GLASS FROGS: Centrolenidae

PHYSICAL CHARACTERISTICS

In many of the species of glass frogs, the beating heart, other working organs, blood vessels, and the bones inside are clearly visible through their see-through, or transparent, undersides. Even from the top in many species, the frogs' bodies have the look of frosted glass and sometimes provide a glimpse of the animals' inner workings. In fact, the descriptions of some species include the size of their organs. The view from the topside is not as good as it is from the bottom, because the frog has thick muscles on its back that hide the organs from sight.

Most of the glass frogs are shades of green, although some are brown. Many have tiny spots, which are called ocelli (oh-CELL-ee). These ocelli in frogs should not be confused with the ocelli in insects. In insects, ocelli look like the spots on a frog's back, but they are actually tiny eyes. Lynch's Cochran frog, for instance, may be brownish green or tan and has black ocelli that are tipped in orange or yellow. It also has white warts. The Nicaraguan glass frog is green, sometimes with numerous black spots on its head, back, and legs. In the Atrato glass frog, the back is yellow green with small brown spots in all but a few large, round patches on its head, back, and legs. Regardless of their color or pattern, most glass frogs are extremely hard to see when they sit on green leaves. They almost look as if they melt into the leaves and become a part of them.

Since the bones are visible from the outside in most glass frogs, the color of the bones also helps to tell some species apart. The bones in the Pichincha glass frog, Pacific giant glass frog, Ecuador Cochran frog, and many others, are green. In the Atrato glass frog, Fleischmann's glass frog, and La Palma glass frog, among others, the bones are white.

The typical glass frog has a delicate body that looks as if it would easily break if handled. These slender, and often very smooth, bodies have thin front and rear legs. Some species, like the grainy Cochran frog, have hundreds of tiny bumps on their heads and backs. All of their toe bones are T-shaped at the ends. On the outside, the toes are tipped with wide, rounded pads. Thin, transparent webs stretch between their toes.

The head in the average glass frog has large bulging eyes that face mostly forward rather than to the sides and are located more toward the top of the head than the eyes in most other frogs. In many frog species, the head blends into the body and does not appear to have a neck between the head and body. The typical glass frog's head, on the other hand, is obvious, even looking round when viewed from above.

Most members of this family are small, reaching 0.7 to 1.2 inches (1.8 to 3 centimeters) from the tip of the snout to the end of the rump. Males in many species are smaller than females. In the Pichincha glass frog, for example, females grow to 1.3 inches (3.23 centimeters) long, while males reach 1.1 to 1.2 inches (2.68 to 3.15 centimeters) in length. The Nicaragua glass frog is similar. The female in this species reaches 1.0 to 1.1 inches (2.54 to 2.68 centimeters) long, while the male grows to 0.9 to 1.1 inches (2.17 to 2.68 centimeters) in length. In other species, like the Ecuador Cochran frog, the males are the larger of the two. Female Ecuador Cochran frogs reach 0.8 to 1.0 inches (2 to 2.54 centimeters) in length, while males can grow to 1.9 inches (4.83 centimeters) long. The Pacific giant glass frog is truly a giant among glass frogs. Although not large when compared to frogs in some other families, the male's 3.2-inch (8.13-centimeter) body makes it the biggest of the glass frogs. Female Pacific giant glass frogs are not quite as large, but they are still giants compared to most other glass frogs. Females can reach 2.4 to 2.9 inches (6.09 to 7.36 centimeters) in length.

GEOGRAPHIC RANGE

Glass frogs can be found in North, Central, and South America. In North America, they only live in southern Mexico. Many species are found throughout Central America and in many parts of South America as far south as southern Brazil and northern Argentina.

