<p>The <a href="http://animals.nationalgeographic.com/animals/mammals/african-lion/">African lion</a> (<i>Panthera leo</i>) is the only wild cat that lives in large family groups. Two thousand years ago, more than a million lions roamed the earth, but today, African and <a href="http://animals.nationalgeographic.com/animals/mammals/asian-lion/">Asiatic lions</a> together number fewer than 30,000 individuals. </p>

Monarchs of the Savanna

The African lion (Panthera leo) is the only wild cat that lives in large family groups. Two thousand years ago, more than a million lions roamed the earth, but today, African and Asiatic lions together number fewer than 30,000 individuals.

Photograph by Vincent J. Musi

Our Most Stunning Pictures of Big Cats

From lions to tigers to leopards, see National Geographic's most beautiful portraits of wild felines in honor of Big Cat Week.

Lions, and tigers, and pumas—oh my.

Humans have long been fascinated with big cats, the eight largest of Earth’s 37 wild cat species.

“Of all the great creatures, they are perhaps the most magnificent, iconic, and fabled,” says conservation scientist Luke Dollar, a National Geographic Emerging Explorer.

Their size and power makes big cats top predators, helping maintain the balance of the ecosystem by controlling prey populations, says Dollar. (Check out the lineup for National Geographic Channel's Big Cat Week.)

But the big cats’ predatory skills and massive sizes also make them easy targets. All big cats are losing habitat, much of it eaten away by human sprawl. Many get killed by herders retaliating against deaths of their livestock.

And poachers continually chip away at their numbers, killing species such as tigers for their skin and other hunting trophies.

In other words, says Dollar, the danger to cats can be summarized in one word.

“People.”

But people can also be part of the solution. Dollar and other conservationists are hopeful that targeted, cost-effective interventions—such as fortifying the fences around livestock-grazing areas—can preserve species for future generations. (Read more about National Geographic's Big Cats Initiative.)

After all, the stakes are high. As field biologist George B. Schaller wrote for National Geographic magazine, "The great cats represent the ultimate test of our willingness to share this planet with other species."

This slideshow showcases National Geographic’s favorite pictures of big cats, snapped by Vincent J. Musi at zoos in Virginia and Texas.

Big Cat Week returns Friday, November 27 at 9/8c.

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