Frog Sex - Part Two

In a previous post ("Frog Sex is a Strange Affair") I discussed the shenanigans that mountain yellow-legged frogs engage in during the breeding season. During a recent trip into the backcountry to survey frog populations, I was reminded of the peculiar sexual behaviors that these frogs display in the fall when frogs are obviously not breeding. With the arrival of shorter days and cooling water temperatures in late-August and September frogs start engaging behaviors normally associated with breeding, including calling and amplexus (above photo was taken two days ago). Seeing frogs engaging in these behaviors in the fall is rather odd since actual breeding is still nine months away. So, what are these frogs doing?

My guess is that they are "practicing" for the breeding season that will occur next spring. This guess is based on the fact that most of the individuals showing these behaviors appear to be small males. Their small size suggests that they may have just attained sexual maturity and therefore have not yet participated in a breeding season. If true, it would be valuable to be able to "practice" calling and amplexing before next spring's breeding season. The individuals I watched a few days ago who were trying to amplex other frogs were certainly clumsy in their efforts. For example, the male shown in the above photo has placed his forearms in front of the forearms of the other frog instead of behind them. I watched this pair for several minutes, and it seemed that the amplexing individual needed all the practice he could get - what a bumbler!

With the return of ice and snow just a month or so away, the frogs may be using this short window in the fall to practice their breeding skills and thereby improve their chances of mating successfully in the spring.

I wonder if other amphibian species show similar pre-breeding behaviors....

Back to The Mountain Yellow-legged Frog Site.

Color Variation in Mountain Yellow-legged Frogs

I've visited a lot of mountain yellow-legged frog populations this summer during my surveys conducted throughout the Sierra Nevada, and have been amazed (as always) at the variation in frog color patterns that I've seen. Sierra Nevada yellow-legged frogs (Rana sierrae) seem to have relatively invariant color patterns, with their characteristic dorsal dark blotches and smaller cream-colored spots on a dark brown background, yellow bellies, and dark speckling under the chin. In some populations the cream-colored spots are absent and the dark blotches are more obvious, but this variation is pretty subtle. In contrast, Southern mountain yellow-legged frogs (Rana muscosa) have wildly variable colorations. The most common dorsal color pattern is one with prominent dark blotches on a tan background, cream-colored bellies, and no spots under the chin. However, I just surveyed an R. muscosa population in which the frogs had an R. sierrae-like dorsal coloration and bellies that were markedly orange and semi-transparent (to the point where internal organs were distinctly visible through the skin!). At another site, the R. muscosa were highly variable within a population, some frogs having cream-colored or silverish vermiculations across their backs and others having the dorsal colorations more typical of R. sierrae.

What is the source of this variation? Does it have a genetic basis, and if so why the occasionally high levels of within-site variation? Given that the ranges of R. muscosa and R. sierrae abut each other along the Monarch Divide and Cirque Crest in the southern Sierra Nevada (R. sierrae north of the divide/crest, R. muscosa to the south), why the color variation despite virtually identical habitat conditions? Clearly these differences in coloration are driven not simply by natural selection for effective camouflage. Instead, I suspect that the color variation we see across the landscape is a consequence of the post-glacial frog colonization histories of these different parts of the Sierra Nevada.

If only the frogs could talk, they'd undoubtedly have quite a tale to tell....

Back to The Mountain Yellow-legged Frog Site.

Aquatic Ecosystem Restoration in Yosemite National Park

During the past century, trout were introduced to thousands of naturally fishless lakes and streams throughout California's Sierra Nevada to create recreational fisheries. These introductions profoundly changed these aquatic ecosystems, often resulting in the elimination of numerous native species, including amphibians and large-bodied invertebrates. Today there is hardly a single watershed in the Sierra Nevada that still remains in its historic fishless condition.

Reversing some of the impacts caused by nonnative trout is a difficult challenge, and one that Sequoia-Kings Canyon National Park began to tackle a couple of years ago with the preparation of a Park-wide aquatic restoration plan. A draft version of this plan is scheduled for release to the public sometime this fall. Now Yosemite National Park is following suit. According to a recent news release, Yosemite will soon be preparing a "High-Elevation Aquatic Resources Management Plan and Environmental Assessment", the purpose of which will be to "guide management actions by the National Park Service to protect Yosemite's diverse high-elevation aquatic ecosystems and to restore natural composition, structure and function to systems that have been disturbed by past or ongoing human activities".

The document will consider the removal of nonnative fish from selected areas of the Park, but will not include removal of fish populations using chemical methods (e.g., rotenone). This will be an interesting process to watch. Public comments are being accepted until July 25. More information is available at www.nps.gov/yose/parkmgmt/aquatic.htm.

Back to The Mountain Yellow-legged Frog Site.

Frog Sex is a Strange Affair

I've been in the field for most of the last month and given that this is frog breeding season I've seen some pretty bizarre things related to this reproductive frenzy. But first, some background on mating in frogs. To mate, a male clasps a female with his powerful front legs that he wraps around the female just behind her front legs. To strengthen that grip, the male has nuptial pads on the thumbs of each hand that make it very difficult for the male to be dislodged when the digits from the front legs are interlocked around the female. This clasping of the female by a male is called "amplexus", and in frogs serves two purposes. First, it allows the male to "guard" the female from other males, thereby ensuring the reproductive success of the amplexing male. Second, it positions the male to allow him to fertilize the eggs as the female releases them.

