生命之旅第四集:共生 Living Together

[00:00.00] [00:12.50] [00:14.50]Antarctica... the coldest place on earth... [00:17.83] [00:18.17]with an air temperature of minus 50 degrees Fahrenheit. [00:21.90] [00:23.47]You'd think that nothing could survive here - [00:26.00] [00:26.64]Yet huddling together these Emperor Penguins [00:29.40] [00:29.61]have evolved to cope with the battle for life in the big freeze. [00:33.21] [00:55.27]But cold isn't the penguins' biggest enemy [00:58.14] [00:59.71]...this is! [01:00.87] [01:07.21]The leopard seal - four meters long - a Super predator. [01:13.02] [01:14.42]No wonder everyone's reluctant to be the first to take the plunge! [01:17.69] [01:30.47]Penguin and seal have a relationship forged over millions of years, [01:34.77] [01:35.18]that of predator and prey. [01:37.97] [01:47.86]It's easy to see nature as red in tooth and claw, [01:51.45] [01:51.66]where predators dictate who lives and dies and only the fastest and strongest survive [01:57.49] [02:01.27]The truth is, predators and prey are just part of life's story, [02:05.73] [02:06.37]our world is made up of a vast network of complex relationships. [02:10.40] [02:13.68]Animals -including us - have to find mates, [02:17.31] [02:17.52]look after families and keep up with the neighbors. [02:20.32] [02:25.56]All these relationships affect how we look and behave. [02:28.86] [02:38.67]This is the surprising story of how the relationships [02:41.77] [02:41.98]between all living things have shaped the whole Journey of Life. [02:46.34] [03:01.30]Life on earth wasn't always as complicated as it is today... [03:04.96] [03:05.47]There was a time when there were no relationships at all. [03:08.40] [03:09.50]No violence, practically no competition and no sex. [03:14.13] [03:19.25]Three billion years ago [03:20.87] [03:21.08]the pinnacle of evolution was these strange lumps on the ocean floor - [03:25.21] [03:26.25]stromatolites, [03:27.74] [03:27.96]made up of ancient blue green algae. [03:30.48] [03:39.53]They might not look much but stromatolites had the power to change the world. [03:44.49] [03:46.17]They produced the first oxygen in earth's history and totally transformed the planet's atmosphere. [03:52.20] [03:54.78]Not only did they generate the air we breathe, [03:57.22] [03:57.48]they set the scene for all life as we know it today. [04:00.68] [04:02.26]Because of them, a brand new type of cell evolved, [04:05.75] [04:05.99]perhaps the most important life has ever known. [04:08.83] [04:13.20]These revolutionary, oxygen-fuelled cells spawned our ancestors. [04:18.47] [04:22.44]They were 20 times more energetic than the older models - [04:25.67] [04:26.18]And some then discovered that the quickest way to get the extra energy [04:29.67] [04:29.88]they needed was to steal it - by engulfing other cells. [04:37.55] [04:41.06]A new force was unleashed - the predator. [04:45.36] [04:46.67]And the struggle between predators and prey [04:48.96] [04:49.17]has been going on for hundreds of millions of years. [04:52.23] [04:53.91]As predators find better ways to catch and kill, their prey have to improve their self-defense. [04:59.81] [05:03.48]The two are locked in an intense relationship which forces both sides to change, [05:08.39] [05:08.76]because neither can afford to be left behind. [05:11.09] [05:12.69]Nature selects only the winners' genes the losers genes will disappear. [05:17.53] [05:19.20]This arms race has helped shape the lives and bodies of just about everything alive today [05:25.00] [05:25.31]and pushed some creatures to extremes. [05:28.71] [05:50.40]This cricket might think it's on a flower. [05:53.30] [05:59.64]Until the flower bites it in the neck. [06:01.83] [06:05.31]It's actually a mantis that's evolved to look just like an orchid - [06:09.01] [06:09.48]beauty turns out to be a beast! [06:11.71] [06:19.19]Other hunting strategies are even more bizarre [06:21.59] [06:23.30]Over millennia the snapping turtle's efforts [06:25.63] [06:25.83]to reel in a fish have turned its tongue into a juicy worm! [06:30.57] [06:42.38]Secret weapons aren't the only winning formula sometimes more subtle tactics may be needed. [06:48.18] [06:51.59]Meet jumping spider Portia, one of the world's smallest assassins [06:56.09] [06:57.56]She's hunting the much larger web spider [07:00.03] [07:01.50]She might be tiny, but she has a big appetite [07:04.60] [07:11.34]Her first direct - approach alerts her target, [07:14.78] [07:14.98]who just shakes the web and forces Portia to retreat. [07:18.21] [07:22.32]But Portia has evolved a Plan B, a maneuver straight out of Mission Impossible! [07:27.06] [07:41.27]She slowly lowers herself within striking range - [07:44.47] [07:48.32]she leaps - bites - and leaves the poison from her tiny fangs to do the rest. [07:55.28] [08:01.53]This time it isn't size or secrecy that counts - [08:04.43] [08:04.63]but strategy - Portia may be small, but she's as deadly as they come. [08:10.30] [08:13.57]The relationship between hunter and hunted doesn't only shape the way the predators attack, [08:18.88] [08:19.25]it also drives the way that prey fight back. [08:22.18] [08:24.62]Sometimes it's just about using natural assets in a different way. [08:28.38] [08:32.29]When a hungry raven eyes their chicks, field fares fight dirty! [08:36.56] [08:45.07]Dropping bombs! [08:46.37] [08:52.41]What a way to win! [08:53.81] [08:58.49]After millions of years of predators and prey slugging it out pretty much anything is possible. [09:03.98] [09:10.13]The only problem with an evolutionary arms race is we don't see it develop, [09:14.66] [09:15.20]all we see is the end result. [09:17.47] [09:21.04]The story of the passion flower vine is a rare exception. [09:24.67] [09:28.11]Just like any other prey passion vines are evolving to protect themselves [09:32.81] [09:33.02]from hungry predators. [09:34.58] [09:37.42]What makes them unique is that you can pretty much watch them doing it. [09:41.52] [09:43.30]At home in tropical America, [09:45.03] [09:45.23]they're under constant attack from lethal predators that lurk deep in the jungle. [09:51.14] [09:52.84]Helliconius butterflies! [09:54.70] [09:59.01]The adults are harmless enough - it's what they leave behind that's the problem. [10:03.47] [10:08.32]Mother butterflies lay their eggs on the passion vine's leaves. [10:12.02] [10:12.33]Tiny time bombs which soon hatch out into very hungry caterpillars. [10:16.42] [10:23.30]The fate of this plant is already sealed. [10:25.46] [10:26.01]In a few days there'll be nothing left. [10:28.20] [10:40.52]If a passion vine already has a batch of eggs installed [10:43.89] [10:44.09]it's pointless for a mother butterfly to lay more. [10:47.32] [10:47.73]There'd be nothing for her caterpillars to eat. [10:49.66] [10:50.56]This gives the passionflower a chance to fight back. [10:53.43] [10:55.64]Today not all the eggs are what they appear - [10:58.60] [10:59.07]some are in fact just yellow spots grown by the passion vine itself as a deterrent to butterflies [11:05.48] [11:11.25]But as in every arms race, not every attack is foiled. [11:14.85] [11:21.76]So there's pressure for better and better mimics to evolve [11:25.16] [11:25.43]and some are now so good it's hard to tell which eggs are real! [11:29.39] [11:31.50]False! [11:32.49] [11:33.84]Real! [11:34.93] [11:36.41]Real! [11:37.34] [11:38.38]False! [11:39.37] [11:40.38]Real! [11:41.37] [11:42.98]Who'd have thought the relationship between a butterfly and a vine [11:46.15] [11:46.35]could have produced such a sophisticated bluff. [11:49.05] [11:53.76]Even when you're safe from predators things don't always go your way, [11:57.82] [11:58.03]there's competition to contend with. [11:59.90] [12:04.40]In Africa lots of hungry meat eaters are on the lookout for a free meal. [12:08.70] [12:11.31]A