The rainiest inhabited place on Earth is Cherrapunjee in the Meghalaya district of north-east India. In July, the average rainfall is 9.6 metres. The source is the south-western monsoon, which forms in May in the Bay of Bengal. Giant waterfalls pour down the hillsides, and the rivers swell to twice their normal size. So how do you cross a raging torrent when man-made structures of wood or concrete simply wash away? The Khasi people of Meghalaya have an organic solution: they grow bridges from trees. The fig Ficus elastica, known as the Indian rubber-tree plant, is found in the gorges and along stream banks throughout Meghalaya. Its strong roots thrive in the rocky soil of riverbeds, plunging deep into the ground and allowing the trees to live perched on top of boulders, while hundreds of secondary tendrils grope towards the earth in search of greater stability and better purchase.
The Khasi look for a particularly well-situated fig tree and begin to train its roots in the long, slow process of spanning a river. The tree has to be high enough off the river bed to keep from washing away during the 15 or so years needed for the roots to strengthen and take hold. Inside the logs, soil and leaves are carefully placed to feed the shoots as they creep over and across the water below.
In the space of a human lifetime – 80 years, say – a root-bridge will solidify from a web of woven tendrils into fully grown horizontal tree. The Khasi say that an individual bridge, lovingly cared for and maintained, can last for 500 years.
The most glamorous husband
As the planet warms, the rains are becoming less and less predictable. In the African Sahel, Lake Chad is drying up, and for the Suudu Suka'el Wodaabe nomads, life is getting tougher. But perhaps once every seven years, good rains bring enough water and new grass to sustain a gathering of the clans. After surviving in their small, isolated communities for years, passions rise in anticipation of the chance for a young person to meet a lover who might last a night or a lifetime.
It is then that they come together for jeerewol. It's a competition of dancing and beauty, and it begins with natural make-up – but for the men only. Their faces are painted with a saffron-coloured earth collected from a specific desert mountain many days walk from the festival site. The young men also use red earth and charcoal black to ring their eyes, with white make-up from doobal – the dried and crushed excrement of the Sudan bustard. The young men then begin a dance marathon – the boisterous, welcoming Raume dance, which goes on until dawn.
The most beautiful man is chosen by a jury of three girls, as an opinionated crowd looks on. Every Wodaabe girl and boy is betrothed in marriage, but there is always the possibility of a second love marriage. So jeerewol is a dangerous time. Not only may flirtation lead to romance, it may also be when your husband is stolen by another woman. Such love marriages give a man enhanced sexual and social status. They might last for a week, a month or a lifetime and carry no social stigma.
The fish cornucopia
South-east Asia's greatest liquid asset is the Mekong. The river begins its epic journey among the glaciated peaks of Tibet and flows southwards for 4,800km before emptying into the South China Sea. The Mekong is multinational, crossing China, bordering Burma, Thailand and Laos and winding through Cambodia and Vietnam. The river's basin is almost the size of France and Germany combined.
This water nourishes large tracts of forest and wetlands, providing habitats for thousands of species of plants and animals. It also supports an inland fishery with an estimated value of £1.3bn a year – which the fishermen of Laos depend on for both sustenance and profit. The annual harvest, including that from fish farms, is 1 to 2 million tonnes – about twice the catch from the North Sea. In Laos, seven out of 10 rural households rely on fish as a vital source of protein and an income supplement.
The local Lao fishermen watch the weather carefully and adapt their fishing techniques accordingly. During the dry season, they fish from the exposed rocks using nets and small fish traps. But they also take advantage of the low water levels to prepare for the fishing event of the year, building big wooden traps in the middle of the Mekong's channels.
With the arrival of the monsoon comes a deluge of fish. The traps are ramp-like bamboo constructions that capture the fish on the platforms but let the turbid water rush through. At peak fish-run time, massive shoals of catfish arrive, keeping the locals busy day and night.
