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'Field Notes': The Tale Of Montana's Strangest Frog

Ascaphus montanus tadpole.
Ascaphus montanus tadpole.

Several decades back while working as a biologist in Oregon, I was picking rocks off the bottom of a rushing stream. While investigating the underlying aquatic insects, I encountered an odd animal. It was what appeared to be a tadpole stuck to the bottom of the rock! Assuming all frogs and tadpoles occur in swamps, not in high elevation rushing streams, I wondered what it was doing there.

I picked it off the rock and it immediately attached itself to my hand. I frantically scraped it off assuming that I’d somehow mistaken a tadpole for leech! While it had the sucking mouth of a leech, it certainly looked like a tadpole. But if it was a tadpole, what was it doing in a rushing stream?

A year or two later, I was sitting at my desk catching up on research and literature. In my pile of literature was a reference to Ascaphus montanus, or the tailed frog, which according to the article, live in rushing streams. As I read further, it mentioned that the tadpoles are adapted with a sucking mouth that allows them to attach to the bottom side of rocks in fast moving water. There it was, end of the mystery!

Since the eye-opening article, I’ve come to appreciate the tailed frogs as our strangest frog! Tailed frogs are like no other.

Most frogs and toads have a very short larval or tadpole phase. Because frog eggs and tadpoles are a delicacy for many predators, including fish, birds, snakes, and even predatory insects, the tadpoles of most frogs mature very rapidly. For most frogs, the time from egg to adult is a few weeks or months. Not so in the tailed frog.

Because tailed frog tadpoles live under rocks, they're unavailable to most predators. Consequently they take their sweet time in metamorphosing into adult frogs. According to Bryce Maxell, Program Coordinator of the Montana Natural Heritage Program with a PhD in Fish & Wildlife Biology, tailed frogs stay in the larval stage for an average of five years in the northern Rockies.

We also think of frogs as having to lay thousands of eggs so that a few will make it to maturity. Again, the tailed frog is an exception. Because the juveniles and adults live in a habitat protected from predators, the tailed frogs reproduce by laying only about 25 eggs per year — a fraction of what other frogs lay.

Adult Ascaphus montanus, tailed frog.
Credit Gary Nafis (CC-BY-NC-ND-3)
Adult Ascaphus montanus, tailed frog.

Perhaps the oddest thing about the tailed frog it its tail. Yes, the adult male does have a permanent tail. The tail functions as a reproductive organ allowing internal fertilization. This improves the chances for successful breeding in the tailed frog's rushing stream environment.

Incidentally, tailed frogs aren’t totally free from predation. During research on American Dippers in the Bitterroot, former UM student Sophie Osborne noted that birds occasionally prey on tailed frog tadpoles. This is an example of a highly specialized bird preying on a very specialized amphibian!

Because tailed frogs require cold, clean water, their presence or absence may be a good barometer for riparian zone health. And here's the good news: stream surveys suggest that tailed frog populations, determined by the presence of tailed frog tadpoles, may be fairly stable in western Montana. But as climate change continues to affect our snowpack levels and stream temperatures, this may not continue to be the case for these highly adapted and interesting amphibians.

"Fieldnotes," airs Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or listen any time via podcast.