This post concludes the hypothetical discussion of my previous three posts, the first of which you can find here.

By the end of this long thought-exercise it would seem that the most feasible explanation for the barring and display is that the male turacos developed the barring to signal their quality—both to females and other males—not because the barring is expensive to produce, but because the standardized bars allow for close comparison of individuals. By relating directly with growth, bars should allow females to assess the males’ age and relative fitness; they also probably accentuate indicators such as body symmetry, foraging ability, and health. If snake and raptor predators do in fact rely on the ultraviolet pigments to hunt the male turacos, then the barring is probably not only an indicator of (or proxy for) fitness but also an honest handicap that allows males to demonstrate their higher quality (e.g. their low feather wear despite increased need for predator avoidance).

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This post continues the hypothetical situation described in my previous posts, the first of which you can find here.

What if the turaco barring and display were a different sort of quality indicator? They could be amplifiers that emphasize a trait perceived by other turacos as directly related to high fitness. For example, some birds’ feathers, such as those of the European jay, are colored in stripes that develop similarly to, or perhaps accentuate, growth bars. As the turaco’s barring correlates directly to its age, the bars must have typical widths at certain ages. These widths, along with the display, probably serve as a standard by which females can compare males, and males assess competitors. The bars are likely indicators similar to the stripes on jays and other birds—that is, they may portray the bird’s growth regularity, or accentuate flight behavior, body size, feather wear, symmetry, and other potential quality indicators. Should the turaco barring amplify any of these possible traits, then displaying the bars is not a deceptive signal, because it genuinely reflects quality. For instance, if a three-year-old male turaco has successfully avoided predators and found the best food throughout his life, his barring should reflect such facts, and portray his success through low feather wear, larger body size, or more regular bars; a one-year-old male with similar traits but different barring would presumably be less preferable to females. Continue reading →

This post continues the hypothetical situation described in my previous post, which you can find here. I left off claiming that the most likely explanation for the barring and display of the male turacos was sexual selection; below I try to support this hypothesis in greater detail:

We know that the male turacos limit themselves to intraspecific displays, so the pigmentation and display must be signals for other turacos. It also seems likely that some element of the barring is, or is at least perceived as, evidence of quality. Depending on how elaborate and visible the display is, it may create a handicap for male turacos. The handicap theory posits that sexually attractive traits are frequently impediments to an organism’s ability to survive, and thus that an organism’s continued survival despite such a hindrance portrays its higher quality. For example, some male birds, peacocks being an immediate example, have displays of quality reflected by an honest handicap: their conspicuous and unwieldy tail feathers. Although the turaco’s ultraviolet pigment is inexpensive to produce and invisible to mammalian predators, it may be noticeable to some reptilian predators and is probably visible to avian predators, making it possible that the turaco barring display is an honest handicap signal.

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Let’s say you’ve just found out something new about a species of bird, a certain turaco that is a very dark cryptic green, in the forests of the Congo. The species hasn’t been studied much, and you’ve discovered that the male birds have a special pigmentation only visible in the ultraviolet spectrum, and this pigment is found in a barred pattern across their undersides, which they display to females and other males in their dense colonies. Based on certain test results, you can positively claim that the pigment is easy and inexpensive to produce, and you’ve also ascertained that the bars of ultravioletly-pigmented feathers are closely correlated with the bird’s growth—that is, older birds have wider bars. But apart from these few facts, you know very little about the turaco species, and your job is to wonder how and why the ultraviolet barring and its display evolved.

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