Before he steps from the airboat, walt rhodes leans over and pokes around in the muddy brown water with a chewed-up boat oar. "Just making sure mama gator's not here," he says, squinting across the marsh against the South Carolina sun. "She could be lying out here, and I don't want to step on her." He waves off a cloud of mosquitoes and swings his big rubber boot over the side of the boat. A few feet away, fire ants swarm an alligator's nest cordgrass heaped three feet high and baking in the late-August heat. The warm center is filled with eggs.
|A 1-week-old gator, 10 inches long, could be a meal for a heron, a snake, or a big fish. But in one year it will triple in size and easily escape most predators.|
Photo by Grant Delin
Warily, Rhodes wades calf-deep in the muddy water and sticks his hands into the nest, releasing a funky smell as he digs into the heap and uncovers eggs a few inches down. He yelps as the venomous fire ants begin to sting. Sweat runs down his arms, adding to the pain, but this is no time to notice much. He quickly lines a plastic bin with moist nest material, lays in 44 beige eggs, and covers them with more of the cordgrass. He pops on a lid and labels it with the year, nest number, and location, then sloshes back to the aluminum airboat. The engine roars and he's off to another nest, the Teflon-coated hull skimming over land and water alike, flushing marsh wrens and bitterns as it races across the Santee Coastal Reserve 30 miles north of Charleston. Rhodes looks pleased.
Every summer, as alligator project supervisor for the South Carolina Department of Natural Resources, Rhodes collects hundreds of alligator eggs and incubates them under the puzzled eyes of hunting dogs in the shade of his backyard live oaks. When the eggs hatch in mid-August, he and his research partner, Jeffrey Lang, a biology professor at the University of North Dakota who has traveled the world studying alligators and other crocodilians, determine the gators' sex, size, and weight. They take blood samples for DNA identification to help monitor kinship relations, and then return the hatchlings to their original nests in the marsh once again fending off mother alligators as they hear their youngsters grunting.
Alligators seem to be trapped in an ancient and mindless way of life. "They don't do much," says Rhodes with a shrug. "They're cold-blooded, so they sit in the sun to warm up. They get hot, and they crawl into the water and cool off. Once in a while they eat. But they were here before the dinosaurs and with the dinosaurs. And they're still here."
|Biologist Walt Rhodes cautiously collects eggs from a nest a female gator spent a week building by scraping up grass with her legs.|
Photo by Grant Delin
Very few members of the animal kingdom have survived as long 200 million years as crocodilians. That fact alone drives Rhodes, Lang, and many other scientists in their search for data and understanding. How, they wonder, did crocodilians manage to survive apocalyptic environmental and climatic shifts, including whatever killed the dinosaurs? How did they differ from their cousins who perished? And most important of all, what can their stunning adaptability tell us about the long-term prospects of survival for Homo sapiens?
Ironically, just 30 years ago the American gator Alligator mississippiensis
was a threatened species: hunted, harassed, and crowded to the brink of endangerment by people who saw them as little more than dangerous pests on swampland that could be drained and developed. Now, protected by state and federal laws, alligators are back, basking on golf courses, splashing in backyard pools, and devouring the occasional family pet. They are finally recognized, and managed, as ecological, economic, and aesthetic resources. More important, scientists increasingly see alligators' remarkable sensitivity to environmental poisons as evidence that they can serve as early- warning sentries for humans. Scientists have discovered, for instance, that pesticides containing environmental estrogens can disrupt the endocrine system of male alligators and make it difficult for them to breed. "Are there human consequences?" asks University of Florida zoology professor Louis Guillette. "We don't know. But we'd better start finding out."
Alligators evolved their thick hides, heavy tails, and long rows of teeth in a swampy, dangerous, prehistoric world where armor made sense. Maybe it still does. Only 10 percent of alligator hatchlings survive to measure 4 feet long. But then they're home free, unless they are hit by a car or shot. A male can grow to 14 feet and weigh 1,000 pounds, produce and discard 3,000 teeth in a lifetime, and still be growing at age 40. And yet, says Lang, when you look at how an alligator's sex is determined, you might think they'd all have been extinct ages ago.
