Scientists search for clues to the origin of the zebras' urge to migrate. With lifespans of just 15 years, signs point to a genetic urge, rather than a taught behavior.

Thousands of zebra face lions, hunting dogs, drought and starvation as they undertake the world's most recently discovered migration. Braving one of Botswana's most brutal wildernesses, they make Africa's longest land migration to reach the rich grasslands of Nxai Pan National Park.

As the zebra gather to leave, the crew gets a chance to meet the individuals. Some have given birth, some are still pregnant, but the start of the great 150 mile migration is just about to start.

As the zebras get close, lions and dirt fields await them at the migration site.

A hundred thousand caribou must face hungry bears and wolves, deadly frozen rivers and rugged mountains as they undertake the world's longest land migration - 3,000 miles through the frozen Arctic wilderness of Canada and Alaska.

The grizzles have just woken up from 6 months of hibernation, and they're starving. They badly need protein, and when the caribou pass through, they try to make the most of the opportunity to supplement their diet of roots and berries. The result is an area with one of the highest concentrations of grizzly bears on the planet.

In open landscape, a bear has a hard time catching caribou - but not humans. The camera crew must carefully remove themselves from the scene as the bear meanders, then charges, toward them.

With just 13 days left until much of the herd is due to give birth, this herd still has 180 miles to go. But the herd finally arrives where the team predicted. They descend from the mountains, spilling into the valley by the thousands.

A thousand elephants, from magnificent bulls to vulnerable orphans, come together in a crucial annual gathering in northern Kenya, whilst facing deadly threats from both predators and poachers.

Follow three iconic animals on three of the world’s most breath-taking wildlife adventures. Across hundreds of miles of Canadian arctic wilderness and African bush, we follow every life and death decision these herds make as they overcome immense obstacles and face fearless predators in a race to reach their destination.

In an act never before filmed, a group of elephants gorges themselves on weaver bird nests. Their behavior raises a compelling question: have they acquired a taste for eggs, chicks or the vegetation of the nests?

Using the information from a satellite collar, cameraman Bob Poole tracks down a musth bull elephant. As Bob gets close, he attracts some unwanted attention.