1 00:00:13,580 --> 00:00:16,447 This is a story about a hungry mother... 2 00:00:16,616 --> 00:00:19,050 A female tiger shark... 3 00:00:19,219 --> 00:00:22,655 ...and the extra-ordinary world she lives in. 4 00:00:26,926 --> 00:00:29,292 Right now she's got to find a meal... 5 00:00:29,929 --> 00:00:33,626 she's pregnant - there are forty babies in her belly... 6 00:00:33,800 --> 00:00:37,201 and if they are to survive, she must eat soon. 7 00:00:39,406 --> 00:00:41,169 But she has rivals... 8 00:00:42,809 --> 00:00:45,039 ...other tiger sharks... 9 00:00:46,546 --> 00:00:49,242 ...they're chasing the same food. 10 00:00:49,916 --> 00:00:52,578 And there are other hunters out today... 11 00:00:55,422 --> 00:00:59,358 ...sharks with a range of senses we can barely comprehend. 12 00:01:03,196 --> 00:01:07,098 They're all competing in a world beyond our perception. 13 00:01:13,273 --> 00:01:14,570 This is a story, 14 00:01:14,741 --> 00:01:18,233 but everything you're about to see is based on fact. 15 00:01:19,946 --> 00:01:25,111 For the next few months these waters will become a Shark Battlefield. 16 00:01:35,929 --> 00:01:39,763 You're about to see a tropical island like you've never seen it before. 17 00:01:43,269 --> 00:01:47,729 A virtual world - based on fifty years of scientific study. 18 00:01:51,678 --> 00:01:55,239 Within it, we can travel to any point on the island... 19 00:01:57,417 --> 00:02:01,854 ...revealing our shark in ways that ordinary cameras cannot show. 20 00:02:06,292 --> 00:02:11,355 From the outer reef edge, we take you to... the channel... 21 00:02:13,466 --> 00:02:17,596 ...a highway between the open sea and sheltered lagoons. 22 00:02:22,408 --> 00:02:25,241 All around the island is a ring of coral... 23 00:02:26,579 --> 00:02:28,740 ...the reef flats... 24 00:02:31,317 --> 00:02:35,583 ...there's no richer concentration of marine life anywhere on earth. 25 00:02:38,424 --> 00:02:43,794 Protected from the stormy ocean by these reefs, there's a large, shallow lagoon. 26 00:02:47,934 --> 00:02:53,395 lt's a feeding and breeding area for residents and ocean visitors alike. 27 00:02:57,677 --> 00:03:00,145 And where sand has built up on the reef flats 28 00:03:00,313 --> 00:03:05,376 there are coral cays... nest sites for millions of seabirds. 29 00:03:14,327 --> 00:03:18,093 These are the critical zones in this natural battlefield. 30 00:03:24,938 --> 00:03:26,701 And for the next three months 31 00:03:26,873 --> 00:03:30,775 they'll be the hunting ground of our female tiger shark. 32 00:03:33,746 --> 00:03:37,204 She's about four metres long and twenty years old... 33 00:03:37,550 --> 00:03:40,678 her tiger stripes have begun to fade with age. 34 00:03:43,990 --> 00:03:46,049 Along with her there's a gang of remoras 35 00:03:46,226 --> 00:03:49,923 or sucker fish that come and go looking for scraps. 36 00:03:54,100 --> 00:03:56,295 She's now nine months pregnant... 37 00:03:56,469 --> 00:03:58,596 three months from giving birth - 38 00:03:58,771 --> 00:04:01,706 but her developing pups are in danger. 39 00:04:03,776 --> 00:04:05,801 She hasn't fed for some time 40 00:04:05,979 --> 00:04:11,474 and if she doesn't find a meal soon, some may die before they're born. 41 00:04:17,857 --> 00:04:19,654 Finding food isn't easy. 42 00:04:19,826 --> 00:04:25,025 The water is thick with plankton and for us visibility is restricted... 43 00:04:27,800 --> 00:04:32,169 ...but a tiger shark's eyes are ten times more sensitive than ours. 44 00:04:40,013 --> 00:04:42,004 Vision alone won't help her anyway, 45 00:04:42,181 --> 00:04:46,845 especially when the nearest easy meal is ten kilometres away. 46 00:04:52,759 --> 00:04:55,887 A dead turtle floats near a breeding beach... 47 00:04:56,062 --> 00:04:57,723 a hundred kilos of food - 48 00:04:57,897 --> 00:05:00,695 just what our pregnant mother needs... 49 00:05:01,534 --> 00:05:04,230 ...but she'll have to be quick. 50 00:05:10,376 --> 00:05:13,174 Other tiger sharks are in the area... 51 00:05:15,815 --> 00:05:19,410 ...each one is focused on the need for food. 52 00:05:22,922 --> 00:05:27,791 To track down this meal, eyesight isn't going to be much use. 53 00:05:29,128 --> 00:05:32,564 We've now entered the shark's sensory world. 54 00:05:36,269 --> 00:05:39,670 We see the turtle as a source of tantalising smells. 55 00:05:46,679 --> 00:05:51,878 As it decomposes, telltale scents diffuse into the water. 