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"Crocodiles!"

PBS Airdate: April 28, 1998
Go to the companion Web site

ANNOUNCER: Tonight on NOVA, come face to face with the most dangerous reptile predator on the planet. For over 70 million years, the crocodile has remained virtually unchanged—but not unchallenged. How have they survived so long? Now, get up close and personal. "Crocodiles."

Major funding for NOVA is provided by the Park Foundation. Dedicated to education and quality television.

This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from the Quiet Company? Northwestern Mutual Life.

And by Iomega, makers of personal storage solutions for your computer, so you can create more, share more, save more and do more of whatever it is you do. Iomega. Because it's your stuff.

And by the Corporation for Public Broadcasting. And viewers like you.

NARRATOR: Crocodiles hail from the great age of reptiles well over 70 million years ago. Since then, the world has changed dramatically around them, but amazingly, they have changed little. They are the only great reptile predator to have outlived the dinosaurs. Ancient they may be, but 70 million years on, they are still the most successful freshwater predator. Most of the waterways in the tropics these days have their own species of crocodile. How come that they've been so successful for so long? Well, it seems that their design, when it first appeared, was already very advanced, and it's kept them successful ever since. This film reveals the nature of that success. Crocodiles may have scored an early evolutionary bull's eye, but there's much more to their staying power than brute strength. Only now, by exploring their complete world—above and below water, by day and night—can we fully appreciate just how sophisticated they are. Crocodiles can float or sink at will, finely tuning their buoyancy. They can control their buoyancy like a submarine. The liver is squeezed back to make more room for the expanded lungs. A crocodile can stay submerged for up to three hours. While submerged, a protective membrane closes over their eyes, like swimming goggles. Crocodiles are cold-blooded. They rely on the temperature of the sun and water to heat themselves up or to cool down. They are sluggish, but when they are warm enough, crocodiles are capable of breathtaking speed. They're the most dangerous freshwater predator on the planet. It's a status gained thanks to adaptations so sophisticated that ancient crocodile survived when their close relatives, the mighty dinosaurs, perished. It's a design that has allowed them to spread around the tropical world. This extended family includes alligators of temperate waterways, gharials of the Indian subcontinent, and caiman of tropical South America. They've even invaded the sea. Special glands in the mouth allow crocodiles to tolerate salt water long enough to reach the next river. This is how they spread along the coasts. But only a few species can migrate across oceans. The real specialists, the saltwater crocodiles, are affectionately known as "salties." They've been seen more than 600 miles from land, encrusted with barnacles, as they search for new island homes. This pioneering spirit and their unique character has made them a universal threat. Crocodiles deserve their tough reputation, but they aren't just solitary killers. In the calm of the Florida swamps, we can begin to see them in a new light. They are reptiles with social skills, and American alligators are particularly vocal. During the breeding season, an expressive chorus of bellows, grunts, and hisses rises from their dense swamps. Having settled himself into a suitable lagoon, a territorial male sets about fending off rivals. Such a perfect breeding pond is hot property. Other males put in an unwelcome bid for occupancy. The resident male only shows a bit of muscle if the rival fails to get the message. He's made his point, and he can now focus on finding a mate. Vibrations within his muscle walls send sound waves rippling out into the swamp, powerful enough to make the water dance. Part of his roar radiates at such low frequency that human ears can't hear it. But these subsonic calls have astounding penetration, travelling four times faster, and further, under the water. Only with special low frequency recordings can we appreciate what the female alligator hears. When she hears his rumbling invitation, she wastes no time, and the low frequency calls guide her through the murk and vegetation. Female alligators move freely from the territory of one male into that of another's. Even though the call may come from the remotest part of the swamp, she soon finds him. Then, things get intimate. After lengthy calling, the physical part of the courtship begins. Alligators may mate with several partners during the short summer breeding season. Crocodiles are extremely sensitive to touch. Their armor-plated toughness would appear to be only skin-deep. Such complicated language and behavior is exceptional among today's reptiles, but crocodiles might provide a tantalizing rear window on the social life of their ancient relatives, the dinosaurs. Nile crocodiles are the most fraternal crocs of all. Once a year, they hang out together, soaking up the sun at traditional mating places. Compared to Florida's noisy alligators, African Nile crocodiles are fairly quiet. These are far from being lazy afternoons. But the group is very organized, and individuals keep a careful eye on the movements of others within it. A healthy, 15-foot adult male attracts the attention of smaller females on the lookout for a fit partner. Up to ten females spend much of their time in the water soliciting him. They bare their throats as a gesture of appeasement. He'll mate with them all in turn during the breeding season. The male also uses body signals and shows off with noisy water displays. But these postures don't always keep rival males waiting in the wings. The top male flaunts to his female audience as the challenger tries to get in on the act. The rival's change of mind comes too late. Lives are rarely lost, but toes, limbs, and even tails can sometimes get ripped off. A victorious fountain of water re-asserts the top male's authority. The biggest males are usually the most successful when it comes to attracting mates. The females can also be influenced by individual temperaments. It's they who often take the lead, by rubbing themselves along the length of his body. At first, he snubs both of them, but the rubbing