Vulture has the aerial advantage of being able to spot the first signs of a kill. [12:16.01] [12:24.16]Though it won't feed undisturbed for long. [12:26.25] [12:27.53]Soon every vulture in the area is fighting for a beak full! [12:30.52] [12:50.28]At this buffet, only the toughest get to eat. [12:52.91] [12:56.86]Lappet-faced vultures are the biggest bully boys of all, [12:59.55] [12:59.96]they knock the smaller griffons flying. [13:01.86] [13:12.31]But a free lunch attracts other hopefuls too - [13:14.87] [13:20.35]more feathers fly as even Lappets have to make way for the mammal heavy mob. [13:25.18] [13:29.39]Like predators and prey, this kind of competition is a real force to be reckoned with - [13:34.16] [13:34.49]another influence on evolution which selects only the toughest to survive. [13:38.62] [13:54.88]Competition is so crucial to animals' lives, [13:57.41] [13:57.62]that it can have a massive impact on their body shape. [14:00.61] [14:03.59]Many animals in Africa feed on Acacia trees [14:06.65] [14:06.99]why don't they fight like vultures at a kill? [14:09.43] [14:10.43]Because these diners have been molded into different shapes and sizes [14:13.99] [14:14.20]so they don't tread on each other's toes. [14:16.57] [14:18.67]Tiny Dik diks can nibble around the thorny lower branches in a way no bigger animals can - [14:24.51] [14:31.25]Taller Impala have control over the acacia's middle zone [14:34.78] [14:37.42]But even they are kept in their place - by the Gerenuk - [14:40.69] [14:40.96]which has a poise no other antelope can match. [14:43.76] [14:47.43]Gerenuks look more like ballet dancers than gazelles - [14:50.53] [14:50.74]they even balance on points! [14:52.50] [14:59.68]Their specially adapted hips and spines enable them to swivel [15:03.67] [15:03.88]while on their hind legs. [15:05.21] [15:08.42]But there's one animal which can feed higher still -up to 6 meters off the ground. [15:13.95] [15:20.27]It has a flexible, leathery tongue [15:22.10] [15:22.30]it uses just like a hand to pluck the highest twigs and leaves - [15:26.43] [15:27.81]thanks to the longest neck in the world. [15:30.00] [15:35.21]The Giraffe [15:36.34] [15:40.19]So competition hasn't just pushed animals into developing different eating habits, [15:44.99] [15:45.22]it's helped create entirely different species that can live in harmony [15:49.25] [15:58.34]But what happens when neighbors want exactly the same thing? [16:02.03] [16:12.32]Ring-tailed Lemurs live in troops on the island of Madagascar. [16:16.08] [16:18.26]Each group needs their own territory to survive, [16:20.85] [16:21.09]for shelter, food and a safe place to rear babies. [16:24.59] [16:26.63]They're often under threat from other lemur gangs, [16:28.79] [16:29.00]who'll steal the whole thing if they can [16:30.94] [16:42.62]The alarm sounds - Time to rally the Troops [16:45.48] [16:51.49]These intruders look like they mean business. [16:53.82] [16:59.37]Lemur society is run by females - [17:01.83] [17:02.14]it's they who lead the band into the fray - soldiers with babies on their backs! [17:07.23] [17:29.73]When competition gets this intense, the Lemurs' answer is teamwork. [17:34.72] [17:57.29]Once animals are driven to work together, [17:59.72] [18:00.03]the close bonds that evolve can open up a whole new way of life [18:04.29] [18:08.40]Meerkats are high on the menu for many predators [18:11.43] [18:11.70]so they need to work together to protect themselves. [18:14.70] [18:35.33]They're one of the few mammals that take turns at doing different jobs. [18:39.20] [18:44.47]While the rest of the family starts digging for breakfast, [18:47.34] [18:48.31]one member stands guard, on the lookout for predators. [18:54.91] [19:03.46]The others can carry on feeding safe in the knowledge [19:06.69] [19:06.96]that someone is watching their backs. [19:08.48] [19:31.65]It's not just lookout duty that meerkats