Control and Function of Carotenoid Coloration in Birds: Selected Case Studies - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 500

readiness and ability to i ght, which ensures that not all contests escalate to the point of injury or
death (Maynard-Smith and Harper 2003). The dominance status signaling hypothesis was originally
devised for male birds that live in nonbreeding foraging l ocks, where individuals regularly encounter
new, unfamiliar foes and could benei t from using signals that reveal their aggressive nature (Rohwer
1975). We now know that carotenoid coloration, among other sexually selected traits, can serve as a
signal of social status in various species of birds and i sh (reviewed by Blount and McGraw (2008)).
One group of birds has been especially well studied in the i eld of carotenoid coloration as a social
status signal—the widowbirds ( Euplectes spp). Widowbirds exhibit a long tail, and, in many spe-
cies, also striking yellow or red color patches on their body or shoulders (epaulettes). Long tails and
carotenoid coloration are two examples of sexually selected plumage traits, prompting the question
of what selection pressures have maintained the elaboration of two, distinct and costly ornaments
(Andersson et al. 2002). In his classic sexual selection experiment, Malte Andersson demonstrated
that female long-tailed widowbirds ( E. progne ) preferred to mate with males that had had their tails
experimentally elongated (rather than males where the tail was left unmanipulated or was of reduced
length) (Andersson 1982). This female preference for longer-tailed males has also been shown in
Jackson's widowbirds ( E. jacksoni ), red-collared widowbirds ( E. ardens ), and red-shouldered wid-
owbirds ( E. axillaries ) (Andersson 1992, Pryke and Andersson 2002, 2005, Pryke et al. 2001a),
indicating a generalized female bias for long tails in widowbirds (Pryke and Andersson 2002).
Carotenoid coloration in males of these widowbird species, however, plays no role in female
mate choice (Pryke and Andersson 2003a, Pryke et al. 2001a). In a series of elegant studies using
experimental removal and replacement of territory holding males in the wild, and experimental
manipulations of the size and redness of color patches, coupled with captive studies of aggressive
interactions, Sarah Pryke, Staffan Andersson and coworkers have shown that carotenoid coloration
functions in male-male competition, with males that have larger and/or redder epaulettes outcom-
peting males with smaller and/or less red signals (Pryke and Andersson 2003a,b, Pryke et al. 2001b,
2002; Figure 23.6). Territory acquisition and defense is a prerequisite for reproductive success in
males, because territories are in limited supply and are used to display to females, which base their

0.8

P < 0.001

0.4

0

-0.4

P < 0.001

-0.8

Red-collared
(11)

Control
(11)

Orange-collared
(7)

Collar manipulation group

FIGURE 23.6 Effects of experimentally manipulated collar color on changes in territory size of red-collared
widowbirds ( Euplectes ardens ), i.e., territory size before collar treatment subtracted from territory size after
treatment. Red collar males had their collar feathers bleached before being repainted with average-sized red
collars; control collar males had their feathers bleached before being repainted to their original size and color;
orange collar males had their feathers bleached before being repainted with average-sized orange collars.