Earlier this year, Toni, one of the National Zoo’s resident elephants, was euthanized after a two-decade-long struggle with arthritis in her legs. While zoo officials maintained that Toni’s condition was pre-existent, animal rights activists claimed that the tight quarters of the zoo’s Elephant House only worsened her mental and physical health. Though an isolated incident, Toni’s death brought to light an issue that has long afflicted zoos across the country — are certain animals meant…