Boy or girl? It’s the eternal question. But that question may have more than two answers. Science reveals the line between male and female is blurred. And the battle of the sexes is happening deep within our cells. Are there more than two sexes?

The most famous – and perhaps most misunderstood – episodes in the epic history of the Holy Wars that are known to history as The Crusades – the clash between Richard the Lionheart and Saladin. In July 1192, Richard the Lionheart stood poised for a strike on Jerusalem, while Saladin – mighty sultan of Islam – readied his troops inside the city, preparing for the inevitable attack. During a year-long campaign across Palestine, these two legendary leaders had fought each other to a stand-still. Thousands had perished, appalling atrocities had been perpetrated on both sides. And now they faced each other in a battle for their sacred objective.

David Attenborough continues his search for the origins of flying animals, as he charts the rise of new, larger creatures – extinct reptiles that flew on wings of skin, dinosaurs that sprouted feathers to become the ancestors of birds, and gliding mammals that emerged from a life in the trees to dominate the night as bats. His journey spans the globe – from the rainforests of Borneo to see bizarre gliding animals, to China and its new discoveries of dinosaur fossils. Stunning CGI brings to life creatures that disappeared over 200 million years ago, and new filming techniques bring us closer than ever to the secrets of flight itself.

David Attenborough concludes his epic history of the evolution of flight with an exploration of the highly advanced fliers that dominate our skies today – the extraordinarily diverse skills of the birds, and the sonar-guided precision of the bats. He encounters some of our planet’s most remarkable fliers – from peregrine falcons dive-bombing starlings over Rome, to hummingbirds hovering in the cloud forests of Ecuador. In a spectacular finale, he visits Gomantong cave in Borneo to witness the mass exodus of a million bats.

In this bird's-eye view of two continents, demoiselle cranes negotiate a dangerous Himalayan pass on their way to India while high-flying bar-headed geese take the fast track five miles above. In Rajasthan, vultures watch hunting tigers hoping for a meal and pigeons visit a temple dedicated solely to sacred rats. Pigeons are also our guide to the greatest gatherings of camels on Earth and learn to dodge buzzards around the battlements of Jodhpur Fort. 9,000 cranes overwinter in the most unlikely of spots - a barbed wire compound in the centre of a desert town. In Australia, rainbow lorikeets drop in on Sydney and patrol Australia's Gold Coast. In the outback, white cockatoos swirl in thousands and budgerigars pass Uluru (Ayers Rock) and gather in the biggest flocks ever recorded. In China, swallows and swifts visit the Great Wall and the Forbidden City of Beijing. In Japan, the country's most revered birds - Japanese cranes are fed fish by appreciative locals and are joined in strange, momentary harmony by hungry red foxes, white-tailed eagles and Steller's eagles. As peace descends, Japanese cranes dance beautifully in the snow.

On the coast, two worlds collide. Coasts are the most dynamic and challenging habitats in the ocean - that brings great rewards but also great danger. The extraordinary animals that live here must find ingenious ways to cope with two very different worlds. This episode is a rollercoaster ride of heart-stopping action and epic drama, peopled with characters from the beautiful to the bizarre. We meet fish that live on dry land and puffins that must travel 60 miles or more for a single meal, and witness a life-and-death struggle in a technicolour rock pool. In a secluded cove in the Galapagos, sea lions feast on 60kg tuna. It should be impossible - tuna are usually far too fast for sea lions to catch. But here the sea lions club together to herd their prey inshore. Once trapped in the shallows, these huge fish are easy pickings. As the tide recedes in Brazil, lightfoot crabs leap from rock to rock, desperately avoiding the water - their lives depend on it. Moray eels launch themselves from rock pools, jaw gaping. Then octopuses, too. Both crawl across dry rock to set their ambush. Elsewhere, the ever-changing tides create rock pools. But these temporary worlds are a battleground. Predatory starfish turn a magical garden into the stuff of nightmares.