What are the extermal Characteristics of frogs? The behaviors, breeding and feeding of frogs. Information on frogs
CHARACTERISTICS OF FROGS
Externally frogs, like birds, all conform to one major body form. Like other vertebrates, the frog has a skull, backbone, and bones for the attachment of forelimbs and hind limbs. The anuran vertebral column, and therefore the body, is short.
Frogs do not vary much in size. No species is known to be larger than 8 to 10 inches (20-25 cm) from snout to vent. The Goliath frog (Gigantorana goliath), some bullfrogs (Rana), and some toads (Bu/o) may reach this size, but most other species are considerably smaller. There are many tiny species of frogs, ranging from V2 to % of an inch (1.2-1.8 cm) from snout to vent. These include many species of tree frogs, some aquatic species, and some burrowing species.
Frogs typically have relatively long, sturdy hind limbs that make them well adapted for hopping. This characteristic, however, varies from species to species. Ptychadena oxyrhyncha, a 2-inch (5 cm) frog of South Africa, with hind limbs nearly three times its body length, has been reported to jump 10 feet (3 meters). Many other frogs, particularly poisonous ones, never jump at all and have relatively short, thin hind limbs. The tiny Central and South American atelopodids and some toads climb or walk on all four of their limbs. Aquatic frogs, such as pipids, usually have strong hind limbs that they use when swimming.
Frog forelimbs are generally short and in jumping frogs serve as shock absorbers. Frogs that live in or frequently enter water have webbed toes and, in some cases, webbed fingers. One tree frog of Malaysia also has webbing both between toes and fingers. When this frog jumps from a tree, its spread-out toes and fingers serve as parachutes, enabling it to descend lightly to the ground or to a tree trunk. Together with salamanders, frogs have the trait, peculiar for vertebrates, of having only four fingers.
Skin and Glands.
The skin of frogs is typically moist and smooth. The moisture is produced in part by’mucous glands that secrete a slimy substance. A moist skin is important for frogs since it functions as a kind of external lung. Much of the frog’s gas exchange, particularly the excretion of carbon dioxide, takes place across the skin instead of in the lungs, and this process cannot occur across a dry barrier. The skin of some frogs, such as toads, appears to be dry but is not completely dry, even though it is not slimy like that of most frogs. In drier-skinned frogs, a larger proportion of gas exchange takes place in the lungs.
Other glands are also found in the skin, notably various types of poison glands. In toads and some tree frogs (Hylidae), for example, there are local swellings just behind the head on each side. These swellings form the parotoid glands, a complex of poison-secreting glands.
When disturbed, some frogs, such as the North American pickerel frog (Rana palustris) and the mink frog (R. septentrionalis), secrete substances that have quite pungent odors. Mam-species are distasteful to dogs, which will show obvious discomfort after taking one in the mouth.
Other Sense Organs.
Frog eyes are characteristically bulging, presumably allowing each one a 180° field of vision. Most species also have an external eardrum, or tympanum, behind each eye. The tympanum is often larger in males and is clearly associated with the use of calls of various sorts. The senses of smell and taste are well developed in aquatic frogs, and touch is an important sense in all frogs. All tadpoles and many adult frogs, especially aquatic pipids, have lateral-line sense organs that function £s pressure sensors, detecting vibrations and movements in the water.
Both sexes produce low croaks, but only males produce loud calls. When calling, they inflate either a single or a pair of vocal sacs located around the throat. These sacs serve as reverberating chambers to amplify the calls, which may carry for long distances. The aquatic pipids, however, can call softly underwater even though they lack vocal sacs.
Many poisonous frogs are brilliantly colored. This is thought to serve as a warning for predators that can distinguish colors. Bold color patterns may also serve as disruptive coloration, tending to obscure the actual shape of the animals. Most species are rather cryptically colored and tend to blend into the background. This tendency, plus their habit of moving only intermittently and spending long periods motionless, makes them inconspicuous.
Although the various species of frogs differ in adaptations and specific aspects of behavior, some patterns do emerge.
Movement and Orientation.
Frogs, like all amphibians, are rather sluggish. They remain immobile for long periods, and this, combined with their cryptic colors, probably functions to conceal them. Studies of semiaquatic riparian (shoreline) frogs have given some insight into how frogs orient themselves. Tadpoles tend to orient parallel to the shoreline. Adult frogs tend to orient toward the shoreline after entering the water, but they orient toward the water when they are sitting on the shore. Given a choice, frogs will jump toward blue rather than any other color. This tendency gets them into bodies of water that reflect the sky.
The orientations seem to involve a kind of sun-compass mechanism. If a frog is removed from a shoreline, blindfolded, and carried away before being released, it will hop in the direction where the water would be if the frog were still sitting on the shore. If moved without a blindfold, the frog will head back toward the pond, having somehow kept track visually of its general direction.
Riparian frogs usually have a small home range centering on a tuft of grass or root that serves as a customary hiding place. If disturbed, they head for this place either directly or after having leaped into the water. Some frogs, such as bullfrogs, are known to defend their home ranges as territories. Individual leopard frogs may occupy a home range for more than one season. Travel is more frequent in young leopard frogs before they begin occupying a home range. Such travel is confined to rainy or misty days and nights, as is foraging activity away from water by settled adults.
