An Unnatural Quiet

This summer I returned again to one of the Sierra's most remote lake basins to resurvey the amphibian populations there. This basin holds a special place in my heart due to its spectacular beauty, isolation, and at one time, an incredible Sierra Nevada yellow-legged frog population. The first time I visited this basin was in 1997 when my assistants and I conducted our initial surveys of its frogs, fish, and invertebrates. Back then, the sheer number of frogs was overwhelming. 

I remember counting 10,000 tadpoles in a single lake. The frantic swimming of these tadpole masses at my approach turned the tranquil water surface to a muddy froth. Adult frogs seemed to occupy every possible bit of water; nearly every lake, pond, stream, and puddle was full of them. As I walked along one lake shore, a frog dove off of a damp ledge ten feet above my head and with legs splayed wide splashed into the water at my feet. The abundant bears in this basin seemed just as fond of the frogs as I was. In several spots where tadpoles aggregated by the thousands, I saw where bears had waded out into the water to catch them. One evening I watched a Clark's Nutcracker land at the waters edge and spear a tadpole. With the tadpole in its beak it flew to a nearby snag, snipped off the tadpole's tail, and fed the body to a begging fledgling. Transfixed, I sat down and watched the show repeat itself several more times. When the adult and fledgling had flown away I walked to the snag and saw that the branch the fledgling had been perched on was covered with 50 or more tadpole tails, some dry and shriveled, some still wet. What an incredible web of life these frogs were a part of!

Back then the world was unaware of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") and its devastating effects on amphibians around the globe. I took it for granted that given the paucity of trout in this basin the frogs would always be there, for the bears, for me. A few years later Bd arrived in the basin, causing the collapse of one frog population after another. Within five years the frog numbers had decreased by 95% but I still held out the hope that maybe a few would survive to repopulate the lakes. 

Our resurveys this summer showed that even that modest hope was too optimistic. Most water bodies were now entirely frogless. A few contained only a single adult frog. We found some tadpoles, but too few to give us much hope that the population would still be there the next time we did our resurveys. 

Sitting on a lakeside boulder eating my lunch in the summer sun, I couldn't help notice how quiet the basin had become. No more frothing masses of tadpoles. No frogs jumping into the water by the 10s and 100s in front of me as I walked along the shoreline. Bear sign, once so common, was far less in evidence, although I did see one set of bear tracks that went into the lake where a few tadpoles still basked in the warm shallows, but the paucity of animals would hardly have made a snack much less a meal. No Clark's Nutcrackers eating tadpoles. The mayflies, caddisflies, and beetles were still there, but the once-dominant frogs and the web of life they once supported were gone.

When I first happened upon this place it was defined by the frogs, frogs as predators, frogs as prey, frogs as inhabitants. Now it was different. In the span of less than a decade, a shangri la had become a place of ghosts.


Back to The Mountain Yellow-legged Frog Site.

Effects of Frog Declines on Bears in the Sierra Nevada

A few years ago, Dave Graber (Chief Scientist, Pacific Southwest Region of the National Park Service) asked me a question that I've never been able to get out of my head. Dave, who studied black bears in the Sierra Nevada for many years, asked me if there was a connection between the dramatic decline of mountain yellow-legged frogs in the 1970s and the coincident increase in the number of bears frequenting campgrounds and mountain towns. As an example, he mentioned that black bears were essentially absent from the eastern Sierra until the 1970s, and today the town of Mammoth Lakes alone is home to dozens of dumpster-diving ursines. I'm afraid my answer was little more than "we know that bears feed on frogs so the potential exists for frog declines to affect bears". Unfortunately we'll never know the answer to Dave's question because mountain yellow-legged frogs no longer play the important ecological role they once did as the most abundant vertebrate in the Sierra Nevada.

But what do we know about this frog-bear interaction? Over the years we've made numerous observations of bears feeding on frogs. Those I made in a fishless basin in Kings Canyon National Park in 1997 still haunt me today. This unnamed basin has never been stocked with fish and in 1997 virtually every lake was full of mountain yellow-legged frogs and tadpoles. Frog counts from that 1997 visit were 11,330 frogs and 107,750 tadpoles. During that time it was rare to walk around a lake without stumbling across numerous bear shits. In a shallow cove of one lake, tadpoles aggregated by the thousands to take advantage of the warm water found there every afternoon. Bear tracks ringed the cove shoreline and frequently veered into the lake where tadpoles were particularly dense. The only way to make sense of these observations was if bears were feeding on the abundant tadpoles. The frog population in this basin crashed about seven years ago due to chytridiomycosis, and during recent visits bear sign seems much scarcer than it was 12 years ago.

