As it was, 55 of 96 baby rats came back dead, victims of maternal neglect."When you initially see this, you say, oh, no' then we started treating. I guess the veterinarian in me kicked in,"Richard Linnehan said late on Sunday after the space shuttle landed, ending a 16-day mission that was the most in-depth study ever into how the brain operates in space.

Scientists want that question answered before planning possible Moon colonies or a trip to Mars. Linnehan said the mission also provided immediate lessons for the international space station, to be assembled in orbit beginning this year.

The mortality rate was even higher among the young swordtail fish that flew on the mission: 200 dead out of 225, most likely because their water was too warm. But the astronauts didn't learn about that untilafter they landed they didn't have access to the fish in orbit.

They did have access to the rats, however, and did everything in their power to save the tiny, emaciated rodents once it became clear they were quickly dying of dehydration.

To the crew's astonishment and dismay, the surrogate mother rats couldn't or wouldn't feed the young animals.

As the astronaut in charge of Columbia's lab and the only veterinarian on board, Linnehan immediately assembled a critical care unit. Night after night, after their other work was done, he and his crewmates tended to the rats: Because of the crew's heroic efforts, enough rats survived to fulfill the primary objectives of the brain developmental experiments, NASA said.