Why We Run: A Natural History - Bernd Heinrich (2002)

Chapter 11. Athletic Frogs

In Mark Twain’s famous story about the Calaveras County jumping frog, the frog’s owner, the “infamous Jim Smiley,” was challenged by a stranger who asked, “Well, what’s he good for?” Smiley replied, “easy and careless,” that “he’s good enough for one thing, I should judge—he can outjump any frog in Calaveras county.” Of course, we all know the outcome: the frog didn’t budge, because when Smiley went out into the swamp to find a challenger for the bets that had been placed, the stranger stuffed the notorious jumping frog full of lead quail shot.

Since Mark Twain’s time, frogs have budged a lot. When unencumbered, they’re quite impressive at performing a rapid series of long jumps in a short time, that is, sprinting, using both legs at the same time. The current champion is a bullfrog named Rosie the Ribiter, who competed in the now-annual Jumping Frog Jubilee at the Angel’s Camp fairgrounds in Calaveras County. She (?) set an all-time world record for a three-leap total, which is how the record is judged, of 21 feet 5.74 inches. That’s not quite up to par with Bob Beamon’s famous Olympic record of 29 feet 2.5 inches in the long jump event, but for a frog, covering more than 21 feet is impressive, even if it does take her three jumps.

A frog’s leg muscles are designed for quick, explosive releases of energy. Frogs, just like sprinting cheetahs or humans, burn carbohydrates without the immediate use of oxygen, that is, anaerobically. Anaerobic performance never goes unpunished—in seconds the critter gets laced full of lactic acid and its muscles tie up. Undoubtedly, if Rosie had continued with more than three successive mighty jumps, there is good scientific evidence that each successive jump would have become very much shorter.

People do some pretty odd things to satisfy their curiosity. A colleague of mine at the University of California used to chase lizards on a miniature racetrack until they could go no farther. Then he grabbed them and ground them up in a blender and measured how much lactic acid they had generated. By assaying lizards after varying sprint durations, and at varying postsprint intervals, he was able to determine that it takes some lizards an hour or more to get rid of their lactic acid load. Frogs operate under the same constraints, and air-breathing divers who stay underwater without access to oxygen suffer the same lactic acid accumulations. There is no grand discovery here—just a reaffirmation of what we all know from personal experience. We can’t sprint or hold our breath if we want to go far. Sprinting in the middle of a race is not to be recommended. The sprint must come at the end, because you can pay off the oxygen debt when you’re done.

Some frogs, unlike Rosie, are ultraendurance athletes par excellence, and they annually stage their own remarkable contests. These contests are performed strictly by males, and the contested skill is in aerobic shouting, not anaerobic jumping. The prize is copulation with female spectators. The longer a frog can sustain energetically demanding aerobic shouting, the better his chances of getting a prize. Naturally, each male puts out his best effort. The contests are not as open to the general public as the bullfrog jumping contest at the Angel’s Camp fairgrounds in California, because they involve small, highly camouflaged individuals that gather only in or near isolated swamps and that usually don’t reach their full stride until well after dark. The contests also have been staged at the University of Connecticut, where in the labs of Theodore L. Taigen and Kentwood D. Wells, details were divulged that would otherwise have remained secret. One of the first findings these researchers made was that the chorusing of male tree frogs (Hyla versicolor) is achieved at about 60 percent of their imageO2 max, which is close to that of ultrarunners on long races. The frog chorusers maintain their extremely high work outputs for hours each night, apparently until exhaustion sets in near morning. Not surprising, not all contestants last the whole night.

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Bullfrog

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Male tree frog calling

In frogs of some species, there are thousands of competitors per contest, but individual male frogs usually pursue their strenuous efforts out of visual contact with their competitors. However, Taigen and Wells found that as soon as a frog sees a female spectator, he increases his vocal output to near 100 percent aerobic capacity. He can’t keep it up for long, though.

