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Pseudacris illinoensisIllinois Chorus Frog

By Brittany Mason

Geographic Range

Illinois chorus frogs (Pseudacris illinoensis) are native to the Nearctic region. More specifically, they occupy a small range along the Illinois and Mississippi River from southwestern Illinois, through eastern Missouri, to the northeastern tip of Arkansas, near the Obion River. (Lonneker and Boykin, 2015)

Habitat

Illinois chorus frogs inhabit prairies, wetlands, and occasionally agricultural fields. They require habitats with moist, soft, sandy soils. They spend most of the year in underground burrows and emerge during breeding season, at which point they occupy flooded fields, wetlands, ponds, or vernal pools. Illinois chorus frog tadpoles remain in these areas until they metamorphose. Froglets and adults move back to burrowing areas. (Elliott, et al., 2009; Forbes, 2021)

  • Aquatic Biomes
  • lakes and ponds

Physical Description

Illinois chorus frogs are the largest species in the genus Pseudacris. Compared to other chorus frog species, they tend to have thicker forearms. Both sexes have an average snout-vent length (SVL) of 19 mm as froglets and 28 mm as adults, although adults can reach a maximum of 47 mm in length. Females have an average body mass of 7.22 g whereas males average 6.44 g. Illinois chorus frogs have an average mouth width of 9.89 mm, though this ranges from from 6.61 mm to 14.14 mm, depending on the sex.

Adult Illinois chorus frogs are light tan or grey with reddish-brown, irregular, often elongated blotches on their dorsal sides. Typically, they have a distinctive "V" or "Y"-shaped dorsal blotch between their eyes and one small blotch underneath each eye. They also have an elongated lateral blotch on either side of their head that extends from their tympanum, across their eye, to their snout. The blotches of color along the rest of their bodies and limbs differ in size and shape between individuals. They have dark, greenish-black tail stubs and white or cream-colored venters with no clear markings

Illinois chorus frog tadpoles have dark brown dorsal and lateral coloration and tan ventral coloration. They have no clear patterns, high dorsal fins, and are generally larger than tadpoles of other chorus frog species. Froglets are typically grey or brown and lack the patterns present in adults. (Forbes, 2021; Hebenstreit, 2018; Tucker, 1997)

  • Sexual Dimorphism
  • female larger
  • Range mass
    6.44 to 7.22 g
    0.23 to 0.25 oz
  • Range length
    25 to 41 mm
    0.98 to 1.61 in

Development

There is limited information regarding the developmental timing of Illinois chorus frogs. The eggs of other chorus frogs (genus Pseudacris) hatch between 2 and 27 days after they are laid. Eggs and tadpoles develop at different rates depending on water temperature and other environmental conditions. Water temperature also plays a role in determining the sex of tadpoles.

Illinois chorus frog tadpoles complete metamorphosis between 30 and 90 days after hatching. Illinois chorus frogs exhibit indeterminate growth. Froglets grow rapidly for around a year, after which growth rates decline substantially. Within the first year, approximately 90% of females and 78% of males approach their maximum size.

Illinois chorus frogs reach sexual maturity after one year, but they typically do not breed until they are closer to two years old. (Ethier, et al., 2021; Tucker, 2000)

Reproduction

Illinois chorus frogs have a breeding season that begins in February and continues into March. During breeding season, males emerge from brumation and remain within breeding habitats for 4 to 10 weeks. Females are less active, typically emerging for less than two weeks per season. Illinois chorus frogs usually breed in shallow bodies of water, such as ponds, vernal pools, or wetlands

Aside from their breeding season, there is limited information regarding the mating systems of Illinois chorus frogs. However, their mating behaviors are likely similar to those of other chorus frogs (genus Pseudacris). Across the genus, males and females are capable of mating with multiple individuals at a time, and females only lay one clutch of eggs per breeding session. Females select mates based on the quality of their mating calls, approaching males that produce higher-quality calls. Chorus frogs use a mating position called amplexus, in which a male clasps onto the back of a female. Several males may compete for a position on top of a female, and in some cases males will mount other males that are engaged in amplexus. Females release their eggs during amplexus and nearby males release their sperm to fertilize the eggs. Consequently, multiple males may contribute to the fertilization of eggs within a single brood. (Elliott, et al., 2009; Schneider, 2012)

Illinois chorus frogs mate in axillary amplexus, a position in which a male mounts a female from behind, using its arms to clasp the torso of the female. The mating pair then align their cloacae, which stimulates the female to lay eggs. Females typically initiate copulation, selecting their mates based on the quality of male vocalization. In some cases, males can be dissuaded from mating if a female moves away before mounting is complete. Chorus frogs (genus Pseudacris) generally mate at night, but amplexus can last anywhere from a few hours to as long as 40 hours.

