The director of the Detroit zoo hired four new security guards last week, not to contain the wildness within the cages, but to protect the animals from the inhumanity of man. In the past two years, the zoo population has been victimized by deliberate acts of brutality. A baby Australian wallaby left the protection of its mother's pouch and was stoned to death; a duck died with a steel-tipped hunting arrow in its breast. A pregnant reindeer miscarried after firecracker-hurling youths bombed the frantic animal into convulsions. Visitors have been observed dropping lighted cigar butts on the backs of alligators, watching the ashes burn through the reptiles' skin, then breaking into laughter when the alligators reacted to the severe burn. Finally, the zoo's male hippopotamus choked to death last week after someone responded to the hippo's open-mouthed begging for peanuts by rolling a tennis ball down its throat. The zookeepers were left to wonder whether it was their charges or their visitors that really should be caged.