WEBVTT 00:00:12.846 --> 00:00:15.056 Welcome to North America. 00:00:21.271 --> 00:00:23.690 The most diverse continent on Earth. 00:00:27.402 --> 00:00:31.906 The only place where you can find every landscape from polar ice cap... 00:00:33.700 --> 00:00:37.662 to baking desert, and everything in between. 00:00:39.998 --> 00:00:44.669 Whatever the neighborhood, there's a hero who can make it a home. 00:00:51.342 --> 00:00:56.681 Smart, tough, brave, you name it. 00:00:57.474 --> 00:00:59.392 They all have what it takes to thrive 00:01:00.393 --> 00:01:04.522 in the wildest, most beautiful continent on Earth. 00:01:44.813 --> 00:01:47.440 The wild Northwest of America. 00:01:50.652 --> 00:01:55.824 Majestic mountain ranges march north for over 2,000 miles, 00:01:56.908 --> 00:01:58.868 from Wyoming to Alaska, 00:01:59.869 --> 00:02:03.998 and onwards into the great wilderness of the Arctic. 00:02:06.376 --> 00:02:08.545 This is a world of epic winters 00:02:08.670 --> 00:02:11.736 and brief, spectacular summers, 00:02:11.760 --> 00:02:14.634 where timing is everything. 00:02:26.771 --> 00:02:29.149 There's a new ruler in the Arctic. 00:02:36.447 --> 00:02:39.075 The pack's relying on her to find the food they'll need 00:02:39.159 --> 00:02:41.286 to get through the tough months ahead. 00:02:52.338 --> 00:02:57.468 Their home is Ellesmere Island, as far north as you can go. 00:03:02.515 --> 00:03:03.892 Food is scarce. 00:03:04.267 --> 00:03:08.438 The wolves search a territory larger than any other wolf pack on Earth. 00:03:11.858 --> 00:03:14.819 Forty times the size of Washington, D.C. 00:03:19.032 --> 00:03:20.116 They're in luck. 00:03:20.867 --> 00:03:22.994 She's picked up a scent. 00:03:31.127 --> 00:03:34.797 But this prey will test the pack to their limit. 00:03:42.680 --> 00:03:46.059 Musk ox. Relics of the Ice Age. 00:03:47.810 --> 00:03:52.649 Five foot high and 800 pounds of pure attitude. 00:04:02.283 --> 00:04:04.827 An adult ox could definitely kill a wolf. 00:04:06.704 --> 00:04:10.333 But this herd has calves, and with the right game plan, 00:04:10.458 --> 00:04:12.085 the pack could take one down. 00:04:21.594 --> 00:04:24.597 The howl says, "It's time to hunt." 00:04:29.894 --> 00:04:33.147 The alpha female goes out front to spook the herd. 00:04:33.815 --> 00:04:38.152 She wants to get them running, to tire them out and separate the young. 00:04:48.538 --> 00:04:52.583 She chooses the two fastest wolves to help her push the herd 00:04:52.667 --> 00:04:55.295 into a 40-mile-per-hour sprint. 00:05:12.895 --> 00:05:14.314 She's got a calf, 00:05:15.356 --> 00:05:18.151 but her teammates aren't watching her back. 00:05:24.282 --> 00:05:27.910 The wolves' mistake gives the musk ox time to regroup. 00:05:35.293 --> 00:05:38.087 A wall of horns surround the calves. 00:05:39.797 --> 00:05:43.801 Shoulder to shoulder, they're unbreakable. 00:05:51.851 --> 00:05:53.853 This hunt is over. 00:05:55.313 --> 00:05:56.522 It's a stalemate. 00:05:57.607 --> 00:05:59.984 A strong leader knows when to back down. 00:06:03.321 --> 00:06:04.906 With this opportunity lost, 00:06:06.282 --> 00:06:10.411 it's going to be a long and hungry winter for this wolf pack. 00:06:18.086 --> 00:06:22.548 In Alaska, the first snow covers the peaks of the Aleutian Range. 00:06:33.935 --> 00:06:36.062 Where these mountains meet the sea, 00:06:36.562 --> 00:06:40.441 another northerner needs to pack on the pounds before the winter comes. 00:06:45.571 --> 00:06:46.739 A brown bear. 00:06:48.574 --> 00:06:52.245 Survival superpower? An iron stomach. 