Illinois chorus frogs are iteroparous and females lay a single clutch of eggs per breeding season. Each clutch consists of 200 to 1,000 eggs, although females typically lay multiple separate clusters of eggs. Individual clusters can contain anywhere from 8 to 79 eggs.

There is limited information regarding egg size, egg mass, and birth mass for Illinois chorus frogs. Their young are immediately independent upon hatching, and both sexes reach sexual maturity within a year after hatching. (Ethier, et al., 2021; Hebenstreit, 2018; Schneider, 2012)

  • Breeding interval
    Illinois chorus frogs breed multiple times per season
  • Breeding season
    February to March
  • Range number of offspring
    8 to 79
  • Average time to independence
    0 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

There is limited information regarding the parental investment of Illinois chorus frogs specifically. However, investment levels are likely similar to other chorus frog (genus Pseudacris) species, which show no further parental investment beyond the act of mating. (Ethier, et al., 2021)

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

There are no known reports of Illinois chorus frog longevity at the time of writing. Other species in the genus Pseudacris may live as long as 7 years, although most individuals live less than 4 years due to environmental factors, such as predation or disease. (Ethier, et al., 2021)

Behavior

Illinois chorus frogs are typically solitary, but are social during the breeding season. Males produce aggression calls and advertisement calls throughout the breeding season. Aggression calls range from 4 to 7 pulses in length and are used during conflicts with other males, whereas advertisement calls consist of a single note or pulse and are used to attract females. Illinois chorus frogs exhibit lower movement rates when producing advertisement calls and higher movement rates when producing aggressive calls. Typically, some individuals, referred to as calling males, produce advertisement calls while others, referred to as satellite males, remain nearby and intercept females that are attracted by calling males. Satellite males may change strategies depending on the amount of other calling males in the vicinity; they conserve energy by remaining quiet, but may begin calling if a territory becomes available.

Illinois chorus frogs enter brumation and overwinter under rocks, logs, or leaf litter. They may also burrow as deep as 25 cm underground to avoid freezing temperatures.

Newly metamorphosed juveniles stay near their natal ponds for several weeks before migrating as much as 500 m away to reach more suitable overwintering habitats. Illinois chorus frog tadpoles are entirely natatorial, whereas adults are primarily terrestrial outside of breeding season. (Ethier, et al., 2021; McCallum and Trauth, 2001; McCallum, et al., 2006; Schneider, 2012)

  • Range territory size
    1 to 4 m^2

Home Range

Illinois chorus frogs do not have home ranges, as they remain dormant for most of the year. However, they may move as far as 1.5 m away from breeding ponds to reach suitable overwintering habitat. During the breeding season, males defend territories that are 1 to 2 m apart. Data also suggest that Illinois chorus frogs can disperse underground to avoid hazards presented by human activity or predators. (Elliott, et al., 2009; Ethier, et al., 2021; Schneider, 2012)

Communication and Perception

Illinois chorus frogs communicate primarily using acoustic signals. Males produce mating calls, consisting of a single pulse, in areas where females can deposit their eggs. Males defend these areas from other males using aggressive calls, which consist of 4 to 7 pulses. Illinois chorus frogs call more frequently between dusk and dawn. There is no apparent correlation between body size and call frequency or call rate, and there is no apparent correlation between the frequencies of male and female calls. In addition to mating calls and territorial calls, Illinois chorus frogs will produce alert calls if they detect a predator.

Illinois chorus frogs also use visual, chemical, and tactile cues to communicate and perceive their environment. For example, mating pairs use visual and tactile cues to engage in amplexus and align their cloacae. Furthermore, adults and tadpoles regularly rely on chemical and physical stimuli to find food and avoid predators. (Ethier, et al., 2021; Owen, 2003)

Food Habits

Illinois chorus frogs are insectivores. They mostly forage aboveground, but likely consume both fossorial and terrestrial prey. The majority of their diet consists of butterfly and moth larvae (order Lepidoptera), such as dingy cutworms (Feltia jaculifera). However, they also consume a variety of other invertebrates including spiders (order Araneae), true bugs (order Hemiptera), beetles (order Coleoptera), and flies (order Diptera). Unlike other frog species, Illinois chorus frogs are not known to eat earthworms (Lumbricus terrestris). However, they do occasionally eat aquatic worms (order Haplotaxida). There are no apparent differences in the diets of males and females. (Forbes, 2021)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • terrestrial worms

