0:00:03 > 0:00:05Planet Earth.
0:00:06 > 0:00:08Millions of species.
0:00:10 > 0:00:13But a few are special.
0:00:17 > 0:00:19Thriving.
0:00:19 > 0:00:22Dominating.
0:00:27 > 0:00:29These are the opportunists.
0:00:33 > 0:00:34The collaborators.
0:00:38 > 0:00:39The survivors.
0:00:42 > 0:00:45So, what makes these animals so successful?
0:00:48 > 0:00:52In this series, we'll delve deep beneath the skin
0:00:52 > 0:00:56to reveal the unique features that set some species apart.
0:01:01 > 0:01:04New behaviour and the very latest scientific discoveries
0:01:04 > 0:01:09will offer fresh insight into the wonder of animals.
0:01:22 > 0:01:28Over the past 250 million years, continents have shifted.
0:01:28 > 0:01:32ice ages have come and gone...
0:01:33 > 0:01:36..and the climate has changed dramatically.
0:01:39 > 0:01:43Throughout all of this, one animal group has survived.
0:01:46 > 0:01:49It may look like a relic from a prehistoric world,
0:01:49 > 0:01:52but I can tell you that appearances can be deceptive.
0:01:56 > 0:01:59Beneath the crocodile's hard exterior
0:01:59 > 0:02:02lies one of the most efficient and sophisticated
0:02:02 > 0:02:04physiologies on the planet.
0:02:06 > 0:02:09To see how that has allowed crocodiles to endure
0:02:09 > 0:02:13and to succeed, we're going to follow them as they hunt.
0:02:16 > 0:02:19From the warming up to the ambush.
0:02:19 > 0:02:21And finally, to the kill.
0:02:33 > 0:02:36Crocodiles are the largest reptiles on Earth.
0:02:38 > 0:02:41And like all reptiles, these cold-blooded giants
0:02:41 > 0:02:45use their environment to control their body temperature.
0:02:45 > 0:02:48They need to be warm to operate effectively.
0:02:50 > 0:02:53So to maintain their optimum temperature
0:02:53 > 0:02:55of almost 30-degrees centigrade,
0:02:55 > 0:02:59their heavily-armoured body is specialised
0:02:59 > 0:03:01to capture energy from the sun.
0:03:04 > 0:03:08Enlarged scales run along the back and tail,
0:03:08 > 0:03:11maximising the surface area to be warmed.
0:03:14 > 0:03:18Each bony scale is densely packed with a network of capillaries.
0:03:22 > 0:03:26These tiny blood vessels carry the warmed blood from the scales
0:03:26 > 0:03:29throughout the crocodile's body.
0:03:32 > 0:03:35This heating system can be so effective
0:03:35 > 0:03:40that the crocodiles have to find simple means to cool down.
0:03:40 > 0:03:43Like gaping, or entering the water.
0:03:52 > 0:03:54By carefully positioning themselves
0:03:54 > 0:03:56to take advantage of the sun's warmth,
0:03:56 > 0:03:58they reach their optimum temperature.
0:04:00 > 0:04:03And this is when the crocodile's senses sharpen.
0:04:07 > 0:04:11Colour vision...comes into focus.
0:04:15 > 0:04:18Hearing is heightened.
0:04:23 > 0:04:26It's now ready to hunt.
0:04:31 > 0:04:34In harnessing the sun's heat to warm their bodies,
0:04:34 > 0:04:37rather than generating their own, as mammals do,
0:04:37 > 0:04:40crocodiles have lowered their energy demand.
0:04:41 > 0:04:44This means they can afford to be patient
0:04:44 > 0:04:49and wait for the very best feeding opportunities.
0:04:50 > 0:04:54Crocodiles use the most efficient hunting method.
0:04:58 > 0:05:00The ambush.
0:05:00 > 0:05:05It's a strategy which is designed to conserve as much energy as possible.
0:05:05 > 0:05:10There's no stalking, no chasing. There is, simply, waiting.
0:05:17 > 0:05:22Now, I know it might sound basic, but to pull off the perfect ambush
0:05:22 > 0:05:25requires some extraordinary physiology.
0:05:29 > 0:05:32June, in the Serengeti.
0:05:33 > 0:05:37This time of year marks the seasonal wildebeest migration.
