THREE one-majestic Siberian tigers, that had fallen victim to the ills of old age were humanely put to death at the Detroit zoo last week, but not before a hearing was held, testimony taken and a judge's decision handed down allowing veterinarians to do what they felt should be done. What queer admixture of imperatives made the decision of life or death one for a judge?

The zoo director, Steve Graham, says his tigers are too ill to go on, that the humane course is to administer a lethal dose of barbiturates. Some self-proclaimed friends of the tigers--the tigers have disdainfully refused to confirm having had meaningful relationships with any humans other than Felix, their feeder --filed suit to block the mercy-killing.

Ubiquitous lawyers no doubt asserted some constitutional requirement that the zoo hold public hearings and seek written consent from . . . from . . . oh, well. The bemused judge, in a respite from all those murder and heroin cases, will now hear arguments about Siberian tiger mouth diseases and hip ailments.

Closer to home, at the Washington National Zoo's preserve in rural Virginia, the deer flourish. They have flourished so freely, in fact, that there's now a population problem and some of them must be harvested or slaughtered--the particular terminology depends on your perspective. Few people hesitate to capture free, wild beasts and lock them in cages or pits to be gawked at by children of all ages -- a small moral chasm easily bridged. But more people balk at the ensuing series of moral chasms. Having volunteered for the responsibility of acting in loco naturalis, the keepers uncomfortably assert the duty to use nature's method for handling such problems--death.

People certainly have a peculiar way of imitating nature. In Detroit, the methods proposed seem expertly humane enough, but the decision itself got tossed into the hands of inexpert lawyers and courts -- preposterous. At the Smithsonian's zoo here, the legal institutions haven't gotten in the way, but the officials want to turn an unfortunate need to kill deer into a sporting spectacular for check-shirted sportsmen. It's (human) nature's way.