A Milk Frog, Trachycephalus (=Phrynohyas) venulosus calling in Tikal, Guatemala, May 2010.  Note the two vocal pouches that extend upward, sometimes touching when actively calling.  Photo by Shannon Fortenberry.

A Mexican treefrog, Smilisca baudini, calling in Elegans Pond, Chiquibul Education Centre, Las Cuevas, Cayo District, Belize, in May 2012.  This common species has double vocal pouches under its chin.  Photo by Kevin Zansler.

This Dendropsophus (Hyla) microcephala misera has a single vocal pouch, which is the norm in the frog world.  This specimen was calling at Matura Beach, Trinidad, in June 2007.  Photo by Bob Thomas.Middle American burrowing toad, Rhinophrynus dorsalis calling in Tikal, Guatemala, in May 2010.  This species is unusual in many ways, but its air movement during calling is unique.  Instead of vocal pouches lateral to or under the head, it inflates internal sacs toward the rear of the body, and makes its “car alarm” sound as the air moves to internal sacs on the sides of the shoulders.  Photo by Bob Thomas.