00:06:54.956 --> 00:06:58.876 The smell of this rotting whale carcass is attracting bears 00:06:58.960 --> 00:07:01.045 from as far as 20 miles away. 00:07:07.468 --> 00:07:09.554 Males are usually bad-tempered loners. 00:07:12.432 --> 00:07:15.810 But each bear needs up to 60,000 calories a day 00:07:16.060 --> 00:07:19.021 to get fat enough to make it through the winter hibernation. 00:07:21.274 --> 00:07:22.984 So, they've called a truce. 00:07:26.404 --> 00:07:29.365 It's pretty rare to see this many together on a carcass. 00:07:31.159 --> 00:07:35.121 Thirty tons of rotting flesh doesn't just wash up every day. 00:07:52.972 --> 00:07:57.602 When bulking up, brown bears can eat for 20 hours straight. 00:08:04.650 --> 00:08:07.403 Oh, whoa. I think he's overdone it. 00:08:11.574 --> 00:08:12.992 Time for an ice bath. 00:08:15.620 --> 00:08:16.746 Oh, man. 00:08:20.416 --> 00:08:23.252 These guys have timed things just right, 00:08:24.712 --> 00:08:27.507 fattening up before the Alaskan winter strikes. 00:08:48.444 --> 00:08:50.530 In the Teton Mountains of Wyoming, 00:08:51.405 --> 00:08:55.409 another American icon is planning even further ahead. 00:09:12.593 --> 00:09:15.388 This female moose has spent the year alone. 00:09:19.559 --> 00:09:22.687 With fall here, she wants to become a mom. 00:09:24.480 --> 00:09:27.525 And now, she's the center of attention. 00:09:31.237 --> 00:09:34.282 This huge male has been on her for days. 00:09:42.248 --> 00:09:44.792 Sure, he's got five-foot antlers. 00:09:45.585 --> 00:09:48.462 But they can hit six-foot. 00:09:50.715 --> 00:09:54.885 Let's just say she's keeping her options open. 00:10:01.142 --> 00:10:04.645 Now, this guy has got her attention. 00:10:06.397 --> 00:10:08.399 Tall, dark, and handsome. 00:10:11.152 --> 00:10:12.778 He's got moves. 00:10:17.116 --> 00:10:18.200 Impressive! 00:10:20.828 --> 00:10:24.081 These two will fight over the right to mate with the female. 00:10:26.626 --> 00:10:29.337 But only when she decides the time is right. 00:10:31.881 --> 00:10:35.801 If she mates right now, her calf will be born in the middle of winter 00:10:36.135 --> 00:10:37.511 and may not survive. 00:10:38.971 --> 00:10:44.268 If she holds out just a little bit longer, it could be born in the warmth of spring. 00:10:47.897 --> 00:10:50.608 Timing is everything. 00:10:57.907 --> 00:11:03.579 After a month, things get a little... hmm, weird. 00:11:07.166 --> 00:11:08.542 Sniffing... 00:11:12.129 --> 00:11:13.422 thrashing... 00:11:18.010 --> 00:11:20.262 And quite a bit of this. 00:11:22.640 --> 00:11:25.309 Male urine is full of pheromones, 00:11:25.685 --> 00:11:28.771 sort of a moose aphrodisiac. 00:11:30.064 --> 00:11:33.859 She has to resist it, though. It's still too early. 00:11:34.110 --> 00:11:37.530 Just be patient, big guy. 00:11:44.662 --> 00:11:47.790 Two months in, and winter shows its hand. 00:11:52.628 --> 00:11:54.213 With colder temperatures... 00:11:55.673 --> 00:11:57.091 come shorter days. 00:11:58.634 --> 00:12:01.053 That's the cue she's been waiting for. 00:12:03.431 --> 00:12:05.015 Now, she's ready. 00:12:07.017 --> 00:12:08.894 And the bulls know it. 00:12:18.946 --> 00:12:22.324 Their neck muscles have doubled in size over the summer 00:12:22.408 --> 00:12:25.911 in preparation for this exact moment. 00:12:30.166 --> 00:12:33.711 They leverage 1,600 pounds of power... 00:12:37.965 --> 00:12:39.258 to breaking point. 00:12:47.683 --> 00:12:49.560 The antler will grow again. 00:12:50.394 --> 00:12:53.397 Now, his pride? That'll take some healing. 