share. [19:34.75] [19:37.62]When a monitor lizard appears they all gang up to drive it way [19:41.72] [20:05.35]En masse, these little creatures make a formidable force! [20:08.98] [20:11.46]Being such good team players is the meerkats' winning formula... [20:14.72] [20:14.99]it's all for one... and one for all! [20:17.90] [20:27.34]Some animals have taken cooperation to even greater extreme. [20:31.47] [20:33.41]Certain types of insects from bees to termites to ants [20:37.28] [20:37.48]live together in huge social colonies. [20:39.88] [20:42.66]But rather than share duties like the meerkats, [20:45.22] [20:45.52]termites have actually evolved different shapes and sizes [20:48.69] [20:48.93]to make them physically specialized for certain tasks. [20:51.90] [20:53.73]Workers are the general dogsbodies which do a multitude of jobs [20:57.60] [20:57.80]from finding food to looking after the babies, and repairing the nest. [21:02.14] [21:04.58]A soldier's sole purpose is to defend the colony. [21:08.10] [21:08.41]They're twice the size of the others and are armed with pincer claws. [21:12.32] [21:14.65]Both these types have given up the ability to breed, [21:17.45] [21:17.82]that role is played by just one queen a mother of the entire colony. [21:22.82] [21:28.53]She has become an egg producing machine and swollen and immobile, [21:33.37] [21:33.57]without her extended family of servants she would die. [21:36.94] [21:38.31]In fact none of the termites can survive without the others help [21:41.61] [21:42.11]they have become one massive super organism - [21:44.84] [21:46.69]It's easy to see why we'd help our own relations [21:49.68] [21:49.89]but some animals co-operate with different species. [21:52.82] [21:53.23]How could that evolve. [21:54.56] [21:55.66]One answer is found in the sea [21:57.75] [22:00.30]A coral reef is a natural metropolis, [22:02.73] [22:02.94]a city where everyone is trying to get on and make their own way. [22:06.53] [22:10.28]It's full of opportunities, deals to be struck. [22:13.30] [22:16.22]And some entrepreneurs are cleaning up! [22:18.58] [22:24.42]It turns out there's a market niche for professional groomers! [22:27.92] [22:42.74]Cleaners - like these tiny wrasse - [22:44.90] [22:45.11]snaffle up their clients' parasites and dead skin. [22:48.01] [22:49.82]They set up their own cleaning stations [22:51.68] [22:51.92]and their clients come from far and wide for dental hygiene or a gill wash. [22:56.58] [22:57.19]In return, the cleaner gets a meal. [22:59.38] [23:03.33]Cleaner shrimps, too, serve a most discerning clientele. [23:07.13] [23:11.20]And fish are smarter than you might think, [23:13.23] [23:13.51]they have good memories and come back regularly to exactly the same spot to be pampered. [23:19.47] [23:47.51]It isn't just tropical fish though which have discovered that [23:50.37] [23:50.58]hiring a little help gives you the edge. [23:53.10] [23:55.45]Some plants have too. [23:57.01] [23:59.62]Flowers only exist because of an ancient alliance between insects and plants. [24:04.89] [24:10.26]Since plants can't move around, they need help to carry pollen [24:14.36] [24:14.57]between them to produce fertile seeds. [24:17.33] [24:18.60]So they advertise themselves as nectar feeding stations. [24:22.37] [24:24.34]As the insects feed, they get an extra take-away of pollen. [24:28.07] [24:29.32]It's a partnership that has evolved so successfully flowering plants [24:33.41] [24:33.62]now dominate our world. [24:35.59] [24:37.46]The only draw back is some insects may take pollen to the wrong plant. [24:42.16] [24:45.13]To get round this certain flowers have developed an exclusive contract with their pollinators. [24:50.63] [24:53.47]And Orchids for example can be very demanding indeed. [24:57.17] [24:59.78]Some, like this bucket orchid, may seem downright cruel! [25:03.41] [25:06.72]This Euglossine bee needs its perfumed