A zooligical irony: Just south of the panda house at the National Zoo lived Arusha and Joe Smith, no easy-come, easy-go lovers. While Ling-Ling got all the headlines, alternately flirting with Hsing-Hsing and the London import, Chia-Chia, the story of Arusha and Joe went unchronicled. A simple story, theirs.

These humble, contented cohabitants of the hippo tank in the elephant house lived together for 25 years. Once, it is true, Arusha, who weighs in at 7,000 pounds, sank six inches of her tust into Joe's hide. But this was thought by the keepers to be an accident. Their happy liaison ended when Joe Smith died in November 1980.

But not before the two produced 18 calves, setting a record for hippo births in captivity.

"After the disappointment of the recent panda encounters," said Bess Frank, staff zoologist, "it's nice to reflect on a zoo success."

Arusha, a prize Nile hippopotamus, arrived at the zoo in the mid-'50s at the age of 3 from Arusha, a town in Tanzania. Shortly after, Joe Smith arrived, via a New York dealer, at the age of 6 months. Compared to the worldwide waiting for panda progeny, activities in the hippo tank went virtually unnoticed.

And Joe Smith died in obscurity. Headlines at the time were only of Ling-Ling and her cases of fruitless love. Keeper Lee Battle related that for some time afterward, Arusha watched the door to the tank and turned at any noise, possibly hoping to see Joe return. She appeared to be quite listless -- and then . . .

On Jan. 4, 1981, Arusha gave birth to a male calf, Happy. He weighed between 80 and 100 pounds, and like other young hippos, came into the world knowing how to swim.

And what were the pandas doing in January? Not much. Hsing-Hsing's virility was in serious question; an SOS to London was near.

Meanwhile, at the hippo tank, there is nothing but bliss. The water level of the tank is just right for Happy (known as Joe Jr. by the keepers) to stand with his nostrils above water, wriggling his ears. He nurses underwater and soon will be weaned.

While Happy's 15 brothers and sisters who lived were given as goodwill gifts to other zoos, Happy is scheduled to stay on with Arusha.