56 00:05:59,325 --> 00:06:02,624 Some scent molecules sink down into the deeper currents, 57 00:06:02,795 --> 00:06:06,026 while others spread out quickly at the surface. 58 00:06:12,705 --> 00:06:17,540 These smells broadcast news of the turtle's death... and its location... 59 00:06:19,011 --> 00:06:23,812 ...news that travels on the current at about four kilometres per hour. 60 00:06:26,352 --> 00:06:31,085 Soon the dead turtle detection zone covers ten square kilometres... 61 00:06:34,794 --> 00:06:40,027 ...and our female tiger shark is perfectly designed to find it. 62 00:06:42,568 --> 00:06:46,095 She can read signs in the water that we cannot see... 63 00:06:48,474 --> 00:06:52,308 ...unless we switch back into scent world... 64 00:06:54,914 --> 00:06:59,214 Now we can see the clouds of background smells that seep out of the reef, 65 00:06:59,385 --> 00:07:02,252 and dominate our tiger's life. 66 00:07:05,591 --> 00:07:09,027 She channels water through her ultra-sensitive nostrils, 67 00:07:09,195 --> 00:07:13,723 testing it for interesting odours twenty-four hours a day. 68 00:07:22,642 --> 00:07:27,841 Two-thirds of her brain is constantly at work analysing the messages received. 69 00:07:29,582 --> 00:07:34,246 ln effect, she is reading a chemical book dissolved in the water... 70 00:07:34,420 --> 00:07:37,480 a book that tells the story of what lies upstream. 71 00:07:37,657 --> 00:07:41,093 And this is crucial if she's to find the turtle. 72 00:07:44,230 --> 00:07:45,925 But she's not alone. 73 00:07:53,873 --> 00:07:56,774 There's no prize for runners-up... 74 00:08:08,754 --> 00:08:11,245 To increase her chance of picking up a lead, 75 00:08:11,424 --> 00:08:14,689 she zigzags back and forth across the current. 76 00:08:18,331 --> 00:08:21,232 This enables her to test a greater expanse of water 77 00:08:21,400 --> 00:08:23,868 than if she just swam straight ahead. 78 00:08:25,872 --> 00:08:28,500 She checks the deeper water near the sea floor, 79 00:08:28,674 --> 00:08:31,438 where heavier scent molecules may lie... 80 00:08:36,983 --> 00:08:39,417 ...then swims back to the surface... 81 00:08:39,585 --> 00:08:43,316 always covering as large a sample area as possible. 82 00:08:49,028 --> 00:08:52,088 She's getting closer to the turtle's scent trail, 83 00:08:52,265 --> 00:08:54,392 but unless she actually swims across it, 84 00:08:54,567 --> 00:08:56,694 she won't know it's there. 85 00:08:59,705 --> 00:09:03,334 Just a few metres to the left or right and she could miss it... 86 00:09:03,509 --> 00:09:06,171 and miss out on a meal. 87 00:09:15,388 --> 00:09:18,846 She's found it, and she cuts across the flow. 88 00:09:29,669 --> 00:09:32,536 She swims back and forth across the current. 89 00:09:32,705 --> 00:09:37,074 As one nostril leaves the odour trail, she turns back into it... 90 00:09:38,978 --> 00:09:40,172 ...she turns again, 91 00:09:40,346 --> 00:09:42,712 she mustn't lose this vein of scent... 92 00:09:42,882 --> 00:09:47,114 her unborn babies are depending on what may lay at the end of it. 93 00:09:53,159 --> 00:09:57,152 She swims up the scent corridor as it meanders back to its source... 94 00:09:57,330 --> 00:10:00,231 back to the dead turtle. 95 00:10:03,703 --> 00:10:06,536 But is she already too late? 96 00:10:09,108 --> 00:10:12,236 Other tiger sharks have hit the jackpot first. 97 00:10:30,096 --> 00:10:36,126 The tigers' saw-like teeth can slice through flesh, shell, even bones. 98 00:10:39,405 --> 00:10:42,499 More scent cascades into the water... 99 00:10:45,778 --> 00:10:50,875 ...and more sharks arrive to fight for the prime cuts. 100 00:10:52,985 --> 00:10:55,886 One turtle won't last long. 101 00:11:09,335 --> 00:11:12,736 lt's thirty minutes since our female hit the scent trail... 102 00:11:12,905 --> 00:11:16,568 but there's no way she could know if she's following old news. 103 00:11:29,855 --> 00:11:31,220 The meal is over 104 00:11:31,390 --> 00:11:35,918 and the few scraps left behind are snatched by smaller reef sharks. 105 00:11:43,769 --> 00:11:47,637 lt's back to square one for our pregnant female... 106 00:11:48,874 --> 00:11:54,870 She searches around the clock, circling and re-circling the island... 107 00:11:56,182 --> 00:12:00,551 She can swim non-stop up to ninety kilometres a day. 108 00:12:07,893 --> 00:12:11,021 But after a week she's still failed to find food, 109 00:12:11,197 --> 00:12:14,894 so her search has a renewed urgency. 110 00:12:16,335 --> 00:12:19,133 But it's evening, and an hour before dark 111 00:12:19,305 --> 00:12:23,366 there's a race for the safest places to spend the night. 