An individual gator's sex has nothing to do with genetics crocodilians don't have sex chromosomes but with the temperature at which eggs are incubated. Cold or extremely hot nest temperatures produce females, while intermediate temperatures favor males. One nest might hatch mostly females, while another nearby might hatch mostly males. Or the temperature might vary enough inside a single nest to produce males in the center and females near the edges. "We've demonstrated that the sex ratio varies year to year, depending on the weather and the local nesting environment," Lang says.
|Approximately 5 million American alligators are spread out across the southeastern U.S., mostly in freshwater swamps and marshes.|
Graphic by Matt Zang
Evolutionary theory suggests a species can't survive without maintaining a rough numerical parity between the sexes. If weather determines sex in alligators, couldn't an especially hot or cold year produce greatly skewed sex numbers too many females and too few males, for example and create an evolutionary disadvantage? Strangely enough, alligators do not appear to be handicapped by temperature-dependent sex determination. Scientists think nest temperatures determine sex for all 23 species of crocodilians on the planet, and it seems to work for them too. "If all crocodilians have this characteristic," Lang says, "then it must be adaptive."
Lang suspects that temperature-determined sex is related to other environmental features in crocodilians that increase their chances of survival. In birds and mammals, for example, energy requirements are fixed the creature feeds or dies. But a cold-blooded animal can vary its performance and adjust its body temperature to conditions. "It can warm up, eat fast, metabolize, grow, eat more," says Lang. Or, if there's no food, a cold-blooded animal can simply shut down, lie around for months and do nothing, but still survive.
Evidence suggests nest temperatures affect other attributes of crocodilians. Excessive heat, for example, can cause birth defects embryos incubated at high temperatures emerge with a twisted tail and a hugely bulbous cranium that Walt Rhodes calls "helmet head." Lang has discovered that a crocodilian's incubation temperature can determine its growth rate and behavior later on. "In India," he said, "we found that mugger crocodiles incubated at warmer temperatures grew faster and produced eggs sooner than genetically identical hatchlings from the same clutch incubated at a lower temperature. If an animal's juvenile and adult performance capabilities are strongly influenced by its embryonic experience, maybe it's more able to cope later on, responding to an environmental signal like temperature in a more adaptive way. For example, low temperatures may produce a tougher animal, or high temperatures a fast grower. Make those faster-growing animals male, and they'll be bigger and have a breeding advantage."
|A gator's sex organ is hidden in the cloaca, a vent on its belly. |
Photo by Grant Delin
In one Louisiana field study, Lang and his colleagues found a two-to-one ratio of young males to young females, despite the fact that they had counted many more female hatchlings just a few years before. "We had differential survivorship among these younger ages," he says. "More males were surviving." In other words, sex may be fixed at birth, but sex ratios aren't.
As Lang and other researchers learn more, they are uncovering clues that may help solve one of the greatest scientific mysteries of all time: What killed off the dinosaurs 65 million years ago? Until recently, many scientists assumed that nest temperatures determined the sex of dinosaurs, as it still does for many surviving reptiles. Some researchers have even speculated that's what led to their demise: The climate turned cold, the dinosaurs produced all males or all females, and the species couldn't survive. But if that was true, why wouldn't crocodilians have suffered the same fate? "It suggests there was something different about dinosaur behavior or biology that made them susceptible," says Lang.
Scientists have argued that water could be part of the explanation, says Perran Ross of the Crocodile Specialist Group at the Florida Museum of Natural History. When the dinosaurs died out, many of the creatures that survived including turtles and crocodilians were aquatic. And the water they lived in may have ameliorated the effects of whatever killed large, land-dwelling animals. "Whatever it was that did dinosaurs in meteor impact, global winter, or whatever may have been less intense in aquatic environments," says Ross.
Lang agrees that water might have been a factor, but for a different reason. Alligators, turtles, and crocodiles were always tied to the water, just as they are today. For animals that don't live in the water, Lang says, being warm-blooded is more of an option and "a different lifestyle is possible." So it could be, as other scientists have also suggested, that dinosaurs did not have the reptile's cold-blooded biology at all.
|Biologist Jeffrey Lang uses forceps to peer inside the cloaca. "If the organ fills the cloaca, the gator is male," he says. "A female's organ is half that size." |
Photo by Grant Delin
Maybe they were somewhere in between cold-blooded and warm-blooded, with higher metabolic levels than crocodilians and warm, stable body temperatures. That would have "tremendous advantages for activity," says Ross. "Crocodilians never made this transition and remained stodgy old ectotherms, although able to maintain quite stable body temperatures when basking opportunities were available. But they can do just fine for very long periods, like a year or more, at lower temperatures and with little or no food."
