The bison named Shponti was thought at first to be the victim of food poisoning from rotten tomatoes and tangerines. However, closer forensic examination revealed fatal levels of caffeine and amphetamine in the animal's system.
Veterinarian Vyacheslav Vasin told Russia's RIA Novosti news agency the stimulants were found in Shponti's kidney at levels high enough to cause cardiac arrest.
Vasin downplayed suspicions the European bison imported from Germany was deliberately fed the fatal dose. He told the news agency the cost of the amphetamines was probably too high for the animal's overdose to have been deliberate.
The vet suggested Shponti somehow came across the stimulants accidentally.
Shponti died Jan. 7 at a reserve outside Moscow on the same day its female mate also dropped dead of unknown causes.