releases a pungent oil from glands under their jaws. Both sexes are highly sensitive to this crocodile perfume, and the male answers with his own musky message. The scent in the water is particularly potent before mating. The male will eventually court and couple with all the females in the group. But the subtleties of crocodile relationships remain submerged in mystery. Several weeks later, the females are ready to make their nests. Even when basking, adults rarely move far from water. So, it's ironic that the first and most critical part of the crocodilian life is spent on land. Crocodile babies are born encased in eggs, but they must still breathe air. It's essential that this mother lays her clutch well beyond the high water mark. The eggs will be so at risk—from drowning, from predators, and from temperature fluctuations—that she relies on exactly the same nest position year after year. These time-honored sites are a measure of the experienced care that crocodile mothers provide. Once the excavation is complete, she lays her eggs in a trance-like state. The hole will become an incubator. Crocodile eggs will only develop if they remain between a steamy 80 and 94 degrees. So, her choice of site is critical. Nest chambers from equatorial African plains to high Himalayan river valleys all remain within this temperature range. But no one knows how crocodile mothers get it right. Once the nest is sealed, the mothers keep vigil for 90 days. Temperature determines the sex of the developing eggs. A half-degree difference between the top and bottom of the next could markedly affect the ratio of males to females. No one knows yet just how disruptive global climate changes will be in terms of the sex ratio of hatchlings in the future. Right now, these youngsters face more immediate, local threats. The noise and smell coming from the nest could soon attract the attention of hungry raiders. Fortunately, there mother has been waiting nearby, and is the first to hear the muffled newborn cries. Once she digs them out, they must reach the relative security of water as quickly as possible. The babies could probably find their own way, but they're only safe if they all stick together, so their mother transports them to water herself in her mouth. The skin of her lower jaw stretches into a hanging cradle in which she can carry fifteen or so at a time. It's a snug fit—once you're in. Her 40 or so eggs hatch simultaneously, so she'll have to make several trips. The clamorings of the waiting hatchlings constantly remind her to return to the nest, and ensure that no one misses the last bus. She releases them into a nursery pool, where they must make the most of her tender loving care. They have only a few weeks under her watchful eye. Their rate of growth is affected by temperature. Over a lifetime under a tropical sun, they may grow 4,000 times bigger than this. While they're so tiny, they call to each other and to her—conversations that continue into adulthood. The 18 different sounds they'll use represent a vocabulary more typical of mammals than of reptiles. These calls can be of life-saving importance, particularly if one loses contact with the group. This hatchling's adventurous spirit is putting him in great danger, from predatory birds and monitor lizards. The rest of the family is now nearly 50 feet away. But just as their calls fade, the lost hatchling's survival instincts kick in. He utters a loud call of distress. It's answered by his siblings. The stray baby calls again. The other hatchlings reply. Their constant calls guide him back to the security of the family. Vocal contact ensured this youngster's survival. Other threats demand spirited action by their mother. One of the biggest risks is from other crocodiles. The mother successfully defends her own brood, but it's a grim reminder that crocodiles will eat just about anything. Like the adults, the young aren't fussy about food. And, like adults, they are masters of the stealthy hunting technique that all crocodiles are born with, keeping eyes, ears and nose at surface level. These shoreline snacks may be wildebeest or zebra in years to come. Growing hatchlings must eat 5% of their own weight every day. Frogs, grasshoppers, spiders, anything small enough goes. They rely on communal living to survive this tender age, but competition between the youngsters is intense. Many will be picked off by predators. Floods and droughts also take their toll. Only 2% of the hatchlings will make it to adulthood. In Venezuela, the babies of South American caiman also face tough odds. It's the dry season. Only a few pools remain. Even the fish are out of water. The dry season is five months long, but in some years, it's more severe than usual. Unexpectedly, animals can be left high, if not exactly dry. Eventually, some of these watercourses will dry up completely, leaving many caiman exposed and dehydrated. Temporary skin relief is provided by this ultimate of mud masks. Cooling water is at a premium. The remaining pools are becoming over-crowded. At such pressing times, the adults usually tolerate each other. However, babies could be eaten by them, so this is no place for a mother to raise her young. Fortunately, her babies can expect complete maternal attention during this five-month period of danger. The shrinking marsh forces their mother to make a drastic move. If she gets it wrong, the whole family will perish. Night-time cameras reveal that they will make a run for it, under the cover of dark. Their escape will be risky, for it's made over land. Mother leads the way. It's a while before the babies have the courage to join her. Calls of reassurance, and maybe her musky smell, encourage the hatchlings out of the water. She follows good instincts. Night-time temperatures are less dangerous to tiny cold-blooded bodies, and there are fewer threats about. But this is a long march that must be concluded by day-break. Their mother's acute sense of smell brings them to their watery destination, with less than an hour to spare until dawn. There's plenty of water here for her babies to survive the rest of the dry season. But there's no room for their mother. Another female has already had the same idea, and adults won't share the pool. To avoid a fight, the newly arrived mother moves on, leaving her family behind. They'll be safer in the swamp than with her. Others have done the same thing. The foster mother's creche has now swollen to more than 100 babies. Only some are hers. This mother carries her burden