HABITAT

Glass frogs are mainly land species that live in humid mountain forests. Lower on the mountains where the weather is warm, these forests are called rainforests. In colder areas higher up mountainsides, they are called cloud forests. Both areas get a good deal of rain and are very humid. Most of the glass frogs live among trees and plants that line streams. Their tadpoles live and grow in slow-moving waters of the streams. The Pichincha glass frog, for instance, lives in cloud forests high on mountains that are 6,430 to 7,870 feet (1,960 to 2,400 meters) above sea level. The Nicaraguan glass frog chooses humid forests that are not so far up. It lives about 328 to 4,921 feet (100 to 1,500 meters) above sea level. Fleischmann's glass frog goes even lower on mountainsides, down to 200 feet (60 meters) above sea level, but also may live as high as 4,790 feet (1,460 meters) above sea level. Each of these three species makes its home in plants and trees around streams.

Some people think that a few species of glass frogs, especially those that survive in Mexico, may be able to make their homes in places away from streams by living in wet plants, like bromeliads (broh-MEE-lee-ads) that grow on the sides of trees. Bromeliads often have overlapping leaves that form cups and can hold rainwater. If the water is deep enough and does not drain out or dry up, the tadpoles might be able to survive there and develop into froglets. So far, however, the tadpoles of only one species, known by its scientific name of Centrolene buckleyi, has been found living in a bromeliad.

DIET

Although little research has been done on their food habits, scientists think these small frogs probably mainly eat tiny insects. The large Pacific giant glass frog, however, can and does consume larger prey, including fishes and other frogs.

BEHAVIOR AND REPRODUCTION

Glass frogs are usually active at night. This, combined with their transparent bodies, makes them very difficult to spot for people or for predators. A flashlight shown on a glass frog at night reveals little of the frog except its large eyes and a dark smudge where the skull is. During the daylight, the frogs hide among the leaves. Since the rainforest and cloud forests are so full of plants and trees, the tiny green frogs can easily stay out of sight if they sit on a leaf and do not move. The frogs also become even more invisible because they squat their bodies down flat on the leaves. Even from the side, they look much like a slight lump on the leaf rather than a living frog. Only the most careful observers see the frog during the day or at night.

Because they are so well-hidden, most of the information about the glass frogs comes from studies done when the frogs are most noticeable. This happens when they breed. Some species that live in areas where the weather is about the same all year will mate on any night. Others that live in places with changing weather usually mate only at certain times. Like the Nicaraguan glass frogs, which live in Nicaragua, Costa Rica, Panama, Colombia, and Ecuador, they may mate only on nights following heavy rains.

The males of many glass frogs are fussy about the places where they want to mate and have their young. Once they find a good spot, they will often fight other males who try to take it from them. These "good spots" are known as territories. Male Nicaraguan glass frogs set up and defend the territories they will use as calling sites. Like the males of most other species of frogs, male glass frogs call to attract females for mating. In this species, two males may fight over a leaf by grasping onto a side of the leaf or a stem with their back feet, hanging upside down, and wrestling one another. The winner is the one that can knock the other off, or that can manage to scramble onto the leaf's surface and flatten down his body on it. In the Ecuador Cochran frog, males battle, again while hanging upside down but in a belly-to-belly position and with their front legs wrapped around one another's neck. They then pump their hind legs, which causes the wrestling pair to swing up and down and back and forth.

The males of many species have one sharp bony spine on the upper part of each front leg. The bone in this part of the leg is called the humerus (HYOO-mer-us). In a human, the humerus is the long bone in the arm that runs from the shoulder to the elbow. Because the spine is on the humerus, it is called a humeral (HYOO-mer-ul) spine. Males of these glass frog species often have scars on their faces, the backs of the head, and sides of the body, which suggests that the males use their humeral spines when fighting one another. For example, the male Pacific giant glass frogs have powerful and thick front legs, unlike most other glass frogs, and long pointed humeral spines. Although no one has every seen the males of this species fighting, many males have numerous scars that match the marks that would be made if other males had sliced them with their spines. The males of many other species of glass frogs have humeral spines, too. In fact, all species that fall into one group, called the genus Centrolene, have humeral spines. This includes the Pacific giant glass frog, the Nicaraguan glass frog, and the Pichincha glass frog, among others. In species like the Pacific giant glass frog and the Nicaraguan glass frog, the humeral spines are sharp, but in other species, like the Pichincha glass frog, the spines have dull tips.