Since females are attracted to and approach the most energetic callers, for a strictly short-term strategy a male frog who sees a female should call as vigorously as he can. But in the dark, he never knows whether or not a female is near. Therefore, when calling in the dark, he must be hopeful and call for as long and vigorously as possible. Those frogs with the highest aerobic capacities, who can keep up the most vigorous calling for the longest time, are the ones to leave the most descendants. Frog behavior and the anatomy and physiology that is the basis of the behavior are the results of millions of years of evolution, and the optimum results in the conflicting trade-off between maximal energy output and endurance are available for inspection.

The structure and physiology of male frogs is different from that of females, who do not enter the energy-demanding chorusing contests. Both males and females are of equal length and have the same leg muscle mass, but males on average weigh 1.25 grams while females weigh only 1.05 grams. This difference in mass is mainly due to highly hypertrophied body-trunk muscles in the males (0.18 grams for males versus 0.03 grams for females). The males’ trunk muscles are so well developed because they are used to drive air over the vocal cords to produce what are very loud sounds for such tiny frogs. Without their muscle hypertrophy, the males presumably would be able only to whisper rather than shout. The females are silent. Similar sexual dimorphism in behavior and associated muscle anatomy and physiology exist in katydids and crickets, which pursue a similar mating strategy.

The frogs’ trunk muscles, in contrast to the leg muscles, are uniquely adapted for aerobic metabolism. These muscles, like those of geese, antelopes, and human distance runners, are packed with mitochondria, the small power packs in cells within which all aerobic metabolism occurs. In the frogs, the mitochondria contain citrate synthase, a key enzyme for aerobic metabolism, in higher levels than is found in any cold-blooded vertebrate so far examined. The mitochondria in the trunk muscles of male as opposed to female frogs also contain almost twelve times the activity level of the key enzymes phosphofrustokinase and ß-hydroxyacetyl-CoA dehydrogenase for fatty acid metabolism. The conclusion can be drawn that, just as for human endurance running, aerobic fatty acid oxidation plays a key role in the endurance energetics of the frogs’ chorusing. As in all other animals, physiology is closely tied in with behavior, and the frogs’ behavior has evolved to maintain the highest possible rate of activity for very long durations. And as in ultramarathon running, that involves pacing.

In the laboratory, calling rates offer a direct measure of aerobic energy expenditure. In the field, one can estimate energy expenditure simply by measuring calling rates, much as one can deduce human energy expenditure on the track from running speed, after one has measured it on the laboratory treadmill. Comparisons of estimated aerobic output in the field with maximum rates observed in the laboratory make it possible to calculate the percentage of maximum effort that the frogs put out at any one time. This averages, as mentioned already, close to 60 percent, but the frogs start out their shouting matches at a much more leisurely pace.

Male tree frogs, like many ultramarathoners, who also pace themselves by starting out slowly, begin their evening chorusing contest at about six hundred calls per hour, then increase their calling pace (depending on the individual) gradually during the next two hours. They then gradually slow down during the final, predawn hour or so, when many individuals begin to drop out of the chorus. Taigen and Wells ground up whole frogs to measure body lactic acid accumulation and showed that even though the frogs’ initial calling pace is slow, the lactic acid concentrations found in the first half hour are higher than those found later, at peak calling frequency! These results strongly suggest that these animals need a long period of warm-up, when glycogen is used as a fuel, before a switch-over to fat metabolism occurs. This is also the case for locusts during flight, as well as for human distance runners. The lesson to be learned from frogs is, start slow and work into a pace slower than the final pace.

The frogs’ pacing involves not only starting slowly, but also varying the lengths of repetitive work bouts (each call) and rest periods (intercall intervals). Many ultramarathoners who race for 24 hours or longer ask themselves whether it is better to walk for 1 mile out of every 10 while maintaining the same overall pace, or to walk a tenth of every mile. The data from the frogs’ calling behavior may provide a clue.

