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A Breeding Congress


David Scott

Note: An edited version of this article appeared in Natural History magazine (October 1998).

Marbled salamander

Had I given it much thought, I never would have expected to actually hear them -- such sounds are from the realm of cartoons and special effects artists. But there I was, standing in a quiet rain on a cool October night, and I could actually hear the salamanders coming. Over leaves. Around logs. Tumbling off clumps of sedges. A few hundred had even crossed a nearby highway. Sometimes in the course of conducting field research on amphibians my hearing has played tricks -- echoes of calling frog trills reverberate in the mind hours later. Was this my imagination? No. The marbled salamander herd thundered in as only salamanders can thunder (though not at lightning speed), arriving by the thousands at a small, shrubby wetland called Ginger's Bay on the Savannah River Site.

But first, what should we call this approaching horde? "Herd" is a bit misleading, with its connotation of trampling and mayhem. I prefer the term coined by a biologist decades ago: a breeding congress. Forget the trampling, but keep the mayhem. Instead of lobbyist-filled hallways, picture low-lying depressions that fill with water part of the year. Envision marbled salamanders rather than marble floors. "Breaker breaker...I'm standing on the salamander highway and I think we got us a congress." And indeed there was.

Marbled salamanders in pitfall trap

 

Ginger's Bay

Most of us learn about amphibians in elementary school classrooms, and, if we're lucky enough, during childhood romps to explore the neighborhood pond. Nelson Hairston Sr., a researcher at the University of North Carolina, has noted that the general facts about amphibians we learned as kids were a bit misleading. The amphibian life cycle -- eggs laid in water, aquatic larvae, a period of transition, and adult life on land -- that applies generally to frogs, toads, and salamanders is not so general after all. Only one-third or so of the world's salamander species follow this blueprint. The species that I study, however, are relatively typical with only minor exceptions. Typical perhaps, but never dull. Fascinating enough that most of my rainy October nights during the last 12 years have been spent watching one species, the marbled salamander, as individuals move from woodlands to wetlands on annual breeding migrations. Their congress is in session.

Charles Darwin acquainted us with the phrases "struggle for existence" and "survival of the fittest." Individuals have but two concerns, survival and reproduction, the two components of an individual's "fitness." If an animal survives to adulthood, the urge to reproduce is strong. Perhaps The Boss was born to run, but these salamanders were born to breed.

Marbled salamanders inhabit much of the eastern United States, but the Coastal Plain of the Southeast -- home to thousands of Carolina bays -- is a hot spot. How the adults find the breeding sites is a mystery (to us, not them). At the time of the migrations, the "wetlands" oftentimes have no water; pond filling comes later with the onset of winter rains and cooler temperatures. So these pond-breeding salamanders are not cueing in on the presence of water. There are no calling males as in frogs and toads, so acoustic cues are not involved. Some scientists suspect chemical signals are important because many individuals return to the same pond they were born in -- perhaps they act as salmon do. Others suspect the Earth's magnetic field provides a cranial roadmap. I don't have a clue how they do it. Just call it radar love.

Aside from entering a dry pond for breeding, marbled salamanders also differ from their close relatives (mole, tiger, and spotted salamanders) in other aspects. Perhaps because they don't use water as a cue, the annual timing of marbled salamander migrations is relatively predictable from year to year. Forget an explanation using scientific analyses and jargon. A few years ago two British filmmakers asked to come film the migrations of marbled salamanders. With our assurance of salamander regularity, they were able to buy the cheap airfares several months in advance. We suggested that they arrive on 2 October -- they did, about 6 p.m. By 8 p.m. that night the first pulse of several thousand male salamanders began to enter Ginger's Bay, one of several wetland breeding sites we study. There are no other species that I'd venture such a guarantee for, even with someone else's money at stake.

"Spermatophore play"

 

Marbled salamanders mating

I should mention at this point that Lynne Houck, Oregon State University ecologist, is my hero. Lynne, past president of the prestigious Society for the Study of Reptiles and Amphibians, successfully published the phrase "spermatophore play" in a respected scientific journal. Spermatophores, for the uninitiated, are the packets of sperm laid down on a jelly-like base by male salamanders at the peak of sexual stimulation. A female salamander at the appropriate stage of receptivity will straddle a packet, squat, and incorporate the sperm mass into her cloaca. In many species, the courtship activities between male and female are elaborate, with behavioral signals and mating dances lasting for an hour before both individuals are "ready." Such is not the case with marbled salamanders. The frenzied activity of the congress results in spermatophores being deposited everywhere; Lynne calls it the "minefield of spermatophores." Females hardly need to squat, just pick up a spermatophore on the run. I worry that our lab's safety program will declare Ginger's Bay off-limits due to the spermatophore slipping hazard. To me such indiscriminate mating is a bit curious for a salamander species, although not unprecedented in the amphibian world. After all, a male toad "in the mood" will mate with your thumb.

On the right night these marbled salamander males are quite in the mood. In lab analyses of blood samples we learned that these 4-5 inch long males have testosterone levels far exceeding mine. Bummer. Males "dance" (their overtures have been described as a waltz, perhaps with a touch of slam dancing thrown in) with females, and can deposit not just one spermatophore per hour, but more than 10 in 30 minutes! Males court other males, and still deposit spermatophores galore. Chin rubbing with this animal...tail whipping with that one -- gender doesn't seem to matter. What gives? They must be doing something right. Ambystoma opacum has been around far more than a million years. Last year at Ginger's Bay (which is about half the size of a football field), we caught almost 12,000 adults coming in to breed. Mathematical models estimate the local population in the hardwood forest surrounding Ginger's Bay at close to 40,000 animals. Salamanders rule.

