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Wild About Illinois Frogs and Toads!

WFTCover.jpgFrogs and toads are the most conspicuous members of a very secretive group of animals called amphibians. Frogs and toads are well known for their mating call and long, insect-catching tongue. They are excellent gauges of environmental health because of their close contact with aquatic, wetland, and terrestrial environments. They have moist, sensitive skin that allows chemicals in the environment to pass into their bodies. Furthermore, their eggs lack a hard shell so their developing young also are directly exposed to the environment. The puzzling disappearance of some frog and toad species in remote, pristine parts of the world has scientists concerned about overall planetary health.
 
 

 Gallery by Family and Species

 

Family Bufonidae – The true toads are found nearly worldwide. They have dry, bumpy skin. Large parotoid glands are present behind the eyes. Most are nocturnal. They produce a poisonous skin secretion.

     American toad (Anaxyrus americanus)
     Fowler’s toad (Anaxyrus fowleri)


Family Hylidae – Treefrogs, chorus frogs and cricket frogs are small anurans. Most of them have adhesive pads on the tips of their toes to aid in climbing. Females are usually larger than males.

     eastern cricket frog (Acris crepitans)
     bird-voiced treefrog (Hyla avivoca)*
     Cope’s gray treefrog (Hyla chrysoscelis)
     green treefrog (Hyla cinerea)
     gray treefrog (Hyla versicolor)
     spring peeper (Pseudacris crucifer)
     upland chorus frog (Pseudacris feriarum)
     boreal chorus frog (Pseudacris maculata)
     Strecker’s chorus frog (Pseudacris streckeri)*
     western chorus frog (Pseudacris triseriata)


Family Microhylidae – The narrowmouth toads are tiny, burrowing anurans. They have a fold of skin behind the pointed head. Their primary food is ants.

     eastern narrowmouth toad (Gastrophryne carolinensis)*


Family Ranidae – The true frogs have long legs for jumping. Their toes are webbed to aid in swimming. Most of them have a ridge (dorsolateral fold) along each side of their back.

     crawfish frog (Lithobates areolatus)
     plains leopard frog (Lithobates blairi)
     American bullfrog (Lithobates catesbeianus)
     green frog (Lithobates clamitans)
     pickerel frog (Lithobates palustris)
     northern leopard frog (Lithobates pipiens)
     southern leopard frog (Lithobates sphenocephalus)
     wood frog (Lithobates sylvaticus)


Family Scaphiopodidae – Spadefoots have a hard structure on each hind foot which is used to aid them when digging. They spend much time under ground. The pupil of the eye is verticle.

     eastern spadefoot (Scaphiopus holbrookii)

 

* = Illinois threatened species
** = Illinois endangered species 
 

 Anatomy

 
WFTAnatomyBW.JPG
 Illustration from Smith, 1961
 
Frogs and toads are the only members of the Phylum Chordata, Class Amphibia, and Order Anura. Unlike tadpoles, adults do not have a tail. Adult frogs and toads have a squat posture and strong rear legs, which give them the ability to jump long distances. Their long, retractable tongue is used to catch their main food item, insects.
 
The tympanic membrane, or tympanum, aids the process of hearing. This circular structure, located on each side of the head behind the eyes, picks up vibrations in air or water and passes them to the inner ear.
 
Frogs and toads breathe through their lungs like most terrestrial vertebrates, but they also utilize their soft, moist skin as a secondary means of taking in oxygen. This type of breathing requires that the skin be both moist and permeable. Their skin remains permeable because they do not have a protective covering like scales, hair, or feathers. They maintain moisture through mucous gland secretions and by staying near water. Toads have a somewhat more watertight skin than frogs and can therefore venture farther from water.
 
Toads are also characterized by the horny bumps, sometimes called warts, on their skin. The most pronounced of these bumps, called parotoid glands, are found just behind the toad’s eyes and produce a poison that helps in defense against predators. The poison repels animals as large as raccoons because of the mild mouth discomfort they experience when trying to eat a toad. Often they drop the toad, allowing its escape.
 

 Life History

 
WFTLifeHistoryBW.JPG
Illustrations from Mark Sabaj, Illinois State History Survey
 
All Illinois frogs and toads have a complex, biphasic life cycle. Adults are aquatic, semi-aquatic, or terrestrial, but all species lay their eggs in water. During the breeding season, which may start as early as February in Illinois, males congregate at wetlands and begin chorusing. The males’ calls attract females to the wetlands, and soon males and females are paired in amplexus (1). Amplexus ends with fertilization: the female expels her eggs into the water, and the male immediately covers them with sperm.
 
