g, and two additional
pouch tissues exceeded 2000 μg/g; these to our knowledge are
the highest reported concentrations of carotenoids in any bird
tissue. Astaxanthin concentration in throat pouch tissue wasremarkably high compared to the total carotenoid concentrations found in blood plasma. In addition, of the three carotenoids
circulated in great frigatebird blood plasma, only astaxanthin was
deposited in throat pouch tissue. It is rare that pure astaxanthin isdeposited in avian tissues, as it is almost always accompanied byother 4-oxo-carotenoids (Stradi, 1999). This study, to our knowledge, is the third demonstration of pure astaxanthin deposition inbird tissue and the first in bare skin. Interestingly, the two previous

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examples may both be the result of human introduction of astaxanthin into the environment (Negro and Garrido-Fernandez,
2000; McGraw and Hardy, 2006). Astaxanthin present infrigatebird blood plasma suggests that it is acquired directlyfrom a food source, rather than being manufactured fromanother precursor. The dietary source of astaxanthin in great
frigatebirds on Tern Island is not well understood. Greatfrigatebird diet has been reported to consist of primarily
flying fish and squid (Metz and Schreiber, 2002). Initial
analyses of diet samples revealed moderate levels of astaxanthin in some fish (F. Juola and K.J. McGraw pers. obs.).
Astaxanthin is commonly acquired through aquatic food sources
such as fish and crustaceans (Goodwin, 1984), and some squid
species are highly enriched in astaxanthin (K. McGraw, M.
Hipfner, and J. Dale, pers. obs.), making this a likely source forastaxanthin accumulation in great frigatebirds.
Carotenoid concentrations in great frigatebird blood plasma
were similar to those reported for other large, non-passerine
birds, and lower than those reported for small passerine birds(Tella et al., 2004). Carotenoid concentrations in blood plasma
differ widely across avian species (Parker, 1996; Tella et al.,
1998; Slifka et al., 1999; McGraw, 2005), with phylogeny beinga significant predictor of circulating carotenoid levels (Tella
et al., 2004). Additional factors such as diet, (Tella et al., 2004),
occurrence of ornamental coloration (Tella et al., 2004), andefficiency in assimilating consumed carotenoids into the blood
system (McGraw, 2005) have been used to explain higher levelsof circulating carotenoids in passerine birds. In addition, bodysize may play a role. In vertebrates, food consumption is proportional to body mass raised to the 3/4 power (Peters, 1983).
The increase in blood volume is directly proportional to theincrease in species body mass. Thus, larger animals eat lessfood, and presumably fewer carotenoids, per body mass unit
than smaller species, and incorporate relatively fewer carotenoids into their proportionally equal blood volume (Tella et al.,
2004; McGraw, 2005).
As has been found in other dichromatic species (Hill, 1995;
Bortolotti et al., 1996; Negro et al., 1998; McGraw and Ardia,
2005), male great frigatebirds circulate higher concentrationsof carotenoids then do females. Dietary differences ordifferences in the absorption capacities of males and females
could mechanistically account for this difference in carotenoid
levels. Interestingly, incubating and non-breeding males
exhibited higher levels of circulating carotenoids comparedwith displaying males. A prediction of higher circulating
carotenoid levels in more colorful males (while displaying)seems intuitive if the assumption is that males flood theirsystem with more carotenoids during times of peak color
production. However, circulating carotenoids diverted into
tissues such as skin, beaks, or feathers may deplete carotenoid
levels circulating in the blood. For this reason, predictions ofeither higher or lower circulating carotenoid levels in morecolorful males seem plausible. Lower carotenoid levels in
displaying males lend support to the idea that carotenoids are
depleted by diversion into throat pouch tissue. Courting females had higher circulating carotenoid levels than incubating
and non-breeding females, a pattern similar to that reported in

375

American kestrels (Negro et al., 1998). It's possible that females circulated higher carotenoid levels during courtship in
preparation for egg-laying, as carotenoids are a known component of egg yolk (Fletcher 1992; Royle et al., 1999).
The average spectral curve presented here is the first reported
for any frigatebird throat pouch. A spectral curve derived solely
from astaxanthin-based red tissue would predict a single absorption valley at approximately 470 nm, followed by a steeper
upslope and plateau at around 550 nm. However, gular pouch
tissue is highly vascular. Blood filled capillaries are visible in
inflated pouches (F. Juola pers. obs.) The presence of oxyhemoglobin produces a characteristic spectral pattern of absorption bands at 542 nm and 577 nm (Zonios et al., 2001).
Great Frigatebird throat pouches exhibited absorption bands atthese locations. This suggests that increased blood flow and thepresence of hemoglobin may contribute substantially to theoverall coloration and spectral properties of these throat pouches during the display period. This notion is supported by the
rapid fading of pouch color from red to orange once a male has
secured a mate, something that could be accomplished throughdiversion of blood flow. Furthermore, one key difference between the spectral curves of displaying males and those of
incubating males was the lack of absorption bands expectedwith the presence of hemoglobin in incubating males. This
supports the notion that gular pouch coloration is produced
through a combination of carotenoid deposition and increased
blood flow.
A secondary peak appeared in the UV-range of the pouch
reflectance spectrum. UV-signaling in birds has been the focus
of a number studies in recent years, since the advent of moresophisticated analytical methods. It has been demonstrated thatfemales of some species use UV-reflecting feathers to assessmale quality and prefer those males that exhibit greater UV
reflection (Bennett et al., 1996; Hunt et al., 1999). UV-reflectance in avian skin parts is not well documented. Some examples have been reported in passerine species (Hunt et al.,
2003; Jourdie et al., 2004) and in the orbital combs of the redgrouse (Lagopus lagopus scoticus; Mougeot and Redpath,2004). Interestingly, UV brightness in the red orbital combs of
the red grouse was negatively correlated with parasite infection
rates (Mougeot et al., 2005), suggesting that UV reflectancecould serve as a reliable signal of parasite resistance. It has been
demonstrated that feathers with carotenoid-based colors createsuch UV spectral patterns by absorbing light from a structural
white tissue that in which the carotenoids are embedded (Shawkey
and Hill, 2005). Frigatebird pouch tissue was comprised of two
tissue layers, an inner tissue layer that was pink or white, and anouter dermal layer that contained the astaxanthin pigments(K. McGraw pers. obs.). It is possible that, as in grouse (Mougeot
et al., 2007), the pouch tissue gets its UV reflectance from thewhite background layer.
Acknowledgements
We thank Angela Anders, Beth Flint, Dominique Horvathand the U.S. Fish and Wildlife Service for logistical support andaccess to Tern Island, Morgan Gilmour for assistance in the376

F.A. Juola et al. / Comparative Biochemistry and Physiology, Part B 149 (2008) 370–377

field, and William A. Searcy for comments on the original
manuscript. Funding sources included the University of Miami,
Department of Biology's Kushlan/Frohring Graduate Research
Support Fund, and the Maytag endowment.
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