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 awaiting transfer on a layover flight. While on the tarmac and loading ramps, pets are more susceptible to stress from freezing temperatures or sweltering heat absorbed by the asphalt paving a plane's runway.
Anne Martin, the rabbit society's executive director, added that other stressors, such as noises or stimuli, could also have caused the demise of Simon, who belonged to a breed called the continental giant.
"Rabbits are prey animals. If there are barking dogs, that might be enough to stress them to the point of heart attack," says Martin.
Both Parshall and Martin recommended flying with rabbits tucked under seats in the plane's cabin or transporting rabbits via other means such as by car.
Animals bred for specific physical features—such as large size—often have more genetic defects, which may have made Simon more fragile, Martin notes.
In an interview with British tabloid The Sun, Simon's owner Annette Edwards claimed that a vet's visit three hours prior to the flight diagnosed the rabbit as "fit as a fiddle."
The ten-month-old, which was destined to become a pet in the U.S., was likely going to outgrow its father, Darius, which at 4.4 feet long holds the Guinness World Record for longest rabbit.
Continental rabbits are recognized throughout Europe; however, the American Rabbit Breeders Association recognizes breeds such as Simon's as Flemish giants. Early records of the breed in Europe date to the 16th century, and it was imported from Belgium to the U.S. in the 1890s.
Dubbed "gentle giants," the docile breed is considered an excellent pet, particularly for small children, according to the Maryland Zoo at Baltimore.As of the this article's publication, United Airlines had not released Simon's official cause of death.