oils to attract a mate [25:10.92] [25:15.29]But as he gathers the oil, he loses his grip. [25:18.26] [25:20.20]Exactly what the orchid wants! [25:22.46] [25:23.67]It has evolved a complex trap which press-gangs male bees into service [25:28.30] [25:30.94]The only way out is a tight squeeze via the back door [25:34.17] [25:34.38]and the orchid's pollen sacks [25:36.31] [25:57.27]The pollen sticks to the delivery bee's back. [26:00.24] [26:03.41]Once free, and dried off surely he'll never go near a bucket orchid again! [26:08.11] [26:18.46]But perhaps male bees have short memories [26:20.65] [26:21.03]because here he is falling for the same ploy all over again! [26:24.19] [26:30.47]But the only difference this time is that he's delivering pollen instead. [26:34.74] [26:39.45]No question who's in charge in this relationship! [26:41.97] [26:45.68]But there is another invisible partnership which is even more important to plants. [26:50.14] [26:53.43]Including the tallest trees on the planet. [26:55.99] [26:58.33]California Redwoods. [26:59.89] [27:01.20]They can live for more than 2000 years and grow more than a hundred meters tall. [27:07.14] [27:08.91]Although these giants are some of the largest of all living things [27:12.43] [27:12.64]they rely on the tiniest of partners which live hidden underground. [27:16.58] [27:18.18]These secret accomplices are fungi. [27:20.91] [27:21.22]They penetrate the roots and then stretch out through the soil as a network of tiny threads. [27:26.75] [27:30.63]The giant trees can't get enough water and nutrients on their own, [27:34.46] [27:34.93]but the fungal network can. [27:36.53] [27:38.54]Its massive surface area sucks up the fluid and minerals [27:42.20] [27:42.41]and transports them back to the trees roots. [27:44.97] [27:47.31]And in return trees feed the fungi sugars produced by their leaves. [27:51.65] [27:53.79]90% of all plants live with fungal partners, [27:56.85] [27:57.06]from the redwoods to the tropical rainforests, it's a 400 million year old marriage. [28:02.99] [28:07.13]Without it, plants might never have been able to colonize the land. [28:11.09] [28:12.34]And without all this green stuff, where would all we animals be? [28:17.00] [28:22.28]The close relationship between plants and fungi [28:25.11] [28:25.35]is only one of the great alliances that have shaped life. [28:28.72] [28:37.63]There is one more - you'd need a microscope to see it [28:41.09] [28:41.43]but it is perhaps the most important partnership on earth. [28:44.49] [28:47.57]Because it's what gives animals their energy. [28:50.06] [29:10.36]Scattered throughout our cells are billions of structures known as Mitochondria. [29:15.89] [29:17.10]Each is a tiny furnace that releases energy from our food [29:21.06] [29:21.41]and generates the life force that our bodies need. [29:24.10] [29:25.28]It's a crucial relationship one that started out in a very unusual way. [29:29.98] [29:31.62]Billions of years ago, the mitochondria lived independent lives, [29:35.58] [29:35.79]as free-floating bacteria. [29:37.38] [29:41.33]Then our ancestor cell engulfed one, but instead of digesting it, [29:45.92] [29:46.13]formed an alliance with its prey. [29:48.22] [29:51.67]The two joined forces and were transformed. [29:54.14] [29:54.74]Bacteria became mitochondria creating a new type of hybrid super-cell. [30:00.44] [30:03.18]A super-cell which was the basic building block for almost everything alive today. [30:08.35] [30:19.20]So there are some evolutionary partnerships where all benefit. [30:23.26] [30:28.57]But as we know relationships aren't always that straightforward. [30:32.07] [30:41.25]Most grazing animals in Africa are plagued by biting insects and ticks. [30:46.05] [30:47.76]They have what seem to be whole squads of helpful hangers-on. [30:51.63] [30:55.30]Oxpeckers - insect-eating birds that reach the parts the animals themselves