Denver, Colorado -- It's a day of mourning at the Denver Zoo after the death of one of their tigers.

Martin, a 3-year-old Amur Tiger brought in from Russia recently, died of an extremely aggressive form of leukemia on Friday.

The tiger started showing signs of lack of appetite and decreased activity back on October 22, prompting animal care and vet staff to begin examining the tiger, the Denver Zoo said in a news release announcing the tiger's passing.

The exams showed the tiger had severe anemia. Despite this, zoo staff on Thursday conducted a blood transfusion with another tiger at the zoo - Nikolai - but it was no use.

Martin's test results came back Friday and showed the worst: he was suffering from an extremely aggressive form of leukemia, with cancer cells present in his blood, bone marrow and spleen.

The zoo brought in consultants from the Colorado State University's Veterinary Oncology Department and staffers were told the cancer was terminal.

Having lost his quality of life, the zoo said the decision was made to humanely euthanize him on Friday.

Martin's Life

He was born at the Moscow Zoo back in 2014 and arrived in Denver from there on June 1 of this year. No one had met or seen him since he came from Russia, as he was still acclimating to his new environment, zoo staff said.

Martin was brought to the U.S. to help support the Association of Zoos and Aquariums' (AZA) Species Survival Plan. The plan was created in 1981 by the AZA to help repopulate threatened and endangered animal species.

He wasn't related to any North American tigers; he was to be a great help to the efforts - and still will be.

Before he was euthanized, zoo staff collected a sample of his reproductive material for pass on "his valuable genetics," the zoo said.

Records from the Moscow Zoo showed a clean bill of health and didn't show any significant conditions prior to his shipping to the U.S. Staff said they believe his decline was rapid.

Keepers said Martin was "shy but flirtatious."

Amur tigers have a median age of 16 years for boys and 14.3 for girls. The Denver Zoo has three other tigers, 7-year-old boys Nikolai and Thimbu and 6-year-old girl Nikita.

The International Union for Conservation of Nature classifies Amur tigers as 'endangered,' meaning that there's less than 400 of them remaining in the wild.

You might know these tigers as 'Siberian tigers' because they were once found throughout the territory.

However, they're now almost completely confined to the Far East portion of Asia along the Amur River, hence their name.

Besides habitat loss, their biggest threats come from poachers - for fur and other body parts - used in traditional Asian medicine, the zoo said.