No one knows why Simon, the three-foot-long rabbit that could have become the world's longest, died on a United Airlines flight from London to Chicago.

But rabbit experts told National Geographic they suspect the animal's cause of death stems from stressful conditions during traveling.

Though airlines generally outfit planes' cargo holds to be comfortable for animals, it's not always consistent, says Sandy Parshall, a program manager with the House Rabbit Society, a California-based nonprofit that promotes rabbit rescue and welfare. (Read why Easter is bad for bunnies.)

"The entire aircraft is pressurized, and not all pieces of the aircraft are heated," says Parshall, who is also a commercial pilot. "It generally gets pretty cold."

What's more, a bigger risk comes from animals