can't. [31:00.67] [31:05.24]But oxpeckers aren't quite as innocent as they appear. [31:07.94] [31:09.82]They don't just eat up ticks, they peck holes in the animals' skin. [31:14.78] [31:23.40]What appear to be honest little helpers, in fact turn out to be blood-sucking parasites. [31:29.06] [31:32.40]A way of life which is surprisingly common. [31:35.20] [31:40.41]Take the rabbit for example, [31:42.21] [31:42.65]you might expect predators to have the biggest impact on their lives. [31:46.31] [31:48.22]After all, they're hunted by a whole army of killers. [31:51.21] [31:58.30]But with predators at least they can run away. [32:00.99] [32:01.90]Rabbits face another threat they can't see, smell or hear - [32:06.63] [32:07.31]parasites. [32:08.64] [32:12.31]Each rabbit is infested with thousands of fleas, ticks and minuscule Mites, [32:17.44] [32:17.75]which may be small but pack a powerful bite. [32:20.58] [32:22.19]These tiny bloodsuckers sap the rabbits' strength and within them [32:26.52] [32:26.76]they harbor an even deadlier threat. [32:29.02] [32:30.40]A fatal virus -myxamatosis - which kills far more rabbits [32:34.53] [32:34.73]every year than all predators combined. [32:37.07] [32:42.24]A virus is a nightmare straight out of science fiction. [32:46.30] [32:47.48]It's an invader with one aim - to replicate. [32:51.07] [32:51.38]It's made up of special components it needs to fulfill its mission. [32:54.72] [32:57.42]A potential victim - the outer surface of a body cell. [33:01.09] [33:03.16]On first contact the virus docks and merges with the cell membrane. [33:07.66] [33:12.34]Its inner pod enters the cell, seeks out the command centre, [33:16.24] [33:16.44]the nucleus and then injects its secret weapon - a snake of virus genes. [33:22.21] [33:26.45]These then corrupt the cell making it into a virus factory. [33:30.48] [33:32.12]Eventually the cell dies and bursts, [33:34.82] [33:35.03]releasing thousands of new viruses and the real destruction begins. [33:40.40] [33:43.50]Each year just about every baby rabbit born in the wild will catch this deadly virus. [33:49.30] [33:49.81]And only one in five will survive so it's a massive selection pressure. [33:55.41] [33:58.55]It's not just rabbits that viruses affect. [34:01.02] [34:04.29]In 1918, at the end of the Great War, a flu virus struck Europe. [34:09.06] [34:13.67]The war killed 25 million people over four years, that's three times the population of London, [34:19.76] [34:20.67]the flu epidemic wiped out the same number in just four months. [34:25.30] [34:34.42]Against such a powerful enemy, how can anything possibly fight back? [34:38.69] [34:41.13]The answer is a word we're all familiar with SEX! [34:44.93] [35:09.62]Sexual relationships aren't just about reproduction [35:13.22] [35:17.40]In fact it's perfectly possible to reproduce without sex -by cloning. [35:22.53] [35:24.60]Aphids are baby machines - they're all female and don't need a mate. [35:29.91] [35:30.28]They can pump out new aphids all by themselves. [35:33.27] [35:34.25]One every 20 minutes - each already stocked with it's own daughters! [35:38.65] [35:40.69]Cloning is great for increasing numbers fast [35:43.02] [35:43.22]but it has one flaw: As all cloned babies are identical, [35:47.89] [35:48.26]a killer disease that wipes out one wipes them all out. [35:52.49] [35:57.10]The key thing sex does is to create variety. [36:00.63] [36:03.51]Although these puppies all have the same parents, they're individuals. [36:07.77] [36:08.65]During sexual reproduction genetic material is reshuffled [36:12.71] [36:13.05]so that each sperm and egg is unique [36:15.71] [36:16.52]and when the 2 join together to make a puppy, each puppy is unique too. [36:21.25] [36:22.09]What's critical in the fight against parasites though is that [36:25.59] [36:25.80]the puppies are unique on the inside. [36:28.63] [36:29.90]This one might be more resistant to parvo