112 00:12:32,718 --> 00:12:36,051 Our tiger shark's too big to nose about among the corals, 113 00:12:36,222 --> 00:12:39,988 so she heads out over the reef edge to deeper water. 114 00:12:41,861 --> 00:12:44,694 The reef fish are safe from our tiger... 115 00:12:44,864 --> 00:12:46,855 but what will the night bring? 116 00:12:50,136 --> 00:12:53,105 On moonless nights, there's little light down here. 117 00:12:53,272 --> 00:12:55,832 Even with the latest night-vision technology, 118 00:12:56,008 --> 00:12:58,875 it's hard for us to see anything. 119 00:12:59,044 --> 00:13:02,980 But there's a shark that's perfectly at ease in these black waters... 120 00:13:18,130 --> 00:13:24,194 lt hunts in total darkness... using senses hard for us to understand. 121 00:13:38,918 --> 00:13:43,787 lt's a whitetip reef shark - a specialist reef hunter. 122 00:13:44,023 --> 00:13:45,354 Even with no light at all, 123 00:13:45,524 --> 00:13:49,654 it weaves its way between the jagged corals without a scratch. 124 00:13:50,796 --> 00:13:54,129 lt's guided by a sixth sense that we don't have. 125 00:14:02,241 --> 00:14:07,076 lt has sensors that detect movements in the water from a couple of metres away. 126 00:14:07,413 --> 00:14:12,077 lt's as if it is surrounded... by a pressure sensitive bubble. 127 00:14:12,685 --> 00:14:16,849 The sensory cells run down each side of its body and over its head... 128 00:14:17,022 --> 00:14:19,718 and they're known as the lateral line. 129 00:14:26,265 --> 00:14:28,733 lt's a kind of 'touch-at-a-distance' - 130 00:14:28,901 --> 00:14:32,667 like having a second skin a short distance from its body. 131 00:14:34,273 --> 00:14:35,399 The shark can even feel 132 00:14:35,574 --> 00:14:39,567 when its own pressure wave becomes distorted by the coral walls. 133 00:14:39,745 --> 00:14:42,873 So it can hug the reef without colliding with it... 134 00:14:43,315 --> 00:14:46,944 ...investigating every nook and cranny for hidden fish. 135 00:15:06,171 --> 00:15:09,766 Even if they've wriggled out of sight deep in the coral fortress, 136 00:15:09,942 --> 00:15:12,843 fish still give themselves away... 137 00:15:13,145 --> 00:15:15,909 they can't do anything about their scent. 138 00:15:19,184 --> 00:15:21,084 Odours build up in their hideaways 139 00:15:21,253 --> 00:15:24,518 and tiny clues seep out across the reef... 140 00:15:28,427 --> 00:15:30,292 Courtesy of its lateral line, 141 00:15:30,462 --> 00:15:34,296 a whitetip can press its nostrils close in to the corals. 142 00:15:38,337 --> 00:15:41,431 lt's missed this tiny trail of fish odour. 143 00:15:47,613 --> 00:15:50,446 But there's more than one shark hunting tonight... 144 00:15:52,685 --> 00:15:55,313 ...whitetips hunt in gangs. 145 00:15:56,989 --> 00:15:59,150 When they do detect the whiff of food, 146 00:15:59,325 --> 00:16:02,624 they've got what it takes to get in really close. 147 00:16:06,065 --> 00:16:10,365 The whitetip's wedge-shaped head can squeeze into any crevice... 148 00:16:14,373 --> 00:16:17,706 lts thick skin, covered with millions of tiny teeth, 149 00:16:17,876 --> 00:16:19,707 acts like a chain mail suit, 150 00:16:19,878 --> 00:16:22,847 protection from the sharpest corals. 151 00:16:23,983 --> 00:16:26,850 By folding its large dorsal fin against its back, 152 00:16:27,019 --> 00:16:29,988 it can squirm even further into holes... 153 00:16:30,589 --> 00:16:33,820 and by manoeuvring along the seabed with its pectoral fins, 154 00:16:33,993 --> 00:16:36,393 it can reverse back out again. 155 00:16:48,073 --> 00:16:50,337 Our tiger shark's not so flexible... 156 00:16:50,509 --> 00:16:53,410 but then, she is considerably larger. 157 00:16:54,513 --> 00:16:56,981 A whitetip that strays too far from the reef 158 00:16:57,149 --> 00:16:59,811 could end up as her long-awaited meal... 159 00:17:05,824 --> 00:17:10,818 ...but now the whitetip gang has picked up the scent from a potential target... 160 00:17:15,267 --> 00:17:18,327 ...and they flush out a fish... 161 00:17:21,707 --> 00:17:23,038 ...a rock cod. 162 00:17:24,243 --> 00:17:26,541 lt's now caught in a game of cat and mouse - 163 00:17:26,712 --> 00:17:30,648 and don't forget this is all taking place in complete darkness. 164 00:17:40,893 --> 00:17:42,451 As it makes a dash for cover, 165 00:17:42,628 --> 00:17:45,028 the fish leaves something behind, 166 00:17:45,230 --> 00:17:47,494 and the shark follows it. 167 00:17:54,273 --> 00:17:58,676 Each time it thrashes its tail, it leaves a trail of disturbance... 