0:05:39 > 0:05:42A great herd of up to 2 million animals is on the move,
0:05:42 > 0:05:46in search of new feeding grounds.
0:05:49 > 0:05:53They stop to drink at watering holes like this as they pass through.
0:05:58 > 0:06:03And these Nile crocodiles...are waiting.
0:06:16 > 0:06:20With only their eyes, ears and nose above the surface,
0:06:20 > 0:06:23they can track their prey's every move...
0:06:24 > 0:06:26..without arousing suspicion.
0:06:29 > 0:06:32The wildebeests are in their sights.
0:06:37 > 0:06:41These stealthy hunters must now disappear completely.
0:06:43 > 0:06:49But the very best place to launch an ambush from is beneath the surface.
0:06:51 > 0:06:55Taking one last breath, it submerges.
0:07:00 > 0:07:05If necessary, a crocodile can remain here for more than an hour.
0:07:08 > 0:07:12This is where some of its remarkable physiology comes into play.
0:07:14 > 0:07:17Allowing the crocodile to ration its oxygen.
0:07:19 > 0:07:22Not just by controlling how quickly it's used...
0:07:24 > 0:07:28..but by controlling WHERE in the body it's used.
0:07:34 > 0:07:37The crocodile's heart is one of the most sophisticated
0:07:37 > 0:07:39in the animal kingdom.
0:07:39 > 0:07:41It has an extra aorta,
0:07:41 > 0:07:44which allows a crocodile to control where its blood flows.
0:07:47 > 0:07:50When submerged, oxygen-rich blood is pumped to the brain
0:07:50 > 0:07:52and to the vital organs.
0:07:55 > 0:07:59But as oxygen-depleted blood returns to the heart,
0:07:59 > 0:08:01an unique valve closes,
0:08:01 > 0:08:05preventing that blood from flowing to the lungs.
0:08:05 > 0:08:10Underwater, those lungs contain no new oxygen.
0:08:10 > 0:08:13Pumping it here would be a waste of energy.
0:08:15 > 0:08:18So the heart bypasses the lungs
0:08:18 > 0:08:21and recirculates this less-oxygen-rich blood
0:08:21 > 0:08:23to the less important muscles.
0:08:31 > 0:08:34Crocodiles can remain submerged for so long
0:08:34 > 0:08:39because of this ability to pump oxygen where it's needed most.
0:08:40 > 0:08:44But to make their single breath last even longer,
0:08:44 > 0:08:48they also have specialised blood
0:08:48 > 0:08:53which carries and releases more oxygen than any other animal.
0:08:59 > 0:09:03Human blood keeps hold of a large percentage of the oxygen it carries.
0:09:05 > 0:09:10Even when the body is in a state of hypoxia, or oxygen starvation.
0:09:13 > 0:09:18But crocodile blood releases all of the oxygen molecules it carries.
0:09:19 > 0:09:24As the crocodile uses up its breath, the blood chemistry changes.
0:09:25 > 0:09:29Causing its haemoglobin, the oxygen-carrying molecule,
0:09:29 > 0:09:31to give up all the oxygen.
0:09:37 > 0:09:40Underwater, the crocodile's ears and nostrils are closed.
0:09:45 > 0:09:48And their eyes are protected by an additional eyelid.
0:09:50 > 0:09:54Whilst they can still detect shadows and shapes,
0:09:54 > 0:09:59for a successful ambush, they need to precisely locate their prey.
0:10:02 > 0:10:06And to do it, they have a rather unexpected sense.
0:10:08 > 0:10:10It's perhaps somewhat surprising,
0:10:10 > 0:10:13considering the heavy-armoured exterior,
0:10:13 > 0:10:17but crocodiles have exceptionally sensitive skin.
0:10:19 > 0:10:20Buried within each scale,
0:10:20 > 0:10:25these tiny black dots are bundles of nerve endings.
0:10:26 > 0:10:29They detect touch, pressure
0:10:29 > 0:10:32and the most minute vibrations.
0:10:34 > 0:10:38There are as many as 9,000 of these sensory organs
0:10:38 > 0:10:41across a crocodile's body.
0:10:41 > 0:10:464,000 of them are concentrated around the jaws and teeth alone.
0:10:48 > 0:10:53Making the crocodile's jaws 10 times more sensitive to touch
0:10:53 > 0:10:56than our own fingertips.
