The monarch butterfly is one of the most recognizable and well-studied butterflies. Western monarchs have declined by more than 99 percent since the 1980s. Eastern monarchs have declined by an estimated 80 percent.

Black rhinos are browsers rather than grazers, and their pointed lip helps them feed on leaves from bushes and trees. Between 1960 and 1995, black rhino numbers dropped by a sobering 98%, to less than 2,500.

Cotton-top tamarins are roughly the size of a squirrel and are named for the shock of white hair encircling their heads, a look reminiscent of Albert Einstein. Cotton-headed tamarins are critically endangered. In the 1960s and 1970s, 20,000 to 30,000 cotton-top tamarins were used for biomedical research.

African elephants are the largest land animals on Earth and they are a keystone species, meaning they play a critical role in their ecosystem. Poaching for the ivory trade is the biggest threat. As few as 400,000 remain today, vulnerable to extinction.

Grizzly bears once lived across much of western North America until aggressive hunting eliminated most of the population. In the 1960s, it was estimated that there were only 600 to 800 remaining in the wild.

Whales roam throughout all of the world's oceans, communicating with complex and mysterious sounds. Their sheer size amazes us: the blue whale can reach lengths of more than 100 feet and weigh up to 200 tons—as much as 33 elephants. With only 5,000-15,000 mature individuals left, their status is endangered.

Chimpanzees are great apes found across central and West Africa. Along with bonobos, they are our closest living relatives, sharing 98.7 percent of our genetic blueprint. The International Union for the Conservation of Nature has declared the chimpanzee an endangered species—and the booming human population is primarily to blame.

Once numbering 30 million to 60 million in North America, their numbers were decimated in just a few decades as expansion pressed westward. No other species on Earth has declined so quickly. With only 11,248 to 12,123 mature individuals there are near threatened.

Polar bears are classified as marine mammals because they spend most of their lives on the sea ice of the Arctic Ocean. Because of ongoing and potential loss of their sea ice habitat resulting from climate change, polar bears were listed as a threatened species and are classified vulnerable.