Mouse mourned: Yoda dies at age 4.

The age-defying laboratory mouse known by his keepers as Yoda died peacefully in his cage in Ann Arbor, Mich., on April 22. The cause of death is unknown. Autopsy results are pending. The oldest known living member of his species at the time of his death, Yoda was 4 years and 12 days old.

Yoda carried a mutation that disables the production of three hormones required for normal growth. Scientists were studying the diminutive mouse to learn how hormonal irregularities enabled him to postpone signs of aging and associated diseases.

Yoda weighed between 10 and 15 grams during his adult life. Most lab mice of his strain grow to about 30 or 35 grams, but they typically live only 2 to 2.5 years. While small size and extreme longevity in some lab animals can be attributed to low-calorie diets, Yoda's food intake was never restricted.

A mutation in the gene Pit1 accounts for Yoda's dwarfism, long life span, and other unusual traits, says Richard Miller of the University of Michigan, who supervised the mouse's care. Mice with this mutation seem to delay aging and develop diseases such as cancer about 40 percent later than related mice with normal Pit1 do.

Infertile because of his missing hormones, Yoda fathered no offspring. He leaves behind Princess Leia, his constant companion of the past 18 months, without her body warmth, Yoda might have died earlier, because maintaining normal temperature requires the hormones he lacked. Yoda outlived three previous cage mates, including one with the Pit1 mutation.