Males of different species have different calls, but most are some type of a whistling sound. Male Fleischmann's glass frogs call with a short trill that they repeat again and again. Male La Palma glass frogs also have a short call, but it does not trill like that of the Fleischmann's glass frog. The male Nicaraguan glass frog's call is made up of three short beeps. They may make this call as often as forty-three times an hour or as little as just once in an hour. Most males call at night and from leaves in plants or trees above streams. Some males, like the Ecuador Cochran frog, prefer spots over streams that are rushing downhill. Male Pacific giant glass frogs make their loud, trilled calls from behind waterfalls or on boulders in fast waters.

When a male glass frog attracts a female with his call and she approaches him, he climbs onto her back. This piggyback position is called amplexus (am-PLEK-sus). The male wraps his front legs around her and hangs on just behind her front legs. He remains there until she lays her eggs. As she does, he releases a fluid containing microscopic cells, called sperm, that trigger the eggs to start growing.

NEW GLASS FROGS

People are still discovering new species of frogs, including glass frogs. In 2004, for instance, researchers from the University of Kansas announced a new species from northwestern Ecuador. In 2003, scientists from the University of Texas described a new species from western Guyana. Both of these species are green with tiny yellow dots.

Usually, glass frogs mate at or near the place where the male was calling. This is the case with the Ecuador Cochran frog. She lays her eggs on the tip of the same or a nearby leaf where the male was calling. In a few species, like the Nicaraguan glass frog, the male may lead the female away from his calling site and to another place where they actually mate. In this species, the female lays her eggs on the top of a leaf near the ground on in a plant up to 10 feet (3 meters) above the ground. Sometimes, she will instead lay her eggs on mossy rocks or branches. While they are mating and even for a short time after she lays her eggs, the male continues to call. The vast majority of species in this family lay their sticky eggs either on top of or on the bottom of leaves. The Ecuador Cochran frog is one species that lays eggs on the tops of leaves. Some of the species that lay their eggs on the bottom surfaces of leaves include the Atrato glass frog and the Fleischmann's glass frog. The only member of this family that does not follow this pattern of laying eggs on leaves is the Pacific giant glass frog. This species mates in the male's calling site, which is on a wet, splashed rock behind a waterfall or sticking up next to rapids.

The typical number of eggs laid by a female glass frog is about two or three dozen. Female Fleischmann's glass frogs, for instance, lay about eighteen to thirty eggs, the Nicaraguan glass frog lays about twenty, and the Atrato glass frog lays twenty to twenty-five eggs in a clutch. Eggs come in different colors, depending on the species. Some, like the Nicaraguan glass frogs, have black eggs, while others, like the Atrato glass frog, lay transparent green eggs.

AN OUT-OF-THE-WAY SPOT

The glass frog, known only by its scientific name Cochranella saxiscandens, makes its home in what was once an out-of-the-way spot: the stream at the bottom of a steep gorge in the mountains of northern Peru. People, however, have discovered the area and have begun cutting down the nearby forests for farms and for wood.

It is common to see an adult staying with the eggs for at least a short time. The female Nicaraguan frog stays with her clutch for at least the first night. In the Atrato glass frog, one of the adults either sits next to or on top of the eggs. In the Fleischmann's glass frog, it is usually the male that stays with the clutch. He sits nearby during the day, but covers them with his body at night. Despite his care, fruit flies often manage to land on the frog eggs and lay their eggs on them. The fly eggs hatch into maggots that eat the frog eggs, sometimes destroying almost all of them. In La Palma glass frogs, the males are the caregivers. A male will stay with his young day in and day out. Interestingly, the pattern on the adult frog's back looks very much like the pile of eggs and may confuse predators enough to cause them to leave alone both the male and his eggs.

Glass frog eggs hatch into tadpoles, which usually slide off the leaves and drop into the water below. Sometimes, a tadpole may slide off in the wrong direction and wind up on the shore instead of in the water. Fortunately, most tadpoles have strong tails that are powerful enough to flip them into the stream. The typical tadpole is long and thin with eyes on the top of its head. In Fleischmann's glass frogs, the tadpoles are bright red in color. The red is not, however, the color of the skin. It is the color of the tadpole's blood, which shows through the skin. Once a glass frog's eggs hatch into tadpoles, the adult leaves the clutch, and the tadpoles continue their development, eventually turning into froglets, on their own.