virus [36:32.87] [36:33.17]and this one to distemper virus it's the same for human diseases too. [36:39.11] [36:45.65]Resistant individuals have cell membranes that lock the killer virus out. [36:50.59] [36:50.89]So its lethal life cycle can't get started. [36:54.42] [36:57.80]Sex is crucial it's evolution's way of ensuring the success of future generations. [37:03.93] [37:05.00]No wonder animals are so desperate to have sex. [37:07.97] [37:12.44]Once a year in autumn, American Elk gather to breed. [37:16.64] [37:24.46]It's called the rut - [37:25.86] [37:26.19]a spectacle where stags compete for breeding rights. [37:29.02] [37:50.28]This rut is played out in a most unusual location. [37:53.55] [37:59.12]In Yellowstone National Park [38:00.75] [38:01.03]manicured lawns attract the herds right into the centre of Yellowstone village itself. [38:06.56] [38:13.34]Here humans get a ringside seat. [38:15.57] [38:32.42]Each stag has two aims - to impress the females and frighten off rivals. [38:37.89] [38:42.37]Although the males are bigger and more dominant, it's the females who really run this show. [38:47.27] [38:49.37]As they'll be left holding the babies they're very choosy about whom they mate with. [38:53.67] [38:55.68]Only the stags with the most impressive antlers and fighting ability will do. [38:59.41] [39:02.72]Which forces the males to fight to prove themselves. [39:05.92] [39:09.46]It's a knock-out contest - where only winners get the right to breed. [39:13.09] [39:14.00]So over generations the battles get harder and the stags stronger. [39:17.99] [39:20.44]Female choice means that sexual relationships [39:23.41] [39:23.61]can shape the bodies and behavior of their mates so they are driving evolution. [39:29.24] [39:35.35]The overwhelming urge to win and pass on their genes [39:38.41] [39:38.69]can push the stags meanwhile right over the edge. [39:41.66] [40:15.33]Thankfully for other males, [40:16.69] [40:17.00]there are other less violent ways of courting favor. [40:19.62] [40:21.23]Sometimes it's all just a matter of showing off. [40:23.72] [40:25.44]So some species put all their effort into how they look. [40:28.89] [40:31.51]Posers can be just as successful as fighters! [40:34.21] [40:50.40]Instead of proving their worth on the battleground, [40:52.66] [40:52.93]these peacocks flaunt fancy dress instead. [40:55.66] [41:03.31]The females will only mate with the ones they find the most attractive. [41:06.84] [41:11.82]What does all this ostentation and display actually mean? [41:14.88] [41:20.43]How can a female be sure she's going to have good strong healthy babies? [41:24.66] [41:27.43]Remarkably a little finch proves that masculine beauty isn't just skin deep. [41:33.46] [41:34.97]Zebra Finch males have orange legs and cheeks and bright red beaks. [41:39.93] [41:43.51]The females are much plainer. [41:44.98] [41:50.29]So why are the males so colorful. [41:52.26] [41:53.16]To find out we tried a little experiment. [41:55.25] [42:00.57]The males' red legs and beaks are full of color pigments called carotenoids, [42:05.13] [42:05.34]found naturally in carrots certain fruits and seeds [42:09.10] [42:09.97]Half the males were fed a diet high in carotenoids to make them redder. [42:14.27] [42:15.78]Then they were put into a special choice chamber [42:18.11] [42:18.45]next to a duller male who hadn't been fed the pigments. [42:21.68] [42:22.62]Two males in their own designer bedrooms at the back, [42:25.38] [42:25.69]one female at the front. [42:27.49] [42:27.86]The sexes were segregated by a pane of glass. [42:30.42] [42:31.20]The males couldn't leave their rooms, [42:32.66] [42:32.96]but the female at the front could move from the left to the right. [42:36.42] [42:37.87]She could then choose a male by perching by his window. [42:40.90] [42:43.37]Tiny cameras were installed to record what happened throughout the Little Bird House. [42:47.64] [42:53.22]The