lightly. Crocodiles are more social than all other reptiles and grow up in company. As hunting adults, this may prepare them for dedicated teamwork—especially if there's a rare dish on the menu. Migrating zebras cross the Mara River in Kenya only a few times each year, travelling between traditional grazing grounds. Resident Nile crocodiles cannot afford to miss these fleeting chances. So, they cooperate. From different places under the riverbanks, they spend days waiting and watching the nervous herds. Only when the zebra have committed themselves to a departure point do the crocodiles take up their final positions. The current drags the zebra downstream, so five or six crocodiles cover the crossing from different angles. It's hard work for crocodiles too, but they can submerge, taking advantage of deeper, slower currents to move closer. Their approach is stealthy, measured and tactical, as if moving in on a chosen victim. The current is flowing in this zebra's favor. It buffets the crocodiles out of position and they can't get a grip. The zebra's wide rump gives it a big advantage. Adult zebras also have a vicious kick, which could break a crocodile's jaw. That might mean starvation, so crocodiles are also taking risks. A lot of effort results in little success for the crocodiles—but only until smaller, more vulnerable animals enter the water. At any time, the zebras can be ambushed by frightening power and lightening speed. The baby zebra faces several waves of attack. Crocodiles work alone, or in groups, depending on the size of the prey. This time, they were rewarded. Farther south, in Zambia, the parched Luangwa Valley is the scene of some remarkable behavior. In the dry season, it has one of the highest concentrations of crocodiles in the world. Hundreds crowd together, forced to share what little water remains. Even the hippos can't find water deep enough to wet their backs. In the heat of the day, crocodiles save their energy. There is little to do and, it seems, little to eat. But even in these extreme conditions, crocodiles excel at making the most of their circumstances. Far from being dependent on large mammals such as wildebeest or zebra, crocodiles have extraordinarily adaptable diets. We've not known this until now, because we've never seen them hunting at night. What crocodiles do at night has remained a mystery because they will only move around naturally in total darkness. So, only with the use of invisible infrared light can we see their nocturnal habits for the first time. In the Luangwa River shallows, a Nile crocodile hunts for less glamorous prey. It's stalking fish. Over the year, crocodiles fatten up on fish and mollusks, rather than mammals. Mammals come and go. But the muddy river bottom is always a good place to stir up snails, for instance. The crocodile is a flexible feeder in its methods, too. It uses its armor-plated body as sinuously as a length of fishing net to corral fish towards the shore. Midnight feasting is not uncommon at this time of year. But even the fish are running out. By day, the temperatures soar. The only ripples now are those shimmering in the scorching air. For the crocs, things appear to be getting desperate. Many animals have to resort to the scavenging habits of vultures. Even hippos, normally grazers, are forced to supplement their meager rations with the grassy stomach contents of a dead buffalo. But crocodiles still thrive on the edge. And, once again, infrared cameras allow their night habits to be discreetly filmed. Vultures have to wait until daybreak, but other scavengers and hunters are out in force. Hyenas are doing particularly well at the expense of others. Some crocs just couldn't find enough to live on. Others, though, hang onto life. But they look danger in the eye every night. Only when the dry season becomes desperate, are these two great hunters of land and water likely to come face to face. The crocodile is too hungry to be intimidated by a lion, and the smell of food wafting towards the river on the night air is just too strong. But it walks into an unlikely dinner party. A hungry hippo is formidable company. Even the lions know that. Unsure at first of its welcome, the crocodile soon takes its place at this most unusual mutually tolerant gathering. For the lions, there's no such thing as eating in peace. No sooner is the hippo out of the way than the crocodile actively takes on the whole pride. The lions, though, have had enough of the intrusion. Though lions have been known to kill crocodiles, this croc won't take the hint. There's not a lot the lions can do, especially when the smell of their supper coaxes other crocodiles out of the water. Crocodiles have thick skins. The lions are forced to defend their meal by moving to a different table. They just leave nothing but scraps behind. The reptiles have stronger stomachs than the mammals. Lions can only cope with smaller tissues. Crocodile digestive juices are so acidic that they can break down skin, bone, and horn. There's no such thing as crocodile left-overs. Their exceptional sense of smell enables them quickly to capitalize on the demise of others. And at the end of the dry season, the smell of death is everywhere. Sickness, starvation, and injury take many victims. One of this hippo's group has died. The crocodiles have good reason to be wary. A provoked adult hippo could easily inflict a mortal wound. So, they wait for it to go before closing in on the carcass. Food, any food, is hard to find. Crocodiles from up to four miles away recognize the whiff of opportunity. Each crocodile patiently waits for the chance to feed. The reason for this patient approach? Crocodiles have a powerful grip and sharp teeth, but their jaws don't move sideways. They can't chew. So, they have to eat large animals together. One or two crocodiles effectively brace the carcass in the jaws, while others wrench off smaller pieces of flesh by spinning their bodies around. The largest male usually initiates the communal meal. How the rest of the crocodiles organize themselves to feed, no one knows. It can take 24 hours to satisfy every diner. Too readily, perhaps, we have cast crocodiles as ruthless predators, feared them, misunderstood them, attacked and exploited them. But they are great survivors from an ancient line that goes back more than 70 million years. And perhaps, in millions of years to come, crocodiles will live on, successful still, looking and behaving much the same as they do today—one of the world's great survivors.