00:12:58.277 --> 00:13:00.488 Our female's got her timing right. 00:13:01.030 --> 00:13:03.574 Her calf will be born eight months from now, 00:13:03.824 --> 00:13:05.451 at the best possible time. 00:13:10.539 --> 00:13:13.751 But first, there's winter to deal with. 00:13:26.597 --> 00:13:30.267 The mountains of the Northwest are the tallest in North America. 00:13:32.853 --> 00:13:35.272 Many over 14,000 feet. 00:13:37.191 --> 00:13:41.612 And they see some of the most extreme winter weather on Earth. 00:13:47.326 --> 00:13:50.287 Humid air blows in from the Pacific. 00:13:51.705 --> 00:13:54.250 It crashes against the frozen peaks. 00:13:58.838 --> 00:14:04.927 Resulting in epic snowfall. Up to 100 feet in a single year. 00:14:15.104 --> 00:14:18.858 The further north you go, the harder the winter hits. 00:14:29.076 --> 00:14:32.246 Alaska is the northernmost state in the U.S., 00:14:33.747 --> 00:14:37.960 though even here, there's one place which doesn't freeze. 00:14:39.128 --> 00:14:44.008 The Chilkat River, heated from below by geothermal vents. 00:14:46.969 --> 00:14:49.680 Chilkat means "salmon storehouse" 00:14:49.847 --> 00:14:52.224 in the language of the local Tlingit people. 00:14:53.517 --> 00:14:57.062 And one hungry American hero knows just why. 00:15:03.527 --> 00:15:07.031 This young bald eagle's been struggling to find food. 00:15:08.991 --> 00:15:11.744 Most eagles don't make it through their first winter. 00:15:13.037 --> 00:15:15.998 But here, because the river doesn't freeze, 00:15:16.749 --> 00:15:19.960 he can access dead fish from the salmon run... 00:15:21.337 --> 00:15:23.422 ...to boost his chances of survival. 00:15:32.264 --> 00:15:35.225 Only problem is, he's got company. 00:15:41.023 --> 00:15:43.359 Well, that's intimidating. 00:15:45.778 --> 00:15:50.449 Almost 4,000 bald eagles come to feed here every winter. 00:15:52.701 --> 00:15:54.828 The largest gathering in the world. 00:15:56.288 --> 00:15:58.624 And it's every eagle for itself. 00:16:06.465 --> 00:16:08.676 With a wingspan of eight feet, 00:16:08.759 --> 00:16:13.389 the females are the largest and the most aggressive. 00:16:27.069 --> 00:16:28.654 That head tilt tells him, 00:16:28.988 --> 00:16:33.701 "You don't want any problems. Back off. The fish is mine." 00:16:37.830 --> 00:16:41.792 He's hungry, but he's also afraid of getting hurt. 00:16:48.590 --> 00:16:51.051 From the sidelines, he can see that around here, 00:16:51.385 --> 00:16:53.178 you have to fight to survive. 00:17:37.931 --> 00:17:42.561 He could hold back, stay safe, and face starvation. 00:17:44.271 --> 00:17:46.356 But he decides to risk it all. 00:18:19.640 --> 00:18:21.391 Through sheer determination, 00:18:21.892 --> 00:18:25.771 he’s fought off the elders and staked his claim. 00:18:32.778 --> 00:18:36.740 These vital calories will help him through the hard months ahead. 00:18:45.833 --> 00:18:48.460 Nowhere does winter like here. 00:18:49.044 --> 00:18:52.798 Denali, the highest mountain in North America, 00:18:53.215 --> 00:18:57.386 and at minus 100 degrees, the coldest on Earth. 00:19:00.681 --> 00:19:03.934 With the last light disappearing over the horizon, 00:19:04.184 --> 00:19:09.565 the clock starts on the hardest, darkest months of the year. 00:19:14.069 --> 00:19:16.905 Though sometimes, there's magic in the air. 00:19:23.120 --> 00:19:27.875 The absolute dark makes Alaska one of the best places on Earth 00:19:28.125 --> 00:19:33.964 to witness the aurora borealis. 00:20:11.793 --> 00:20:15.672 In Ellesmere, polar winds rip through the barren hills. 