Bat City, Texas
By day, Austin – the capital of Texas – bustles with the business of an average American city. Legislators and bureaucrats pour into the city to the imposing dome of the neo-classical, white-marble state capitol building. Commuters clog the arteries of the city's freeway system, travelling from air-conditioned homes in air-conditioned cars to air-conditioned offices. Above their heads in the dark sky, millions of creatures of the night are on the wing.
Bats have always frequented the city regions, but in the 1980s their numbers started to increase, dramatically. In 1980, the Congress Avenue Bridge spanning Lady Bird Lake was reconstructed, with cracks designed to expand and contract with the annual temperature fluctuations. The engineers had unwittingly provided the perfect bat roost. Mexican free-tailed bats, returning from their winter quarters south of the border, began to pack into the tiny spaces by the thousands.
But when the citizens of Austin realised they had a bat colony, many reacted in fear, viewing them as disease- and vermin-ridden pests. Into the maelstrom stepped biologist Merlin Tuttle, who had moved to Austin and in 1982 set up Bat Conservation International. He began educating people about the benefits of bats, how they eat agricultural pests by the millions, saving farmers huge amounts of money on pesticides, and how they keep down the city's mosquitoes. Now Austin proclaims itself to be Bat City USA, with upwards of 1.5 million Mexican free-tailed bats.
Bat-inspired murals and public art dot the cityscape. Hotels offer bat packages, bars offer bat-tini cocktails, and tour-boats float under the bridge. Bat tourism generates an estimated $10m a year for local businesses. There is even a bat hotline to check the forecast for bat activity. The city that once hated bats now embraces them.V
Herds, homes and horses
VMongolian nomads carry their homes – gers – with them as they and their herds follow the grazing. They owe their success to another animal of the grasslands, the horse. They are ridden but also milked, and fermented mare's milk is still an essential summer food. First, though, you have to catch your mare.
In late June, when new foals are about a month old and the spring grass is vibrant green, riders from every family saddle up and head out to round up their horses. In Chuluun's family, six riders round up more than 100 horses. Ulaana is equipped with a larch pole about six metres long with a simple leather loop lasso at the end. He and his horse cut and twist through the herd, aiming to loop the delicate lasso around a foal's neck. Tungaa then drags the foal to a rope strung between two posts at ankle level and ties it up. They repeat the process until all the foals are tied up. The mares, meanwhile, stay close by, and now the boys turn their mounts in pursuit of them. The chase is wild, the dust flies and the mares lean as they race ahead of the herd. Ulaana reaches forward, both hands on the pole while also holding the reins, and stands up in the saddle at a full gallop. With incredible horsemanship he lassos the mare. But fear makes her strong, and Ulaana needs all his strength to hold the pole. When she pulls free, the mare bolts back to the herd with the pole trailing along beside her. They have to start again.
This time, the pursuit is swift and the catch is clean, but it takes a second noose before the trio of horses comes to a halt. Esee approaches her on foot. He talks softly, with a halter held at arm's length. Slowly, calmly, he slips the strapping around her head. Only then does he remove the lassos. With help from Chuluun he straps up her forelimbs and hobbles her back legs so she can only lurch towards her tethered foal. The two are left alone for a few hours before milking is attempted. With encouraging murmuring, they bring the foal to suckle, then interrupt its drinking to milk the hobbled mare. With that done, the foal drinks again, and Tungaa takes the milk away to make the most treasured summer food, arak – a 24-hour fermented yogurt.
Taming game and fighting rivals
In the remote Omo Valley of southern Ethiopia live the Suri and their arch enemies, the Nyangatom. Their cultures are distinct, but both tribes are tied to their cattle and the grasslands that sustain them. Cattle bring wealth and status but also conflict and danger. To marry, a man needs to have a big enough herd of cattle. Today, many Suri carry AK47s bartered for cattle from conflicts across the border in Sudan, but for those without guns or bullets, courage must do if they are to protect their herds.
Good grazing is in short supply, and when the Suri men go in search of the best grass, they risk encountering Nyangatom cattle-raiders as well as lions and leopards. Either way, defending their cattle may become a fight to the death. So young Suri men must learn to master their fears and become brave, which they do through violent donga stick-fighting contests.
