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<p>Researchers have long assumed that black widow coloration functions as a warning
signal to avian predators. However adult female black widow coloration does not resemble
typical warning coloration in two distinct ways. First, black widows are less colorful
than most other documented aposematic species. Second, the hourglass shape of an adult
female varies both between species and within a site. Here I examine the ultimate
causes and consequences of North American black widow coloration. </p><p>In chapter
two I present data that suggest that black widow coloration not only functions as
an aposematic signal to avian predators, but has also been selected to be inconspicuous
to insect prey. In choice experiments with wild birds, I found that the red-and-black
coloration of black widows deters potential predators: wild birds were ~3 times less
likely to attack a black widow model with a red hourglass than one without. Using
visual-system appropriate models, I also found that a black widow's red-and-black
color combo is more apparent to a typical bird than typical insect (Euclidean color
distance ~2.2 times greater for birds than insects). Additionally, an ancestral reconstruction
revealed that red dorsal coloration is ancestral in black widows and that at some
point some North American black widows lost their red dorsal coloration (while maintaining
the ventral hourglass). Behaviorally, differences in red dorsal coloration between
two North American species are accompanied by differences in microhabitat that affects
how often a bird will view a black widow's dorsal region. All observations are consistent
with a cost-benefit tradeoff of being conspicuous to potential predators while being
inconspicuous to prey. I suggest that avoiding detection by prey --- combined with
Müllerian mimicry --- may help explain why red-and-black aposematic signals occur
frequently in nature.</p><p>In chapter three, I examine the variation in hourglass
shape. Classical aposematic theory predicts near uniformity in warning signal appearance
because a uniform signal is easier to learn to avoid than a variable signal. However
the shape of the hourglass of North American black widows appears to vary both within
and between sites in ways that are inconsistent with classical aposematic theory.
Using 133 black widows of three different species from nine sites across the United
States, I quantified the variation in hourglass shape and examined how Müllerian mimicry,
species type, and condition each influenced hourglass shape. A principle components
analysis revealed that 84.5% of the variation in hourglass shape can be explained
by principle components 1, 2, and 3, which corresponded to hourglass size (PC1), the
separation between hourglass parts (PC2), and the slenderness of the hourglass (PC3).
Both a black widow's condition and species significantly predicted hourglass shape;
however I found no support for localized Müllerian mimicry within different geographical
regions. My results suggest a relaxed role for selection on hourglass shape. I discuss
several hypotheses that could explain the variation in hourglass morphology including
that potential predators may avoid any red markings rather than an exact shape (categorical
rather than continuous perception).</p><p>In chapter four I expand on my work from
chapter two to examine the eavesdropper's perspective on private communication channels.
Private communication may benefit signalers by reducing the costs imposed by potential
eavesdroppers such as parasites, predators, prey, or rivals. It is likely that private
communication channels are influenced by the evolution of signalers, intended receivers,
and potential eavesdroppers, but most studies only examine how private communication
benefits signalers. Here, I address this shortcoming by examining visual private communication
from a potential eavesdropper's perspective. Specifically, I ask if a signaler would
face fitness consequences if a potential eavesdropper could detect its signal more
clearly. By integrating studies on private communication with those on the evolution
of vision, I suggest that published studies find few taxon-based constraints that
could keep potential eavesdroppers from detecting most hypothesized forms of visual
private communication. However, I find that private signals may persist over evolutionary
time if the benefits of detecting a particular signal do not outweigh the functional
costs a potential eavesdropper would suffer from evolving the ability to detect it.</p>
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