True Frogs

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Ranidae – True Frogs


 Ranid frogs are sometimes called true frogs because Linnaeus applied the Latin name for frog, Rana, to virtually all the frogs he described in the mid-18th century.  Ranids have the greatest distribution of any frog family and are represented in North America, South America, Europe, Asia, the Indonesian Archipelago, Madagascar, Africa, and Australasia.  The family has more than 1000 species that range in size from some of the smallest anurans (10 mm in some Phrynobatrachus and Arthroleptella) to the largest frog, Conraua goliath (adult size up to 320 mm). Bossuyt et al. (2006) found a close association between the major ranid clades and Gondwanan plates suggesting that the breakup of Gondwana played a major role in the current distribution of ranid frogs.

Ranid frogs are the sister to the Narrowmouths in the family Microhylidae, as the two families last shared an ancestor about 111.9 MYA.

There is controversy over which generic name should be applied to most North American species.  For more than two centuries they were placed in the genus Rana, along with many other species found over much of the planet.  However, the North American species are obviously isolated from the European, African, and Southeast Asian species—and have been for a long time.  Thus, the name Lithobates was used for most (but not all) of the North American frogs formerly placed in the genus Rana.  There are now some biologists returning to using the name Rana as a worldwide genus.  Here we retain Lithobates for most of the North American ranids and all Arizona ranid frogs.

Leopard frogs are icons of the North American wetlands.  At one time they were frequently encountered over much of the country.  However, humans have exploited them for more than a century as model vertebrates for dissection in classrooms and degraded their environment with agricultural chemicals and the draining of wetlands.  About 20 species of leopard frogs range from northern Canada to southern Mexico.  In the United States, Arizona is the epicenter of species divergence. Most ranids have smooth, moist skin, muscular legs, and extensive webbing between their toes.

The Northern Leopard Frog was once considered a single widespread species that ranged from Canada to Panama.  Now, it is known to be a complex of species that are morphologically, behaviorally, and ecologically distinct.  There are still undescribed species within the clade, and the relationships between described species require clarification.  Arizona has at least seven native species, all of which are protected by law.  Leopard frogs are brown to green and usually have large dorsal spots or blotches; they are very similar to one another and are sometimes difficult to differentiate.  The Tarahumara Frog is similar in body form but lacks the spots.  Two ranid frogs are invasive species in Arizona – the American Bullfrog and the Rio Grande Leopard Frog.

Populations in Arizona and elsewhere have been declining for at least the last half century.  In addition to habitat loss and predation by invasive American Bullfrogs, many Lithobates are disappearing because of the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), the ranavirus, the red-leg syndrome, and pesticides, endocrine disrupters and lowering the water table. In the last few decades leopard frogs have been the subjects for many studies on the effects of heavy metals, pesticides, herbicides, and other organic chemicals on anurans.

Mead (2005) summarized the fossil record for remains of ranid species in Arizona. They have been recovered from Deadman Cave and Papago Springs Cave. He notes that all species of Rana are associated with nearby permanent water and that no Rana live adjacent to Deadman Cave today.