00:20:27.726 --> 00:20:29.645 You can’t complain about the cold 00:20:29.895 --> 00:20:32.481 when your ancestors survived the Ice Age. 00:20:35.525 --> 00:20:41.281 Under their long guard hairs, the musk oxen have grown a fine underwool, 00:20:41.907 --> 00:20:44.451 the warmest fiber in the world. 00:20:47.287 --> 00:20:52.209 In the dead of winter, they slow their metabolism by 30 percent. 00:20:59.299 --> 00:21:01.843 They're the ultimate arctic survivors. 00:21:02.386 --> 00:21:05.472 Staying slow to stay alive. 00:21:12.980 --> 00:21:16.692 Winter takes northern heroes to their limits. 00:21:20.153 --> 00:21:23.657 But eventually, the good times return. 00:21:26.952 --> 00:21:28.036 Spring. 00:21:41.174 --> 00:21:44.386 The warmth sweeps northwards into Alaska. 00:21:58.900 --> 00:22:01.361 A wake-up call for brown bears. 00:22:03.196 --> 00:22:06.074 A mom and her two new cubs. 00:22:09.453 --> 00:22:12.164 She nursed them through the winter inside her den... 00:22:14.958 --> 00:22:17.669 but it cost 30 percent of her body weight. 00:22:18.503 --> 00:22:20.213 She needs to eat fast. 00:22:23.300 --> 00:22:26.553 The cubs are just enjoying their newfound freedom. 00:22:31.600 --> 00:22:35.270 A snow day. It's the ultimate cure for cabin fever. 00:22:36.063 --> 00:22:37.230 Gotcha! 00:22:55.999 --> 00:22:59.586 The only way to get these little guys' attention is milk. 00:23:08.011 --> 00:23:11.223 Out of the den, their intake quadruples. 00:23:18.313 --> 00:23:20.816 They’ll need over a half gallon a day. 00:23:27.239 --> 00:23:30.492 To keep the family fed, she follows her nose. 00:23:31.618 --> 00:23:34.246 Two thousand times more sensitive than ours. 00:23:42.087 --> 00:23:47.676 Jackpot! A termite nest, full of protein-packed insects. 00:23:58.687 --> 00:24:01.231 It’s their first taste of solid food. 00:24:04.860 --> 00:24:06.987 Though this meal bites back. 00:24:13.452 --> 00:24:16.339 This snack buys Mom some time, 00:24:16.363 --> 00:24:19.249 but there’s not much else up here to eat. 00:24:23.879 --> 00:24:29.551 She knows that if she can just get them down the mountain, a feast awaits. 00:24:47.110 --> 00:24:51.323 She’s not the only new mom who needs to step up for her family. 00:25:03.210 --> 00:25:06.296 The Ellesmere wolf pack made it through the winter. 00:25:10.008 --> 00:25:11.760 They got four new pups to feed. 00:25:24.397 --> 00:25:27.317 A musk ox calf won’t feed them all this time. 00:25:30.445 --> 00:25:32.948 Taking on an adult is high risk. 00:25:36.117 --> 00:25:39.496 But the alpha knows there’s no other choice. 00:25:41.665 --> 00:25:45.043 In 24-hour daylight, there’s nowhere to hide. 00:25:46.086 --> 00:25:49.756 So, the pack stalks downwind to hide their scent. 00:26:04.563 --> 00:26:07.524 Like before, the fastest split the herd 00:26:07.691 --> 00:26:09.943 and single out a weak adult. 00:26:16.658 --> 00:26:19.911 This time, the rest of the pack closes in. 00:26:28.086 --> 00:26:32.424 It’s an ancient battle between two heroes of the frozen north. 00:26:37.804 --> 00:26:41.933 And the pups, they're learning the art of the hunt through their elders. 00:27:05.457 --> 00:27:07.375 Thanks to the alpha's leadership, 00:27:07.459 --> 00:27:11.504 the pack will thrive on the arctic edge of America. 00:27:19.137 --> 00:27:24.768 On the Cascade Range in Washington State, the snow has almost disappeared. 00:27:27.145 --> 00:27:28.647 Even Mount Baker, 00:27:29.689 --> 00:27:33.485 holder of the world record for highest snowfall in a single winter, 00:27:34.235 --> 00:27:35.612 is shedding its coat. 00:27:38.281 --> 00:27:42.952 The 50-foot winter snowpack feeds huge glaciers. 