Frog Calling

Spring has arrived bringing rising temperatures, warm winds, and spring rains to the upper Midwest.  This is the time of year when many amphibians, most notably salamanders, frogs, and toads, start their annual breeding periods.  Illinois has nineteen native frog species.  They are secretive and difficult to find most of the year; but spring is the season for mating, and males will leave their woodland homes and make their way to the nearest pond on nights when temperatures stay in the forties or above.

Frogs use a variety of different calls.  Advertisement calls, the loudest calls voiced by male frogs, advertises their presence and availability to potential mates.  The courtship call, a slight variation of the advertisement call, is used as a female gets close to direct her to the exact location where the male is waiting.  If a male mounts an unreceptive female, or mounts another male, which can happen by mistake in the dark, a release call is emitted to inform the male to stop and look for another potential mate.  Aggressive calls may be used by one male towards another who gets too close.  And if any frogs are attacked by a predator, a loud distress call is made.  This may be to startle the attacker or warn other frogs in the area.

Calls are created by filling the lungs with air, and then blowing out a steady stream of air over the vocal chords.  When calling, the mouth and nostrils are normally closed.  The air is passed from the lungs, over the vocal chords in the larynx and into the air sac under the mouth.  This air sac, shown in many frog pictures, amplifies the sound to help it carry farther.  Calling requires considerable energy and may increase the metabolic rate, the rate at which calories are used, from ten to twenty times the resting rate of an individual.  For frogs with breeding periods lasting many weeks, the expenditure of calories will cause a male to lose a significant amount of body weight.

Many different species of frogs lay their eggs in the same ponds.  Males arrive early in the evening and begin advertising, while females arrive a bit later and identify a potential mate based on his call.  When a gravid female approaches, the male clasps her from on top and holds on.  After a time, the female releases her eggs in the water, and the male immediately fertilizes them by releasing sperm into the water around the egg sac.  Depending on the species, all of the eggs may be deposited in one egg sac or in several separate egg sacs.  Egg laying can last several minutes to several hours.  After each group of eggs is laid, the male and female may separate or rest for a brief period and then move to another spot to deposit more eggs.

Their skin is moist and cool to the touch.  It is permeable, allowing air to pass directly through it into their circulatory system, helping to remove some of the workload from the lungs and heart.  This can be especially useful during inclement weather.  However, the skin’s permeability makes them susceptible to drying out, so they live in or close to water or high humidity environments.  Their lifestyle includes being active mostly at night or on overcast days when it is cooler and more humid.

^{From left: Spring peeper calling by Andrew Hoffman, Jan 2000; Ornate chorus frog calling by FWC Fish & Wildlife, Jan 2014; Green tree frog calling by Eyeweed, Dec 2009}

Many frog populations have been impacted by losses of both wetland and forest habitat.  Wetlands are used for breeding and forests provide sheltered areas with shade, ground cover, and higher humidity than areas open to bright sunlight.  During spring evenings or overcast days, listen closely as you pass by these shady areas and nearby ponds to catch sound or sight of these wonderous small creatures.