Amphibians
Narrow-mouthed Frog
Microhylidae · also called narrowmouth toad, microhylid
The biggest family of frogs is built around a small idea: eat ants. Narrow-mouthed frogs have traded the wide gape of a typical frog for a tiny mouth and pointed head suited to picking ants and termites out of the leaf litter and pushing into their nests. That specialisation has carried the family to more than 700 species — and to the smallest animal with a backbone.
Overview
Narrow-mouthed frogs are the family Microhylidae, over 700 species, about a tenth of all frogs, across the Americas, Africa, Madagascar, southern Asia, New Guinea, and northern Australia. Most are small, often plump, short-legged frogs of forest floor and soil, with a small pointed head and a fold of skin behind the eyes. Nearly all are ant and termite specialists.
The smallest vertebrate
Paedophryne amauensis, a New Guinea microhylid described in 2012, averages about 7.7 mm — smaller than a housefly — and is the smallest known vertebrate, with several close relatives nearly as tiny. They live in wet leaf litter, eat mites and springtails, and hatch straight into froglets.
Skipping the pond
Many microhylids have unusual tadpoles that filter-feed in midwater with no rasping mouth, and a large fraction — especially the New Guinea species — have done away with the free-swimming stage entirely, laying large yolky eggs on land that hatch as miniature adults.
Strange company and sticky skin
In several regions small narrow-mouthed frogs share burrows with tarantulas, protected by the spider while eating ants that would raid its eggs. Handled roughly, many produce a sticky rubbery mucus that gums up ants and snakes; the tomato frog of Madagascar oozes a thick white secretion that can cause an allergic reaction.
Status
Narrow-mouthed frogs as a family are Not Evaluated, trend decreasing. Many are common, but a large number are narrow-range specialists of single mountains, forests, or islands in New Guinea, Madagascar, and the Western Ghats, threatened by deforestation, the chytrid fungus, and climate change.
Narrow-mouthed Frog: common questions
Why is it called a narrow-mouthed frog?
Because the mouth really is small and narrow compared with a typical frog. Most frogs have a wide gape that lets them take large, varied prey, but narrow-mouthed frogs have specialised on tiny, abundant prey, chiefly ants and termites, and their mouth, jaw muscles, and tongue are built for grabbing many small items quickly rather than one big one. The narrow head that goes with this is also useful for pushing into the confined spaces where ants and termites live. A few members of the family have secondarily broadened the mouth again, but the small mouth is the ancestral and defining feature.
Is the smallest animal with a backbone really a frog?
The smallest known vertebrate is a narrow-mouthed frog, Paedophryne amauensis, from the rainforest floor of Papua New Guinea, described in 2012 with an average length of about 7.7 millimetres, which is small enough to sit comfortably on a coin. Several of its close relatives in the same genus are only slightly larger. These frogs live among wet leaf litter, are active by day, eat mites and springtails too small for most frogs, and skip the tadpole stage, hatching directly as miniature adults. The title of smallest vertebrate has passed back and forth between tiny frogs and tiny fish as new species are found, but a microhylid frog currently holds it.
Do narrow-mouthed frogs have tadpoles?
It varies across the family. The ancestral and still common pattern is an aquatic tadpole, but many microhylid tadpoles are unusual, being filter feeders that hang in midwater with no beak or rasping teeth, quite unlike the algae-scraping tadpoles of most frogs. A large fraction of the family, particularly the New Guinea and some Asian and African species, has done away with the free-swimming stage altogether, laying a small clutch of large yolky eggs in a moist sheltered spot that hatch directly into fully formed froglets. This frees those species from any need for standing water.
Why do some narrow-mouthed frogs live with tarantulas?
In several parts of the world small microhylid frogs have been found sharing burrows with large spiders, most famously the eastern narrow-mouthed toad with tarantulas in the southern United States and various microhylids with tarantulas in South America. The arrangement appears to benefit both. The frog gets a safe, humid retreat and protection, since the spider drives off snakes and other frog predators and the frog is small enough that the spider does not treat it as food, perhaps recognising it by chemical cues. The spider may benefit because the frog eats ants and small insects that would otherwise attack the spider's eggs and young.