Diversity and Sexual Dichromatism in Treefrog Throat Coloration: Potential Signal Function?
Sexual dichromatism, where males and females of the same species differ in coloration, is best studied in diurnal animals. Nocturnal animals such as frogs do not seem good candidates for widespread sexual dichromatism, or for the use of visual signals in social communication in general. Yet, up to 25% of treefrog species (Anura: Hylidae) show some degree of sexual difference in their dorsal coloration. The ventral side of frogs is hidden during rest, but the throat region becomes visible in active individuals, especially in calling males. We compiled a database of 249 treefrog species that include members of all seven subfamilies of the Family Hylidae. We document that the throat region of male frogs is frequently colorful and of a different color than the abdomen (ventral dichromatism), that there is interspecific variation in throat coloration, and that males and females differ in throat coloration (sexual dichromatism). We also examined intraspecific variation in throat coloration, using Eastern Gray Treefrogs, Dryophytes versicolor (Hyla versicolor) as the focal species. Here, we document throat color is sexually dimorphic, that it varies among males, is associated with better body condition, and on average is darker in mated compared to unmated males. Our study finds that throat color dichromatism is the most prevalent type of sexual dichromatism in treefrogs, and that throat coloration may have sexual signal function.ABSTRACT
Representative examples of variation in throat coloration of male Hylid frogs. Dendrosophus reticulatus (photo credit: K. H. Jungfer; A); Boana boans (photo credit: F. Kok; B); Osteocephalus mutabor (photo credit: K. H. Jungfer; C); Boana punctatus (photo credit: D. R. Neira; D).
Throat and abdomen color variation in treefrogs (Family Hylidae). Distribution of throat colors in males and females (A). Distribution of abdomen colors in males and females (B). Difference in throat colors between species where males have a subgular vocal sac (VS), where the vocal sac is an integral part of the throat area, and species where males have lateral or absent vocal sacs (C). Shown are sample sizes and color categories (white, gray, yellow, green). Note that female throats designated as “gray” generally consist of cream backgrounds with dark dots, thus being darker than their abdomens, yet much brighter than male throats.
Distribution of throat colors in males (top) and females (bottom) in each of the seven subfamilies of Hylidae. Shown are sample sizes and color categories (white, gray, yellow, green). Note that female throats designated as “gray” generally consist of cream backgrounds with dark dots, thus being darker than their abdomens, yet much brighter than male throats.
Intraspecific variation and sexual dichromatism in throat color of Eastern Gray Treefrogs. Males have darker throats than females (A, B). Abdomen color is similar between the sexes (A, C). In addition, there are differences in the spatial distribution of pigmentation: In males the pigment is distributed more uniformly and appears as the background against small, lighter spots (A, top), whereas in females the background tends to be light in color, with small, irregular dark spots (A, bottom). The squares in (A) indicate the size and location of the standardized areas over which the color (as mean gray value) was measured. Top histograms show male data; bottom histograms show female data. The x axis in histograms in (B, C) show the mean gray value (0–255; bin size 20).
Factors affecting ventral color of male Eastern Gray Treefrogs. The top row shows the effect of body length (SVL) on throat (A) and abdomen (B) coloration, and the magnitude of the color contrast between throat and abdomen (C). The bottom row shows the effect of body condition (SMI) on throat (D) and abdomen (E) coloration, and the magnitude of the color contrast between throat and abdomen (F). Absolute color is measured as mean gray value, where a pixel with a brightness intensity of 0 is black, a pixel with a value of 255 is white, and everything in between is a shade of gray (A, B, D, E). For relative color measures (ventral contrast), higher values indicate darker throats (C, F). Trend lines indicate statistical significant differences.
Difference in throat (A) and abdomen (B) coloration, and the magnitude of the color contrast between throat and abdomen (C) between mated and unmated males of Eastern Gray Treefrogs. Color is measured as mean gray value, where a pixel with a brightness intensity of 0 is black, a pixel with a value of 255 is white, and everything in between is a shade of gray. Symbols indicate the mean, box lines correspond to the top, middle, and bottom quartile, and whiskers indicate the top and bottom decile. Asterisks indicate statistically significant comparisons.
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