Monday, March 9, 2009

Frog Blog (1)

In this nature blog so far, we have mostly written about the things we know pretty well. However, I was thinking recently about this: no naturalist, no matter how experienced, knows everything. There is always plenty more to learn. And so it is for us.

My newest enthusiasm is that I want to learn more about our local amphibians. I know very little about these amazing animals. For me, it is important that my knowledge is based on formal training, supplemented by lots of hours of "dirt time" aka field experience. We have neither with amphibians. And yet they are CALLING to me (literally, from the nighttime ponds). So I thought I would write about my explorations, so other people can learn about how we learn about nature.

I decided to focus first on Pacific tree frogs. I have been fascinated for years by the idea of how they migrate in spring. Late one January, on a warm, wet night, Glen and I were heading to Millersylvania State Park for a weekend singing camp. As we headed down the narrow access road to the camp, we saw ahead of us in our headlights several frogs jumping across the road. I slammed on the brakes and we got out to check them out: Pacific Tree Frogs were hopping across the road, heading to the breeding ponds. I was enthralled.

Look at the picture of this frog: these frogs generally do not exceed 2 inches. They are often green, but can vary from deep brown to tan; from my reading it appears that they can change their color, depending on the habitat they live in, though these color changes may take several days or months. Two key field marks you can see in this picture: all Pacific tree frogs have a dark line/mask running from the tip of the nostril, over the shoulder, duplicated on both sides. They also are frogs which can climb, so they have small suction pads on the tips of their toes, clearly visible in this photo. This is how you know it is a Pacific tree frog.

I've been interested in the timing of migration of these frogs; typically their winter life is spent in upland woods, but when the time comes to breed, they move to the vernal (spring/breeding) ponds. I sent an email around a nature listserve asking about migration: one fellow near Bremerton noticed a mass migration across his road on Feb. 22nd. This fits, as my friends Barbara and Rain who both live near wetlands, reported the beginning of the deafening choruses about 2 weeks ago.

The females follow the calls of the males, and then mates. She begins to lay her eggs in a mass of gel: she attaches the egg mass to a floating stem. There may be as many as 10-70 dark eggs inside. From this picture you can see some of the early round eggs, but most of this eggs have developed into tiny tadpoles and are close to emerging. When they emerge, they leave the gel, and are free-swimming in the pond, where they are vegetarians.

Frogs are famous for using temporary ponds as their breeding grounds. Sometimes they even choose mudpuddles. I was puzzled about that; why use a mudpuddle that might dry up when you likely have a permanent lake nearby? It turns out the frogs know what they are doing: permanent water bodies tend to have fish, which eat frog eggs. A temporary pond does not, so there is better survival of the young.
Many amphibians also display something called site fidelity: like salmon, they remember the waters of their birth and it is to these waters they return when the times comes to breed.

So right now, the males are at their breeding ponds. You can hear them whooping it up much of the night. Soon the females will follow and egg masses and tadpoles will, too. I look forward to seeing these things, and I will share them with you when I do.


Washington State Department of Fish & Wildlife photos
Amphibians of Washington & Oregon by Leonard, Brown, Jones, McAllister and Storm
Amphibians of Oregon, Washington & Oregon by Corkran & Thoms
A Natural History of Amphibians by Stebbins & Cohen

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