Wild About Illinois Frogs and Toads!

Frogs 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.

Kingdom: Animalia - Animals are multicellular organisms that rely on other organisms for nourishment. There cells do not have cell walls. Most animals are capable of movement at least in some portion of their life cycle. Reproduction is generally sexual, but in some animals asexual reproduction may be utilized at certain times.
Phylum: Chordata - The Phylum Chordata contains the vertebrate animals. Mammals, birds, reptiles, amphibians and fishes are included in this phylum. These animals have a notochord at some point in their development. They have a tubular nerve cord along the back. Gill slits and a tail are present at some point in their life cycle. They have an internal skeleton.
Class: Amphibia (frogs, toads and salamanders) - Most of its members have four feet. Most amphibians must return to water to reproduce. The aquatic larval stage is the tadpole. Most amphibians must live in a moist environment. The skin and lungs are used for gas exchange in the adults while the larvae breathe with gills. Most amphibians produce toxins in the skin. Amphibians do not have scales, feathers or hair.
Order: Anura (frogs and toads) - Frogs and toads have moist skin or are active when the environment is moist. They must return to water to reproduce even if they normally do not live in it. Males produce a breeding call. The adults do not have a tail.
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.
     Blanchard's cricket frog (Acris blanchardi)
     bird-voiced treefrog (Hyla avivoca) [state threatened]
     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)
     Illinois chorus frog (Pseudacris illinoensis) [state threatened]
     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 narrow-mouthed toad (Gastrophryne carolinensis) [state threatened]

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 vertical.
     eastern spadefoot (Scaphiopus holbrookii)

 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.

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 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.

​Cricket 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.

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.

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

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