168 00:17:58,844 --> 00:18:02,974 and this is detected by the shark's lateral line sensors. 169 00:18:25,304 --> 00:18:28,205 The rock cod's best chance is to stay calm 170 00:18:28,373 --> 00:18:32,002 and move slowly until it can find another hideaway. 171 00:18:36,281 --> 00:18:38,681 The whitetips probe the darkness. 172 00:18:41,520 --> 00:18:42,987 They know there's something here, 173 00:18:43,155 --> 00:18:46,249 if only they could pinpoint it. 174 00:18:54,366 --> 00:18:56,266 The sharks that follow in the fish's wake 175 00:18:56,435 --> 00:19:01,338 can also smell its scent trapped in the turbulence it leaves behind. 176 00:19:05,144 --> 00:19:07,442 The rock cod seems to live a charmed life, 177 00:19:07,613 --> 00:19:10,582 but the odds are stacking up against it. 178 00:19:15,787 --> 00:19:18,813 All it will take is one false move... 179 00:19:26,098 --> 00:19:29,499 The commotion trips the lateral lines of any sharks nearby, 180 00:19:29,668 --> 00:19:32,501 and they join the frenzy. 181 00:19:48,086 --> 00:19:50,077 But they need to watch their backs... 182 00:19:50,255 --> 00:19:55,022 like all sharks, tiger sharks have lateral lines too. 183 00:19:56,361 --> 00:19:58,329 Predators can soon become prey - 184 00:19:58,497 --> 00:20:01,989 the balance shifts throughout the night. 185 00:20:06,338 --> 00:20:10,104 By morning our tiger has still failed to find food. 186 00:20:11,643 --> 00:20:15,943 She's moved into the lagoon, heading for the fringe of mangroves... 187 00:20:16,114 --> 00:20:18,480 ...and she's in a hurry... 188 00:20:22,955 --> 00:20:25,753 She's detected an irresistible smell... 189 00:20:25,924 --> 00:20:29,883 and it seems to be coming from the shallow waters around the mangroves. 190 00:20:34,866 --> 00:20:37,164 But now she can't go any further... 191 00:20:38,937 --> 00:20:40,529 ...it's too shallow... 192 00:20:40,706 --> 00:20:44,142 just another a dead end in her search for food. 193 00:20:48,080 --> 00:20:50,378 The smell is coming from another mother - 194 00:20:50,549 --> 00:20:54,508 a much smaller lemon shark - giving birth. 195 00:21:00,926 --> 00:21:04,384 Like most reef sharks, she's swum in as shallow as she can 196 00:21:04,563 --> 00:21:08,363 so that her newborn pups are out of reach of larger sharks. 197 00:21:16,341 --> 00:21:17,603 Minutes after birth, 198 00:21:17,776 --> 00:21:21,803 each pup makes a beeline for the safety of the mangroves. 199 00:21:29,087 --> 00:21:33,251 For the moment, these youngsters are safe here from the tiger shark. 200 00:21:43,902 --> 00:21:45,529 But on the other side of the lagoon, 201 00:21:45,704 --> 00:21:48,639 there's more potential food arriving. 202 00:22:01,286 --> 00:22:04,619 They're stingrays, riding the incoming tide... 203 00:22:04,790 --> 00:22:08,487 heading for the island's central lagoon. 204 00:22:18,303 --> 00:22:22,467 Here, they search for worms and shellfish buried in the sand... 205 00:22:25,377 --> 00:22:27,971 ...and then disperse to find a resting place 206 00:22:28,146 --> 00:22:31,809 where they can simply disappear. 207 00:22:38,290 --> 00:22:42,249 Their sandy camouflage breaks up the outline of their bodies. 208 00:22:48,533 --> 00:22:50,933 Nothing will find them here... 209 00:22:51,103 --> 00:22:53,765 or will it? 210 00:22:56,007 --> 00:23:00,671 An even more sinister visitor is entering the lagoon. 211 00:23:03,682 --> 00:23:05,274 lt's hammerhead shark - 212 00:23:05,450 --> 00:23:09,011 a great hammerhead, more than three metres long. 213 00:23:19,598 --> 00:23:21,896 lts head swings from side to side, 214 00:23:22,067 --> 00:23:24,763 producing a strange double vision... 215 00:23:24,936 --> 00:23:29,236 first one eye points forward, and then the other. 216 00:23:31,309 --> 00:23:35,541 lts hammer-shaped head also works like the hydroplanes of a submarine, 217 00:23:35,714 --> 00:23:37,739 so it can turn on a sixpence, 218 00:23:37,916 --> 00:23:40,908 or dive rapidly to the bottom. 219 00:23:44,022 --> 00:23:48,982 Unlike our tiger shark, its speciality is hunting on the ocean floor. 220 00:23:49,227 --> 00:23:51,661 With smaller pectoral fins than other sharks, 221 00:23:51,830 --> 00:23:55,266 its belly almost skims the sand. 222 00:23:56,067 --> 00:24:00,060 And to detect its prey it has an extraordinary seventh sense, 223 00:24:00,238 --> 00:24:03,765 one that can find stingrays hiding under the sand. 224 00:24:05,143 --> 00:24:09,375 The underside of its head is lined with receptors that detect electricity... 