Chemistry Reference
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Much like the parallel work on house i nches, researchers began to investigate mechanisms
underlying color variation among males (i.e., what factors make some males more attractive and
worth mating with more than others?). Because these animals were housed under identical condi-
tions, given identical foods, and yet displayed variable beak colors, initial emphases were placed
on how physiological and genetic parameters should impact beak color expression. Burley et al.
(1992) provided an excellent i rst description of the degree to which bill color changed over varying
environmental contexts; intense reproduction and social crowding, for example, appeared to have
strongest effects on color, in these cases fading the beak from red to orange. Birkhead et al. (1998)
showed that birds with redder beaks were in better body condition. Parasite loads also were oddly
positively correlated with bill redness (Burley et al. 1991), however, unlike the traditional prediction
that parasites should debilitate birds and fade beak color. Nonetheless, these initial studies were
suggestive of physiological control agents of coloration, whereby the hormones and energetic invest-
ments associated with breeding and competing in groups, not diet, are key in shaping carotenoid
accumulation and coloration.
Little was done to test these energy/hormone hypotheses for many years, however, and instead
attention was devoted next to genetic mechanisms. For sexual traits like coloration to persist over
evolutionary time, they must have heritable bases; however, virtually no studies, aside from those in
chickens and aquacultured i sh (where the sexual selection role in maintaining coloration is unclear;
e.g., McGraw and Klasing [2006]), had investigated the genetic architecture underlying carotenoid
pigmentation. Still some of the best work on this topic in birds was done by Price and colleagues
using domesticated zebra i nches. In their controlled breeding and selection experiments, they found
strong heritability of male beak redness (and also orange coloration of the female beak; Price 1996,
Price and Burley 1993) as well as signii cantly positive selection differentials and gradient coefi -
cients, such that redder males had higher reproductive rates (Price and Burley 1994). Very recently,
this issue has been revisited in the context of the heritability of other information that beak redness
might reveal, including health and condition (Birkhead et al. 2006; also see more below in this sec-
tion). These researchers found strong additive genetic variation in and genetic correlations among
male beak redness, condition, and health (as measured by response to a tetanus immunization), such
that females gain good genes by mating with males in good condition and a redder beak. Despite
consistent support for this genetic model of color control, it is important to point out that in none of
these studies were differential maternal effects—specii cally, varying amounts of yolk carotenoids
contributed by mothers—carefully controlled for; in the study by Birkhead et al. (2006), females
were paired twice, each time with a different male, to increase chances of detecting maternal effects,
but it is possible that many of the effects seen here are linked to carotenoid accumulation patterns,
with more carotenoid-replete birds depositing greater amounts of carotenoids in yolk and thus hav-
ing offspring with more carotenoids and redder beaks, independent of their genes. In fact, in zebra
i nches, McGraw et al. (2005) found such a positive correlation between maternal beak redness, yolk
carotenoid investment, and the redness of the beak developed by sons when sexually mature.
This trans-generational view of carotenoids raises the prospects of the direct physiological
actions that carotenoids have on animals, as it relates to their color development. In another subsec-
tion, we detail the antioxidant role of carotenoids in another avian species, but zebra i nches have
been among the best-studied birds in the context of the immunoenhancing roles of carotenoids.
The humoral and cell-mediated aspects of immunocompetence have been shown to increase in
response to increased dietary carotenoid intake (Blount et al. 2003, McGraw and Ardia 2003).
Alonso-Alvarez et al. (2004) also found that circulating carotenoid levels in zebra i nches changed
in parallel with a measure of antioxidant defense (resistance of red blood cells to free radical attack).
Taken together, these studies reveal key health roles of carotenoids, consistent with the view that
there are strong physiological inputs into the carotenoid color signaling system of this species. But
might diet also play a role in coloration then? Clearly in many studies (the three listed above) pro-
viding experimental supplements with carotenoids can enhance beak coloration, but Birkhead et al.
(1999) i rst showed experimentally that raising nestling zebra i nches on a poor-quality diet did not
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