Background for Teachers
Amphibians are remarkable creatures. Most begin their lives as an egg laid in water, hatch into gilled larvae, and eventually emerge on to land as an air-breathing adult. In fact the word amphibian itself is formed from two words from ancient Greek, amphi meaning "both houses" and bios meaning "life."
Amphibians are vertebrates that belong to a group of animals that consist of frogs and toads, called Anurans, salamanders, and, an even odder looking group called caecilians. Both frogs and toads have strong hind legs, used for leaping or hopping as well as long tongue tongues that can be used to capture prey. Frogs have moist, smooth skin, while toads, a form of frog, have dry skin covered with wart-like bumps. Most salamanders look like small lizards, but they do not have claws on the end of their toes and have the typical smooth, moist skin of most amphibians. Caecilians look somewhat like worms. With the exception of toads, amphibians have cool, moist skin.
Amphibians can be found on every continent except Antarctica. There are nearly 6,000 species of amphibians world wide. Texas hosts over 60 species: 9 species of toads, 31 species of frogs, and 22 species of salamanders. Amphibians, along with reptiles, belong to a group of animals called herptiles. Sometimes this word is shortened to herps. The study of amphibians and reptiles is called herpetology.
Amphibians come in a vast assortment of body designs, patterns, and coloration. They range in size from less than half an inch, the Brazilian Golden Frog at 0.37 inches, to the Chinese Giant Salamander at close to six feet long.
Frogs and toads can be immediately identified by their powerful hind limbs. These legs allow them to make a series of small hops (toads) or incredible leaps (frogs) to escape enemies or to capture food.
Frogs and toads are also equipped with particularly long tongues to gather food, consisting primarily of small invertebrates. When prey is detected, the tongue is lobbed in its direction. The sticky tongue adheres to the food item and the Anuran "reels" the tongue back into its mouth. When swallowing food, amphibians have to blink their eyes. Their eye balls are literally used to help push food down their throat.
The Skinny on Amphibian Skin
With the exception of toads, most amphibians are covered with a moist, smooth skin. Although most adult amphibians breathe air with lungs, they also use their skin to absorb oxygen from the air. Should a frog's skin grow too dry, it will have a difficult time surviving. A mucous coating helps protect amphibian skin and keeps it moist.
Many amphibian species use secretions through their skin as defense mechanisms. Some have poisonous glands that, when attacked by a predator, will extrude a noxious, bad-tasting substance that can cause nausea or numbness. The secretions of the brilliantly colored dart frogs of tropical Central and South America can cause death. The Fire Salamander of central Europe can squirt secreted toxins at enemies. Some salamanders from Asia have poison glands along the sides of their bodies which are aligned the salamander's short but sharp ribs. When shot through the skin and they can deliver a dose of a toxic substance directly into a predator's mouth. The Slimy Salamander of Central Texas lives in caves, ravines, and cliffs. They can secret a substance that acts like a glue, causing the predator’s jaws to stick together and perhaps allowing the amphibian to escape.
Conversely, toad skin is dry to the touch, feeling something like leather, and is covered in small wart-like bumps. Toads also have bean-shaped warts on the back of their heads. When an animal tries to eat them, they secrete a milky-white liquid, called Bufotoxin.
While most adult amphibians live on the land, almost all begin life as an egg laid in the water. When they hatch from their jelly-like eggs, larval amphibians breathe the oxygen within the water they live in using gills. At this time they look and behave more like small, strange fish than amphibians.
As they mature, they become air breathers. Frogs and toads loose their tails, their intestines change from being able to digest plants to being able to digest animal matter. They develop lungs that can breathe air, legs that can better move on the land, and wide mouths that can swallow prey.
A number of salamanders mature while still in the larval state. These forms are entirely aquatic and as such retain their gills. When this occurs it is referred to as neotency.
Amphibian reproduction is prompted by two things, temperature and rainfall. In tropics, where the weather is always warm, amphibians are driven to reproduce when the seasonal rains fall. In temperate areas, places where there are distinct seasons, breeding occurs with an increase of temperature and increased rainfall.
Frogs and toads in Texas generally breed from late winter through spring, with some species breeding throughout the spring and summer. The male frogs and toads arrive at vernal breeding pools during the wet season.
Male frogs and toads “sing to their ladies;” although the human ear may not recognize their calls as music. The males inflate their throats with air and blow it out. The sound of their voice depends on their species, which can range from clicks (Cricket Frogs), to a buzz (Narrowmouth Toads), to peeps and chirps (Tree Frogs), to trills and bleats (Toads), to the deep bellows of the American Bullfrog. The volume and length of the call indicates to the female something about the strength of his genetics.