225 00:24:09,548 --> 00:24:14,110 including minute pulses radiating from the gills of its prey. 226 00:24:23,829 --> 00:24:26,889 The stingrays may be lying still and out of sight 227 00:24:27,065 --> 00:24:31,695 but they will give themselves away by the very thing that keeps them alive. 228 00:24:33,605 --> 00:24:37,666 When a hammerhead's about, breathing can be fatal. 229 00:24:45,283 --> 00:24:48,810 ...three stingrays, hiding in an area the size of a football pitch, 230 00:24:48,987 --> 00:24:50,784 will not be easy to locate, 231 00:24:50,956 --> 00:24:54,414 especially as the shark must detect the very weak electric fields 232 00:24:54,593 --> 00:24:57,619 generated by the activity of their gills... 233 00:24:59,931 --> 00:25:04,561 ...these electric fields can be picked up from twenty centimetres above the sand... 234 00:25:05,303 --> 00:25:08,602 ...so the shark must be directly above its target... 235 00:25:10,475 --> 00:25:13,501 ...the hammerhead is systematic in its search... 236 00:25:15,146 --> 00:25:18,479 ...it sweeps its head from side to side, like a metal detector, 237 00:25:18,650 --> 00:25:21,585 covering as much sea floor as possible... 238 00:25:21,753 --> 00:25:24,221 its electrical sensors are so sensitive, 239 00:25:24,389 --> 00:25:27,017 it's the equivalent of detecting a household battery 240 00:25:27,192 --> 00:25:30,025 from halfway across the Pacific Ocean... 241 00:25:39,971 --> 00:25:44,271 ...effective quartering will ensure that the stingrays have no place to hide... 242 00:25:47,212 --> 00:25:49,908 lt's now close to a target. 243 00:26:00,358 --> 00:26:02,519 lt tightens its search pattern. 244 00:26:18,276 --> 00:26:21,837 The shark uses its hammer to pin down the ray. 245 00:26:23,915 --> 00:26:27,749 lt disables its prey with a single bite out of its wing. 246 00:26:29,988 --> 00:26:32,821 The commotion in the water alerts our tiger shark... 247 00:26:32,991 --> 00:26:35,653 and she comes in to investigate. 248 00:26:37,162 --> 00:26:40,393 Scent oozes from the dying prey. 249 00:26:42,267 --> 00:26:45,168 The tiger shark locks in. 250 00:26:48,773 --> 00:26:52,573 A pregnant female might do better to avoid this clash of titans. 251 00:26:52,744 --> 00:26:56,077 But the smell of food is just too tempting... 252 00:27:04,122 --> 00:27:05,817 Anyway she's bigger - 253 00:27:05,991 --> 00:27:09,950 usually the last word in shark body language. 254 00:27:10,695 --> 00:27:13,721 The hammerhead gives way. 255 00:27:21,906 --> 00:27:25,137 Our tiger cautiously nudges the stingray's body. 256 00:27:25,477 --> 00:27:30,039 Taste receptors in her mouth check out if it's edible before the first bite. 257 00:27:40,058 --> 00:27:42,390 lt's a successful heist... 258 00:27:46,798 --> 00:27:48,322 ...though no more than a snack, 259 00:27:48,500 --> 00:27:53,437 it's enough to be converted into some food for her forty unborn babies. 260 00:27:56,875 --> 00:28:00,242 A nutritious fluid oozes from the lining of the mother's womb, 261 00:28:00,412 --> 00:28:02,903 bathing the babies in a fat-rich milk. 262 00:28:03,081 --> 00:28:05,379 They drink and grow... 263 00:28:08,920 --> 00:28:12,287 ...but only if their mother manages to eat for forty... 264 00:28:24,035 --> 00:28:28,665 Food is what drew our female tiger to the shark battlefield in the first place. 265 00:28:29,541 --> 00:28:31,304 lt's the three-month breeding season 266 00:28:31,476 --> 00:28:34,673 and the island literally teems with life. 267 00:28:58,603 --> 00:29:00,833 Millions of seabirds come here... 268 00:29:01,005 --> 00:29:02,939 with no predators on land at least, 269 00:29:03,108 --> 00:29:06,373 the island is a perfect place to nest. 270 00:29:09,914 --> 00:29:12,109 As more prospective parents fly in, 271 00:29:12,283 --> 00:29:14,774 there's hardly room to touch down... 272 00:29:20,759 --> 00:29:22,727 ...but this carpet of sooty terns 273 00:29:22,894 --> 00:29:26,159 provides surprising benefits for sharks. 274 00:29:30,101 --> 00:29:35,129 Each day the parent birds bring back hundreds of tons of fish and squid. 275 00:29:41,479 --> 00:29:43,413 They and their babies process it 276 00:29:43,581 --> 00:29:48,416 and turn it into something else that helps keep everyone well fed. 277 00:29:49,454 --> 00:29:51,513 lt's guano! 278 00:29:52,891 --> 00:29:56,019 Layer upon layer of seabird fertiliser. 