By contrast, animals with a high metabolism have to eat a lot. If a global winter enveloped Earth, Ross speculates, "perhaps the crocodilians just toughed it out while all those high-speed dinos starved to death." The crocodilians' long-lived nature would have helped. "It really doesn't matter what sex the offspring from a single crocodile nest are all male, all female, some of each," Ross explains. "A female crocodile may live 30 to 60 years and nest 15 to 45 times. As long as she gets it right once in her life, then there's no problem." Her genes have been passed on and the species survives.
Lang and Rhodes learned the importance of taking a long-term perspective in 1997, when El Niño produced unusually cool temperatures in the South Carolina marsh and the entire gator-hatch turned out to be female. "If we'd just looked at one year, we might have come out with different conclusions and never known this was possible," Lang says. It suggests that what happens in a single year may not be crucial. In long-term turtle studies, some beaches are thought to produce 100 percent of a single sex, notes Valentine Lance, of the Center for Reproduction of Endangered Species at the San Diego Zoo. "But another beach may produce the other sex, or in other years there may be a mix."
Having just flown to South Carolina from North Dakota, Lang is hunched on a chair on Rhodes's screened porch, surrounded by plastic bins of chirping, newly hatched gator babies. For the past week or so, Rhodes has monitored the temperature of the eggs, hosing them down when they got too hot, covering them with a tarp when the nights turned cool. Now he has placed the incubating bins on the porch of his log house, and Lang is sexing the hatchlings. They're about 9 inches long, black-green with yellow stripes, and they look perky, with little snub noses. One wriggles as Lang picks it up, taps it lightly against the side of the bin to shake off excess water, and spread-eagles it in the light under his big magnifier. He bends to the lens and squints. "Female," he says.
|A gator hatchling will chirp loudly to make its presence known.|
Photo by Grant Delin
Less than a decade ago, scientists couldn't sex a young crocodilian without killing it and then examining its reproductive organs under a microscope. But while studying protected species in India in the early 1990s, Lang worked out a method of peering into a hatchling's cloacal opening with tweezers and an illuminated magnifying glass. He learned to identify sex by the size, shape, and color of the rudimentary organ. "People had tried before, but there was just too much ambiguity," he says, dropping another hatchling male into a plastic bucket. His method is now used worldwide, making crocodilian study easier and more humane.
Rhodes picks up another hatchling, measures it against a tape nailed to the table, and weighs it in a plastic soda cup resting on a scale. Then he passes the squeaking gator to graduate student Lisa Davis. Every year, Rhodes collects eggs from 20 to 50 nests, but he doesn't know which mother laid the eggs or which gator was the father. Davis is trying to answer these questions with DNA. "We don't know which females are at which nest, we don't know if every female nests every year, and that's crucial to understanding the population dynamics," she says, reaching for a syringe. She pokes the baby gator with the needle and extracts blood. "Does the same female return to the same nest area?" From studies in Louisiana, Davis knows that some clutches have two or three fathers. "Is that always so? Is it true here too? Is it the same males or different ones?"
Answering those questions requires DNA from the mother gators, too, which are harder to sample because they're seldom seen except at a distance. Davis has to collect excrement from their nests and hope it contains enough epithelial cells.
"There's still a lot we don't know," Lang says. That includes exactly how temperature determines sex something his fellow researcher Valentine Lance is studying. Early speculation was that temperature might have a mediating effect on hormones and hormone-receptor levels, but that research has fizzled, and Lance now says he's looking at the molecular level: "We still haven't figured it out."
