Color Pattern Variation in a Cryptic Amphibian, Anaxyrus fowleri
Many species of animals employ camouflage to render them inconspicuous. Selection to precisely match cryptic color patterns to the background substrate should result in geographic variation in relation to substrate type. We tested this premise by examining color pattern variation in relation to substrate surface in Fowler's Toad (Anaxyrus fowleri), a noxious and cryptically colored amphibian that is widespread in eastern North America and frequently associated with sandy habitats. We quantified total dorsal spot area, number of spots, and size of the four largest dorsal spots among 330 specimens of Fowler's Toads (89 live, 241 preserved) in 14 samples from Canada and the United States. We found no significant difference in the extent or number of spots between males vs. females or between living vs. preserved specimens after compensating for variation in snout–vent length. However, toads from freshwater habitats with extensive areas of open sandy terrain had significantly smaller and fewer dorsal spots than toads from either seacoast localities with open sands present or toads from freshwater habitats with open sands absent. Because saltwater seaside beaches and sand dunes are generally uninhabitable by amphibians, we take this as evidence consistent with the presence of adaptive background pattern matching coloration in this species.Abstract
Sample localities (circles) within the range of Fowler's Toad (gray shading). The type locality of Danvers, Massachusetts, is indicated by a black dot. Specimens from some adjacent collection localities were pooled (shown linked by black lines) to derive the 14 samples used for analysis (see Table 1). Sites were characterized according to salinity and the presence of open sands.
Representative images used for analysis of dorsal spot variation in Fowler's Toads. (A) Original image of the specimen, a female toad from Iroquois County, Illinois (FMNH 271821). (B) Two-color reduction of the original image portraying the dorsal spots (black) alone against a white background.
Fowler's Toads from (A) the North Carolina Outer Banks near Kitty Hawk and (B) Long Point, Ontario, illustrating the extent of dorsal color pattern variation (photos taken in situ by D. M. Green).
Spot Area comparison between samples of Fowler's Toads from across the species' range, plus the neotype specimen. The scale is the size-adjusted (residual) square-root of the total area of spots per individual. Box and whisker plots portray the mean (outer tick marks), median (horizontal inner line), 25% and 75% quartiles (box), and minimum and maximum (whiskers) for each sample.
Variation in dorsal spot Size (area of the four largest spots) vs. spot Number among samples of Fowler's Toads.
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