0:10:58 > 0:11:01These bundles of receptors are also grouped along
0:11:01 > 0:11:03the outermost digits of the forearms.
0:11:06 > 0:11:09So that in still water, a crocodile can detect
0:11:09 > 0:11:14the vibrations of a wildebeest drinking from 20 metres away.
0:11:18 > 0:11:20The crocodile's heart and blood
0:11:20 > 0:11:24allow it to remain hidden underwater.
0:11:24 > 0:11:29Whilst its skin detects what's going on above the water.
0:11:32 > 0:11:37It's this combination of unusual physiology and acute senses
0:11:37 > 0:11:40which allow the crocodile
0:11:40 > 0:11:44to position itself perfectly for an attack.
0:12:12 > 0:12:17Crocodiles have the strongest known bite of any animal on Earth.
0:12:20 > 0:12:22More than 60 cone-shaped teeth
0:12:22 > 0:12:25pierce and clamp down on their prey.
0:12:29 > 0:12:33Exerting over 2,000 kilos of force.
0:12:35 > 0:12:37And they don't let go.
0:12:41 > 0:12:44With muscular legs and claws for grip,
0:12:44 > 0:12:49the crocodile drags its kill into the water and drowns it.
0:13:02 > 0:13:04Whilst crocodiles are accomplished predators
0:13:04 > 0:13:07in the light and heat of the day...
0:13:11 > 0:13:14..they can also turn their extraordinary senses
0:13:14 > 0:13:17to ambush in complete darkness.
0:13:17 > 0:13:22And capture a very different type of food.
0:13:24 > 0:13:26Australia's Northern Territory.
0:13:28 > 0:13:32At unusually high tides, this river breaches its barrage.
0:13:33 > 0:13:38Introducing a deluge of saltwater into the river system.
0:13:40 > 0:13:43With it, a whole tide of grey mullet.
0:13:49 > 0:13:52It's a welcome feast for these saltwater crocodiles.
0:13:54 > 0:13:58But the fish are fast and extremely difficult to catch.
0:14:01 > 0:14:04To snare such fidgety prey in moving water,
0:14:04 > 0:14:07they must draw on their sensory arsenal.
0:14:11 > 0:14:15For a successful ambush, they each take their optimal position.
0:14:18 > 0:14:22Now, saltwater crocodiles are the most aggressive
0:14:22 > 0:14:25and territorial of all the crocodile species.
0:14:28 > 0:14:31But faced with such a bounty of food,
0:14:31 > 0:14:33they are willing to tolerate one another.
0:14:36 > 0:14:37Just about.
0:14:47 > 0:14:50With water temperatures of 28 degrees,
0:14:50 > 0:14:54the crocodiles are warm and ready to hunt.
0:14:58 > 0:15:01The darkness is not an issue either.
0:15:01 > 0:15:04Because crocodiles have excellent night vision.
0:15:07 > 0:15:11Their retinas contain extremely dense concentrations
0:15:11 > 0:15:13of light-sensitive cells.
0:15:18 > 0:15:21And to maximise the light reaching these receptors,
0:15:21 > 0:15:27their normally-slit pupils open wide, into a full circle.
0:15:30 > 0:15:32Their vision is also boosted by
0:15:32 > 0:15:36a layer of tissue called the tapetum lucidum.
0:15:40 > 0:15:43Sitting behind the retina, this reflects the light back
0:15:43 > 0:15:45onto its light receptors,
0:15:45 > 0:15:49giving these cells a second chance to react.
0:15:53 > 0:15:55In the glare of a torch,
0:15:55 > 0:15:59this layer glows like a cat's eyes caught in the headlights.
0:16:09 > 0:16:12With their mouths gaping, they lie in wait.
0:16:12 > 0:16:17The touch receptors on their jaws scanning for pressure waves.
0:16:21 > 0:16:24Able to differentiate between the touch of a neighbour
0:16:24 > 0:16:27and the movement of a fish.
0:16:34 > 0:16:36And when these sensors fire,
0:16:36 > 0:16:40the jaws clamp shut in just 15 milliseconds.
0:16:43 > 0:16:46Six times faster than you can blink.
0:17:07 > 0:17:10Physiology enables the crocodiles to hunt
0:17:10 > 0:17:14large, slow-moving prey like wildebeest,
0:17:14 > 0:17:17as well as smaller, faster animals like fish.