As you might expect, all the frenzy of searching for mates and U-whipping tails (a courtship behavior exhibited be males) during the congress is energetically expensive. When male and female marbled salamanders enter the breeding site they are literally fat and ready. Many have close to 15% body fat, which appears to be used to provision the clutch of 30-150 eggs in females, and to fuel the searching and courtship activities of males. These body fats also provide maintenance for both sexes during the 6-8 weeks they may stay at the wetland site without feeding. A high proportion of animals apparently don't survive; in some years the mortality during the breeding season is close to 50%. Reasons for this high mortality are not obvious, as skin toxins make adult marbled salamanders distasteful to many predators. For some it's old age--at 8, 9, or 10 years of age they've gone about as far as they can go. Adults that do survive there breeding season generally depart the wetland as lean, hungry animals. Because future reproduction is contingent on accumulating adequate fat stores, it may take some animals 2 or more years to be ready to breed again.

Marbled salamander larva

You know, mama always said "Life is like a box..." No wait... my mama always said "Drink your milk. And eat your cereal." Because you are what you eat. Remember, we're talking about salamanders, so for their young (larvae) the food of choice is zooplankton. And there's not always enough to go around in the pond habitat, either. Numerous experiments have demonstrated that larval salamanders compete with each other for food. Jim Petranka at the University of North Carolina in Asheville manipulated numbers of larvae in entire ponds (have you ever tried to catch several thousand slippery baby salamanders the size of shoelace tips?) and found that more larvae means smaller larvae. A higher density of babies in a pond results in slower growth, a longer development time before they emerge from the pond (the larval period), a smaller size when they do emerge, and lower survival. I was amazed that salamanders that left their natal pond (metamorphosed) at a small body size were still relatively small five or six years later when they returned as prospective parents. Their larval environment had set a course for the rest of their 10-year lives. Sound familiar? Oh man, my mom was right again.

I decided years ago that my personal goal in ecological research was to truly understand a single species well enough to make meaningful predictions. JUST ONE SPECIES! We have studied marbled salamanders from eggs to adulthood and beyond. In experiments we have manipulated females on their nests, numbers of larvae, aquatic predators such as dragonflies and spiders, prey levels, timing of hatching, water levels, and numbers of coexisting amphibian species. We have studied salamander nest sites, salamander genetics, salamander metabolism, and salamander fat. At Rainbow Bay, another ephemeral-wetland study site, we have collected data for 27 years on the natural population fluctuations of the 25 amphibian species that breed at the bay. It is the longest running observational study of an entire amphibian community in the world. At a time when many amphibian species are thought to be in serious trouble, the Rainbow Bay study provides a model for long-term ecological studies. Using the observations of natural populations in conjunction with our manipulative experiments, we have developed insights into the driving forces behind the commonly observed ups and downs of amphibian populations.

Female marbled salamander on eggs

So you would think I would know them by now. On October nights I dream of marbled salamanders. "Be the salamander, Grasshopper." So why do I owe Joe Pechmann, my ex-office mate, more pitchers of beer due to errant predictions than I could repay in ...? Well, let's not go there.

Some theoretical physicists and philosophers speak of "choice" and "consciousness" at the sub-atomic level. Pour a few gazillion particles together in an organismal bowl, stir thousands of individuals in a population, blend numerous species in a community, and mix with an incredibly fickle environment, and you have a recipe for chaos. Predictable chaos? What is that, anyway? And I hope to understand an entire species?

It is encouraging to have learned so much. It is discouraging to still know so little, and disheartening that time and money are in short supply. I lived in a dream world a few years back -- my job was to study a fascinating creature. I was paid (not much of course, we are talking whole-organism biology here) to continue my interests developed in childhood. I was paid to discover basic answers to basic questions about ecology that today's kids can read about in encyclopedias. Folks today (my Congress) attach too little importance to those basic questions of ecology. If a critter is contaminated with mercury or arsenic, or exposed to radioactivity, or eliminated from vast stretches of habitat so that we have but a few left, then it may get some attention, especially if it is furry or pretty. As well it should. But what about the knowledge necessary to maintain "healthy" habitats? What about the value of understanding animal and plant populations not yet suffering the impacts of human society? What about having the data to avoid the necessity of costly fix-it plans in the first place? Wouldn't that be great? Where's a Keystone beer when you need one?

Many questions remain, even for a species not in apparent decline. How do salamanders find that wetland, anyway? What happens, as is sadly still often the case, when their wetland is destroyed? When they leave the aquatic environment, how far do they go? How many never come back, but instead search for a new wetland? We know for this species that a forested area around the wetland is essential, but how big must this "buffer" strip be? How "connected" must populations be to each other if all are to persist? Under natural conditions do larvae eat each other? Do males arrive at the site first for some evolutionary reason, or are fat egg-laden females just slower? Do most males sire offspring, or are there stud males? Should salamander licking be banned in South Carolina? If marbled salamanders had a Web site, would it be marble.uwhip.org? And would it be banned in South Carolina?

I hope I'll be able to help others answer some of these questions. But in the meantime, for the foreseeable future (pay or no pay), you'll know where to find me on a rainy October night. Watching hormonally charged salamanders marching into Ginger's Bay (yeah, I know -- "Get a life"). Once across the highway, they reach the safety of the wetland, and the tail-slapping mayhem begins. Finally, a congress I can relate to.

 
 
 
 
 
 
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