Predation

Illinois chorus frogs have high mortality rates within one year after hatching, in large part due to predation. Froglets have a survival rate of around 4% and adults have a survival rate of around 52%. As tadpoles, Illinois chorus frogs serve as prey for many vertebrate predators, including American bullfrogs (Lithobates catesbeianus), tiger salamander larvae (Ambystoma tigrinum), smallmouth salamander larvae (Ambystoma texanum), and many species of fish. Invertebrate predators include the larvae of dragonflies and damselflies (order Odonata), diving beetles (family Dytiscidae), weevils (superfamily Curculionoidea), and water striders (family Gerridae). Illinois chorus frog tadpoles also begin to cannibalize smaller individuals between 28 and 54 days after hatching. Cannibalism tends to occur more often in low-nutrient environments, suggesting that it is a response to overcrowding of natal ponds. As adults, Illinois chorus frogs are most susceptible to predation by reptiles, such as western hognose snakes (Heterodon nasicus) and Illinois mud turtles (Kinosternon flavescens), although they are likely eaten by other vertebrate predators as well.

Illinois chorus frogs exhibit cryptic coloration that helps them avoid predation, and their nocturnal behavior also reduces the number of predators they encounter. Illinois chorus frogs also use alarm calls to warn others of nearby predators. ("Conservation guidance for Species in Greatest Need of Conservation (SGNC)", 2016; Dirrigl, Jr. and Hammerson, 2002; Ethier, et al., 2021; McCallum and Trauth, 2001)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Illinois chorus frogs are insectivores, and likely play a role in controlling invertebrate populations, including those of butterflies and moths (order Lepidoptera). Illinois chorus frog tadpoles serve as prey for aquatic predators, such as amphibians, reptiles, fish, and invertebrates. Adults also serve as prey for terrestrial vertebrates, such as snakes and turtles.

There is limited information regarding parasites that use Illinois chorus frogs as hosts, although populations in Arkansas have been reported to host the Apicomplexan parasite species Isospora delicatus. ("Conservation guidance for Species in Greatest Need of Conservation (SGNC)", 2016; McAllister, et al., 2017; McCallum and Trauth, 2001)

Commensal/Parasitic Species
  • Apicomplexans (Isospora delicatus)

Economic Importance for Humans: Positive

There are no known positive economic impacts of Illinois chorus frogs.

Economic Importance for Humans: Negative

There are no known negative economic impacts of Illinois chorus frogs.

Conservation Status

Illinois chorus frogs have no special status on the IUCN Red List. They have no special status on the US Federal list, CITES, or the State of Michigan List. Illinois chorus frogs were evaluated by the Fish and Wildlife Service to be federally listed in 2015, but were not listed because more information on their ecology was needed to make a decision. Illinois chorus frogs are considered to be "Vulnerable" by NatureServe, "Imperiled" by both Illinois and Missouri state departments, and "Critically Imperiled" by the Arkansas state department; these three states constitute the entirety of their geographic range.

Major threats to Illinois chorus frogs include habitat loss due to anthropogenic development. Open wetlands and bottomland hardwoods (which become seasonal wetlands) are frequently drained and cleared for agricultural use or suburban housing. Furthermore, road-building leads to habitat fragmentation, which can lead to habitat degradation and reduced gene flow between populations in separate fragments. Other threats to Illinois chorus frogs include water pollution, predation by invasive fish and American bullfrogs (Lithobates catesbeianus), and potential exposure to chytrid fungus, which is present in the Midwest and impacts other Pseudacris species.

There are currently no conservation efforts directed towards Illinois chorus frogs specifically. Some populations in Illinois are indirectly protected because they inhabit protected state landholdings. Preservation of suitable breeding habitat appears to be a key conservation method, and it is also critical to maintain habitat corridors to facilitate migration to breeding pools from upland habitats, and vice versa. Future federal protection may help the states of concern accomplish such conservation efforts. (Dirrigl, Jr. and Hammerson, 2002)

Contributors

Brittany Mason (author), Radford University, Candice Amick (editor), Radford University, Katherine Gorman (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltatorial

specialized for leaping or bounding locomotion; jumps or hops.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

References

Illinois Department of Natural Resources, Division of Natural Heritage. Conservation guidance for Species in Greatest Need of Conservation (SGNC). INHS Technical Report 2016 (09). Champaign, Illinois: Illinois Natural History Survey. 2016.

