Hylidae

• 1. Introduction

  ›

  Hylidae, the family of frogs commonly called "tree frogs and their allies," is far stranger than its name suggests. Most people picture a bright green frog clinging to a branch. Yet a substantial number of hylids never climb at all. They burrow into soil, swim in streams, or carry their eggs on their own backs. So what exactly ties this sprawling family together, and how did one group of frogs end up on nearly every continent? The answers run through deep geological time, a frozen continent that once was not frozen, and dozens of genera with names that hint at warrior frogs, harlequins, and lime-colored treetop dwellers.

• 2. Ancient Roots

  ›

  The earliest fossils that scientists can confidently assign to Hylidae come from two very distant locations: the Cretaceous rocks of India and the state of Wyoming in the United States. That pairing alone signals how old and geographically scattered this lineage is.

  Two of the three subfamilies, Pelodryadinae and Phyllomedusinae, sit so far apart from the rest of the family on the evolutionary tree that researchers sometimes treat them as distinct families entirely. The source of that divergence is Pelodryadinae's origin story: this group colonized Australia during the Eocene epoch by crossing what is now Antarctica, at a time when the Antarctic land bridge had not yet frozen over. Pelodryadinae is the sister group to Phyllomedusinae, and together they represent a lineage that charted a course across what would become the most inhospitable terrain on Earth. Both subfamilies are currently retained within Hylidae despite the scientific debate about their status.

• 3. A Family of Many Names

  ›

  Acris, the cricket frogs; Pseudacris, the chorus frogs; Triprion, the shovel-headed tree frogs; Aparasphenodon, the casque-headed frogs. The genus names inside Hylidae read like a bestiary, and together they number in the dozens across three subfamilies. Hylinae is the largest, organized into tribes with names such as Cophomantini, Dendropsophini, Hylini, Lophiohylini, Pseudini, Scinaxini, and Sphaenorhynchini.

  The subfamily Pelodryadinae, the Australian tree frogs, includes genera such as Litoria, Nyctimystes, and Ranoidea. Several species formerly placed in Litoria sit in taxonomic limbo, listed under quotation marks to signal uncertain placement. Phyllomedusinae, the leaf frogs, contains genera such as Agalychnis, Phyllomedusa, and Pithecopus, along with Phrynomedusa, the colored leaf frogs, and Hylomantis, the rough leaf frogs.

  The common name "treefrog" causes its own confusion. It belongs not only to Hylidae but also to many species in the entirely separate family Rhacophoridae. A name can travel across family lines while the animals bearing it remain only distant cousins.

• 4. Built for the Canopy

  ›

  Forward-facing eyes are unusual in frogs. Most species have eyes placed on the sides of the head, giving them a wide field of view for spotting predators. Hylids, by contrast, typically carry their eyes facing forward, which produces binocular overlap and helps them judge distances when leaping between branches. Adhesive pads on the fingers and toes complete the arboreal toolkit, gripping bark and leaves through a combination of mucus and microscopic surface geometry.

  In hylid species that have abandoned trees for a life on the ground or in water, these same features can be reduced or absent entirely. Evolution does not maintain structures that cost energy but serve no purpose. The result is a family where the classic tree frog look is really just one end of a continuum, with terrestrial and semiaquatic species occupying the other.

• 5. Where They Live

  ›

  Hyla arborea, the European tree frog, ranges across the middle and south of Europe and extends into Asia and North Africa. North America holds a dense concentration of hylids: Hyla versicolor, the gray tree frog, shares the continent with Hyla cinerea, the American green tree frog. Pseudacris crucifer, the spring peeper, is widespread across the eastern United States, and its call is commonly heard on spring and summer evenings.

  Species in the genus Cyclorana take a different approach to habitat altogether. These burrowing frogs spend much of their lives underground, surfacing only when conditions favor activity. Their lifestyle makes them nearly invisible for long stretches and illustrates just how far the family has wandered from its tree-dwelling reputation.

• 6. Breeding Strategies

  ›

  Hylids lay their eggs in an unusually wide range of settings. Many species choose ponds, or use small pools that collect inside tree hollows. Others deposit eggs in bromeliads or other plants that hold water in their leaf axils. A third approach involves laying eggs on leaves that hang directly over water, so tadpoles drop into the pond the moment they hatch.

  Species that breed in fast-moving streams face a different problem: current. These frogs attach their eggs firmly to rocks or substrate to prevent them from washing away. Their tadpoles hatch with suckers that let them hold position on rock surfaces in the flow. Some South American hylids go further still, brooding eggs directly on the female's back rather than placing them in water at all.

  Tadpoles of most hylid species share a recognizable silhouette: eyes placed on the sides of the head and broad tails that narrow to filamentous tips. The casque-headed genera of tribe Lophiohylini, such as Aparasphenodon, Corythomantis, and Trachycephalus, carry hardened bony structures on their skulls and point toward yet another ecological niche this family has found a way to fill.