I've witnessed something similar in Humphreys Basin following fish removal and subsequent frog recovery. In 1997, Marmot Lake contained lots of small trout and a few mountain yellow-legged frogs. Following fish removal, the frog population exploded and more than 3,000 frogs exist at the lake today. As the numbers of frogs increased so did my observations of bear sign which had been nonexistent during all of my previous years spent at this lake. Marmot Lake doesn't seem like typical bear habitat given its very high elevation but it is now not uncommon to see bear tracks all around the lake shoreline. I'm guessing that the motivation for such shoreline ambulations are the little frog snacks found there.

Given the historic abundance of mountain yellow-legged frogs in the Sierra Nevada I suspect that frogs were often included in bear diets. And maybe, just maybe, the decline of frogs has driven bears into campgrounds and towns in search of alternate food sources. Maybe someday those backcountry lakes will once again contain an abundance of frogs sufficient to interest a hungry bear.

So, as I've said many times before, although the frog restoration issue is often couched simplistically as "fish versus frogs", the issue is in fact much broader.

Back to The Mountain Yellow-legged Frog Site.

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.

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.

Garter Snakes - The Anaconda of the Frog World

In last week's post, I wrote about the relative lack of food for mountain yellow-legged frogs at high elevation sites, and some of the tactics they use to fill their bellies despite this scarcity. Despite this disadvantage of high elevation sites compared to lower elevation sites where food is more plentiful, the high elevation habitats also have a distinct advantage - a virtual lack of predators, especially garter snakes.

Three species of garter snakes inhabit the range of the mountain yellow-legged frog, each in a very specific habitat niche and all with a strong affinity for water and for amphibian prey. The Valley garter snake (Thamnophis sirtalis fitchi) is found at elevations up to about 5,000'. The Sierra garter snake (Thamnophis couchi) is found at elevations of 5,000' to 8,000', and the mountain garter snake (Thamnophis elegans elegans - photo above) is found at elevations of 8,000' to 11,000'. These snakes congregate at water bodies containing amphibians, and mountain yellow-legged frogs are a major component of their diet.

Habitats above 11,000' are generally free of snakes, and it is in these highest elevation lakes that the densities of mountain yellow-legged frogs reach their peak. Densities of three frogs per foot of lake shoreline are not uncommon, and at such sites tadpoles by the thousands often aggregate in shallow, warm waters. These densities are a remarkable sight to behold when all other life forms are so scarce, and are in large part a consequence of a lack of predators. Not only are garter snakes absent, but so too are invertebrate predators such as large beetles that prey on tadpoles. Bears and coyotes might come by on occasion and eat a few frogs but their visits are relatively rare. And so, frogs have these highest elevation alpine lakes largely to themselves.

Mountain yellow-legged frogs have evolved some interesting behaviors to reduce the risk of predation. For example, tadpoles reduce their swimming activity when confronted by the smell of a garter snake. In other amphibian species, the presence of predator scents causes tadpoles to develop a more streamlined body shape and a more powerful tail. It remains to be seen whether mountain yellow-legged frogs show similar developmental responses to predators. Given that mountain yellow-legged frogs don't move much during their lives, I've often wondered whether frogs from low-elevation habitats have evolved anti-predator behaviors that are absent in frogs from the highest elevations.

Given that mountain yellow-legged frogs have coexisted with predators for millenia, you can bet that they have a host of tricks that we have not yet discovered to reduce their chances of become someone else's meal.

Back to The Mountain Yellow-legged Frog Site.

Frogs at 12,000 feet?

In the back of our minds, I think most of us have a picture of what frog habitat looks like, perhaps a warm pond filled with lily pads and logs. Given this preconception, it comes as quite a surprise to see mountain yellow-legged frogs basking by the thousands at lakes in the Sierra Nevada that lie well above timberline, some at elevations as high as 12,000' (3,660 m). How do they survive in these cold, stark conditions?

The fact that frogs are ectothermic, as described in my 4/25/08 post, plays an important role in allowing mountain yellow-legged frogs to flourish at high elevations. Given the cold water and air temperatures even in mid-summer, food requirements are relatively modest, allowing frogs to add the necessary fat reserves to get them through the long winter despite low densities of prey.