A gelatinous coating usually covers anuran eggs (2), which may be laid singly, in sheets, strings, or globs, depending on the species. Some species’ eggs are attached to sticks or leaves; others simply float freely in the water. Fertilized eggs develop and hatch into free-swimming larvae, or tadpoles (3 & 4), within a few days. Tadpoles have a horny, beaklike mouth designed for scraping algae, and they begin feeding almost immediately after hatching. A few weeks later, hind limbs appear (5), and within a month or two the tadpoles have undergone metamorphosis. Except for a temporary tail stub, the newly transformed froglets look much like the adults. They usually leave the water and take up existence on land until cold weather prompts them to seek out a safe place, such as the muddy bottom of a pond, for hibernation.
 
Although the larvae are herbivorous, all adult frogs and toads in Illinois are carnivorous. The most common food items are flying insects, such as flies and mosquitoes. Although adults are primarily nocturnal, they may be seen or heard during the day, especially if the weather is overcast or rainy.

 

 Frog Calls

 

WFTCallsBW.JPGAnimals produce sounds for a variety of reasons, but the primary function of vocalization is announcing the presence of one individual to others of the same species. Frogs and toads have well-developed voices that are used to attract mates, proclaim feeding territories, and signal alarm. Of these functions, the mating calls of the males are probably the most familiar to us. Just as in birds, each species of frog or toad has a unique mating song, and biologists can identify an individual species by its call alone. Anurans produce sounds in a manner similar to that of most other vertebrates, by passing air over the vocal cords. Frogs and toads take the process a step further by using the vocal sac—loose skin usually located under the lower jaw—to resonate or amplify the sound. Both males and females possess functional vocal cords, but they are better developed in males. Only males have vocal sacs, which accounts for the difference in sound production between the sexes.

 
Frog calls can be “described” in scientific terms by playing a recording of a call through an electronic sound analyzer. The resulting graph, called a sonogram, is a visual representation of the call. Scientists use sonograms to identify species whose calls cannot be distinguished with the human ear. For this reason, sonograms are used to distinguish between the two species of gray treefrog that occur in Illinois. The eastern gray treefrog, Hyla versicolor, and Cope’s gray treefrog, Hyla chrysoscelis, are identical in outward appearance and were considered to be of the same species until biologists detected differences in their mating calls. Sonograms of the two mating calls show that the call of Cope’s gray treefrog has a higher pitch and more pulses per second than that of the eastern gray treefrog. However, the differences that appear obvious in the sound analysis are actually very subtle in nature and can be picked up only by a trained ear.
 
 
 
 
 

 

 Conservation

 

WFTConservationBW.pngCricket frog (Acris crepitans), from Slime, Scales and Mudpuppy Tails, 1997, Illinois Department of Natural Resources, Office of Public Services, Division of Education, Springfield

Over the last few years there has been increased concern about worldwide population declines and even extinctions of frog and toad species. Habitat destruction and degradation are clearly responsible in some cases, but the most alarming declines have occurred in relatively pristine environments such as the rainforests of eastern Australia and the cloud forests of Costa Rica. Closer to home, biologists in the upper Midwest have been noticing the disappearance of our smallest frog, the cricket frog (Acris crepitans). Until the early 1980s, cricket frogs were among the most common anurans in Illinois, and they could be found in almost every county.  Recently, biologists have noticed a decline in the number of cricket frog populations in the Chicago region but not downstate. Among the causes that have been suggested for this decline is build-up of toxic substances (pesticides, heavy metals) in the frogs’ breeding ponds, leading to endocrine disruption. A survey of museum specimens collected in Illinois over the past 100 years showed that more frogs from the Chicago region displayed anatomical signs of endocrine disruption compared to the rest of the state. In the heavily industrial and urban landscape of the Chicago region, habitat destruction and fragmentation are also likely involved.
 