00:27:47.415 --> 00:27:53.046 And as the spring sun shines down, the snow and ice begin to melt. 00:28:07.811 --> 00:28:12.232 Drops become trickles, trickles become streams, 00:28:12.691 --> 00:28:16.319 until a flood of water lubricates the glacier. 00:28:21.700 --> 00:28:24.411 A river of ice slides down the mountain... 00:28:33.920 --> 00:28:36.339 ...forming condo-sized crevasses, 00:28:38.174 --> 00:28:41.261 and relentlessly grinding stone to rubble. 00:28:53.022 --> 00:28:57.444 Glaciers carve some of America’s most iconic landscapes. 00:29:02.240 --> 00:29:05.994 From the Alaskan Range to the Cascades, 00:29:07.287 --> 00:29:12.542 where 26 glaciers flow down the volcanic slopes of Mount Rainier. 00:29:16.880 --> 00:29:21.050 As the weather warms, more and more ice melts, 00:29:21.342 --> 00:29:23.386 pouring down the mountainside. 00:29:27.265 --> 00:29:29.726 Transforming alpine valleys. 00:29:38.401 --> 00:29:42.405 After eight months of snow, the meadows erupt with color. 00:29:44.908 --> 00:29:47.577 As thousands of wildflowers bloom... 00:29:52.081 --> 00:29:55.919 the slopes of Mount Rainier are known as “Paradise.” 00:29:56.336 --> 00:30:00.006 The perfect home if flowers are your favorite food. 00:30:03.968 --> 00:30:06.513 A marmot and his mate. 00:30:07.931 --> 00:30:11.476 Just awake after nearly eight months of hibernation. 00:30:14.103 --> 00:30:18.358 Like the bears, they’ve lost a third of their body weight underground, 00:30:18.441 --> 00:30:19.776 and need to eat. 00:30:20.485 --> 00:30:23.112 Right now though, they’re running on empty 00:30:23.613 --> 00:30:27.784 and just the thought of a flower is... exhausting. 00:30:33.498 --> 00:30:37.752 Cool mountain mornings make it extra tough to get going. 00:30:42.465 --> 00:30:45.301 The grown-ups catch some rays to warm up. 00:30:56.187 --> 00:30:58.857 And the kids beat them down to breakfast. 00:31:07.448 --> 00:31:11.494 These growing boys have to do all their eating for the whole year 00:31:11.619 --> 00:31:13.997 in the four months while they’re above ground. 00:31:15.915 --> 00:31:19.377 They can eat half their body weight in flowers every day. 00:31:21.838 --> 00:31:24.465 Those flowers are packed with sweet nectar, 00:31:25.508 --> 00:31:29.262 so it’s not long before the sugar rush. 00:31:33.975 --> 00:31:37.103 Roughhousing provides a long-overdue workout. 00:31:39.689 --> 00:31:42.400 It's the fastest way to build muscle tone and strength. 00:32:01.085 --> 00:32:03.630 Oh, come on! No biting! 00:32:06.966 --> 00:32:09.594 Some nice moves! Not bad. 00:32:22.857 --> 00:32:24.067 A clear winner. 00:32:24.609 --> 00:32:27.320 Now, back to the buffet. 00:32:28.655 --> 00:32:32.825 The marmots will make every moment and every mouthful count, 00:32:33.743 --> 00:32:35.370 while paradise blooms. 00:32:38.164 --> 00:32:43.002 On the neighboring mountain, a cousin of the marmots is even busier. 00:32:46.547 --> 00:32:48.591 An all-American architect. 00:32:56.516 --> 00:32:57.684 A beaver. 00:32:58.893 --> 00:33:00.979 He's doing what beavers do best. 00:33:03.815 --> 00:33:06.734 He’s building a dam to create a pond. 00:33:11.322 --> 00:33:15.368 A family of four can build almost five feet of dam a day. 00:33:19.747 --> 00:33:23.835 Their pond is a barrier, keeping predators away from their lodge, 00:33:24.836 --> 00:33:26.170 their family home. 00:33:35.805 --> 00:33:38.766 And pond-building here has other significance, 00:33:38.850 --> 00:33:40.852 as this is no ordinary mountain. 00:33:42.311 --> 00:33:44.981 This is Mount Saint Helens. 