Bay Nature magazineWinter 2017


Pacific Chorus Frogs Use Their Famous Calls to Stake Out Territory

January 1, 2017

It’s February, and the night is soft. A high stratus blanket filters the moonlight and moistens your eyelashes. Wet season has been under way for three months now, and soils are saturated. Shallow water stands in boggy slumps, roadside ditches, and semi-permanent mud puddles. Such are locales for a great late-winter expression of amphibian fecundity: the annual gathering of countless Pacific chorus frogs to breed.

A small but iconic member of our bioregion, the tiny frog is widely recognized and loved for its “rib-it” call. While its name may have changed (from Hyla regilla to Pseudacris regilla and from Pacific tree frog, which many people still say, to Pacific chorus frog), the species’ presence in our winter soundscape holds a timeless resonance for humans.

When hundreds or thousands of males occupy a pond, to compete for the company of somewhat fewer females, and night temperatures dip no lower than 51 degrees Fahrenheit, a burst of new rainfall will set off a pulse of tree frog calling and breeding. If you are listening (often haplessly if you live nearby), you are apt to hear a sudden drop to utter silence. Something has alarmed the singers. Then, soon enough, a single frog voice chimes, then several more, then quickly dozens to many hundreds or several thousand—and all at once the collective din swells to vibrate your diaphragm and rib cage as well as your ear bones.

For a creature that weighs only about two grams, this little amphibian—collectively—has plenty of heft. The males emit their two-note “advertisement” calls to try and attract the silent females. Essentially they are using sound to establish how much space each male can claim around a pond. A flashlight might reveal a dense lineup of bright pearlescent globes along the water’s margin—the inflated throats of male tree frogs holding air for the next chorus.

Each female lurking in or near the water makes her choice of a calling male and approaches him to mate. But the close-packed males may continue competing, using a second kind of call to repel their rivals. Their “encounter” call is a shrill trill—and while males use it to establish territory amid the crowd, it actually repels females! Adding to the chaos, physical conflict can occur between males, with individuals grappling for space and dominance.

Ultimately, the purpose for all this activity rules the night. Once a female approaches a male, he abandons his song territory and wraps her from behind, in a position called amplexus; then the female propels them both into quiet, shallow water, where she deposits batches of eggs on aquatic vegetation or other substrates. The male emits sperm to fertilize the multiple clusters of eggs—hundreds of prospective new chorus frogs. Then the female leaves the party (the pond), and the male resumes advertising for another chance to mate.

This seasonal crowd-sourcing of next-generation froglets occurs after each burst of saturating rain that’s followed by warm night temperatures. There can be two such pulses (each several nights long) of chorusing and breeding in February, another two in March, and possibly one in April.

Pacific chorus frog abundance is due to the species’ successful breeding strategy and ability to survive in developed areas with water, its resilience in the face of fungal disease afflicting other amphibians, and the survival of wetland habitat despite massive land-use change. The frogs are found from British Columbia to Baja California, from the coast eastward to Montana and Arizona, and from sea level to 10,000 feet in the Sierra—where there is gradation among forms currently recognized as separate species.

Where do the chorus frog crowds go when ponds begin drying up? Outward to a distance of several hundred yards—or up to a half-mile away! Supposing that each tiny individual travels just a few inches in a single bound, how many reps must he/she perform to travel 1,000 yards?

As dry season advances, skin-moist creatures such as salamanders and frogs must surround themselves with dampness. Like other amphibians adapted to our Mediterranean climate, chorus frogs seek out wet niches in the landscape. Many make use of rodent burrows, where soil moisture persists and invertebrates—their food—abound.

And true to the species’ long-standing common-name identity, these tiny frogs do occasionally climb high in the coastal conifer forest. Web cameras placed in bald eagle nests atop trees so tall they scrape the fog for moisture have documented chorus frogs hopping about capturing insects.

Individual frogs may continue to sing occasionally throughout the year, with a lone voice emanating from the woodshed or a folded deck chair, a sweet reminder of the frog frenzy that marks a mild California winter.

About the Author

Claire Peaslee is a writer, naturalist, and improviser who lives in Point Reyes Station.