Chemistry Reference
In-Depth Information
10
8
6
4
log
10
gut carotenoid concentration (μg/g)
FIGURE 23.1
Relationship between the redness of growing carotenoid-containing feathers in wild, adult,
male house i nches (
Carpodacus mexicanus
) captured in San Jose, CA and the concentration of carotenoids
measured from their gut contents (e.g., crop, proventriculus, gizzard). Plumage coloration was scored by visual
comparison to color chips in the
Methuen Handbook of Colour
. Diet samples were ground in the presence of
organic solvent and carotenoid concentration was determined with visible-light spectrophotometry.
of carotenoids accumulated by house i nches have coni rmed the strong predictive power of b-cryp-
toxanthin in the development of red feathers in males (McGraw et al. 2006b). Males with more
b-cryptoxanthin in blood circulation and in liver tissue during the molt period were more likely
to grow red, ketocarotenoid-containing feathers (McGraw et al. 2006b); it was not apparent from
this correlational study, however, if liver carotenoids played a key coloring role or were just corre-
lated with other factors (like plasma carotenoids) to determine color. Additional challenges that lie
ahead in this line of work include understanding the natural sources of this pigment in house i nch
food during molt as well as testing whether house i nches adopt a specialized foraging strategy for
acquiring this pigment at this time of year.
It is noteworthy that there has been an extensive parallel line of inquiry on the effects of parasites
and health on house i nch coloration (reviewed in Hill (2002), also see Hill et al. (2004)). Some
studies have gone as far as saying that one disease—the avian pox—may have been the initial driv-
ing force behind the unusual range of intrapopulational color variability seen among males in this
species (Zahn and Rothstein 1999; but see Hill (2001) for a critique). This well-studied system of
mechanisms is a perfect example of the complexities of carotenoid intake and use, especially among
wild animals, and the difi culties in isolating the relative importance of competing control mecha-
nisms without detailed, comparative, experimental approaches within a naturalistic context.
23.3 PHYSIOLOGICAL AND GENETIC CONTROL OF CAROTENOID
COLORATION IN A DOMESTIC SONGBIRD
MODEL, THE ZEBRA FINCH
Even before Hill's extensive work on house i nch carotenoids had been published, there was clear
evidence in one other avian species—the zebra i nch (
Taeniopygia guttata
)—that females used
variation in an elaborate carotenoid-based color signal to make mate selection decisions. Male
zebra i nches, an Australian grassland passerine species from an entirely different family of i nches
(
Estrildidae
) than house i nches, display an intense red beak that derives its color from carotenoids
(McGraw et al. 2002). These birds are ideal for controlled studies because they have been domesti-
cated for centuries and freely mate and breed under laboratory conditions. Burley and Coopersmith
(1987) gave females the choice of assessing and spending time with males that varied in beak col-
oration and showed that females signii cantly preferred to associate with red males; this pattern has
recently been corroborated by Blount et al. (2003), though there have been some objections to the
strength of mate selection on this trait under certain conditions or relative to other traits (Collins
et al. 1994).
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