00:33:47.608 --> 00:33:50.903 In 1980, this volcano erupted 00:33:51.529 --> 00:33:55.867 with the power of 25,000 atomic bombs. 00:34:03.833 --> 00:34:06.794 Thirteen hundred feet blew off the summit... 00:34:10.423 --> 00:34:13.885 leaving a crater the size of a thousand football fields. 00:34:16.929 --> 00:34:19.140 The explosion ripped through the forest... 00:34:21.434 --> 00:34:24.312 leveling an area as large as Chicago. 00:34:28.775 --> 00:34:31.402 Thousands of trees were blown from the hillside 00:34:31.944 --> 00:34:33.863 and plunged into Spirit Lake. 00:34:40.536 --> 00:34:42.080 Nothing survived. 00:34:46.459 --> 00:34:48.669 It looked like a wasteland. 00:34:51.881 --> 00:34:55.676 But amongst the devastation, the beavers saw potential. 00:34:57.512 --> 00:35:01.682 With running water and plenty of shattered wood for building materials, 00:35:02.809 --> 00:35:05.686 they moved in and got to work... 00:35:08.064 --> 00:35:13.069 the population soon growing as their network of dams and ponds spread. 00:35:23.996 --> 00:35:28.835 In just 40 years, these eco-engineers transformed Mount Saint Helens 00:35:28.960 --> 00:35:30.878 into a wetland paradise. 00:35:37.927 --> 00:35:39.300 Their dams hold back water 00:35:39.324 --> 00:35:42.014 that would otherwise flow away. 00:35:47.228 --> 00:35:51.941 They provide water for plants and a home for hundreds of animal species. 00:36:08.082 --> 00:36:11.919 The beavers have brought nature back, big time. 00:36:19.635 --> 00:36:21.387 And the benefits of their hard work 00:36:21.470 --> 00:36:24.056 are felt far beyond this neighborhood. 00:36:28.853 --> 00:36:31.439 By slowing the flow of water down the mountain, 00:36:32.273 --> 00:36:34.775 their ponds keep rivers running year-round... 00:36:37.236 --> 00:36:40.156 feeding lush forests with nutrient-rich water 00:36:40.239 --> 00:36:42.241 all the way to the Pacific... 00:36:48.956 --> 00:36:52.960 where one of the greatest shows on Earth is about to begin. 00:36:59.050 --> 00:37:02.063 Every fall, the Pacific Northwest 00:37:02.087 --> 00:37:04.722 is the setting for the salmon run. 00:37:07.892 --> 00:37:11.437 After years at sea, millions of salmon gather, 00:37:11.938 --> 00:37:15.483 driven by the instinct to swim upriver and breed. 00:37:20.488 --> 00:37:21.948 In Northern California, 00:37:22.198 --> 00:37:25.993 they must squeeze through the narrow mouth of the Klamath River. 00:37:31.791 --> 00:37:34.877 Predators swim in from hundreds of miles away, 00:37:35.628 --> 00:37:37.755 timing their arrival for this event. 00:37:47.598 --> 00:37:52.103 Sea lions, they’re just waiting for the fish to make the first move. 00:38:00.778 --> 00:38:05.116 As the tide rises, the salmon surge towards the river's mouth. 00:38:17.295 --> 00:38:20.673 Squeezed together, they have no room to maneuver. 00:38:24.385 --> 00:38:28.055 The sea lions use the waves to turbo-boost their chase. 00:38:33.019 --> 00:38:35.104 They single out individual salmon... 00:38:37.273 --> 00:38:38.858 and drive them onto the beach. 00:38:44.572 --> 00:38:47.074 Around here, you gotta eat fast. 00:38:48.075 --> 00:38:51.245 There could be a larger sea lion on the next wave. 00:38:55.541 --> 00:38:57.251 A thousand-pound thief 00:38:57.335 --> 00:39:00.379 looking to muscle in and steal your hard-earned lunch. 00:39:28.324 --> 00:39:31.827 For a month, every high tide is a feeding frenzy... 00:39:34.288 --> 00:39:37.416 as sea lions and seabirds gorge themselves. 00:39:52.932 --> 00:39:56.602 Every sea lion eats up to 35 pounds of fish a day. 00:40:03.317 --> 00:40:07.071 But there are so many salmon that 98 percent make it through. 