0:17:17 > 0:17:22But it also lets them tackle a very different type of food.
0:17:22 > 0:17:24One which poses new challenges
0:17:24 > 0:17:28and allows them to feed in a truly surprising way.
0:17:36 > 0:17:39Summer in Zambia.
0:17:41 > 0:17:44For the predators living around this water hole,
0:17:44 > 0:17:47this dead hippopotamus is a feast for the taking.
0:17:51 > 0:17:54But suspiciously, it's been left untouched.
0:17:58 > 0:18:02An outbreak of anthrax has killed this and many other hippos here.
0:18:10 > 0:18:15Contaminated carrion like this would be hazardous to most other animals.
0:18:20 > 0:18:23But not the crocodile.
0:18:29 > 0:18:32They can eat rotten or diseased meat with little risk.
0:18:36 > 0:18:40Proteins in the blood bind to pathogens such as anthrax...
0:18:42 > 0:18:45..and kill them before they take hold.
0:18:47 > 0:18:51Antibacterial blood is an unique trait,
0:18:51 > 0:18:55which opens up a raft of easy feeding opportunities
0:18:55 > 0:18:57for the crocodile.
0:19:02 > 0:19:04Having smelled the rotting flesh,
0:19:04 > 0:19:08more than 40 crocodiles move in for the feast.
0:19:12 > 0:19:15Thrashing their powerful tails, they spin themselves in the water.
0:19:19 > 0:19:22These death rolls swiftly dismember the carcass.
0:19:28 > 0:19:30They tear into their meal.
0:19:33 > 0:19:35Chunks of meat are swallowed whole.
0:19:39 > 0:19:43With such a powerful grip on the flesh and bone,
0:19:43 > 0:19:46crocodiles often lose their teeth.
0:19:47 > 0:19:50But it's not a problem. They're quickly replaced.
0:19:52 > 0:19:56Up to 3,000 new teeth grow in a lifetime.
0:20:06 > 0:20:12Unaffected by this hazardous meal, the crocodiles gorge themselves.
0:20:14 > 0:20:17Following a feed, crocodiles' stomachs can contain
0:20:17 > 0:20:21up to 25 kilos of prey.
0:20:21 > 0:20:25Which is great, but then they've got to digest it.
0:20:27 > 0:20:31Once again, the crocodiles have got some ingenious physiology
0:20:31 > 0:20:32to meet the challenge.
0:20:44 > 0:20:45With their stomachs full,
0:20:45 > 0:20:48they return to the banks to find a basking spot.
0:20:58 > 0:21:01The first task is breaking down the heavy load
0:21:01 > 0:21:04into smaller, digestible pieces.
0:21:06 > 0:21:10And for this, crocodiles use an unusual digestive aid.
0:21:18 > 0:21:22Throughout their lives, they ingest small stones.
0:21:32 > 0:21:35Known as gastroliths, these stones remain
0:21:35 > 0:21:38in the first section of the crocodile's stomach.
0:21:38 > 0:21:42And they're thought to work in the same way as a bird's gizzard,
0:21:42 > 0:21:45to mechanically grind up the contents.
0:21:48 > 0:21:51This shared feature may seem odd,
0:21:51 > 0:21:55but birds are the closest living relatives of crocodiles.
0:21:55 > 0:21:59And the two groups still share common physiology.
0:22:02 > 0:22:05Another relative of the crocodiles, the dinosaurs,
0:22:05 > 0:22:08also used gastroliths to process their food.
0:22:10 > 0:22:15This theropod dinosaur also displays these distinctive stomach stones.
0:22:17 > 0:22:22The more-manageable load now passes to the second stomach chamber.
0:22:24 > 0:22:29Here, bone, horn and hooves are chemically broken down.
0:22:32 > 0:22:35Basking speeds up this stage of digestion.
0:22:38 > 0:22:43The hotter the crocodile gets, the more efficient the breakdown.
0:22:43 > 0:22:47But the process can still take days.
0:22:50 > 0:22:52And whilst the contents sit in the stomach,
0:22:52 > 0:22:54they're at risk of putrefying.
0:22:56 > 0:23:00Fortunately, this stage of digestion doesn't just rely on the sun.
0:23:04 > 0:23:08Crocodiles have the most acidic stomach in the animal kingdom.
0:23:14 > 0:23:17And it's all down to their unique heart.