Male anurans tend to call at night from flooded ditches, pools, swamps, ponds, lakes, oxbows, streams, and rivers. The noise produced by these males can be near deafening. It is the intensity of these calls that help scientists and volunteer citizen scientists, called monitors, to determine the health of amphibian populations
Females select males based on the volume and duration of their calls. Should she select a male she will approach him. The male then grasps the female from behind and wraps his forearms around her waist. She will release her eggs into the water and then he fertilizes them externally. As she lays her eggs, the male provides sperm to fertilize them. This act is known to scientist as amplexis. Frogs have a tendency to lay their eggs in clumps, while toads appear to lay their eggs in a long, string-like formation. The eggs are contained within a clear jelly-like substance.
Some amphibians never meet in order to breed. As with many salamander species, such as the Marbled Salamander, the males arrive at vernal pools well ahead of the females, and deposits a sperm packet on the floor of the pool. Weeks later the females enters the same pool and pick up a sperm packet from one or more males with her cloaca. Her eggs are then fertilized internally, something rather rare in the world of amphibians. The female salamander deposits her eggs individually. They quickly adhere to submerged surfaces, such as vegetation, logs, or rocks.
There are amphibians that developed unique strategies to help their young survive. Many frogs in tropical rainforests lay their eggs on a broad leaf over hanging the water. Large raindrops knock the eggs off the leaf and into the water. The male African Bullfrog guards its eggs and developing young against all predators and have been known to take on lions.
Eggs hatch in a matter of days or weeks, depending on the species. Most amphibian larvae are vegetarians, consuming plant matter and the material or organisms that accumulate on plants, such as hydra. Some amphibian larvae are carnivorous, and may eat tadpoles of their species or other species.
Some species of amphibians have very unusual means of caring for their young. The now extinct Gastric-Brooding Frog of Australia was unique in that the mother frog swallowed her eggs. Her body would stop the production of digestive acids while her eggs brooded. This species died out sometime during the 1980s.
Another example of bizarre amphibian reproductive behavior is that if the Surinam Toad. The female expels several eggs as the male and females swim in somersaults so that the fertilized eggs are captured within pockets in the skin of her back. There the eggs hatch out and complete metamorphosis within the protective skin of the female's back before releasing into the water.
Some amphibian species never experience the larval stage and complete development while within the egg.
Eating or Being Eaten
Amphibians eat enormous amounts of insects, spiders, and worms. There are even species that grow so large that can consume small birds and mice as well as smaller reptiles and other amphibians, including their own kind. Many species can eat anything they can fit into their mouths.
Despite their appetites, amphibians remain very close to the bottom of the food chain. Larvae and adults can fall victim to larger invertebrates, fish, amphibians, reptiles, birds, and mammals.
The First Amphibians
Amphibians predate dinosaurs, which later gave rise to reptiles, mammals, and birds. They are thought to have come into existence in the Devonian Period, which began almost 400 million years ago. It was at this time that some species of fish developed lobed fins that could temporarily function as legs. The amount of dissolved oxygen in both fresh and saltwater may have led to this development. Additionally, seed-bearing plants developed during this period. Arthropods (early relatives of insects) had previously begun to colonize the land, providing a rich food source. After fish, amphibians were the first vertebrates on the planet Earth. The earliest forms were really fish with club-shaped fins that allowed them to crawl across the mudflats of low tides. Amphibians developed almost 400 million years ago and long, long before the dinosaurs. Despite the fact that they still must return to the water in order to bear young, they were the first animals to colonize the land.
Why Should I Care About Amphibians?
Amphibians have much to offer us. They consume large amounts of agricultural and other pests. They also serve as an indicator species. The decline or absence of amphibians can suggest degradation of the local environment. Their uniquely absorbent skin can quickly take in heavy metals, pesticides, and other toxic compounds. If these contaminants or others are in our drinking water they might do use equal harm.
Recently it has been suggested that unused or expired medication flushed down toilets has had a detrimental effect on amphibians as well as fish. Scientists are studying these effects, also with the question of what affect this may have on humans. (Humans and wildlife share sources of drinking water. Although humans drink treated water not all chemicals are filtered out.)
The skin secretions of some amphibians may prove to be useful as medicine. Scientists are currently studying these skin secretions in a hope of developing new medications. The secretion of one frog may lead to the development of a pain-killing medication more effective than morphine and without the addictive side effects. Approximately 10% of Nobel prizes in physiology and medicine have resulted from investigations that used frogs.
Amphibians are in trouble world-wide and their populations are declining.
Household chemicals are causing them serious problems; pesticides kill or contaminant amphibian food resources. Unneeded medications, flushed down the toilet, have resulted in male frogs turning into females. This skews the sexual ratio of populations.
As if these problems weren't enough, there is now a fungus called Chritrid threatening amphibians across the globe. Chritid is deadly with no known cause, nor cure. It has already led to the extinction to a number of frog species, including the Panamanian Golden Toad.
In some places of the world, amphibians that have been released into parts of the world where they are not native have cause problems themselves. The Cane Toad of Mexico, Central America, and northern South America is one such problem.
They were introduced to southern Florida and Australia with the hopes that they might control beetles that feed on the sugar cane. The powerful skin secretions can cause death to predators that attack them. Without a predator that can with stand its Bufotoxins the Cane Toad has over run Australia and has now become the problem.