Male-male competition brings out profound changes in calling behavior, as does the presence of females. The frogs give either long calls or short calls. Females prefer males with the longest calls. Males chorusing in crowds, or those who are duped with playback from a tape recorder to simulate nearby calling males, give calls about twice as long as isolated males, who give primarily short calls.

Long calls are energetically more costly to make than short ones, but as the individual call lengths increase, the frogs adjust by reducing the call rate so as to keep the energy expenditure about the same. These results are not what one would predict from evolutionary logic, because if it costs no more to give the more attractive long calls than the less preferred short calls, then why make any short calls at all? Is there a penalty for making the long calls? There is, indeed. The penalty is, quite unexpectedly, a reduction in stamina. Even while calling at given rates of energy expenditure, males making long calls had dramatically lower endurance. For example, those frogs giving calls with durations averaging 350 milliseconds called for 3.75 hours per night on average, while those whose call durations averaged 500 milliseconds called for only 2.25 hours.

To date, nobody knows why making longer rather than shorter calls, at a given rate of energy expenditure, results in a dramatic reduction in the frogs’ stamina. Can it be related to glycogen depletion? From studies in human runners, we know that although the fuel for long runs is primarily fat, we still “hit the wall” when muscle glycogen (a carbohydrate) has been depleted, even though fats may still be plentifully available. One hypothesis is that carbohydrates and possibly proteins help replenish the constituents of the central biochemical cycle, the so-called Krebs cycle, so that it can continue to burn fat. I suspect that making the long rather than the short calls results in a more rapid depletion of muscle glycogen because each individual call, though less than one second in duration, is a very intense energetic effort that favors the utilization of carbohydrate over fat. Possibly, slightly more muscle glycogen is used during the production of long as opposed to short calls. Glycogen is wholly or partly restored in the short intercall interval. By prolonging the call duration, there might be a shift in the equilibrium that results in faster depletion of critical glycogen stores, with the cumulative effect over many thousands of calls being that glycogen is drained sooner. As a result, the duration of exercise is reduced, even though the fat reserves are still abundant.

I wondered if the same principle might apply to human runners taking short rather than long steps. Most ultrarunners take short steps, and I would do so as well; long steps cover ground faster but tire one out more quickly.

It is still too soon to come to conclusions, but if I were planning strategy to set a record in a six-day race during which it would be necessary to alternate walking with running, I would shorten my stride and make my run/walk intervals very short rather than taking long runs and long rests. Obviously, I have no idea what constitutes “short” or “long” intervals. That could only be determined empirically, because we do not have the data on humans, much less specific individuals, to come to conclusions. Nevertheless, ultramarathoner Kevin Setnes told me of adapting a precise walking/running routine into his 1993 Olander Park 24-hour-run championship race, in which he extended his personal record by 35 miles and set an American road record of 160.4 miles. When I asked about his strategy, he told me his schedule of alternating walking with running was “the single most important factor in achieving that total [mileage].” He admits that he got his racing idea from a preliminary article I had written about frogs for Ultrarunning magazine, after I had read Taigen and Wells’s original work on the tree frog. Maybe I should take the same advice. But I can’t.

In Chicago, given the time in which I want to finish my anticipated 100-kilometer run, I will not be able to stop for even one second. The 100-kilometer race is like the 100 meter is to sprints and the 10 kilometer is to middle-distance running. It’s the premier international ultrarunning standard. Nevertheless, it is still far too short a distance to permit any stopping if you are planning to win or set a record. It is a fast-paced race every step of the way. My pacing in that race will have to refer to overall running speed and possibly stride length, not to schedules of stopping and starting. Such running may not seem natural for humans now, given our very recent lifestyles. What most of us do now may not be a good indicator of what we had to do millions and at least many hundreds of thousands of years ago and still can do, given the right conditions. We can’t be sure what that was, but as in other animals, our bodies still yield clues of what shaped us. They show what is still possible for us now, given the appropriate conditions.