As the photo above vividly shows, amplexus is not a simple matter of a male gently holding on to a female for several minutes while she looks for a place to lay her eggs. In the photo, a male-female pair is in amplexus and three additional males are doing their darnedest to pull the pair apart. If successful, this would afford one of the other males a chance to amplex the female. These battles can go on for hours, and often result in deep abrasions on the ventrum of amplexed females from the male nuptial pads. In rare cases, during extended battles the female can actually be drowned by all of the competing males.

Wrestling with other males for hours for access to females is only the beginning of what goes on in the frog world during the mating season. On my last backcountry trip, a male I was measuring had new amplexus scars, suggesting that he was recently amplexed by another male and that additional males were battling it out for access to him! My guess is that the original male was so jacked up on hormones that he failed to realize that he had amplexed another male instead of a female, and seeing an amplexing pair the other males piled on. Even more bizarre is the not uncommon observation of males amplexing dead frogs, often frogs that have been dead for weeks or even months.

Just another example of how sex on the brain causes all kinds of bizarre behaviors....

Back to The Mountain Yellow-legged Frog Site.

A Hopeful Find

In this age of declines in global amphibian populations, signs of hope are often hard to come by. On my recent research trip into Yosemite National Park, I made one of those rare hopeful finds. My field crew and I were hiking through a forested low elevation portion of the Park and approached a small stream flowing through a meadow. As I neared the stream, a Sierra Nevada yellow-legged frog jumped from the shore into a nearby pool and disappeared. I'd crossed the stream at this very spot numerous times before and had never seen an amphibian. Had I imagined the frog? A quick search upstream and downstream turned up numerous other adults, subadults, and tadpoles. Had they been here all along? If so, why had we never found them previously? If they hadn't been here all along, where did they come from? In all of our survey work, we'd found no other Sierra Nevada yellow-legged frog populations anywhere in the vicinity. Regardless of the answers to these questions, finding a new frog population is always a rare and welcome discovery. It certainly made my day.

More in a week when I'm back from my next trip into the backcountry.

Back to The Mountain Yellow-legged Frog Site.

Another Day, Another Frog Die-off

I'm out in the field this week and won't be able to post anything substantive as a result. However, this photo gives you an indication of what we found on our first trip - lots of dead frogs killed by chytridiomycosis. I've seen these die-offs many times, but still get a knot in my stomach whenever I stumble across another one. More next week.

Back to The Mountain Yellow-legged Frog Site.

The Increasing Inadequacy of the Endangered Species Act

The Endangered Species Act (ESA) was signed into law in 1973, and is responsible for bringing numerous species back from the brink of extinction. However, 35 years after its signing the ESA is increasingly showing its age. In 1973, most species endangerment was the result of local impacts that were relatively easy to identify and the resolution of which was generally straightforward. For example, Brown Pelicans were driven into decline by increased DDT concentrations in the environment. DDT was banned in the U.S. in 1972, and Brown Pelican populations have rebounded dramatically.

Although the current declines of many species have similarly identifiable causes, an increasing number have complex causes that are challenging the relevance of the ESA. The polar bear and the mountain yellow-legged frog provide two examples of the problems posed by this complexity. The polar bear was recently listed as "threatened" under the ESA due to current and projected declines resulting from global warming. Unfortunately, the ESA was designed to deal with local threats such as habitat destruction, not global threats like climate change, and therefore lacks the tools necessary to deal with challenges as complex and far-reaching as global warming. Even if the ESA contained tools that would aid in reducing carbon dioxide emissions in the U.S., the ESA of course lacks the ability to regulate emissions in other countries.

The mountain yellow-legged frog (Rana muscosa, Rana sierrae) that inhabits the mountains of California presents a related but different challenge for the ESA. These two species have declined by more than 90% during the past century. Once the most common vertebrate across much of their range, they are now one of the rarest. R. muscosa in southern California was listed as "endangered" in 2002 and the listing of R. muscosa and R. sierrae in the Sierra Nevada was found to be "warranted" in 2003. The primary threat to these species through the 1970s was likely the introduction of nonnative trout into historically fishless lakes and streams. That threat is now been compounded by the emergence of the disease, chytridiomycosis, that is decimating mountain yellow-legged frog populations once thought to be secure because of the local absence of introduced fish.

If introduced fish were the only threat to mountain yellow-legged frogs, a recovery plan presumably would identify water bodies from which fish should be removed, and those removals would allow frog populations to rebound. Simple, right? Given the known importance of disease in driving current declines, what tools does the ESA give us to deal with this threat? Very few, if any. The biggest challenge caused by chytridiomycosis is that we don't even know what caused the emergence of this disease. Was it human-caused (e.g., climate change) or was it caused by the natural evolution of a more virulent strain? Disentangling "natural" from human-caused stressors is already extremely challenging, and will become much more so in coming decades as subtle effects of climate change, contaminants, and ecosystem changes become ever-more pervasive.

In the case of chytridiomycosis, even if disease emergence was human-caused, we currently have no means of eradicating the disease. Therefore, many species being pushed toward extinction by chytridiomycosis are doomed, and yet the ESA provides no mechanism for triage. Alternatively, if chytridiomycosis is the result of a natural evolutionary event (which will be very difficult to show conclusively), should we try to counteract the effect of that natural event? If yes, how should we allocate scarce resources between species being driven to extinction by anthropogenic causes versus apparently natural causes?

Our inability to distinguish between natural and human-caused species declines and questions on how to respond to these types of declines will increasingly compromise our ability to halt the impending flood of species extinctions. In our ever more complex world, our only hope of slowing the rate of human-caused extinctions may be an international "Endangered Ecosystems Act". More about that pipe dream some other time.

Back to The Mountain Yellow-legged Frog Site.