Natural Notes: A brief introduction to the frogs of Northern Illinois

Introduction
Do you know anyone who loves to frolic in 45 degree weather in the pouring rain?  I do!  The early spring frogs of Illinois love this weather, and as temperature warms up and as ice and snow melts away, various species of frogs come out of hibernation and begin mating and feeding as warmer, rainier nights set in.

In many woodland areas in the spring in Northern Illinois, the ice covering low-lying depressions and shallow ponds melts and forms ephemeral ponds that eventually dry up as summer arrives.  These "temporary" ponds and wetlands are void of fish and many other large predators, making it an ideal spot for amphibians like frogs and salamanders to breed and lay their eggs (figure 1).
Figure 1:  An ephemeral wetland forming from melting ice and snow.  

Early spring frogs must have a quick life cycle so that they can mature from their aquatic larval stage to an air-breathing adult before the pond dries up.  Other species of frogs have longer lifecycles (a year or more) and as such can only thrive in more permanent bodies of water such as shallow lakes and marshes.   Frogs in Illinois reproduce externally, and such as the embryos develop in the open water, usually in masses of gel that either float or are attached to debris (see figure 2).

Figure 2:  Frogs in Illinois reproduce externally, often leaving an egg mass in the water.  The blog of jelly in the middle of this photo is a mass of American Toad eggs attached to vegetation in a pond.  

Common Frogs of Northern Illinois
Northern Illinois has many species of frogs, and as the weather warms up careful observers will likely hear their mating calls and perhaps witness them breeding as they break from their winter hibernation and seek to pass their genetic material on to the next generation.

Wood Frog
Image source:  Wikimedia Commons
One of the first frogs to emerge is the wood frog (Rana sylvatica), emerging sometimes as early as late February or early march, even when there is still some ice or snow in the pond!  Wood frogs having a mating call that sound like raspy croaks and is often heard alongside louder frogs such as spring peepers.  Wood frogs one of the coldest heartiest frog species in the world, and they are the only frog whose range extends north of the Arctic Circle!  Wood frogs prefer ephemeral ponds in undisturbed woodland areas, and as such are becoming less common in Northern Illinois's urban landscape.

Western Chorus Frog
Photo credits:  Carol Kim, Forest Preserves of Cook County
Shortly after wood frogs, Western Chorus Frogs (Pseudacris triseriata) starting emerging from their winter hibernation.  Their mating call is actually quite easy to recognize.  If you've ever stroked your fingers across a hair comb, then you will recognize the mating call of a western chorus frog. Right around March into early April you might hear hundreds or even thousands of chorus frogs in wetlands around Northern Illinois.  Chorus frogs typically prefer shallow areas of marshes as opposed to woodland ponds or open water.  They are locally common around northern Illinois, and seem to be tolerant of a mild amount of pollution.

Spring Peeper
Photo Credits:  USGS/Wikimedia Commons
Spring peepers (Pseudacris crucifer) often inhabit the same areas as wood frogs (woodland ephemeral ponds) although I have heard them calling in ditches and swales.  Their mating call is a loud "ping", and do I mean LOUD!  It's amazing to think how strong the call is for an animal that is not much larger than your fingernail (about 1/4 inch long).  Now imagine hearing hundreds of them calling all at once on a balmy night in April!  In Northern Illinois, spring peepers are more locally common, and it is unclear to me why they are not more widely distributed across the northern part of the state.  Perhaps soil contamination and habitat fragmentation are partly to blame, but it is difficult to speculate why they are not very common in certain areas.

Gray Tree Frog
Photo source:  Wikimedia Commons
The Gray Tree Frog (Hyla versicolor) has a loud trill that increases in pitch as nights become warmer and balmy, usually in May and early June in Northern Illinois.  You are more likely to hear them than see them, as their small size and drab color allows them to easily camouflage.  As the name implies, adult tree frogs will climb onto the trunks and branches of large trees to perform their mating call.  Tree frogs have suction-like cups at the ends of their toes that allow them to cling to tree trunks and sometimes even windows!  The closely related Cope's Gray Tree Frog (H. chrysoscelis) has a faster trill, and is just north of its range in Northern Illinois. 