Frogs have teeth only in the upper jaw. When feeding, the typical insectivorous frog flips its sticky tongue out of its mouth. The tongue is elastic and can stretch a considerable distance to contact a target. As the tongue is pulled back into the mouth, it takes the entrapped prey with it. If any small moving object enters their field of vision, these frogs turn toward it and appear to aim while tensing their muscles. Their accuracy leaves something to be desired, but since their environment is usually crowded with prey, this is not really a problem. The tongueless pipids feed by scooping prey into their mouths with their forelimbs, the fingers of which have taste buds.
Reaction to Temperature.
Frogs are poikilo-therms, that is, so-called cold-blooded animals whose temperature fluctuates with that of the environment and is not regulated internally as it is in birds and mammals. As environmental temperature decreases, frogs become increasingly sluggish. In temperate regions in the winter, they become dormant and hibernate. Conversely, as temperature increases, they become increasingly active and effective in their movements.
Each species has a range of temperatures to which it can adjust. For example, tropical species can adjust to higher temperatures than can species from boreal regions. As a rule, amphibians, even tropical ones, cannot stand really high temperatures, and characteristically live in cooler places—in forests, near waterfalls, and in high mountains.
As a rule, frogs must return to open water for breeding, although there are some interesting exceptions that will be discussed later. The return to water is a seasonal phenomenon, triggered by complex stimuli, including the beginning of rains in the arid tropics and the combination of rain and warmer temperatures in temperate climates. In temperate climates a succession of species typically comes to a given body of water to breed. This implies that the stimuli involved in initiating breeding migrations differ somewhat in the various species. This situation may have evolved as a mechanism for avoiding competition between the tadpoles of different species.
The orientation of frogs toward the breeding waters may involve a number of factors. Some frogs estivate at pond sites, and no migration is involved. Orientation also may be due to a general movement downhill toward water. In many species, individuals return to the same pond year after year. The senses of smell and taste in the skin, and possibly the sun-compass mechanism, also operate to lead the frog to a specific body of water.
In tropical species with no special breeding season, individuals become sexually active periodically throughout the year, so that there is always some breeding activity going on. In these cases, lone males take up breeding stations at which they call, eventually attracting a ripe female or two.
In many species the breeding calls of males at the breeding site are very important. These calls sometimes involve a chorus of hundreds of individuals, producing a deafening din that carries over long distances. The calling normally takes place at night or on cloudy days.
Breeding calls serve three functions: to call the population to the site from the surrounding countryside; to establish a small territory for each male; and to attract the females. Mating calls range from shrill shrieks to rolling snores, depending partly on body size. In all cases where more than a single species breeds at one time in a given body of water, the calls are clearly different from one another. In some species the females do not respond to the calls of males of other species. In one case this is partly because the female eardrum is most sensitive to the frequency of the calls of conspecific males —that is, she hears only the calls of males of her own species. Female discrimination, however, has not been demonstrated in every species tested. In a few cases the mating call has been lost, and the male actively searches for the female.
In many species breeding males will clasp any individual that comes near. They can distinguish other males by their characteristic grunts or by their thin waists, whereas females carrying eggs are generally plumper than usual.
Males have thickened thumbs with eornified thumb pads, often with rugosities or even spikes, which they use to secure a firm hold on the females. The females then swim about with the usually smaller males on their backs. Other males will sometimes attempt to dislodge a successful one, but a mated pair is not easily separated. As the female discharges eggs, the eggs bounce against the male’s cloaca and apparently stimulate the ejaculation of milt.
Egg-Laying in Water.
Many species simply deposit eggs at random in the water where they happen to be swimming. Other frogs seek out special places and attach their eggs to tufts of grass or to twigs. Still others deposit eggs under aquatic objects, such as stones. In some tropical species the males construct little circular walled pools on mud flats or sandbanks. They then station themselves on the rim and call the females to deposit eggs in the pool. A few other species deposit their eggs in tiny pools in the water reservoirs of air plants.
An interesting adaptation in some species of several families is the construction of a foam nest. In these species, the outer jelly coats on the eggs are very soft and fluid. During mating, the male and female slowly tread their legs and whip this substance into a foam nest within which the eggs are contained. The nest may be put under an object on land, placed to float on the pond, or attached to tree branches .overhanging water. In the latter case, the outer layer of foam may harden, forming a protective crust, and the tadpoles, when ready to hatch, digest their way through the crust and fall into the water below. The terrestrial nesting places of certain species of frogs are guarded by one of the parents.
Various species of frogs have evolved ways of carrying their eggs until hatching. In one species the male carries the eggs in his vocal pouch. In the European midwife toad, the male carries a string of eggs wrapped around his hind limbs, and the eggs hatch into advanced tadpoles as the male swims through shallow water. In many other species, either the male or the female carries the eggs on the back. In aquatic pipids, the eggs stick to the female’s back and gradually sink into the skin, which becomes soft and full of blood vessels, forming, in effect, a kind of placenta.