GLASS FROGS AND PEOPLE

People rarely see glass frogs in the wild or in the pet trade. Conservationists are especially interested in these little frogs because they may be good bioindicators (bie-oh-IN-dih-KAY-torz). A bioindicator is a living thing that provides clues about the health of the place where it lives. Glass frogs live in rainforests and cloud forests that are affected by global warming. As the Earth's weather changes, some of these forests are becoming too dry and making life difficult for the frogs, as well as other plants and animals. By watching the frogs, scientists can learn how much of a problem global warming might cause.

CONSERVATION STATUS

Of the 134 species in this family, the World Conservation Union (IUCN) considers sixty to be at risk, and another forty-nine to be Data Deficient, which means too little information is available to make a judgment about the threat of extinction. Of the sixty at-risk species, six are Critically Endangered and face an extremely high risk of extinction in the wild. These include Centrolene ballux, Centrolene gemmatum, Centrolene heloderma, Centrolene puyoense, Cochranella anomala, and Hyalinobatrachium crybetes. Many of the glass frogs are little known and have no common names in the English language. Two of these, Centrolene ballux and Centrolene heloderma, have lost eighty percent of their total number in a very short time. Centrolene ballux, which lives in Colombia and Ecuador, has become very rare in both countries and has not been seen at all in Ecuador since 1989. Centrolene heloderma also lives in Ecuador and Colombia, but has not been seen in Ecuador since 1979. The disappearance of both frogs may be tied to global warming. As the temperatures have changed, the sky is no longer as cloudy as it once was in the frog's habitat. Without the clouds, the weather may be becoming too dry for the frogs. In addition, people are cutting down the frog's forests to build homes, create farms, or to take the logs, and fires are also destroying the forest.

The other four Critically Endangered glass frogs live in very small areas. One makes its home in Honduras, and the other three in Ecuador. In each case, the frog's forest has been destroyed for purposes as farming or logging. The forests where the only known population of Centrolene puyoense lived, for example, was cut down and the land cleared out in 1996.

Besides the Critically Endangered species, sixteen are Endangered and face a very high risk of extinction in the wild, twenty-nine are Vulnerable and face a high risk of extinction in the wild, and nine are Near Threatened and at risk of becoming threatened with extinction in the future. Habitat loss and possibly infection with a fungus, called chytrid (KIT-rid) fungus, are likely causing many of the problems for these frogs. Others appear to be quite rare, but scientists are unsure about how many individuals actually live in the wild. The glass frogs are difficult to spot at night when they are active and during the day when they sit still on leaves. In addition, many of the species spend almost all of their time high up in trees and other hard-to-reach spots. An example is the species known by its scientific name of Cochranella luminosa. This glass frog is found on the western side of the Andes Mountains in Colombia, where it lives at the tops of trees. Recently, however, scientists have begun studying the tops of trees, called a forest's canopy (CAN-oh-pee), using ropes and tall platforms. With these new methods, they will likely learn much more about tree-living frogs, as well as other species of plants and animals.

LYNCH'S COCHRAN FROG (Cochranella ignota): SPECIES ACCOUNTS

Physical characteristics: Unlike most glass frogs, Lynch's Cochran frog is not green. This small frog is usually tan, although it is sometimes greenish brown, and has small, orange- or yellow-centered black spots on its back, head, and legs. Its body and all four legs are thin. The back legs are quite long and have webbed toes. The toes on both the front and back legs end in large, round pads. Its skin is smooth, except for numerous low, white warts. Its head has very large eyes pointed toward the front and a wide, rounded snout. Its bones are light green. Females are usually about an inch (2.42 to 2.44 centimeters) long from snout to rump. Males grow to 0.9 to 1.0 inches (2.23 to 2.54 centimeters) in length.

Geographic range: Lynch's Cochran frog lives in the western Andes Mountains of Colombia.