279 00:30:01,499 --> 00:30:05,196 All it needs is water and a good stir. 280 00:30:18,349 --> 00:30:23,343 The guano run-off mixes with nutrient-rich upwellings from the deep sea floor... 281 00:30:27,692 --> 00:30:30,126 ...creating exceptionally fertile waters - 282 00:30:30,295 --> 00:30:33,787 the basis of a busy food chain. 283 00:30:35,133 --> 00:30:39,593 The nutritious soup feeds plankton... microscopic plants and animals, 284 00:30:39,771 --> 00:30:42,433 which are the staple diet of small fish - 285 00:30:42,907 --> 00:30:45,432 which in turn are eaten by larger fish - 286 00:30:45,610 --> 00:30:47,805 hundreds of millions of them... 287 00:30:47,979 --> 00:30:50,812 and at the top of the chain - sharks - 288 00:30:50,982 --> 00:30:52,950 hundreds of them. 289 00:30:55,253 --> 00:30:59,349 Tiger, black-tip and bronze whaler sharks invade the outer battlefield, 290 00:30:59,524 --> 00:31:02,425 after a gigantic shoal of anchovies. 291 00:31:03,828 --> 00:31:06,422 Bronze whaler sharks are speed merchants. 292 00:31:08,433 --> 00:31:11,061 Lightning attacks shatter the shoal's defences - 293 00:31:11,236 --> 00:31:14,330 a confusion of flashing scales. 294 00:31:37,161 --> 00:31:41,188 The tiger sharks are not fast enough to keep up with this crowd. 295 00:31:50,241 --> 00:31:54,735 But these new arrivals are about to show the lot of them how it's done... 296 00:31:56,614 --> 00:32:01,074 Bryde's whales... while sharks take a single fish in each attack, 297 00:32:01,786 --> 00:32:06,416 these monsters take a hundred kilos of fish with a single lunge. 298 00:32:18,403 --> 00:32:21,304 From guano to huge ocean predators... 299 00:32:21,472 --> 00:32:24,441 it's one of the most fruitful food chains on the planet - 300 00:32:24,609 --> 00:32:28,409 and it underpins the whole shark battlefield. 301 00:32:39,557 --> 00:32:42,253 Across the island there's another gathering. 302 00:32:47,265 --> 00:32:49,324 lt's in the main channel. 303 00:32:52,971 --> 00:32:54,962 These are grey reef sharks - 304 00:32:55,139 --> 00:32:58,233 and they meet at this spot every day. 305 00:33:15,159 --> 00:33:18,151 They've got to keep moving or they'll sink and drown, 306 00:33:18,329 --> 00:33:21,696 but they can 'rest' by hanging in the current. 307 00:33:22,333 --> 00:33:26,667 As the tide moves in and out, they just go with the flow. 308 00:33:29,574 --> 00:33:33,169 To our hungry female tiger, it's a tantalising sight... 309 00:33:33,344 --> 00:33:36,780 but grey reef sharks have numbers on their side... 310 00:33:39,017 --> 00:33:41,451 One spots the cruising tiger... 311 00:33:41,652 --> 00:33:44,746 and warns her to keep her distance. 312 00:33:52,397 --> 00:33:56,163 The body language of the smaller shark is crystal clear. 313 00:33:56,401 --> 00:33:58,494 By swimming in this strange, stiff manner - 314 00:33:58,669 --> 00:34:01,399 snout up, pectoral fins down - 315 00:34:01,572 --> 00:34:05,030 it signals that the tiger should back off. 316 00:34:07,178 --> 00:34:09,703 And although it's just a quarter of the tiger's size, 317 00:34:09,881 --> 00:34:14,511 a grey reef shark could seriously damage her by slashing at her gills. 318 00:34:23,728 --> 00:34:25,889 The stand-off is interrupted... 319 00:34:26,264 --> 00:34:28,562 and it could mean food. 320 00:34:47,452 --> 00:34:50,046 lt's a shoal of spawning parrot fish. 321 00:34:53,157 --> 00:34:56,615 Every time they dart up to mate they produce a sound, 322 00:34:56,794 --> 00:34:59,695 and sounds travel further in water than in air - 323 00:34:59,864 --> 00:35:02,526 it's a dinner bell for sharks. 324 00:35:20,585 --> 00:35:24,749 Reef sharks are sensitive to just this kind of low-frequency sound... 325 00:35:29,360 --> 00:35:32,591 Our tiger registers the racket too... 326 00:35:33,030 --> 00:35:35,521 And whitetips join the scramble. 327 00:35:44,108 --> 00:35:47,134 A whitetip is first on the scene... 328 00:35:56,654 --> 00:36:00,454 ...the parrotfish are in a 'spawning stupor' and slow to react... 329 00:36:01,859 --> 00:36:03,622 The whitetip catches one... 330 00:36:03,794 --> 00:36:08,128 but with a larger grey reef shark arriving, can it keep its catch? 331 00:36:22,446 --> 00:36:25,711 The grey reef shark has made off with the whitetip's prey, 332 00:36:25,883 --> 00:36:29,250 but it attracts too much attention... 