Frog Blog

Vertebrate Diversity

The Animal Kingdom is broken down into a number of different groups called phyla, one of which is Chordata – animals that have a backbone and a spinal cord that runs through it. The common name for these animals is vertebrates. 

There are five main classes of vertebrate creatures in the world.  Bass, salmon, catfish and trout belong to the Class Osteichthyes, the bony fish.  Snakes, lizards, turtles and crocodiles belong to the Class Reptilia, the reptiles.  Robins, crows, hawks and eagles belong to the Class Aves, the birds.  Sea lions, bats, lions, elephants and human beings belong to the Class Mammalia, the mammals.  And frogs, toads, salamanders, newts, hellbenders, mudpuppies, sirens, amphiuma, axolotls and caecilians belong to the Class Amphibia, the amphibians.

If we consider the number of living species, the largest by far of the five classes is Osteichthyes with  about 20,000 different kinds of fish.  Aves is the next most numerous with over 10,000 species of birds.  Reptiles rank third with more than 8,000 species, and amphibians and mammals bring up the rear with approximately 6,000 and 5,000 species, respectively. 

Every day, scientists discover and describe new species of all kinds, so the numbers above will increase as researchers continue to explore little known parts of the planet.

Photo courtesy of KOCP 

Amphibian Diversity

Biologists tell us that just over six thousand species of amphibians inhabit our planet.  That’s how many scientists have officially described to date.  However, new species are being found every day, so don’t be surprised if the total climbs to seven thousand or more by the time we reach the year 2015.

Some amphibians are totally aquatic – they live their entire lives in the water.  Some are terrestrial – they make their homes mainly on land.  Most species are amphibious – living part of the time in the water and part of the time on land.  They make the best of both worlds.  In doing so they go through a transformation called metamorphosis, changing from immature, gill-breathing creatures to lung-breathing adults. 

Amphibians that inhabit the edges of streams and rivers live in riparian habitats.  Those that live and burrow underground are referred to as fossorial.  When toads hop we call them saltatorial and when frogs climb we say they are scansorial.  Those that dwell high up in the trees are arboreal. 

Amphibian larvae are largely herbivorous – subsisting principally on plant matter.  All adult amphibians, except for one, are carnivorous – they eat living animal prey.  The insectivorous ones eat mainly insects, as you might have guessed from the name.  The one exception to the rule is a frugivorous, or fruit-eating, species. 

Diurnal amphibians are ones that are active during the day, but they are in the minority.  Most species are nocturnal – active after dark. Amphibians that inhabit temperate regions, where seasons change throughout the year, must hibernate during the colder winter months.  However, the greatest number of species can be found in the tropical or equatorial regions, where the climate remains relatively constant.  During periods of high heat and little rain, some amphibians burrow below ground and become dormant.  We say that they aestivate.

As you can see, as a group, amphibians have incredibly diverse lifestyles.

         

Amphibians and Human Medicine

Amphibians have been used as a source of traditional medicines for centuries and are prominent subjects of modern biomedical research. The African clawed frog, for example, was captured and raised in large quantities in the mid-20^th century for use in human pregnancy tests. Female clawed frogs, injected with a woman’s urine, will release their eggs if the woman is pregnant. 

More than 200 beneficial chemicals known as alkaloids have been extracted from frog and toad skin. For the amphibians, these poisonous alkaloids serve as a natural chemical defense, affecting the muscles and nerves of predators.   Medical research, however, has uncovered numerous beneficial uses for these same substances.  One alkaloid produced by amphibian skin is a highly effective painkiller, 200 times stronger than morphine.  Yet it is not addictive like morphine. Skin secretions from the green treefrog have been shown to stimulate activity in the human pancreas and intestine, and commercial drugs are now available based on these compounds. The large parotoid glands of toads, located just behind the eyes, produce two substances that affect the adrenal and cardiovascular systems in humans. A third secretion from these same glands is a powerful hallucinogen.  Further research may very well yield new medicines from these compounds.

Frog skin secretions also can have powerful antibiotic properties. The skin of the African clawed frog, once again, produces protein-like chemicals called  peptides that help heal cuts and bruises, which may provide doctors with a whole new class of antibiotics in the years ahead. Research suggests that skin secretions of some frogs may also help repair human internal organs following surgery.

Who would have guessed that the thin skin of our shy and elusive amphibian neighbors could hold so much promise as medicines for our own species?

Vital Part of the Food Web

Even before amphibians enter the world, they are food for many other species.  Frog, toad and  salamander eggs are greedily gobbled up by aquatic invertebrates, fish, turtles, snakes and even adults of their own species. They also fall victim to bacteria, molds and fungus.  After hatching, the larvae and tadpoles still fall easy prey to some of these same creatures, such that thousands of eggs may result in only a few dozen young actually surviving to adulthood. 

Amphibian larvae dine on algae, insects, bits of dead animal matter and tiny living creatures called zooplankton that they find in their aquatic environment. As adults, amphibians lose their vegetarian tastes and become strict carnivores.  The list of living creatures consumed by amphibians is a long one that includes mollusks, worms, crustaceans, fish, birds and mammals.  However, insects top the list of their favorite prey and many of these are what you and I would call pest species.  For example, one field study conducted in Wisconsin estimated that 100 Blanchard's cricket frogs living around a small pond would consume as many as 480,000 insects in a single season.   So ask yourself, what would happen to those insects if the cricket frogs disappeared?  Would other species pick up the slack of eating all of these bugs if amphibians were not around?

Amphibians are important members of the food web.  Because they are predators and prey in many different ecosystems around the world, declines in their populations can potentially affect the ecology of entire wildlife communities.  Predators such as fish, snakes, birds and mammals may be especially affected by declining amphibian populations. Numbers of aquatic insects and mosquitoes are prone to increase in areas where amphibian numbers drop. This, in turn, could increase the risk of humans contracting mosquito-born illnesses such as malaria and West Nile Virus.

So the next time you notice a toad hopping along the path or a frog basking in a pond, remember that it’s providing a valuable service to the local ecology.

Photo courtesy of Bill Konstant 

Lungless Salamanders

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 blog 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.

Photo courtesy of Tim Herman

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.