high-speed CCTV footage shows what happens next. [42:56.88] [42:59.22]Watch the female at the bottom of the screen. [43:01.28] [43:02.96]She seems to have chosen the male on the left. [43:05.02] [43:06.60]And the camera in his bedroom shows she spent most of her time with him. [43:10.19] [43:12.14]While another reveals the other male was ignored. [43:15.50] [43:17.24]The most popular was the one with the brightest red beak - every time. [43:21.37] [43:26.08]In tests 9 out of 10 females preferred him. [43:29.85] [43:32.16]The question is why? [43:34.22] [43:36.33]The same red pigments not only make the male look good, [43:39.56] [43:39.76]they help his body fight diseases, too. [43:42.53] [43:43.40]A dull male is probably weaker and not a good bet as a mate [43:47.24] [43:57.25]A brightly colored male is not just likely to be healthy - [44:00.08] [44:00.39]he'll also pass on his parasite - fighting genes to his young. [44:03.75] [44:08.16]So it pays for a female to demand the best. [44:11.25] [44:17.54]And this male zebra finch has passed his medical with flying colors! [44:21.20] [44:24.64]But there's an extra twist, [44:26.08] [44:26.74]researchers now think female choice even creates new species. [44:30.91] [44:33.48]This is a zebra cichlid and experiments have shown that [44:37.05] [44:37.26]he's a dedicated follower of fashion. [44:39.19] [44:42.53]These fish are only found wild in Lake Malawi in Africa. [44:46.93] [44:48.13]What makes them special is that males have a variety of coats of many colors [44:52.66] [44:52.87]which is extremely unusual in a single species. [44:55.81] [44:57.18]Why the males come in so many different colors was a mystery. [45:00.37] [45:01.61]To investigate the fish were put in choice chambers. [45:04.91] [45:08.39]The larger, more colorful males were confined [45:11.18] [45:11.39]but the little brown females could go where they liked. [45:14.12] [45:15.79]All they had to do was swim in with the male they liked the most, [45:18.92] [45:19.13]and their choice was made. [45:20.43] [45:23.23]The tests proved some rather surprising results. [45:25.86] [45:29.14]It turns out even females of the same species have different ideas about what is sexy. [45:34.60] [45:35.68]Some preferred white males, some yellow, others blotchy and so on. [45:40.17] [45:44.19]Which explains why Zebra cichlid males come in so many different colors [45:48.25] [45:51.43]But how does that help to create new cichlid species? [45:54.42] [45:58.44]If a group of females gang together and decide that one color [46:01.96] [46:02.17]is now highly desirable, they'll only mate with those particular males. [46:06.27] [46:14.22]This preference is passed on to their daughters [46:16.74] [46:17.39]over generations an entirely separate breeding group is formed - [46:20.82] [46:21.16]and eventually a whole new type of cichlid. [46:24.32] [46:28.67]So what exactly did happen in the wild? [46:31.50] [46:35.21]Lake Malawi and the other African great lakes started out [46:38.33] [46:38.54]with just a handful of cichlid species. [46:40.98] [46:48.12]Now they're home to one in 10 of all kinds of freshwater fish on the planet. [46:54.29] [46:55.16]An incredible 1700 cichlids species. [46:58.29] [46:59.33]This is the greatest explosion of new species ever discovered. [47:02.86] [47:03.30]All created by the relationship between females and their males [47:07.40] [47:29.43]Whether with families and friends, competitors or enemies - [47:33.09] [47:33.30]living together isn't always easy. [47:35.79] [47:44.38]But relationships have been a massive force for change throughout the whole journey of life [47:50.01] [47:57.22]And ultimately it's our connections with each other [48:00.52] [48:00.86]each other that have created the spectacular variety of life we see today. [48:05.80] [48:08.80]The End [49:06.80]

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