333 00:36:30,488 --> 00:36:34,151 ...and not just from other members of its own gang. 334 00:36:38,062 --> 00:36:42,761 Our tiger shark's back and she's picked up the smell of sliced fish. 335 00:37:03,754 --> 00:37:08,953 But with forty babies inside, she's just too slow. 336 00:37:09,493 --> 00:37:12,360 The reef shark keeps its fish. 337 00:37:13,197 --> 00:37:16,462 The pregnant female will have to draw upon her own fat reserves 338 00:37:16,634 --> 00:37:19,569 so her unborn pups can grow. 339 00:37:20,471 --> 00:37:22,336 They're now six weeks from birth, 340 00:37:22,506 --> 00:37:23,939 but they'll need to develop some more 341 00:37:24,108 --> 00:37:26,770 before they're ready for the outside world. 342 00:37:30,114 --> 00:37:33,208 Their mother heads again for deeper water. 343 00:37:46,697 --> 00:37:50,861 For now, the islands and other prey are safe... 344 00:37:51,269 --> 00:37:53,703 ...but even sharks have problems. 345 00:37:53,871 --> 00:37:56,738 There are times when they must call a truce... 346 00:37:56,907 --> 00:38:00,138 moments when food is not the top priority. 347 00:38:04,815 --> 00:38:09,275 Clinging to this whitetip's skin there are hundreds of tiny sea lice. 348 00:38:16,360 --> 00:38:19,989 They suck its blood and damage its chain mail suit. 349 00:38:30,808 --> 00:38:32,901 The whitetip can't get rid of them... 350 00:38:33,077 --> 00:38:35,341 but it knows a fish that can. 351 00:38:40,918 --> 00:38:43,785 There are fish on the reef that eat such parasites - 352 00:38:43,954 --> 00:38:47,117 and they're just waiting for the chance to get to work. 353 00:38:48,592 --> 00:38:51,459 This turtle drops into their cleaning station. 354 00:39:05,409 --> 00:39:09,277 Each cleaner fish eats over a thousand parasites a day. 355 00:39:09,447 --> 00:39:12,473 Eager clients are prepared to queue for treatment... 356 00:39:12,650 --> 00:39:15,050 some are more patient than others. 357 00:39:20,491 --> 00:39:24,018 Without cleaner fish, the clientele could die of skin disease. 358 00:39:24,395 --> 00:39:27,922 But luckily, they're open at the same address seven days a week, 359 00:39:28,099 --> 00:39:31,000 and all the locals know it. 360 00:39:35,005 --> 00:39:36,632 Reef sharks are on their way... 361 00:39:36,807 --> 00:39:40,743 and cleaning station customers could be vulnerable... 362 00:39:48,886 --> 00:39:51,218 Hammerheads are heading there too. 363 00:40:08,873 --> 00:40:11,273 But the sharks aren't hunting. 364 00:40:11,442 --> 00:40:14,206 Sharks are also patrons of the cleaning station. 365 00:40:14,378 --> 00:40:19,338 Up to twenty settle into their favourite seats and wait for service. 366 00:40:24,655 --> 00:40:29,524 Like a formula one pit stop team, the shark cleaners race into action... 367 00:40:30,227 --> 00:40:31,353 ...gills... 368 00:40:31,829 --> 00:40:32,955 ...fins... 369 00:40:36,100 --> 00:40:37,533 ...head, mouth and eyes - 370 00:40:37,701 --> 00:40:39,999 they all get a thorough inspection. 371 00:40:46,577 --> 00:40:48,943 The cleaners do the inside too! 372 00:40:49,213 --> 00:40:50,680 They're so important to the sharks 373 00:40:50,848 --> 00:40:53,578 that they can go where other fish would fear to swim. 374 00:40:56,587 --> 00:40:58,885 lt's not only whitetips at the drive-through. 375 00:40:59,056 --> 00:41:01,991 Up above, grey reef sharks join the queue. 376 00:41:05,930 --> 00:41:07,192 They can't stop - 377 00:41:07,364 --> 00:41:09,559 they must keep moving to keep breathing - 378 00:41:09,733 --> 00:41:13,169 so their pit stop team swims along with them. 379 00:41:16,040 --> 00:41:18,099 A hammerhead is next in line. 380 00:41:25,416 --> 00:41:27,714 Fungus is playing havoc with its skin, 381 00:41:27,885 --> 00:41:30,615 and it rolls over to invite attention. 382 00:41:34,325 --> 00:41:36,054 The cleaners pile in... 383 00:41:36,227 --> 00:41:40,596 For a short time, this is neutral ground on the shark battlefield. 384 00:41:45,269 --> 00:41:48,136 The hammerheads are serviced and leave... 385 00:41:49,707 --> 00:41:52,801 but the tiger sharks are nowhere to be seen. 386 00:42:02,953 --> 00:42:05,854 They're gathered in one particular part of the lagoon - 387 00:42:06,090 --> 00:42:08,558 as they've been doing for the past two weeks. 388 00:42:08,726 --> 00:42:10,717 But for what? 389 00:42:11,095 --> 00:42:12,562 Each morning they arrive, 390 00:42:12,730 --> 00:42:16,689 just hang around a few hours, then leave again. 391 00:42:22,406 --> 00:42:25,204 Scents in the water say they're in the right place, 392 00:42:25,376 --> 00:42:29,005 but they're too early and so far nothing's happening. 