0:23:21 > 0:23:22To boost digestion,
0:23:22 > 0:23:26the heart directs deoxygenated blood to the stomach.
0:23:28 > 0:23:31This blood is rich in acidic carbon dioxide
0:23:31 > 0:23:36and stimulates the production of extremely-corrosive gastric juices.
0:23:43 > 0:23:47With this remarkable system, crocodiles can secrete stomach acid
0:23:47 > 0:23:5110 times faster than any other animal.
0:23:54 > 0:23:57Not only does this keep bacteria at bay,
0:23:57 > 0:24:00combined with heat, it enables crocodiles
0:24:00 > 0:24:03to digest a wildebeest in just a matter of days.
0:24:10 > 0:24:14Unique physiology has not only enabled the crocodile to ambush,
0:24:14 > 0:24:18kill and eat in the most efficient way possible,
0:24:18 > 0:24:23its high-performance digestive system saves them energy, too.
0:24:25 > 0:24:29Being cold-blooded and not needing to generate body heat,
0:24:29 > 0:24:33crocodiles use energy stores so sparingly
0:24:33 > 0:24:37that they can go as long as a year between feeds.
0:24:40 > 0:24:43They can do this because they are the most efficient vertebrates
0:24:43 > 0:24:47when it comes to converting food into body mass.
0:25:00 > 0:25:03In India, it's the height of the dry season.
0:25:05 > 0:25:06Lakes are disappearing
0:25:06 > 0:25:10and feeding opportunities are few and far between.
0:25:12 > 0:25:16Drought conditions are enough to kill even the most hardy animals.
0:25:18 > 0:25:21But the crocodile can draw on systems
0:25:21 > 0:25:24which enable it to survive where others can't.
0:25:28 > 0:25:33This mugger crocodile is not on the hunt for food,
0:25:33 > 0:25:35it's on the hunt for shelter.
0:25:36 > 0:25:41Crocodiles exploit the world around them to heat up,
0:25:41 > 0:25:43but they also use it to cool down.
0:25:46 > 0:25:49It's searching for a refuge.
0:25:52 > 0:25:54A cool, sheltered den.
0:26:00 > 0:26:04The crocodile's final and most unexpected adaptation
0:26:04 > 0:26:07is that it can shut its body down
0:26:07 > 0:26:09through the hottest times of the year.
0:26:11 > 0:26:16This period of dormancy, similar to hibernation in mammals,
0:26:16 > 0:26:18is known as aestivation.
0:26:22 > 0:26:25Whilst warm-blooded animals hibernate through the cold months,
0:26:25 > 0:26:29crocodiles use the same physiological process
0:26:29 > 0:26:31to survive the hottest months.
0:26:33 > 0:26:37But where mammals struggle to lower their body temperature
0:26:37 > 0:26:39and slow their metabolic rate,
0:26:39 > 0:26:43crocodiles have no such problems.
0:26:44 > 0:26:49They're already cold-blooded and have a low metabolism,
0:26:49 > 0:26:52so they can easily conserve resources.
0:26:54 > 0:26:58Drawing on its fat reserves which are stored mainly in the tail,
0:26:58 > 0:27:03it can lie here for five months without food and water.
0:27:03 > 0:27:07Only once the monsoon arrives to replenish the lakes and rivers
0:27:07 > 0:27:11will it once more come out to hunt.
0:27:17 > 0:27:20This ability to shut down when times are hard
0:27:20 > 0:27:24is thought to be the key to the crocodile's exceptional longevity.
0:27:28 > 0:27:31Enabling it to avoid the shifts in climate
0:27:31 > 0:27:35that have wiped out so many other species over time.
0:27:37 > 0:27:40But crocodiles haven't endured unchanged.
0:27:43 > 0:27:47They've evolved some of the most surprising physiology
0:27:47 > 0:27:48in the animal kingdom.
0:27:51 > 0:27:55Physiology which allows them to stay submerged.
0:27:57 > 0:28:00To sense the world around them.
0:28:03 > 0:28:07And eat and digest practically anything.
0:28:11 > 0:28:14And it's these unexpected qualities,
0:28:14 > 0:28:16hidden beneath the primitive exterior,
0:28:16 > 0:28:21which allow these remarkable predators not only to survive,
0:28:21 > 0:28:23but also, to prosper.
0:28:23 > 0:28:27And that is the wonder of crocodiles.