Habitat: This frog makes its home around streams in mountain cloud forests from 6,230 to 6,430 feet (1,900 to 1,960 meters) above sea level.

Diet: Its diet is unknown.

Behavior and reproduction: Scientists know little about its behavior, but if it is like many other glass frogs, it probably hides in plants during the day and becomes active at night. To mate, the males attract females with their call, which is a repeated chirping sound. The males call from plants above streams.

Lynch's Cochran frogs and people: Few people have ever seen this frog.

Conservation status: The World Conservation Union (IUCN) lists this frog as Near Threatened, which means that it is at risk of becoming threatened with extinction in the future. This is a common species, but it lives in a small area. Fortunately, most of the area falls within national parks, where the land is protected. Conservationists are still concerned that global warming may affect the future of this frog. A warmer climate may cause weather that is too dry for this species. ∎

LA PALMA GLASS FROG (Hyalinobatrachium valerioi): SPECIES ACCOUNTS

Physical characteristics: Also known as the reticulated glass frog, the La Palma glass frog's head and smooth back are yellow or slightly orange yellow with a net pattern that is green with dark spots. The net pattern forms a series of circles. Looked at another way, the back appears green with large yellow to slightly orange yellow spots. The legs have the same colors and pattern, and the hind legs are especially long and thin. The toes, which are almost completely transparent, have some webbing. Unlike the smooth back, the belly and the thighs are slightly wrinkly. Its bones are white. Males grow to 0.8 to 1.0 inches (2.03 to 2.54 centimeters) from snout to rump. Females are about the same size.

Geographic range: The La Palma glass frog lives in Costa Rica and Panama in Central America and in Ecuador and Colombia in South America.

Habitat: According to the IUCN, it lives in lowland forests below 1,312 feet (400 meters) above sea level, especially in plants and trees that line streams.

Diet: Its diet is unknown.

Behavior and reproduction: Scientists know little about its behavior outside of its breeding, but if the La Palma glass frog is like many other glass frogs, it probably hides in plants during the day and becomes active at night. To mate, the male calls in females with a short "seet" whistle that it repeats again and again. Males and females mate, and the females lay their eggs on leaves above streams. The eggs, which may number about three dozen, are pale green and surrounded with gel. After she lays her eggs, the female leaves, but the male stays behind to provide 24-hour-a-day protection to the eggs. Of all the glass frogs, most of which care for their eggs, the male La Palma glass frog spends the most time with his young. The color and pattern on the male's back looks very much like the clump of eggs he guards. This may confuse predators and cause them to leave both the adult male and the eggs alone.

La Palma glass frogs and people: Few people have ever seen this frog.

Conservation status: The IUCN lists this species as being of Least Concern, which means there is no known threat of extinction and the animal does not qualify for any of the "threatened" categories. The La Palma glass frog lives over a large area and seems to be doing quite well, but conservationists are still watching it carefully. Parts of its forests are disappearing to farming, logging, and land for building, and this may eventually cause problems for the frog. Some of its forest home lies within protected areas, which are off limits to tree-cutting. ∎

PACIFIC GIANT GLASS FROG (Centrolene geckoideum): SPECIES ACCOUNTS

Physical characteristics: The Pacific giant glass frog is the largest species in this family. The typical glass frog is about an inch (2.54 centimeters) long from snout to rump, but the Pacific giant glass frog is about three times as large. Females can reach 2.4 to 2.9 inches (6.09 to 7.36 centimeters) in length, while males can grow to 2.8 to 3.2 inches (7.1 to 8.1 centimeters) long. The Pacific giant glass frog looks different from other glass frogs in other ways, too. Unlike other glass frogs, its eyes are small compared to the size of its head; its legs are rather short and thick; its toes are well-webbed; and its toes have large toe pads that are rectangular shaped instead of round. Pacific giant glass frogs are dark green to lime green in color, and their skin is covered with small bumps, or tubercles (TOO-ber-kulz), and a few small white specks. Their bones are green. Besides being bigger overall, males have stronger front legs than females; they have bony spines, called humeral spines, that stick out of the upper part of each front leg, while females have no spines; and their skin tubercles have tiny spikes that the females lack. Pacific giant glass frog tadpoles are long and thin with two eyes on the top of the head.