 Glossary

 

Amplexus - the mating clasp of frogs and toads; the male holds the female from behind with his forelegs firmly around her chest
Anuran - member of the vertebrate order Anura, characterized by lack of a tail, moist skin, and long hind legs adapted for leaping: a frog or toad
Biphasic - two distinct phases; usually refers to a life cycle that has a land and a water portion
Carnivorous - having a diet that includes mainly animals
Dorsolateral Fold - a line of raised skin along each side of the back (see Anatomy illustration)
Endocrine - ductless glands and their regulatory products
Heavy Metals - potentially poisonous metals such as lead, mercury, and cadmium
Herbivorous - having a diet that includes mainly plants
Larva - (pl. larvae) the newly hatched, immature form of an animal that undergoes metamorphosis, differing markedly in form and appearance from the adult
Metamorphosis - a marked change in the form or structure of an animal occurring after birth or hatching
Mucous Gland Secretion - clear watery liquid that is secreted onto the skin of amphibians through small pores distributed over the body surface
Nocturnal - active primarily at night
Parotoid Glands - paired, wartlike glands in the skin of toads; located behind the eyes or in the neck and shoulders
Snout-to-Vent Length - the distance from the tip of the snout to the excretory opening; the standard measurement for frogs and toads
Terrestrial - land based, or living on land
Tympanum - (tympanic membrane) the externally visible part of the eardrum in most frogs and toads (see Anatomy illustration)
Vertebrate - animal with a backbone

 

 Frog and Toad Facts!

 
Frogs and toads do not cause warts.
 
The largest Illinois anuran is the American bullfrog, which can attain a snout-to-vent length of 15 cm (6 inches).
 
The smallest Illinois anuran is the cricket frog, which averages 2 cm (1 inch) snout-to-vent length.
 
The average frog can leap a distance equal to ten times its body length.
 
Most burrowing frogs dig with their hind legs, but Strecker's chorus frog is one of only a few burrowing species that digs with its front legs.
 
The diet of the eastern narrowmouth toad consists almost exclusively of ants.
 
The long tongue of frogs and toads is folded in half when stored in the mouth. When extended for capturing prey, the tongue unfolds and is flipped out rather than being cast out straight.
 
Strecker's chorus frog can hunt for food while it is underground. It probably eats a variety of insect larvae, such as cutworms (moth larvae), that it finds in its sandy burrows.
 
American bullfrogs are famous for their incredible eating habits. They will attempt to eat anything that moves -- that they can at least partially swallow. They have been known to eat baby ducks, small mammals, other bullfrogs and snakes.
 
The treefrogs of Illinois have a sticky cup at the end of each toe that allows them to climb vertical surfaces such as windows and tree trunks. Treefrogs may be found in branches more than fifty feet off the ground.
 
A female American bullfrog can lay over 20,000 eggs in a thin film measuring two feet by two feet.
 
Some Illinois frogs, such as the wood frog, accumulate glycerol in their body tissues. The glycerol acts as an antifreeze, allowing them to survive subfreezing temperatures.
 

 Bibliography

 
Behler, J.L., and F.W. King. 1979. The Audubon Society field guide to North American reptiles and amphibians. Alfred A. Knopf, New York. 719 pp.

Conant, R., and J.T. Collins. 1991. Reptiles and amphibians of eastern/central North America. Houghton Mifflin Company, Boston. 450 pp.

Dickerson, M.C. 1907. The frog book: North American toads and frogs with a study of the habits and life histories of those of the northeastern United States. Doubleday, Page and Co., New York. 253 pp.

Duellman, W.E., and L. Treub. 1986. Biology of amphibians. McGraw-Hill, Inc., New York. 670 pp.

Halliday, T., and K. Adler (editors). 1986. The encyclopedia of reptiles and amphibians. Facts on File Inc., New York. 143 pp.

Herkert, J.R., editor. 1992. Endangered and threatened species of Illinois: Status and distribution. Vol. 2, Animals. Illinois Endangered Species Protection Board, Springfield. 142 pp.

Johnson, T.R. 2000. The amphibians and reptiles of Missouri. Department of Conservation, Jefferson City, Missouri. 400 pp.

Parmalee, P. A. 1954. Amphibians of Illinois. Story of Illinois, no. 10. Illinois State Museum, Springfield. 38 pp.

Phillips, C.A., R.A. Brandon, and E.O. Moll. 1999. Field guide to amphibians and reptiles of Illinois. Illinois Department of Natural Resources, Illinois Natural History Survey, Champaign, Illinois. Manual 8. 282 pp.

Smith, P.W. 1961. The amphibians and reptiles of Illinois. Illinois Natural History Survey Bulletin 28: 1-298.

Vogt, R.C. 1981. Natural history of the amphibians and reptiles of Wisconsin. The Milwaukee Public Museum, Milwaukee. 205 pp.

Wright, A.H., and A.A. Wright. 1949. Handbook of frogs and toads. Comstock Publishing Co., Ithaca, New York. 640 pp.