#dicroglossidae

Indian Bullfrog (Hoplobatrachus tigerinus), mating, family Dicroglossidae, India

In the breeding season, the males turn bright yellow.

Green pond frogs (Euphlyctis hexadactylus) perch on an Asiatic water buffalo (Bubalus bubalis) in Kumana National Park, Sri Lanka

This is just a Frog™, right?

Wrong. These four frogs belong to four different families (respectively, Dicroglossidae: Limnonectes paramacrodon, Ranidae: Rana temporaria, Mantellidae: Aglyptodactylus madagascariensis, and Pyxicephalidae: Amietia tenuoplicata) from across the world (respectively, I took these photos in Borneo, Denmark, Madagascar, and Tanzania).

These represent just some of the huge number of genera across more than half a dozen families that have independently produced species that look like this:

In Europe: Ranidae: Rana. In North America: Ranidae: Boreorana/Lithobates. In South America: Leptodactylidae: Leptodactylus. In Africa: Pyxicephalidae: Amietia or Arthroleptidae: Arthroleptis. In Madagascar: Mantellidae: Aglyptodactylus. In mainland Asia: any of a dozen or more genera of the family Ranidae; or Ranixalidae: Indirana. In Australia: Pelodryadidae: Rhyaconastes. and more, besides!

Why? Well… that is kind of a fundamental question in evolutionary biology. Why, with the infinite theoretical possibilities of evolutionary processes, do we see the same kind of anatomies evolving again and again? Does this mean that evolution is in some way predictable?

The answer is complex (there are good books on this topic!), but basically, yes: similar pressures can (but do not always) lead to similar solutions. I think this may apply to frogs to an exceptional degree, in part because their body plans are so simple to start with, that the tools available for tinkering are pretty streamlined and refined. These are just one stark example, but you can do this with whole ecological assemblages of frogs in some cases. Madagascar is especially excellent for this, because the island was only naturally colonised by frogs five times, and yet you have practically every main 'type' of frog there, independently derived from one of these colonising lineages.

Yet, each of these generic brown frogs is also subtly different—and their skeletons reveal pretty fundamental differences unique to their lineages. Not to mention their totally distinct tadpoles!! And that is part of the key, too: although some parts of evolution can be predictable, no organism can separate itself from its own evolutionary past. There is always a degree of contingency involved.

I find these frogs to be a really elegant demonstration of this principle. And also, a nice demonstration of why it is fucking impossible to ID frog pictures you guys send me without a more or less precise location.

Can You Imagine the Crab-Eating Frog?

The crab-eating frog (Fejervarya cancrivora) is a species of frog found throughout southeastern Asia, including Taiwan, southern China, the island of Sumatra, and the Philippines. They reside primarily in mangrove swamps, as well as tropical rainforests, estuaries, and near freshwater ponds and streams. They are particularly noted for their ability to tolerate high salinities, and they are the only known amphibian to make excursions into pure salt water.

The crab-eating frog is named for its usual diet which, around the mangrove, estuary, and coastal parts of its distributon, is composed mainly of crabs. This is supplemented with insects and smaller frogs, and near fresh water without crabs they make up the bulk of F. cancrivora's diet. Due to their small size, they have many predators, including birds, snakes, lizards, jungle cats, and larger fogs. Crab-eating frogs avoid being eaten by hiding in grass or under vegetation and leaf litter during the day; they are also more active at night.

Male and female crab-eating frogs are fairly similar in appearence. Both can be tan or brown, with dark mottling to resemble the muddy substrate in which they hunt, and a light underbelly. Females are slightly larger than males, reaching up to 10.7 cm (4.21 in) in length compared to only 8 cm (3.14 in). Males may also have dark throats, while females are bare.

Crab-eating frogs can breed year-round, but is particularly active during the wet season from June to October. Males will gather around bodies of water and call to attract females. Once a female has selected a mate, she will lay her eggs while he grasps her from above and fertilizes them. The eggs remain in the body of water in which they're laid without parental care. After hatching, the tadpoles take about three weeks to develop into adults.

Conservation status: F. cancrivora is considered Least Concern by the IUCN. They are harvested for food, but their primary threat comes from habitat destruction.

Photos

Nick Baker

Elijah Wostl

Blyth's River Frog Limnonectes blythii

Found from Myanmar through western Thailand and the Malay Peninsula (Malaysia, Singapore) to Sumatra and Borneo (Indonesia). These frogs inhabit streams with gravel and rocks in primary and secondary evergreen forest. Males build a nesting hollow in a sandy stream bed area, and the tadpoles develop in streams.

Least Concern