Frog Blog

Salamander Biology

Any one can recognize a frog, they have a relatively standard body form, but Salamanders display a tremendous amount of morphological and ecological variation.For example, take the giant salamanders of Asia and North America.They are large, robust species with wrinkly skin, four limbs, small eyes, and a wide, flat head. Compare this to the Siren’s of the United States who have only two reduced front limbs, external gills and are cylindrical in body shape. One might not guess at first sight that they were both salamanders.

The variation doesn’t stop there, the newts of Europein the genus Triturus are a colorful bunch any time of the year, but during the breeding season they undergo rapid and drastic morphological changes. The males develop crests on top to their backs and they take to a completely aquatic lifestyle. Fire salamanders range all across Europe and depending on different environmental variable have different reproductive strategies. For example, some populations as higher elevations will give birth to juvenile salamanders that have already completed metamorphosis, whereas other populations will deposit larvae of various stages into a pond. Some species of salamander never complete metamorphosis and are known as neotenic. The mudpuppies, waterdogs and olm of the family Proteidae are completely aquatic. Some species completely lack a free-living larval stage. Species in the neotropical genus Bolitoglossa lay eggs that the salamanders complete metamorphosis in before hatching.

How do you tell the difference between a lizard and a salamander?

Lizards and salamanders may look alike, but they represent two separate classes of animals and are as different from one another as you are from an ostrich.

True, the typical lizard and salamander has four legs and a long tail, but the similarities pretty much stop there. The lizard is a reptile and the salamander is an amphibian. If you visit your local zoo, you should be able to observe both creatures in the same building, perhaps even side by side.

If you look closely, starting at the head, it becomes clear very quickly that the lizard is a very attentive animal, turning its head this way and that, trying to follow potential predators and prey with its eyes. The salamander, by contrast, doesn’t seem nearly as interested in its surroundings and tends to stare straight ahead. Lizard legs typically are much longer and more muscular than salamander legs, so lizards can really dart about and put on a burst of speed, while salamanders usually just plod along, step by step.

A lizard’s skin is dry and its body is covered in scales.  Salamander skin is scaleless, smooth and moist to the touch. Lizards are often seen in broad daylight basking in the warm rays of the sun. Salamanders, on the other hand, are secretive, spending much of their time beneath logs and stones, underground or in the water. A good number of salamander species live entirely in the water, but only a handful of lizards have adopted an aquatic lifestyle.

Lizard tails are usually long and thin, often whip-like. Salamander tails tend to be a bit shorter and plumper. Tails of aquatic species may even be flattened vertically into the shape of a paddle or a rudder, which helps its owner swim through the water. There is one interesting thing that the tails of certain lizard and salamander species have in common. They can break off completely at certain points and then re-grow some of what has been lost. We call this process regeneration.

   

The Amphibian Egg

In your mind, the word “egg” probably evokes the image of a chicken’s egg, a critical component of the American breakfast for countless generations. Yet, plant and animal species around the world produce eggs of diverse sizes and shapes, and in quantities that can number into the hundreds, thousands or even millions. 

Amphibians were the first back-boned creatures to invade the land, but the pioneer species continued to lay their eggs in fresh water where they probably hatched unattended.  The eggs of the first amphibians were very much like those of the fish from which amphibians evolved – small and jelly-like - and they were deposited in large numbers in hope that at least a few would avoid being eaten. To this day, amphibian eggs remain clear, soft and lacking shells.  The amniotic egg was the next step in egg evolution. A hard outer shell and several layers of fluid that help to protect and nourish the embryo were added to the amphibian egg, allowing reptiles and birds to reproduce and live entirely on land. 

The egg of a frog begins as a single cell, but it is more than a million times larger than any other cell in the female frog’s body.  When a sperm from the male pierces the egg’s outer membrane, it becomes fertilized, after which the cell begins to divide very rapidly and an embryo takes shape. Over the course of several days or a few weeks, the egg hatches and a free-swimming larva emerges. 

Amphibian Metamorphosis

One reason that amphibians are such special animals is that most of them have two major life stages: a water-dwelling (called aquatic) stage and a land-dwelling stage (called terrestrial). Many amphibians lay their eggs in water and the tadpole, or newborn frog, hatches and begins its life in the water. Tadpoles resemble small fish and are equipped with a tail that allows them to swim just like fish. Tadpoles also have gills that allow them to breathe under water.

If you observe the tadpole closely each day you will begin to notice subtle changes in its appearance. It will undergo what biologists call metamorphosis. As the tadpole grows into a frog, it will lose its gills and develop lungs. It will also lose its tail and develop legs for moving about on land. Most amphibians are equally comfortable walking or hopping about on the ground, climbing in the vegetation, or diving into the water. Other changes in their bodies take place as well. For example, their eyes develop eyelids and adapt to vision outside of the water. The skin changes and develops glands that produce fluids and prevent dehydration.

Of course there are always exceptions to the rule and there are many amphibians that do not go through this “metamorphosis”. There are some salamanders that retain their larval stage and never leave the confines of their watery habitat, but do become sexually mature. This is known as neoteny. A Mexican salamander, the axolotl, is one of the best examples of this lifestyle. Some frogs are direct developers, meaning that they emerge from their egg as tiny frogs and never undergo the tadpole stage. An example of this is the Solomon Island eyelash frog, a beautiful species that can be seen on exhibit in the Houston Zoo’s Reptile House.

