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What Are Amphibians?
Amphibians are the oldest terrestrial* vertebrates* existing in the world today
(even older than dinosaurs). In fact, it has been estimated that the first
amphibian evolved approximately 360 million years ago.
In contrast, Tyrannosaurus rex,
one of the later Dinosaurs to evolve, lived approximately 67 million years ago.
Furthermore, in a time when all vertebrates were fish (or similar to
fish) amphibians were the first vertebrate animals to attempt a life on land.
Because
they were one of the first animals to live, at least, part of their life on
land, most amphibians today still need to have a source of water nearby for
survival. For example, their skin
is incredibly thin, much more so than reptiles or mammals.
So thin, in fact, that most amphibians do not “drink” water, but
absorb it. On the same note,
amphibians can loose water very easily though their skin as well.
Therefore, if they do not have a source of water nearby, they will become
desiccated (or dried-out) and may die. Likewise,
because their skin is so thin, amphibians can cutaneously respire*.
This means that they can essentially “breathe” through their skin.
Furthermore, unlike other egg-laying animals that evolved later than
amphibians (such as reptiles, dinosaurs, and birds) amphibian eggs do not have a
thick shell that keeps them from drying out.
Therefore, amphibians must use a source of water, such as a pond or
wetland, to lay their eggs in.
Amphibians are the only terrestrial vertebrates that have a true larval*
stage. These larvae (or tadpoles)
are completely restricted to life in the water.
They have tails, no legs, and gills.
In most cases, amphibian larvae eat primarily algae and will not begin
consuming insects until they become adults.
After several weeks or months, tadpoles loose their tails, replace their
gills with lungs, and grow legs. At
this time, they can exit the water and live on land, as well as eat insects
instead of algae. Examples
of Amphibians Living Today
There are three main groups of amphibians existing today: 1) frogs, treefrogs, and toads (collectively called Anurans), 2) Salamanders, newts, muddpuppies and sirens (collectively called Caudates), and 3) a very small group that very little is known about called Gymnophiona. Gymnophiona contains strange amphibians called Caecilians. Caecilians are primarily found in Central and South America (not in the U.S.) and, therefore, we will no longer discuss them here. Examples of anurans in the La Crosse area include; leopard frogs, green frogs, gray treefrogs, spring peepers, western chorus frogs, and American toads. Examples of caudates include, tiger salamanders, central newts, and mudpuppies. Morphology*
of Amphibians
Amphibians are tetrapods, meaning that they have four limbs (or legs). They generally have very moist skin. As stated previously, amphibians also have very thin skin. Of all amphibians, anurans are probably the most recognizable. They typically have no tails, large eyes, wide heads, and long powerful hind-legs for jumping. Some anurans, such as treefrogs, can stick to sheer surfaces like as glass. On
the other hand, caudates (i.e. salamanders, mudpuppies, etc.) have small eyes,
their bodies are elongate, they have stubby legs, and also have tails.
In addition, most caudates cannot jump or move as fast as anurans.
Finally, while anurans are frequently seen in the open, caudates are more
secretive and tend to hide beneath leaf litter, under water, or under logs.
Occasionally, salamanders are found in window-wells. A
Brief Natural History of Anurans Anurans have circular lifecycles. This lifecycle starts with breeding adults. Unlike other amphibians, male anurans “call” or “sing” during the breeding season to attract females for mating. Every different type of frog, treefrog, and toad has a call or song that is unique to that species. For example, a leopard frog’s call is much different from a green frog’s call. This helps insure that two different types of frogs do not accidentally try and mate. After the male frog has attracted a female with its call, he grasps her around the waist. This action is called amplexus. Once in amplexus, the female frog releases her eggs, coated with a jelly-like shell, into the water or attaches it to aquatic plants. Sometimes, the eggs are released in strings. Other times, they are released in globular masses, or in small packets (depending on species). As the female releases her eggs, the male releases his sperm into the water, which fertilizes the eggs. After this is complete, the adults leave the eggs to fend for themselves.
Wood frog eggs from a site in Minnesota. Within several weeks, if they have not dried out or been eaten by a predator, the eggs develop and hatch into larvae (tadpoles). These tadpoles are restricted to life in the water. They have tails, no legs, gills, and eat mostly algae. Within, several more weeks, the tadpoles undergo metamorphosis*. During which time, they loose their tails and grow legs. Eventually, they are able to leave the water and live on land as sub-adult* frogs. After a certain amount of time (usually several years, depending on species), these sub-adult frogs become sexually mature adults. Then, when the time is right, the adult male frogs will began calling to attract females to breed with and the cycle starts over.
Pictured above:They are hard to see, but this net is completely full of leopard frog tadpoles. A
Brief Natural History of Caudates Caudates have mating strategies that are different than anurans. For this example, we will use salamanders (as opposed to newts or mudpuppies, whose breeding habits are much different). Generally, males and females of a given species will migrate to ponds or wetlands on moist spring evenings. Here, they begin the mating process. Male salamanders do not "call" to attract females. In some cases, the adult salamanders will perform a very elaborate mating "display", which can involve several different types of body movements, and even biting. When the adults are ready to copulate, the male releases a packet of sperm, called a spermatophore. This spermatophore is sometimes attached to a small twig, or placed directly on the ground (depending on species) for the female to pick up. When she is ready, the female will pick up the spermatophore and insert it into her cloaca and fertilize the eggs. When the time is right, the female lays her eggs in small, loosely connected egg masses and attaches them to aquatic vegetation. Eventually, these hatch into larvae. These larvae have large external gills and are restricted to living in water. Unlike anuran larvae, salamander larvae generally do not eat algae. Most have wide heads and are carnivorous, eating water bugs. As they grow, these larvae will consume larger prey, including tadpoles and even small fish. Many researchers feel that salamander larvae are the top predators in ponds that have no large fish (such as bluegills). After several months, these larvae loose their gills and crawl out of the water. At this time, they migrate to a nice wet burrow and remain here until they reach sexual maturity, when they may leave their burrow to breed.
Pictured
above: a tiger salamander larvae. See the large head, tiny feet, and
external gills? *Glossary: Cutaneous:
A fancy word used by biologists to describe those things dealing with an
organism’s skin. Look at the
following example: cutaneous respiration.
To respire means “to
breath”. Therefore, cutaneous
respiration means “to breath through one’s skin”. Larval:
A juvenile stage in amphibians. Larva is a fancy word for tadpoles. Amphibian larvae have tails, no legs, gills, and are
completely restricted to living in water. These
larvae eventually develop into frogs that can live on land.
Juvenile insects are also called larvae. Metamorphosis:
Change. In the case of frogs, this
change includes developing from a tadpole into a frog.
Butterflies also undergo metamorphosis.
In which case, the caterpillar creates a cocoon and, after a period of
development, hatches out as a butterfly. Morphology:
The form of a given organism,
or the structure of a given organism.
For example, when you discuss if an animal has two legs or four legs, you
are actually discussing its morphology. Sub-adult:
This term is occasionally used to describe individuals that are no longer
juveniles, but are not yet sexually mature adults. Terrestrial:
Something associated with life on land (rather than water).
For example, as far as animals go, humans are terrestrial.
On the other hand, fish are aquatic (or live in water). Vertebrate: Any organism that possesses a backbone. Humans are vertebrates, as are frogs, fish, elephants, etc.. Organisms with no backbones are considered invertebrates, such as insects, clams, jellyfish, etc..
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