ANNOUNCER: Surviving a drought is one thing. But how about mass extinction? Why did crocodiles live when their cousins, the dinosaurs, wound up like this? Log on to NOVA's website.

To order this show for $19.95 plus shipping and handling, call 1(800) 949-8670. And to learn more about how science can solve the mysteries of our world, ask about our many other NOVA videos.

NOVA is a production of WGBH Boston.

Major funding for NOVA is provided by the Park Foundation. Dedicated to education and quality television.

This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from the Quiet Company? Northwestern Mutual Life.

And by Iomega, makers of the zip drive and 100-megabyte zip disks. Your universe is expanding, but you can save it in your own personal space. Iomega. Because it's your stuff.

And by the Corporation for Public Broadcasting. And viewers like you.

This is PBS.



PRODUCTION CREDITS

Crocodiles!

Producer
Karen Bass

Hosted by
David Attenborough

Writer
Susan Western

Assistant Producer
Gavin Maxwell

Photography
Richard Kirby

Additional Photography
Daniel Christen
Martyn Dohrn
Peter Scoones
Kim Wolhuter

Film Editor
Tim Coope

Dubbing Editor
Lucy Rutherford

Dubbing Mixer
Peter Hicks

Music composed by
Nigel Beaham-Powell
Bella Russel

Scientific Consultants
Tony Pooley
John Thorbjarnarson
Kent Vliet

Production Co-ordinators
Fiona Marsh
Sharon Thomas
Pip Lawson

Production Manager
Ruth Flowers

Unit Managers
Nicky Spode
Felicity Mudge

Special Thanks:
Adelaide River Queen
David Blake
Big Cypress Nat'l Reserve
Tomas Blohm
Crocworld
Tony Blums
Florida's Silver Springs
John Bradley
Gatorland
Georgette Douwma
Kakadu National Park
Gustavo Hernandez
Masai Mara Nat'l Reserve
Francois Kruger
South Luangwa Nat'l Park
Penny Lapper
Nat'l Parks and Wildlife Services, Zambia
St Lucia Crocodile Centre
Robin and Jo Pope
Wakulla Springs State Park
Tim Williams
Wildlife Conservation Society

Executive Producer, David Attenborough Specials
Keith Scholey

NOVA Series Graphics
National Ministry of Design

NOVA Theme
Mason Daring
Martin Brody

Closed Captioning
The Caption Center

Production Secretaries
Queene Coyne
Linda Callahan

Publicity
Paul Marotta
Diane Buxton

Unit Manager
Jessica Gilliam Maher

Paralegal
Nancy Marshall

Business Manager
Laurie Cahalane

Post Production Assistant
Pamela B. Jacobson

Associate Producer
Post Production
Kimberly Schaffer

Post Production Editor
Rebecca Nieto

Post Production Supervisor
Mark Geffen

Coproductions Assistant
Andrea Cross

Associate Producer
Coproductions
Stephen Sweigart

Senior Producer
Coproductions and Acquisitions
Melanie Wallace

Series Producer
Beth Hoppe

Managing Director
Alan Ritsko

Executive Producer, NOVA
Paula S. Apsell

A BBC Production in association with WGBH/Boston
© 1998 WGBH Educational Foundation
All rights reserved

 

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