INHS Division of Ecology and Conservation Science Section for Field Stations and Ecosystem Science and Illinois Department of Natural Resources. The Illinois chorus frog (Pseudacris illinoensis) and wetland mitigation: What has worked?. INHS Technical Report 2008 (26). Champaign, Illinois: Illinois Department of Natural Resources and U.S. Fish and Wildlife Service. 2008.

Brown, L., G. Rose. 1988. Distribution, habitat, and calling season of the Illinois chorus frog (Pseudacris steckeri illinoensis) along the Illinois River. Illinois Natural History Survey Biological Notes, 132: 13.

Dirrigl, Jr., F., G. Hammerson. 2002. "Pseudacris illinoensis" (On-line). NatureServe. Accessed May 27, 2023 at https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.869802/Pseudacris_illinoensis.

Elliott, L., C. Gerhardt, C. Davidson. 2009. The Frogs and Toads of North America. Boston, MA and New York, NY: Houghton Mifflin Harcourt.

Ethier, J., A. Fayard, D. Choi, V. Tradeau. 2021. Life history traits and reproductive ecology of North American chorus frogs of the genus Pseudacris (Hylidae). Frontiers in Zoology, 18/40: 2-18.

Forbes, J. 2021. Diet and Prey Preference of the Illinois Chorus Frog (Pseudacris streckeri illinoensis) (Master's Thesis). Edwardsville, Illinois: Southern Illinois University.

Hebenstreit, L. 2018. Conservation Status of the Illinois Chorus Frog (Pseudacris illinoensis) in Madison County, Illinois (Master's Thesis). Edwardsville, Illinois: Southern Illinois University Edwardsville.

Hulin, A., E. Golden, R. Bluett. 2015. Monitoring occupancy of the Illinois chorus frog (Pseudacris streckeri illinoensis): Are plots or ponds the best fine-scaled sampling unit for call surveys?. Transactions of the Illinois State Academy of Science, 108: 55-58.

Lonneker, J., K. Boykin. 2015. "Illinois Chorus Frog (Pseudacris illinoensis) aILGRx_CONUS_2001v1 Range Map" (On-line). ScienceBase-Catalog. Accessed February 02, 2023 at https://www.sciencebase.gov/catalog/item/59f5e22ae4b063d5d307dcff.

McAllister, C., D. Motriuk-Smith, R. Seville, M. Connior, S. Trauth, H. Robison. 2017. Coccidian parasites (Apicomplexa: Eimeriidae) of Arkansas herpetofauna: A summary with two new state records. Journal of the Arkansas Academy of Science, 71: 143-152.

McCallum, M., S. Trauth. 2001. Are tadpoles of the Illinois chorus frog (Pseudacris streckeri illinoensis) cannibalistic?. Transactions of the Illinois State Academy of Science, 94/3: 171-178.

McCallum, M., S. Trauth, R. Neal, T. Klotz. 2006. Calling site characteristics of the Illinois chorus frog (Pseudacris streckeri illinoensis) in northeastern Arkansas. Herpetological Natural History, 9/2: 195-198.

Owen, P. 2003. The Structure, Function, and Evolution of Aggressive Signals in Anuran Amphibians (Ph.D. Dissertation). Mansfield, Connecticut: University of Connecticut.

Schneider, E. 2012. Population Genetics of the Illinois Chorus Frog (Pseudacris illinoensis) (Master's Thesis). Champaign-Urbana, Illinois: University of Illinois.

Trauth, J., S. Trauth, R. Johnson. 2010. Best management practices and drought combine to silence the Illinois chorus frog in Arkansas. Wildlife Society Bulletin, 34/2: 514-518.

Tucker, J. 1997. Description of newly transformed froglets of the Illinois chorus frog (Pseudacris streckeri illinoensis). Transactions of the Illinois State Academy of Science, 90/3-4: 161-166.

Tucker, J. 2000. Growth and survivorship in Illinois chorus Frog (Pseudacris streckeri illinoensis). Transactions of the Illinois Academy of Science, 93/1: 63-68.

Whiting, A. 2004. Population Ecology of the Western Chorus Frog, Pseudacris triseriata (Master's Thesis). Montreal, Canada: McGill University.

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