Frogs are singing for spring in Sonoma County

We humans have come up with some poetic collective nouns for animals. A murder of crows. A parliament of owls. Even a shrewdness of apes (now also the name of a band). So it seems we missed the proverbial lily pad when we came up with the term “army” to describe the millions of frogs that welcome spring with their delightful and often deafening chorus. As few Californians are more joyously vocal about the possible end to the California drought this year, I humbly offer a happiness of frogs as an alternative.

The frogs we hear performing their seasonal symphony right now in the North Bay are Pacific chorus frogs, or sometimes called simply tree frogs. Sticklers for taxonomic accuracy will correctly point out that our local Pacific chorus frog was recently renamed the Sierran chorus frog (Pseudacris sierra) when the genus that was formerly known as Pseudacris regilla was split into three species. Because there remains controversy surrounding the name change, I and many other local naturalists are sticking with Pacific chorus frog as the more descriptive and intuitive moniker.

How do I know it's a Pacific chorus frog?

Pacific chorus frogs range in color through a palette of bright yellow-greens, creamy oranges, reddish tinges, and most commonly a mottled beige or brown. While general coloring of an individual frog does not change, all color varieties have a spectacular ability to adjust their brightness in response to temperature, humidity and even stress. Pacific chorus frogs also vary greatly in size. An adult frog recently transformed from its tadpole form could easily perch on the tip of your pinkie finger. These diminutive “metamorphs” can eventually grow to a maximum of about 2 inches from the tip of their nose to their urostyle, a posterior section of fused vertebrae roughly analogous to a “rump.”

This great variation in both color and size can be confounding for humans. More than a few times, an enthusiastic rainy day hiker has texted me photographs convinced she has discovered at least five different species of frog. Fortunately for budding frog-o-philes, there are two unmistakable features of the Pacific chorus frog, regardless of size or color. All Pacific chorus frogs (indeed all members of the genus Pseudcris) have a characteristic bandit-like dark brown or black stripe across both eyes and all have enlarged toepads, which enable them to climb vertical surfaces.

Our other two native frogs in the North Bay include the medium-sized stream-dwelling foothill yellow-legged frog (Rana boylii) and the larger pond-dwelling California red-legged frog (Rana draytonii). In addition to being larger than the Pacific chorus frogs, neither of these frogs has an eyestripe or bulbous toes. Sadly, both are in serious decline in Northern California because of habitat loss, competition from nonnative bullfrogs and introduced fish and other factors.

Bullfrogs, which are responsible for the decline of many native amphibians worldwide, are easily distinguished from our native frogs by their large size - up to 8 inches long and 1.5 pounds - bulbous eyes, and a green circular external eardrum, called a tympanum, behind the eye.

Although observant hikers may still spot a yellow-legged frog basking on a rock along one of our local streams, few of us are likely to spot a California red-legged frog, as they are confined to a handful of ponds in our area. Despite being named the California State Amphibian by Gov. Jerry Brown in 2014, the California red-legged frog is on the federal list of threatened and endangered species.

Why do frogs sing?

Almost every “why” question in nature boils down to one thing - sex. And like bird song, the frog chorus is an exclusively male club. Each individual male is calling his heart out to lure a female, but en masse the frog chorus is also letting all females know where the good habitat is.

A favorite fake out among frog lovers, which is as instructive as it is fun, is to slowly approach a pond or ditch where frogs are singing and loudly clap. They all stop. A couple minutes later, the “chorus leader” (a large loud frog with prime real estate) starts in with his two-phase call “krick-eek” followed by a second male just a microbeat off sync and then the third. Eventually, all the frogs join in, resulting in the cacophony we call the chorus.

Successful courtship in frogs and other amphibians leads to a kind of mating called amplexus, Latin for “embrace.” Although his precise repertoire of moves vary by species, the general idea is the male climbs on top of and grasps the female with his front legs. She releases eggs in clusters that are bound together in a gelatinous mass, over which he releases his sperm.

A single female may lay from 400 to 750 eggs each season, usually in clusters of 10 to 80 that she attaches to vegetation in still or slow water. The eggs typically hatch into juveniles (tadpoles) within two to three weeks.

Tadpole transformation

Ask any third-grader to describe a frog's lifecycle and you'll get the facts, more or less accurately. Frogs lay eggs in water that hatch into tadpoles that swim around breathing through gills until they grow legs and lose their tail and then they crawl out on land. We are so familiar with the general storyline that we often fail to stop and wonder why. What's the point? Why don't frogs' eggs just hatch into small frogs that grow up to be bigger frogs? Furthermore, why are newly transformed adult frogs so much smaller than the tadpoles they came from?

Just as with butterflies and other insects that undergo a radical life-altering transformation, the answer lies in a sort of division of labor in life stages - juveniles eat and grow, adults mate and reproduce.

Chorus frog tadpoles are vegetarians, consuming mostly algae and other bits of plant matter suspended in water. Like other vegetarians, a tadpole needs an elaborate small intestine to process all those greens (look closely at a tadpole and you can see the twisty digestive system through the semi-transparent skin).

Adult Pacific chorus frogs, on the other hand, are strictly carnivores, eating mostly small invertebrates. How then does a tadpole survive what is literally a gut-wrenching transformation? About the time that the tadpole has sprouted four legs, it stops eating entirely. Far from “losing its tail,” the almost mature froglet reabsorbs the calorie rich muscles of its tail to fuel this transformation. The adult frog thus emerges from its youth much smaller than it was as a teenager.

Probably the most frequent amphibian question I get is “how long does it take for tadpoles to turn into frogs?” Typically, transformation occurs within about eight to 10 weeks, but, it varies based on food availability and temperature.

One of the mad skills of a tadpole is that its metabolism speeds up as temperature rises. In an ephemeral pond, where water temperature increases as the water continues to evaporate, this clever adaptation gives the young frogs a fighting chance to fully transform before the pond disappears.

During our recent drought years, I witnessed many a pond disappear before the tadpoles could make it, leaving an oily slick in the mud when the water was gone. This was so heartbreaking to me at the beginning of the drought, I admit to scooping tadpoles with a soup ladle into mason jars from flooded cow hoofprints in the meadow near my house.

Last year, after a particularly devastating dry spell, a wiser herpetologist friend of mine pointed out that all that nitrogen contained in the bodies of deceased tadpoles would be recycled the following year.

I am forever grateful to be humbled by a happiness of frogs.

For more information about the Pacific chorus frog, including an in-depth discussion of the taxonomy and range differences of California's three chorus frog species,visit californiaherps.com.

Jeanne Wirka is the Director of Stewardship for Audubon Canyon Ranch (ACR). She lives at ACR's Bouverie Preserve near Glen Ellen.