One thing Rhodes and Lang have figured out is how widely alligator sex ratios can vary from year to year. They use that information to help predict population and distribution. But their work may be even more directly useful to humans because alligators, at the top of the food chain, are an indicator species. "If we monitor them for years and see that something has changed in the alligator sex ratio, then we'll know it could be something in the environment," Rhodes says, and that something in the environment could affect humans as well.
|Two-day-old hatchlings from the same clutch all have the same camouflage striping pattern."The hotter the nest, the more stripes they'll have," says Lang. |
Photo by Grant Delin
A case in point was a 1980 pesticide spill in Lake Apopka near Orlando, Florida. When a rainstorm caused a small chemical company's retaining pond to overflow some organochlorine pesticides into the lake, no one was especially alarmed. But certain synthetic chemicals, including organochlorines, mimic estrogens. Simply applying the chemicals to the shell of an alligator egg being incubated at a male-producing temperature can feminize the embryo. That appears to be what happened at Lake Apopka, although scientists didn't know it until years later, in the early 1990s, when zoologist Louis Guillette discovered that the region's entire alligator population had been feminized. The males had low testosterone and abnormally small penises. They couldn't mate.
"If there's a problem with alligators, we should be looking for a problem in humans," says Guillette. "Will it be the same? No, we have different sex-determining mechanisms. But phallic development in alligators is dependent on testosterone, and that's the same thing going on in a baby boy's body."
The most surprising part of Guillette's study was that alligators suffered these serious side effects from pesticide doses of a hundred parts per trillion far below concentrations previously recognized as dangerous or even tested for. He also found that the effects of exposure to one environmental estrogen can be compounded by being exposed to a different one. These revelations have "changed the paradigm that we apply to environmental disruption," says Ross. Earlier research models looked at the environment in terms of additive effects or minimal acceptable contaminant doses, and most environmental regulation is based on such models. But with hormone-disruptive compounds, any amount at all may be damaging. And the effects may not be evident for years, when they show up in the grown offspring of the animal that was originally exposed. "There may be no safe limit, and mixtures of compounds may be many times more potent than the sum of their individual effects," Ross says.
Meanwhile, as people worry whether they can survive the environmental effects they create for themselves, there's little doubt about the alligator's future. Global warming? No problem; alligators will just head north. "If the environment changes, these guys would basically move with it," Rhodes says. "They can't tell us whether Earth is warming, but if we see their range changing, it'll be another indicator." And the alligator has already survived the Ice Age. Fossil records show the gator simply moved south as ice sheets advanced, and then north again when they receded. "Alligators have certainly lived on the Potomac River before, and they might again," Ross says, only half joking. And then, deadpan: "Should be lots of nice coastal marsh habitat when the ice caps melt and the sea level rises." What's Eating You?
Any halfway curious zoo visitor, upon falling into the crocodilian pit, will surely want to know what's having him for lunch alligator, crocodile, or caiman? Luckily, it's easy: The teeth tell the tale. The 23 species of crocodilians are generally classified in three major families Crocodylidae, Gavialidae (called gavials or gharials), and Alligatoridae (which includes alligators and caimans). The crocodile has a more pointed snout than the gator, and the fourth tooth in its lower jaw sets visibly in an upper jaw groove. In the snout of the gator, the same tooth hides in a socket in the upper jaw. The caiman's jaw is often narrow like the croc's, but its fourth tooth is invisible like the gator's.
A half dozen caiman species live in Central and South America, and the largest the black caiman rivals other crocodilians in length at more than 15 feet. Most caimans are smaller, just 4 to 8 feet long versus the alligator's max of 16 feet or so. Caimans have a bony ridge connecting their eye sockets, a feature from which the spectacled caiman gets its common name.
Crocodilians differ, too, in behavior. Slender gavials, with their weirdly long, skinny snouts bristling with snaggly teeth, are mostly fish eaters. The 14 species of crocodile tend to be the most aggressive "quicker, more agile, and very snappy," says Kent Vliet of the University of Florida in Gainesville. One, the Nile crocodile, is a famous man-eater that can grow to more than 18 feet in length and live to be 100.
Alligators are more heavy-bodied and sluggish but resist cold far better than their leaner brethren. And if you think you're safely outside alligator territory, keep in mind that both American and Chinese alligators have been found frozen in winter ice at the northern edge of their range. J.M.
To learn more about crocodilian species, both extant and extinct, and to read about current conservation efforts, visit www.flmnh.ufl.edu/natsci/herpetology/brittoncrocs/csp_amis.htm