BULLFROGS know a familiar voice when they hear it. When
defending their territory, North American male bullfrogs listen
for subtle differences in the croaks of other males to sort out
the newcomers from the old-timers. Using synthetic frog sounds,
biologists Mark Bee and Carl Gerhardt of the University of
Missouri-Columbia played male bullfrogs familiar and unfamiliar
croaks. The frogs reacted more aggressively towards the calls of
strangers, whether they came from previously occupied
territories or from new locations ( To continue reading this
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