00:40:16.872 --> 00:40:21.585 By late summer, the fish are returning to rivers all along the northwest coast. 00:40:25.005 --> 00:40:27.379 Alaska’s Bristol Bay 00:40:27.403 --> 00:40:30.511 hosts the largest salmon run in the world. 00:40:33.889 --> 00:40:38.352 Up to 65 million sockeye salmon are swimming upstream. 00:40:39.687 --> 00:40:42.565 Their journey might be several hundred miles. 00:40:53.534 --> 00:40:57.246 And these amazing navigators use their sense of smell 00:40:57.329 --> 00:40:59.915 to guide them back to the exact patch of river 00:41:00.374 --> 00:41:02.626 where they hatched four years ago. 00:41:14.555 --> 00:41:17.141 But they don’t quite have the place to themselves. 00:41:22.021 --> 00:41:25.357 Mama bear and her two cubs have made it down from the mountains. 00:41:37.786 --> 00:41:40.748 She’s done well to keep both cubs alive. 00:41:40.956 --> 00:41:44.335 Up to 40 percent don’t make it through their first year. 00:41:49.340 --> 00:41:51.717 She’s exhausted though. 00:41:52.885 --> 00:41:56.388 It’s time they learned to stand on their own four paws. 00:42:03.562 --> 00:42:05.773 Time for Fishing 101. 00:42:08.359 --> 00:42:10.444 Just watch me, kids. 00:42:29.547 --> 00:42:31.924 Oh! So close. 00:42:38.472 --> 00:42:39.515 Nope. 00:42:41.183 --> 00:42:43.352 Termites were way easier than this. 00:43:00.703 --> 00:43:01.829 Mom's not worried. 00:43:01.912 --> 00:43:05.040 She knows it will soon be much easier to catch a meal here 00:43:05.124 --> 00:43:06.375 because the salmon are changing. 00:43:09.545 --> 00:43:10.754 They’ve stopped eating, 00:43:12.089 --> 00:43:15.509 and all the males have become mating machines. 00:43:17.511 --> 00:43:21.682 That hunched back? It's kind of like a sockeye six-pack. 00:43:23.934 --> 00:43:25.311 Check it out, ladies. 00:43:27.021 --> 00:43:30.441 And those teeth? Mmm, irresistible. 00:43:35.070 --> 00:43:38.699 While the males show off, the females are being more practical, 00:43:39.408 --> 00:43:42.244 looking for the best patch of riverbed to lay eggs. 00:43:49.126 --> 00:43:54.506 They fight over the shallow spots where the water’s cool and rich in oxygen. 00:43:56.592 --> 00:43:58.427 Then each prepares a nest, 00:44:00.054 --> 00:44:02.806 clearing away silt that could suffocate her eggs. 00:44:12.316 --> 00:44:14.777 She lays over 4,000... 00:44:16.987 --> 00:44:19.198 and the male adds clouds of sperm. 00:44:26.705 --> 00:44:31.960 Across Alaska, millions of salmon nests hold billions of eggs. 00:44:43.430 --> 00:44:45.599 Exhausted and starved, 00:44:46.642 --> 00:44:49.228 the salmon die soon after spawning. 00:44:51.647 --> 00:44:54.775 But even in death, they bring life. 00:44:56.193 --> 00:44:59.154 The decaying bodies merge with the riverbank. 00:45:00.864 --> 00:45:05.619 The biggest transfer of nutrients from ocean to land on Earth. 00:45:09.915 --> 00:45:12.584 Plant food for the great forest of the north, 00:45:13.252 --> 00:45:17.673 where trees grow three times faster along salmon-stocked rivers. 00:45:24.263 --> 00:45:27.433 Dead fish are an easy meal for patient cubs. 00:45:33.689 --> 00:45:35.983 These two will fatten up quickly now, 00:45:37.735 --> 00:45:40.738 just in time to climb the mountain back to their den. 00:45:46.827 --> 00:45:51.415 Winter will come again, hard. But they’ll be ready. 00:45:53.250 --> 00:45:57.588 The snow, though fierce, will be the lifeblood of next summer, 00:45:58.630 --> 00:46:03.218 allowing life to thrive across the northlands of America.