Geographic range: Pacific giant glass frogs live in Ecuador and Colombia.

Habitat: They live high in mountain cloud forests from 5,740 to 9,840 feet (1,750 to 3,000 meters) above sea level. They prefer forests that shade waterfalls or rapids.

Diet: This large species not only eats various insects, but it may also consume fishes or other frogs.

Behavior and reproduction: Like other glass frogs, the Pacific giant glass frog is active at night. But unlike the others, it may spend its days not only among leaves as other members of this family do, but on rocks. It also uses rocks rather than plants when it mates. The male Pacific giant glass frog hops onto wet rocks that are splashed by water from waterfalls or rapids. From there, it makes its call to attract females. The call is a high trill that is loud enough to be heard over the crashing water. It repeats its call every 1.5 to 5 seconds. Many of the male Pacific giant glass frogs have scars on their faces, heads, and sides. Scientists think the scars are the result of injuries suffered when males use their sharp arm spines to fight one another over the places where they call or where they mate. This is just a guess, however, because no one has seen the frogs fighting in this way.

Pacific giant glass frogs and people: Few people have ever seen this frog.

Conservation status: The World Conservation Union (IUCN) lists this species as Vulnerable, which means that it is facing a high risk of extinction in the wild. It already lives in a fairly small area of forests, much of which has already had parts cut down and cleared for farming. More habitat loss will likely occur. Besides the threat from habitat destruction, the frog is also in danger from fishes that have been introduced to the streams where its tadpoles live. The fishes eat tadpoles. In addition, some people have planted illegal crops in land near the forests where the frogs live and spray the crops with chemicals that are dangerous to the frogs. Rain washes the chemicals into the streams, and this can harm the tadpoles. These threats led the IUCN to predict in 2004 that the number of Pacific giant glass frogs would drop by another thirty percent by the year 2014. ∎

FOR MORE INFORMATION

Books:

Beletsky, Les. Costa Rica: The Ecotraveller's Wildlife Guide. San Diego, CA: Academic Press, 1998.

Cogger, Harold G., and Richard G. Zweifel. Encyclopedia of Reptiles and Amphibians. San Diego, CA: Academic Press, 1998.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians (Smithsonian Handbooks). New York: Facts on File, 1991.

Lovett, Sarah. Extremely Weird Frogs. Santa Fe, NM: John Muir Publications, 1996.

Mattison, Chris. Frogs and Toads of the World. New York: Facts on File Publications, 1987.

Meyer, John R., and Carol F. Foster. A Guide to the Frogs and Toads of Belize. Malabar, FL: Krieger Publishing Co., 1996.

Showler, Dave. Frogs and Toads: A Golden Guide. New York: St. Martin's Press, 2004.

Web sites:

Cannatella, David. "Centrolenidae." Texas Memorial Museum, University of Texas.http://www.zo.utexas.edu/research/salientia/centrolenidae/centrolenidae.html (accessed on April 19, 2005).

"Emerald Glass Frog." WildHerps.http://www.wildherps.com/species/C.prosoblepon.html (accessed on April 19, 2005).

"Family Centrolenidae." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/classification/Centrolenidae.html (accessed on April 19, 2005).

Heying, H. "Centrolenidae." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/information/Centrolenidae.html (accessed on April 19, 2005).

Kubicki, Brian. "Chiriqui Glass Frog." Herps of Panama. http://home.earthlink.net/%7Eitec1/Anura/Centronella/Hyalinobatrachium_pulvertum.html (accessed on April 19, 2005).

Kubicki, Brian. "La Palma Glass Frog." Herps of Panama.http://home.earthlink.net/%7Eitec1/Anura/Centronella/Hyalinobatrachium_valerioi.html (accessed on April 19, 2005).

Kubicki, Brian. "White-spotted Cochran Frog." Herps of Panama.http://home.earthlink.net/%7Eitec1/Anura/Centronella/Cochranella_albomaculata.html (accessed on April 19, 2005).