Amphibian Respiration

The most common form of amphibian respiration is through lungs, the same way that you and I breathe.  Well, not exactly. The way that air enters a frog’s or a toad’s lungs is different than the way it enters yours or mine. Amphibians lack a diaphragm, the muscular sheet that lies beneath the human lung. When we inhale, we contract and depress our diaphragm. This creates negative pressure in our chest cavity, expands the lungs and sucks air into them. Amphibian breathing, on the other hand, depends on positive pressure. Muscles in the frog’s throat actually push air to the lungs by a process called buccal pumping.   

In an earlier post you learned that some larval amphibians and some adult salamanders breathe through gills. The gills have lots of blood vessels that directly exchange oxygen and carbon dioxide with the water.  Gill size and structure is often a function of the environment. For example, in streams where the water usually has a high oxygen concentration, gill size tends to be small. However, in habitats where oxygen concentrations are low, such as at the bottom of a pond, gills tend to much larger and bushier.

The other major type of amphibian respiration is cutaneous - through the skin. You might think that this would only serve as a back-up to the use of lungs, but an entire amphibian family – the Plethodontidae – is commonly referred to as the “lungless salamanders.”  Nearly four hundred species are known, most of them from North and South America. Their preferred habitats are mountain woodlands and tropical rainforests. Plethodontid salamanders remain hidden from view much of the time, either in the leaf litter, beneath stones and logs, or high up in the trees, and they seem to get by fine without the use of lungs. 

Amphibians are Ectotherms

Amphibians are ectotherms. This means that they derive body heat from their surroundings. Like reptiles and fish, but unlike mammals and birds, amphibians do not need to consume food to generate metabolic heat. Instead, a large percentage of the food they consume is used directly for growth and reproduction.

Much of an amphibian’s behavior is geared to controlling its body temperature. Most amphibians prefer to live in relatively moist and cool environments.  Hot and arid conditions can easily dry them out.  A frog basking in the full sun while sitting in shallow water is gaining heat energy from above and losing it below.  Young frogs and toads require large amounts of heat energy from the sun in order to grow quickly and reach adulthood.  Some amphibians – blind salamanders that lurk in caves and hellbenders that bury themselves in rocky stream bottoms - prefer environments with more constant temperatures.  The North American wood frog, on the other hand, has evolved adaptations to extreme seasonal fluctuations in temperature.  The wood frog actually freezes along with objects that surround him. At the first touch of ice to its skin, the wood frog’s body chemistry undergoes a drastic change.  It produces ethylene glycol - a natural anti-freeze – to bathe its vital organs.  This allows body fluids to freeze in between cells but prevents damage to the heart, lungs, kidneys and other critical body parts. The wood frog will remain frozen for the duration of the winter, which may last almost half the year in Alaska and other northern parts of its range. Come the spring thaw, these frigid creatures “come alive” again.

Amphibian Skin

Amphibian skin has no hair, scales or feathers, and it tends to be thin, sometimes only a few cell layers thick. This makes their skin very permeable, meaning that air, water and other substances can pass through it quite easily. In fact, the skin of glass frogs is so thin that it is almost transparent – you can actually see a glass frog’s heart and stomach by looking at its belly!

Amphibians absorb water and exchange respiratory gases – oxygen and carbon dioxide - through areas of their skin that contain high concentrations of blood vessels. You might say that amphibians “drink and breathe” through their skin. Some toads have thick skin on their backs, but thin skin in an area of their groins that is called the pelvic patch. They absorb moisture from the surrounding environment through the pelvic patch.

All amphibians possess two types of skin glands, one type that produces poison and another that secretes mucus. Poison glands help protect them from predators can also help keep their skin healthy and clear of harmful bacteria or fungus. In some species, poison glands are concentrated behind the eyes (parotoid glands) or along the sides of the body where they can make easier contact with the sharp bills or teeth of predators. Mucous glands tend to be spread across the body, helping to keep an amphibian’s skin from drying out and preventing its owner from suffocating due to a lack of oxygen. In some species, the mucus also deters predators by coating the animal’s body with a distasteful glue-like substance.

Amphibians are Vertebrates

Amphibians are vertebrates, animals that have a spinal cord enclosed inside a backbone. Vertebrates have a long and amazing history on this earth. They have walked, slithered, hopped and swam on our planet for more than 500 million years, dating back to their appearance in the Cambrian Era. Vertebrates include the classes Mammalia, Aves (birds), Reptilia, Fish and the stars of our show, Amphibia. The first amphibians appeared on earth about 360 million years ago in the Devonian Period.  Many different kinds of amphibian have evolved (and also gone extinct) through the ages.  Today, three basic types exist – frogs and toads, salamanders and newts, and caecilians.

Compared to other modern vertebrates, amphibians have simple skeletons comprised of few bones. Of all the amphibians, salamanders have remained closest in body form to their ancient ancestors and are one of the oldest groups of terrestrial (land-living) vertebrates. Salamanders and newts have a somewhat flexible skeleton with a large amount of cartilage, while the frog or toad skeleton is more rigid and bony, more capable of withstanding the stress of jumping and landing. The frog skeleton also has a broad head, big eye sockets in the skull, a short spine and long hind leg bones. The worm-like caecilians lack legs entirely, their backbones may also possess more than 100 individual vertebrae, and they have a dense, compact and narrow skull that lacks eyes. A caecilian’s unique body shape is ideal for burrowing underground or swimming in ponds or streams.