Chemistry Reference
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Low
High
FIGURE 23.5
Effect of feeding captive male ring-necked pheasant (
Ph. colchicus
) young a high- or low-
protein feed for the i rst three weeks of life on the expression of wattle coloration (mean ± SE) at 20 (open
circles) and 40 (i lled circles) weeks of age. Coloration was determined using a principal components anal-
ysis (PCA) of tristimulus scores (hue, saturation, and brightness) obtained with a Colortron II rel ectance
spectrophotometer.
mitogen injection) was determined at week 20. Antibody production and swelling response were
not associated with early-life nutrition, but Ohlsson et al. (2002) found that higher protein intake
increased wattle size and redness; the very-early-life manipulation was especially strong for wattle
size (Figure 23.5). Though it is not yet apparent
how
protein intake should mechanistically impact
processes associated with carotenoid accumulation and deposition in skin, this study stands as one
of the best demonstrations of an organizational effect on carotenoid coloration in birds. A different
organizational mechanism involving yolk testosterone was experimentally tested in a very recent
study, but had no effect on wattle characteristics (Rubolini et al. 2006).
In follow-up work, emphasis has been placed on factors in adult pheasants that affect their cur-
rent levels of wattle ornamentation. In fact, until this point, the relative roles of early- versus late-life
effects on carotenoid colors have not been wholly apparent in any system yet (i.e., no factor had been
uniformly studied in both developmental and adult life stages). Ohlsson et al. (2003) conducted the
same protein manipulation in adult pheasants and found similar effects of high protein intake on
wattle color (though no effect on wattle size was evident). With respect to wattle color, the ques-
tion now becomes which life phase is more crucial for exaggerated expression of wattle color in
adult male pheasants. Smith et al. (2007) went on to study another adult condition—carotenoid
intake—as it related to wattle coloration and found comparatively stronger effects of carotenoid
consumption, compared to protein intake, on adult wattle coloration. Taken together, these studies
paint a clear picture of nutritional control of l eshy coloration in a precocial bird, both organization-
ally and activationally, but, to fully understand the differential roles of different nutrients in this
system, comprehensive, lifetime experimental manipulations are needed, including more natural-
istic carotenoids (e.g., canthaxanthin was used), to isolate the strongest limitations for becoming
sexually attractive (see Hill (2006) for another detailed discussion of the multivariate forces that
shape carotenoid ornamentation in birds).
23.7 CAROTENOID SIGNALING OF SOCIAL STATUS IN WIDOWBIRDS
Signals that have evolved through sexual selection are either designed to attract the opposite sex,
or to intimidate rivals of the same sex. Males often compete with each other for access to limited
or valuable territories, food or mates, and have been shown to use signals that communicate their
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