Glass Frogs: Centrolenidae

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.

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.

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.