393 00:42:31,115 --> 00:42:34,448 Today, though, could be the day. 394 00:42:37,221 --> 00:42:40,918 lt's some time now since our pregnant female last fed... 395 00:42:41,091 --> 00:42:45,892 and for her unborn pups, the lack of food is reaching crisis point again. 396 00:42:53,904 --> 00:42:56,498 But here's the signal she's been waiting for... 397 00:43:02,346 --> 00:43:04,871 lt's just the sound of waves above - 398 00:43:05,049 --> 00:43:08,883 but what it says is that the wind is picking up. 399 00:43:14,425 --> 00:43:18,885 And the sharks have learned that the wind can mean fresh food at the surface. 400 00:43:19,463 --> 00:43:20,953 The light is blinding 401 00:43:21,131 --> 00:43:25,534 but a special pigment curtain in the shark's eyes enables them to adapt. 402 00:43:28,339 --> 00:43:31,035 And here's what they've been looking for... 403 00:43:45,589 --> 00:43:47,887 A colony of albatrosses... 404 00:43:48,058 --> 00:43:50,891 ...full of chicks about to learn to fly. 405 00:43:53,130 --> 00:43:56,156 All they've been waiting for is wind. 406 00:43:57,801 --> 00:44:02,170 And all the tiger sharks are waiting for is them. 407 00:44:05,776 --> 00:44:07,710 This morning there's a stiff breeze... 408 00:44:07,878 --> 00:44:11,075 and the chicks are ready to set off. 409 00:44:19,923 --> 00:44:22,221 One by one, they make their way to the water, 410 00:44:22,393 --> 00:44:25,260 where they attempt to take off. 411 00:44:34,138 --> 00:44:36,368 ...but every splash could mean a meal... 412 00:44:36,674 --> 00:44:39,108 our female moves in for the kill. 413 00:44:47,651 --> 00:44:50,415 But the chicks are not the sitting ducks they seem... 414 00:44:50,721 --> 00:44:54,782 our shark's own bow wave sweeps them out of harm's way! 415 00:45:10,941 --> 00:45:14,502 Another reason for her failure is that just before attack, 416 00:45:14,678 --> 00:45:19,206 a protective membrane covers her eyes and she swims blind... 417 00:45:20,918 --> 00:45:23,944 or risk serious injury from this sharp bill. 418 00:45:27,658 --> 00:45:30,456 She still has her electrical receptors as a guide... 419 00:45:30,627 --> 00:45:35,496 but in half air, half water, the grab is not precise. 420 00:45:43,240 --> 00:45:46,539 Our pregnant mother can't afford to miss this opportunity. 421 00:45:47,845 --> 00:45:50,439 Soon all the chicks will be gone. 422 00:46:16,006 --> 00:46:18,099 With the breeding season coming to an end, 423 00:46:18,275 --> 00:46:23,008 this could be our tiger's last chance to feed at the island this year. 424 00:46:23,547 --> 00:46:28,575 And back out in the open ocean, finding a meal will be even more difficult. 425 00:46:29,887 --> 00:46:33,345 lf she doesn't get enough food, neither will her babies. 426 00:46:35,292 --> 00:46:37,852 Each one is now sixty centimetres long, 427 00:46:38,028 --> 00:46:41,486 packed tightly in the womb like sardines in a can. 428 00:46:47,304 --> 00:46:49,670 Their mother has to eat - and eat well - 429 00:46:49,840 --> 00:46:52,809 if they are all to survive. 430 00:46:59,283 --> 00:47:03,515 But, today, one last wave of chicks warms up for take-off. 431 00:47:11,795 --> 00:47:14,457 lt's late - they've missed the morning breeze, 432 00:47:14,631 --> 00:47:17,566 and as the wind drops, they flounder in the sea. 433 00:47:17,768 --> 00:47:19,565 They seem oblivious of the danger 434 00:47:19,736 --> 00:47:23,365 and by now the sharks have learned new tricks. 435 00:47:35,085 --> 00:47:36,609 They rush across the surface, 436 00:47:36,787 --> 00:47:38,652 so they don't make bow waves... 437 00:47:38,822 --> 00:47:41,256 and the chicks don't know what's hit them. 438 00:47:58,609 --> 00:48:00,236 Our female has fed well. 439 00:48:00,544 --> 00:48:03,570 Her growing babies can put on their final spurt. 440 00:48:04,715 --> 00:48:10,085 With shark stomachs full, the last few albatrosses have a smooth start. 441 00:48:14,224 --> 00:48:16,158 Soon our mother will give birth, 442 00:48:16,326 --> 00:48:18,817 but where is still a total mystery. 443 00:48:19,029 --> 00:48:21,827 Even with all our technology and scientific knowledge 444 00:48:21,999 --> 00:48:26,163 we still don't know where in the ocean tiger sharks begin their lives. 445 00:48:26,336 --> 00:48:29,464 Nobody has ever witnessed a tiger shark birth. 446 00:48:30,607 --> 00:48:36,944 What we do know is that only two or three will survive their first few years at sea. 447 00:48:37,447 --> 00:48:39,915 And if they do, in five years time 448 00:48:40,083 --> 00:48:45,646 they'll come back to join the hunt at the Shark Battlefield.