The red knot makes one of the longest migrations of any animal. To accomplish this feat, it relies on the eggs of the horseshoe crab. Without these eggs, the red knot is in danger.

Several months ago, Ron began retaining water -- for chimpanzees, a symptom of heart disease. He's being sent to a sanctuary in Florida.

After work in show business, 37-year-old chimpanzee Billy Jo was sent to the lab for use in medical research.

(Airs Sunday, April 18) More than a third of all amphibians have already been lost, and more are disappearing every day. A fungus called chytrid has been identified as the major culprit.