Subzero temperatures force polar bears to bulk up—but as summer returns and the planet grows warmer, they find themselves on thin ice.

Subzero temperatures force polar bears to bulk up—but as summer returns and the planet grows warmer, they find themselves on thin ice.

Monsoon season drives giants toward higher ground, and big cats battle it out to claim dwindling territory in an Indian valley.

Monsoon season drives giants toward higher ground, and big cats battle it out to claim dwindling territory in an Indian valley.

In Borneo, apes and monkeys train their babies in the treetops, but with gravity against them, the stakes are high—and crocodiles lurk below.

In Borneo, apes and monkeys train their babies in the treetops, but with gravity against them, the stakes are high—and crocodiles lurk below.

A blazing sun scorches the African savanna, where the planet's biggest bird protects his eggs and giraffes struggle to fuel their towering bodies.

A blazing sun scorches the African savanna, where the planet's biggest bird protects his eggs and giraffes struggle to fuel their towering bodies.

Rocky Mountain summers are fleeting, leading local bears to gain weight, sharpen defenses, and engage in a dynamic fight for domination.

Rocky Mountain summers are fleeting, leading local bears to gain weight, sharpen defenses, and engage in a dynamic fight for domination.

In Earth's largest inland delta, Africa's deadliest mammal fights to keep his family afloat as the dry season approaches.

In Earth's largest inland delta, Africa's deadliest mammal fights to keep his family afloat as the dry season approaches.

Makumba is one of the best studied gorillas in the world. Now past his prime, he must find new ways to maintain his status as king of the jungle.

Makumba is one of the best studied gorillas in the world. Now past his prime, he must find new ways to maintain his status as king of the jungle.

In the Brazilian Pantanal—a wetland larger than England—a six-foot otter attempts to raise her cubs in the shadow of hungry jaguars.

In the Brazilian Pantanal—a wetland larger than England—a six-foot otter attempts to raise her cubs in the shadow of hungry jaguars.