Burrowing Treefrog
Smilisca fodiens (Boulenger, 1882)
Other names for this frog include Pternohyla fodiens. Adult females reach to 63 mm, and adult males attain 62 mm; tadpoles reach 45 to 50 mm before they transform. The skin on top of head is fused to the skull, and there is a fold of skin behind the head. Toe pads are small, but visible; webbing on the feet is greatly reduced and is absent between the fingers. A white tubercle is present on the hind feet. In adults the dorsum is tan-brown with dark brown spots, which sometimes fuse to form stripes. Juveniles are pale green with scattered flecks on the dorsum. New metamorphs and juveniles are superficially similar in appearance to the Arizona Treefrog. The call is a loud, low-pitched wonk, wonk, wonk, or quack, quack, quack that lasts 0.2 to 0.3 seconds.
The Lowland Burrowing Treefrog occurs from south-central Arizona southward into Mexico and along the Pacific coast to Colima, and west to central Jalisco and Michoacán. It ranges from near sea level to about 1,500 m. It lives in burrows in open mesquite grassland close to washes. The species is part of the Sonoran desert scrub and semi-desert grassland fauna.
This subtropical frog spends much of the year underground. The paradox in its name hints at the story of a lineage of treefrogs that became subterranean during the dry season but could still exploit arboreal habitats when active. The relatively small head, with the skin fused to the skull, is involved in an unusual behavior. This is an adaptation for phragmosis, a behavior whereby the frog flexes its head to plug the opening to its burrow. By closing the burrow opening with its head, the frog reduces water loss and the risk of predation. It has robust limbs with long digits, and a single, large tubercle used for digging is located on each hind foot. Beetles and orthopterans are important in the diet, based on a small sample.
Reproduction occurs in temporary pools formed by monsoonal rains. Sexually mature males have a heavily pigmented throat patch, and large, slightly bilobed vocal sac. The species migrates between aestivation sites and breeding sites. In a monsoonal downpour near Guadalajara one night, I observed hundreds of these frogs moving across the road to breeding ponds. During the day, it takes refuge in holes it excavates or in rodent burrows in hard clay soils, where the humidity is high.
Aestivating Lowland Burrowing Treefrogs form a transparent cocoon made from layers of skin (Ruibal and Hillman, 1981). During cocoon formation, the frog is motionless, with all limbs folded tightly against the body. The presence of the cocoon is not obvious, due to its close adherence to the body and its transparency. The cocoon is perforated at the nostrils but otherwise covers the entire body. The cocoon skin softens when placed in water and the frog’s movements allow it to escape from the chamber. Examination of a cocoon fragment revealed a multi-layered structure with a total thickness of 0.05 mm and about forty-three cell layers visible in the vertical section. The cocoon allows the frog to better control water loss or water uptake during aestivation.