Green Frog
Photo Credits:  Carol Kim, Forest Preserves of Cook County
Once we start feeling that summer heat, you'll start to hear green frogs (Lithobates clamitans) calling. Sometimes you might be hearing their mating calls, especially on a warm night in May or June.  You will also hear them calling during the middle of day, which is likely more of a territorial call or another form of communication.  Green frogs and the closely related bull frog are your typical "pond frogs" that you would likely see resting on a lily pad in a pond or shallow lake.  Green frogs are sometimes found in shallow, weedy shorelines of slow-moving rivers.

Bullfrog
Photo Credits:  Wikimedia Commons
The American Bullfrog (Lithobates catesbeianus) is often found in the same areas a green frogs, but these frogs are often twice the size!  Bullfrogs can reach over 6 inches in length and 4 inches in width, allowing them to quickly snatch a small fish, dragonfly, and sometimes even a small bird!  The call of a bullfrog (mating and for territory) sounds like a low "rrrrr-uhm".  Bullfrogs are very common in Northern Illinois, and are often heard calling in the summer.  In some localities, they can become the dominant frog due to their large size in both the larval and adult stages of growth.

Northern Leopard Frog
Image Source:  Wikimedia Commons
Northern Leopard Frogs (Lithobates pipiens) are similar in shape and size to green frogs, but are distinguished by their large, leopard-like spots and a mating call that sounds similar to someone unzipping a sleeping bag and then lightly tapping a door with a small hammer.   Their habitat is similar to that of green frogs and bull frogs, but leopard frogs seem to prefer a little more diversity of habitat, such as marshes with bullrushes (a type of plant that looks similar to grass) with muddy shorelines.  They are seen and heard most commonly in the summer time.  

Pickerel Frog
Image Source:  Wikimedia Commons
It is unlikely that you will see or hear a pickerel frog in Northern Illinois anymore, mainly due to habitat destruction.  Ideal habitat for pickerel frogs is often clean,  spring-fed streams or wetlands somewhat like the rocky stream pictured in figure 3.  Even the smallest amount of disturbance can alter the water quality of their habitat, and Northern Illinois' highly disturbed landscape makes the future of this animal precarious in the region. 
Figure 3:  Potential pickerel frog habitat.  These sensitive frogs prefer cool sand shaded streams, an  now uncommon habitat in Northern Illinois due to habitat destruction.  
 American Toad
Photo source:  Wikimedia Commons
American toads (Bufo americanus) are perhaps one of the most common amphibians in the Northern Illinois region.  Closely related to frogs, these animals spend much more of their adulthood on drier land than any of the amphibians presented in this blog post.  Their wary and greenish appearance is distinct, and toads can be found in a wide variety of habitats from forests to ponds to even suburban backyards, as long as their as a source of fresh water that is free of fish.  Their mating call can he heard in May as a long and drawn-out trill.  The closely related Fowler's Toad (B. fowleri) resides in similar habitats, but is more common further south.  Although they look nearly identical to the American Toad, their mating call is drastically different.  It sound like a nasally whine.  


Conclusion
The calling of the first frogs is a sure sign of spring!  This blog post hopefully provided you with a good overview of common frog species in Northern Illinois, and I hope you will have the chance to catch some warmer weather soon so you can listen to these amazing animals.

Frogs and other amphibians have been on a steady decline over the years in the Chicago region as well as around the world due to habitat destruction, water pollution, and climate change.  Many species of frogs, such as pickerel frogs, are often considered indicator species as they are sensitive to small changes in the environment.   Hopefully people will pay more attention to the frogs that are present in their area which will help scientists like myself monitor their population status and geographic distribution in Northern Illinois' people-filled landscape.  

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