April 5, 2013

Flirty Spring Frogs

It’s spring sometime around dusk, and as you walk near a pond you hear clucking sounds and peeps. You wonder, what’s making those noises? An insect? A bird? Though those are good guesses, chances are what you’re actually hearing is a frog. After the first warm rains in early spring the male frogs are out, filling the air with their amorous calls in hopes of becoming at least one female’s prince charming.

1. Who’s Who  |  Upon approaching the pond, you get a quick glimpse of a frog before it disappears with a splash of water. But which species was it? In Illinois, wood frogs, spring peepers, and western chorus frogs are some of the most common spring-breeding species. They can be identified by just a few distinguishing characteristics:
  • Wood frogs are generally tan to reddish-brown and have a dark mask on each side of their face, which obscures the lower half of their eyes.
  • Spring peepers have a dark colored “X” on their backs, a narrow dark colored stripe between their eyes, and large toe pads. (Fun Fact: Spring peepers can alter their skin color to camouflage with their environment!)
  • Western chorus frogs have a white stripe on their upper jaw, three thin, dark colored stripes down their back, and small toe pads.

2. Frozen But Not Finished  |  Just a week or two previous to these mild, spring days, temperatures were well below freezing, as evidenced by the chunks of ice that still litter the pond banks. To humans and other endotherms, the freezing temperatures are not much more than a sign of the season because we can regulate our body temperature via internal metabolic processes. Amphibians, however, are ectothermic such that their internal temperature is entirely determined by the temperature of their environment. Consequently, many amphibians hibernate under cover (e.g., leaf litter, logs, bark) over the cold, winter months to maximize their chance at survival. Some frogs, like wood frogs, spring peepers and western chorus frogs are much more tolerant of below-freezing temperatures than other species. In fact, they themselves are able to freeze and then thaw out when the temperature rises above a certain threshold. After a brief recovery phase, these frogs are able to migrate toward breeding ponds, and begin their chorus. Wood frogs, the most cold tolerant of the three species, have been shown to survive freezing 65% of their body for up to 2 weeks without death or permanent damage. This isn’t surprising as wood frogs are one of the few amphibians that live north of the Arctic Circle.

3. A Flirty Chorus  |  As evening progresses, the initial chorus of clucks and peeps grows louder until it is almost deafening. Although wood frogs, spring peepers, and western chorus frogs are tiny (they only reach maximum lengths of 6 cm, 3.5 cm, and 4 cm, respectively,) the cumulative volume of their calls can be great. Each frog species exhibits its own unique call or assortment of calls. For instance, male wood frogs’ mating call consists of five to six clucks. If a male successfully attracts a female, they will dive to the pond bottom where she will lay her eggs while he holds onto her (amplexus.) Afterwards, the male fertilizes the eggs and releases the female. In some cases “mating balls” will form, when several aggressive males grasp a single female. Unfortunately, this often results in the death of the over-desired female. In addition to its mating call, male wood frogs sometimes utter a release call if they are grabbed by another male. This happens occasionally, as wood frogs are unable to differentiate males from females. The mating call of spring peepers, as their common name suggests, is an ascending “peeeep,” that is repeated about 15 to 25 times per minute. Western chorus frogs have the most bizarre sounding call of the three species, as it has often been likened to the sound created by running your finger down the length of a comb.

To see wood frogs, spring peepers, and western chorus frogs in action, watch the video “The Early Frogs Get the Best Spots!  

Written by Danielle M. Ruffatto