As soon as they can lift a stick, young boys practise donga. It is a skilled martial art, and it takes years to perfect. The duelling stick is the donga – two metres long and carved into a phallus at its tip. In contests called sagine, young Suri men from neighbouring villages fight each other in the hope of becoming banzanai – brave men – able to defend their cattle.
The fight ring is formed by the surging crowd. Nervous novices posture and swing their dongas through the air, though without conviction. But when two fighters engage, the tension is electric. If you can take the blows and knock down your opponent, you prove to yourself and your tribe that you have what it takes to protect your cattle.
Here comes the sun
By November, all the Arctic migrations slow down. The icy crown is once again enveloped by 24-hour darkness and extreme cold. People still hunt and fish, but the fading light shortens the days. Niels Gundel and his family live in a pretty A-frame house in Ilulissat, Greenland. In late November, Niels will watch the sun dip behind the horizon for the last time – it won't return until the second week in January.
The darkness isn't always pitch black – during the day, a kind of twilight arises. Even so, the lack of sun can be oppressive.
Arctic people have a simple cure. They socialise and hold coffee and gossip sessions that last hours. Aside from that, life goes on pretty much as if the darkness wasn't there. As January approaches, children make paper suns that they put up in their windows to encourage the sun to come back soon. On 13 January, on the 13th minute before the 13th hour, the locals of Ilulissat gather on the site of the ancient settlement of Sekinarfik to greet the first "dawn" of the year. A choir sings: "The sunlight lit my soul which had been in darkness/And drove away the cold by shining on me."
What will the new dawn bring? Data shows that though temperatures are rising globally, the poles are heating up much faster than anywhere else. Sea ice is melting and sometimes not even forming anymore. Animals are changing their migration patterns, and hunting seasons are disrupted. The knowledge that sustained people for centuries might become obsolete.
Human Planet starts on BBC1 on Thursday 13 January at 8pm. The accompanying book by Dale Templar and Brian Leith with photographs by Timothy Allen is published by BBC Books (£25), also on 13 January.
Human interest: How to see for yourself
Exodus (0845 287 2452; exodus.co.uk) offers a 15-day group trip to the Omo Valley, visiting villages and markets and meeting local tribes. Prices start at £1,939 per person, including flights from London, nine nights' hotel accommodation and four nights camping, most meals, transport and guiding.
Mongolian-based tour operator Nomadic Journeys (00 46 498 487105; nomadicjourneys.com) runs an eight-day Gobi Grasslands Ride with local herdsmen. The price from Ulaanbaatar is US$1,400 (£933) per person, including accommodation in hotels, ger camps and tents; an English-speaking guide, all equipment and most meals. International flights are not included.
Transindus (020-8231 0580; transindus.co.uk) offers a self-guided, 19-day "Following the Mekong" tour, tracing the river through Cambodia and Laos. Prices start at £4,300 per person with international flights, transfers and accommodation with most meals.
Travel the Unknown (0845 053 0352; traveltheunknown.com) offers a six-day Living Root Bridges tour in Meghalaya, taking in hill stations, root bridges and forests. The price of £1,200 per person includes flights from the UK, transfers, accommodation, entrance fees and guiding.
The Foreign Office ( fco.gov.uk) advises against all travel to Niger, with the exception of some southern regions.
Discover the World (01737 214251; discover-the-world.co.uk) offers a seven-day Essential West Greenland trip, based in Ilulissat. The price starts at £1,553 per person, including flights to and from Reykjavik in Iceland, transfers and accommodation with breakfast. Flights to Iceland are not included.
Austin's annual Batfest takes place on the Ann W Richards Congress Avenue Bridge on 4 September ( roadwayevents.com). Flights are offered from Heathrow by BA (0844 493 0787; ba.com) via Chicago or Dallas and Continental (0845 607 6760; continental.com/uk) via Houston or Newark.