Thursday, November 28, 2013

Sub-lethal skin sections in the wrinkled frog effective against snake predation

Frogs and toads are packets of proteins and calories to many predators. In response anurans have evolved a plethora of defenses against their predators, many of these defenses are chemical. Dendrobatid frogs may kill a potential predators with its defensive toxins, but many species of anurans seem to produce toxins that are sub-lethal. Anuran skin secretions may also have other functions, such as antibacterial protection for their skin. But defensive molecules are abundant in a more or less defenseless group of amphibians.
The Japanese striped snake, Elaphe quadrivirgata,, and the prey it usually avoids, Glandirana (Rana) rugosa.
Photo credit: Alpsdake, and Open Cage.
Some molecules produced by frogs alter the predator's behavior. African clawed frogs  induce yawning and gaping movements in the northern water snake Nerodia sipedon as well as two other natural predators, the African aquatic snakes Lycodonomorphus rufulus and L. laevissimus.

In a new study Yoshimura and Kasuya (2013) examine the impact of the molecules produced by the adult wrinkled frog (Glandirana (Rana) rugosa) which has warty skin with a secretion that has a strong and unique odor. The wrinkled frog is rarely found in the natural diet of the Japanese striped snake (Elaphe quadrivirgata), which is considered a general predator of amphibians, mammals, birds, and reptiles. In a previous study, newborn Japanese striped snake with no prey experience ate few wrinkled frogs. When adult Japanese striped snakes  were forced to swallow wrinkled frogs, all the snakes spat out the frogs and opened and closed their mouths in a gaping behavior. The snake did not change its movements or other behaviors and they did not die shortly after contact with the wrinkle frog. These observations suggested that wrinkled frogs are not highly toxic but that they escape from predation by snakes.

Yoshimura and Kasuya (2013) conducted two experiments to examine whether the skin secretion of adult wrinkled frogs is effective for the evasion of predation by snakes. In the first experiment , they compared the proportion of snakes that bit and swallowed wrinkled frogs with the proportion that bit and swallowed the cricket frog, Fejervarya limnocharis, which resembles wrinkled frogs in size and appearance. In the second experiment they coated the natural prey organisms of the snakes with secretions from the wrinkled frog or the cricket frog to examine the effects of the secretions.

They found the wrinkle frog was less frequently bitten or swallowed by snakes. The snakes that bit wrinkled frog spat out the frogs and showed mouth opening (gaping) behavior, while the snakes that bit the cricket frogs did not show gaping behavior. They also compared the responses of the snakes to wrinkled frogs and F. limnocharis secretions. They coated palatable Rana japonica with secretions from wrinkled frogs or cricket frogs. The frogs coated by wrinkled frog's secretion were less frequently bitten or swallowed than those coated by F. limnocharis secretion. The authors concluded that compared to different frog species of similar sizes, the adult wrinkled frog was less frequently preyed upon by, and that its skin secretion was effective in avoiding predation by snakes.

Yoshimura Y, Kasuya E (2013) Odorous and Non-Fatal Skin Secretion of Adult Wrinkled Frog (Rana rugosa) Is Effective in Avoiding Predation by Snakes. PLoS ONE 8(11): e81280. doi:10.1371/journal.pone.0081280

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