Smilisca fodiens (Boulenger, 1882)
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.