Despite the general paucity of food in this harsh environment, food is actually plentiful during short periods and mountain yellow-legged frogs take full advantage of this ephemeral abundance. In early summer (a few weeks after ice-out), mayflies and chironomids ("midges") transform from aquatic larvae into terrestrial winged adults and provide a brief dietary bounty for the frogs. The mayfly, Ameletus edmundsi, thrives in cold, high elevation lakes, and its emergence style makes it particularly vulnerable to frogs. Unlike many mayfly species that emerge well offshore, A. edmundsi crawls up onto a shoreline rock and emerges there. In lakes harboring mountain yellow-legged frog populations, I've watched frogs eyeing A. edmundsi larvae from the second they crawl out of the water and then engulfing the winged adult just as it completes metamorphosis. Midges provide a similar feeding opportunity for frogs. On windy days, chironomid adults aggregate in mating swarms on the lee side of boulders. Within minutes of these aggregations forming, mountain yellow-legged frogs will arrive and spend hours picking off chironomid adults when they land on the rock to rest.

During late summer, most aquatic insect emergences are over and mountain yellow-legged frogs depend more heavily on terrestrial prey. Anything crawling along the lake shore that fits in their mouths is fair game. I've watched grasshoppers get blown into the water, swim back to shore, and be eaten by a frog as soon as they crawl up onto a shoreline rock. Although mountain yellow-legged frogs generally spend their time right at the lake shore, some take advantage of another food source away from water: ants. Frogs will position themselves along ant trails and pick off the hapless ants as they pass by. The metamorphosis of Pacific treefrogs (Pseudacris regilla) from tadpole to froglet in late summer provides another food source. The froglets don't have the well-developed hopping skills of adults and make easy prey for the much larger mountain yellow-legged frog.

Despite the various tricks that mountain yellow-legged frogs employ to fill their bellies, food is scarce in this high elevation environment, especially when there are thousands of frogs at a lake all competing for the same limited food resource. And yet, these high elevation lakes offer the frogs an important advantage over lakes at low elevations: a lack of snake predators. More about that next week....

Back to The Mountain Yellow-legged Frog Site.

For The Frogs, It Is Still Winter

Here in the eastern Sierra Nevada, signs of spring are everywhere. After a winter during which the only common birds were Mountain Chickadees and Dark-eyed Juncos, a host of birds not seen for months are now in every bush and tree. The aspen and cottonwood trees are clothed in catkins, and the trunks of Jeffrey pines smell strongly of vanilla-scented sap. With all of these reminders that summer is on its way, I've been busy preparing for another field season studying mountain yellow-legged frogs up in the High Sierra. On a warm day down here at 7,000' it's easy to forget that for the mountain yellow-legged frogs in the high country it is still winter and will be for another month.

In the fall when the air turns cold and insect prey becomes scarce, mountain yellow-legged frogs retreat into the deep waters of lakes, coming to shore to sun only on the warmest of days. By late October, the lakes are skimmed with ice and by November or December the landscape is clothed in a thick blanket of snow. For the next seven months, the frogs live underwater in a world of near-freezing temperatures and complete darkness, breathing solely through their skin. Like many amphibians, mountain yellow-legged frogs capture prey primarily with their sticky tongues, but this method doesn't work underwater. So, for these seven months the frogs probably don't feed at all, living solely off of the fat reserves they accrued during the previous summer. Most mammals, including humans, die from starvation after a few weeks without feeding, and yet the mountain yellow-legged frog can survive without food for seven or more months! How do they do it?

This survival ability is a direct consequence of amphibians being ectothermic (often called "cold-blooded"), meaning that body temperature is controlled by factors outside of their bodies (e.g., air or water temperature). Body temperature of ectothermic animals controls metabolism, with body temperature and metabolism being positively correlated. During winter, the body temperature of mountain yellow-legged frogs is near the freezing point and their metabolism is therefore extremely low. As a consequence, their need for food is also greatly reduced. The mountain yellow-legged frog pushes this ability to survive without food to an extreme seen in few other amphibians. During winters with unusually heavy snowfall, lakes can thaw as late as August and then freeze over again in October. Under these conditions, mountain yellow-legged frogs can be without food for ten months and have only August-October to replenish their fat reserves for the next winter.

When spring finally comes to the high-elevation haunts of the mountain yellow-legged frog, lakes thaw and frogs crawl to shore in search of warmth, mates, and food. What has always amazed me when I've been at a lake during this time is that the frogs aren't particularly skinny. They haven't eaten anything for months and yet they are only slightly less plump than they were the previous fall.

I wish I could go months without eating. That ability would certainly make my backpack a lot lighter during the summer when I'm doing my frog research in the Sierra Nevada backcountry.

Back to The Mountain Yellow-legged Frog Site.