Usually it’s your ears that tell you that Wood Frogs (Lithobates sylvaticus) have emerged from hibernation. They are one of the first amphibians to announce themselves, often appearing before snow and ice are completely gone. Male frogs congregate in shallow ponds and vernal pools, where their ardent courtship ensues.
Unlike most frogs, which have a single vocal sac below their mouth, Wood Frogs have two paired sacs, called paired lateral vocal sacs, located on either side of their body just behind their head (see photo). With their mouth and nostrils are closed, male frogs pump air back and forth from their lungs to their inflated vocal sacs which vibrates their larynx and produces a duck-like quacking sound. The vocal sacs act as resonating chambers, amplifying the frogs’ calls so as to attract females from far and wide.
Vocal sacs serve a dual purpose for some frogs. As soon as the eggs of Darwin’s Frog (Rhinoderma darwinii), found in Chile and Argentina, hatch, the male scoops the tadpoles into his mouth and they spend the next six weeks metamorphosing inside his vocal sac. The male does not eat until the tadpoles have matured into adults and exited his mouth.
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