Narrow-Mouthed Frogs: Microhylidae

Most of the narrow-mouthed frogs are active at night, but they sometimes come out during the day. Those that live high in mountains are more likely to be active in the day time. Many of the narrow-mouthed frogs are burrowers and use flat scoops, or spades, on the heels of their feet to help them dig backward into the soil. The Bushveld rain frog, for example, has spades on each of its hind feet to help it dig. This frog can burrow as far as 20 inches (50 centimeters) below the surface. Others that burrow do not have the spades and instead use their front feet to dig into the soil head first. A few, like Boulenger's callulops frog, make their homes in burrows that they probably take from other animals, rather than build themselves. Many of the burrowing frogs stay underground during dry periods, only coming out during heavy rains. This behavior allows desert-living frogs to survive, but even the burrowing species that live in rainforests will crawl underground when the weather is dry.

Like many other frogs, most narrow-mouthed frogs defend themselves from predators by quickly jumping off, perhaps into a shrub where they can hide or into a nearby pond where they can dive out of sight. Some will also disappear into a burrow or try to dig down into the soil. A few narrow-mouthed frogs have more unusual defense tactics. One of the most bizarre is the tomato frog, a plump frog that lives on the northeast coast of Madagascar, a large island that lies east of southern Africa. The males are orange and grow to about 2.5 inches (6.5 centimeters), but the females are red and can reach 4 inches (10.1 centimeters) long and weigh half a pound (227 grams). Like many other frogs, the tomato frogs have skin that oozes, or secretes, a poisonous substance that tastes bad to predators. The substance in the tomato frog's skin goes a step farther. When a snake or other predator bites a frog, not only does the predator get a mouthful of white, bad-tasting goop, but the goop is exceptionally sticky—so sticky, in fact, that it can seal shut the mouth and eyelids of the retreating predator for several hours, sometimes days. Studies have shown that the substance is five times stronger than rubber cement.

Another narrow-mouthed frog with an unusual way of protecting itself is a broad-headed frog of New Guinea, near Australia. When it feels threatened, it holds its ground rather than running away, blows up its body, and opens its mouth to show off its bright, blue tongue. If the predator is not already frightened off by this display, the frog clamps its jaws on the predator and hangs on for several minutes. When the frog finally lets go, the predator often has had enough and leaves the frog alone.

Most of the frogs in this family mate when the weather is warm and rainy. In tropical rainforests, where it is almost always wet and warm, some species may be able to mate any time of the year. Those that live in very dry areas, however, mate only during the very short rainy season. The Bushveld rain frog spends most of its life underneath the dry ground of the desert-like areas where it lives. When the rains come, it comes out, usually at night, to search on land for termites and eat until it is fat. This fat helps the frog survive underground until the next rainy season. It also mates during this rainy period.

Depending on the species, narrow-mouthed frogs may mate in or near the water or on land. In both cases, the males begin calling when they are ready to mate. Some call with single or groups of ringing notes, while some have harsher voices. Many males have a bag of skin, called a vocal sac, on the throats. The vocal sac fills with air and deflates when the male calls. Other species have no vocal sacs, but still manage to call. In 2002, scientists reported that one narrow-mouthed frog, called the Borneo tree-hole frog, actually practices and adjusts its call, making it higher or lower to get the best echo from the tree cavity where it does its singing. The scientists said this was the first time any animal had ever been shown to change its call or song based on the place from which it calls. For other narrow-mouthed frogs, the males simply call. The females hear the calls and follow them to find the males. In the species that mate on land, a male's call may not only attract a female, but may also tell other males to find somewhere else to mate. The male horned land frog is such a frog. Males will often call back and forth, apparently to set up and keep their territories.

Species that mate on land usually do so in various hidden-away spots. These may include burrows that the males dig themselves, piles of leaves on the ground, tree holes, or plants that grow on the sides of the trees. The male Fry's whistling frog, for example, moves to the top of the pile of leaves where it lives and calls from there. Boulenger's callulops frog, on the other hand, calls from inside or near the entrance to its burrow, which it does not make but instead takes over from another burrowing animal. More than half of the narrow-mouthed frogs mate in or near water. The water may be a stream or other body of water that remains filled with water all year, or it may be a pool of water that dries up once the rainy season ends. Some frogs use very small pools of water that they find inside tree holes or within plant leaves that grow together to form small cups. Boulenger's climbing frog is an example of a frog that makes use of puddles inside tree holes.

Little information is available about reproduction in most of the 362 species in this family. Scientists assume, however, that the males and females of most if not all narrow-mouthed frogs mate like many other frogs: the male climbs onto the female's back as she lays her eggs. This piggyback position is called amplexus (am-PLEK-sus). In some species, like Bushveld's rain frog, the females are much larger than the males of the same species, which would make it difficult for the male to hold on if he didn't have some help. The help comes in the form of a sticky substance that oozes from the skin. It glues the pair together while they mate. In other species, like Boulenger's climbing frog, the males have sharp spines on one toe of each front foot. They probably use the spine to hang onto the females during mating.

In the species that mate on land, the females lay their eggs in a moist spot. In the Bushveld rain frog, the stuck-together male and female pair dig backward into the soil until they find a moist spot and she lays her eggs there. The eggs of this frog and most other land-mating species of narrow-mouthed frogs develop right into froglets, rather than turning into tadpoles first. The froglets usually look like miniature versions of the adults. Scientists use the term direct development to describe the growth of eggs right into froglets instead of tadpoles and then froglets. The food for each developing egg comes from a large yolk. Because the yolk is so large, the females usually lay only a few eggs at a time. A female Fry's whistling frog lays seven to twelve eggs, while a female Timbo disc frog lays just four to six eggs at a time. The males typically stay behind to watch over the young, sometimes even carrying them around on their backs. The males nab and gulp down insects that would otherwise eat the eggs and may also huddle with the eggs to keep them moist.

In the species of narrow-mouthed frogs that mate in the water, the females of some species drop their eggs in a pond or stream, while others lay their eggs in a pool of water inside a tree hole or within water held in plant leaves. The saffron-bellied frog is a species that mates around small pools that fill with water after a heavy rain. The females lay their eggs in the pools. The eggs of water-mating species may clump together as they do in the Bolivian bleating frog; they may float on the surface as they do in the ornate narrow-mouthed frog, or they may sink or float in other patterns. Frogs that mate in the water often lay hundreds of eggs at a time. One of these frogs, the ornate narrow-mouthed frog, lays several hundred. The Bolivian bleating frog lays about two hundred at a time, but the female may lay several clutches in a single season. One female Bolivian bleating frog was found with more than two thousand eggs in her body.

Once most water-mating narrow-mouthed frogs lay their eggs, both adults leave the eggs to hatch on their own. The females that lay their eggs in larger bodies of water, like streams or ponds, lay small eggs. For instance, two hundred eggs of the Bolivian bleating frog can fit into a cluster just 4 inches (10 centimeters) in diameter. These eggs hatch into tadpoles, sometimes in as little as a day and a half, and the tadpoles search for food in the water. In some species, like the ornate narrow-mouthed frog, the tadpoles have see-through bodies that make them nearly invisible. This helps them avoid predators.

Those water-mating frogs that use small pools of water for mating have eggs with yolks that are larger than the pond or stream species. Even after the tadpoles hatch from the eggs, they continue to rely completely on the egg yolk for food. This is important, because their tiny pools would likely not have enough food in them to keep the tadpoles alive. In some species, like Boulenger's climbing frog, the male stays in the tree hole with the eggs until they develop into froglets.