Creature Feature: The Pacific Treefrog

Tree frogs are pretty ubiquitous in the far Western United States. Photo by tomsimages via flickr.

Twenty-six of the 600 or so known species of treefrogs live in North America. One of these, the Pacific Treefrog, is quite common in the far Western states. If fact, if you've ever heard a frog doing its “ribbet” thing anywhere along the Pacific Coast, it's probably been this one. And if you’ve ever seen one up close, you already know why many consider it one of the most fascinating of all the amphibians. In fact, it has quite a fan following. In 2007 the Pacific chorus treefrog was named Washington's state frog.

The Pacific Treefrog has a remarkably wide geographic distribution. It’s found up and down the Pacific Coast from Baja California to southern British Columbia, as well as inland to parts of Idaho, Nevada, and western Montana. And what an amazingly adaptive creature it is! You can find these frogs in your back yard, along the beach, in the Mojave Desert, in woodlands, in grasslands or pastures, and even at 11,000 feet on Mount Whitney. They inhabit many western national parks, of course, including Channel Islands, Redwood, Olympic, and Crater Lake National Parks, to name a few.

The Pacific Treefrog (Hyla regilla or Pseudacris regilla) is easy to recognize, whether by sight or by sound. The latter is a slam dunk, since it is the only frog in the West that makes the classic “ribbet” vocalization – the sound that Hollywood film producers have made familiar around the world. For a sample of this treefrog music, visit this site.

The treefrog's appearance is distinctive too. Male Pacific Treefrogs are just under 2 inches long and generally green or dark gray, with a creamy underside. Females are gray or brown and a bit larger than the males. The color of both sexes can vary (see below), but you can verify the Pacific Treefrog identification by looking for a dark brown or black band that runs from the eyes to the shoulders and white underside.

Pacific Treefrogs have special “sticky pads” on their feet that help them climb. They are not primarily creatures of the trees, however. You're more likely to find one in a pond or hopping nearby.

The male frog’s dark gold throat is its vocal sack, which puffs out when it vocalizes. Despite its small size, this little frog is a prodigious noisemaker. The calling of the males, which can be heard nearly a mile away, is used to attract mates and ward off other males. Frog music is heard most often in the spring, but treefrogs vocalize all year unless the temperature dips below freezing. Interestingly, the frogs coordinate their vocalizations, creating a frog chorus. One male acts as the “chorus master” and starts calling. Other males in the vicinity gradually join in as the chorus swells.

One of the most fascinating characteristics of this little frog is its ability to change color. Unlike chameleons, which change their color to match their surroundings, the Pacific treefrog changes color based on the air temperature and humidity. The frogs don't control this change; it just happens naturally within a few minutes. The color change is for exactly the reason you think -- a defense mechanism to reduce the likelihood that the treefrog will become a meal for a bullfrog, raccoon, heron, snake, or other predator.

The Pacific Treefrog is primarily nocturnal, spending the night scouring its pond and adjacent leaves, logs, rocks, and crevices for bugs, ants, flies, spiders, and other little crawling or flying things. The frogs hop around as they look for food, landing with all four feet firmly planted. (Because of this, their tracks in mud or soft soil may reveal an indentation of their bodies as well as feet.) A frog flicks out its sticky tongue to catch its prey, which it swallows whole. It sometimes accidentally eats dirt and other debris that is caught on its tongue. Since the frogs don't chew their food, the dirt can clog their digestive systems.

Mating takes place during the rainy season, usually in early- to mid spring. Egg fertilization is external, and the eggs are laid in the traditional frog 'jelly' mass in a seasonal or ephemeral water body, such as a roadside puddle. Each jelly-like mass, which contains up to 70 eggs, is attached to vegetation so it doesn't float away. The eggs, which are not guarded, hatch into tadpoles in roughly a week

By laying their eggs in temporary puddles, treefrogs reduce the chance that predators will come along and gobble up the tadpoles. Of course, if the water dries up too quickly, the tadpoles die anyway.

About three months elapse before tadpoles metamorphose into frogs. In the meantime, the tadpoles use their beak-like mouths to scoop plant matter off the bottom of their puddles. Their growth rate is related to the number of other tadpoles they share their home with – the more tadpoles, the more body heat generated, and the faster they grow (thanks to warmer water).

Curiously, as the tadpoles become frogs, they are unable to eat for several days while their digestive systems transition from vegetarian to carnivorous functioning. The tadpole’s tail also becomes part of its adult frog body as this metamorphosis occurs.

Once it climbs out of its pool, the new froglet can be less than one centimeter long – about as long as your pinkie fingernail. They grow very quickly, however, and can reproduce a year later.

There is some controversy over how the Pacific Treefrog should be categorized. Some scientists refer to it as Hyla regilla, but others call it Pseudacris regilla. This is largely due to the inconsistencies in the frogs' behaviors. Because the Pacific Treefrog is found in so many different places, not all populations act the same. The genus Hyla is primarily treefrogs, the genus Pseudacris includes chorus frogs, and the Pacific Treefrog exhibits both behaviors.

In 1986, DNA tests showed the Pacific Treefrog to be more like Pseudacris, but in 1997 another scientist found that the frog should be placed in the Hyla genus, based on its anatomy. Biologists still aren't entirely sure, since both studies were thoroughly vetted and found to be accurate. For a checklist and guide for the identification of amphibians of the U.S. and Canada, see this site.

Regardless of how you classify it, the Pacific treefrog is important to its ecosystem because it keeps local insect populations in check while providing food for snakes, fish, and birds.

Scientists consider the Pacific Treefrog to be a keystone species as well as an indicator species. It is a keystone species because of the vital role it plays in the food webs through which matter and energy flow. It is an indicator species because its presence, population trends, or absence tells us a lot about the health of the ecosystems it inhabits. The Pacific Treefrog is particularly vulnerable to acid rain and water-borne pollution, especially from agricultural and urban runoff. Although the treefrog isn’t considered threatened or endangered at present, its long term survival remains a matter of concern.