Conspicuousness
As a proxy for larvae conspicuousness to aerial and terrestrial
predators, we estimated the contrast between the tadpole body coloration
and the water background. We estimated conspicuousness from above
because phyllomedusinae larvae are known to congregate at the water
surface during the daytime (Branch 1983; Caramaschi and Jim 1983). Our
experiments (see the previous section) had confirmed this presumption,
at least for large-sized larvae (see Results). We are aware that aquatic
predators, such as arthropod and fish, also prey on phyllomedusinae
larvae (Magnusson & Hero 1991) yet the visibility of these ponds was
below 5 cm, as estimated from the “disappearance” of larvae when they
swim deeper from the surface. Moreover, the metallic and putatively
conspicuous patch was located on the dorsal surface of the tadpole head
and is arguably easier to spot from above the water.
The body-background contrast was measured on lateral and dorsal
photographs of 193 larvae of P. vaillanti , 113 of P.
bicolor , and 30 of C. tomopterna at variable developmental
stages. A Kodak color card was included in each photograph as a control
for white balance. We then used the software ImageJ (Schneider et al.
2017) to measure image red, green, and blue (RGB) brightness on six
areas of the larvae’s body: the interocular patch, the loreal area, the
middle dorsal area, the lateral area, the dorsolateral area, and the
ventral zone (Figure 1-B). For background measurements, we took
photographs of the water surface under daytime natural light and at the
very ponds where we found the tadpoles.
To quantify and compare coloration we used the RGB decomposition method
on the digital images (Endler 1990; Stevens et al. 2007; Amézquita et
al. 2009). Briefly, we estimated relative brightness on each color
channel: red (R* = r/ (r + g + b), green G* = g/ (r + g + b), and blue
B* = b/ (r + g + b). Then, to map those measurements onto a
two-dimensional space, we calculated two scores: LM = R* ̵̶ G*, and B* = B
(Endler 1990; Amézquita et al. 2009). This approach provides a
reasonable estimate (long-medium and short wavelengths) of the
photoreceptor’s response in animals with trichromatic vision (Kebler et
al. 2003). Finally, we used the multivariate distances between each
tadpole area and the background coloration as our proxy for contrast or
conspicuousness (Endler 1990).