Other, more serious problems were afoot, however. On 27 April, NASA revealed that dozens of young (neonatal) mice in the Spacelab module had died unexpectedly; others were in such bad shape that they had to be put to sleep by the astronauts. ''This hit literally out of the blue,'' said Bielitzky. The crew managed to resuscitate a few others with bottle feedings and Linnehan later thanked his crewmates for helping to hand-feed and care for the mice, checking on them each night before bedtime.
At first, Linnehan speculated that the deaths might have been caused by the design of the rodents' cages. ''It may be a function of the way the cages were built and the way the mothers moved around in the cages,'' he said. ''Some of the animals were not able to maintain a hold or get to a mother at certain times they needed to. We saw them free-floating and they appeared to be doing well, but I just don't think they could figure out the environment well enough to get to the mother and nurse; so they became dehydrated. As you know, when you become dehydrated, you become depressed and that is a cyclical thing that builds and builds and you get to the point where some of them just succumb to that.'' In addition to providing fluids for the mice, Linnehan and his colleagues hand-fed the rodents nutritional and subcutaneous water supplements and antibiotics to stem the losses.
Back on Earth, the immense death toll - more than 50% of all the neonatals - had drawn condemnation from animal-rights groups, including People for the Ethical Treatment of Animals (PETA), who charged NASA with abuse. Bielitzky denied the allegation, promising a full investigation, but PETA went further and pressed Congress to ban further animal experiments on the Shuttle. A higher-than-normal death count during a previous rodent experiment on STS-72 in January 1996 should, they argued, have provided a red-flag warning sign to the space agency. ''Certainly,'' Bielitzky acquiesced, ''you would not refly this if you did not have great confidence the mortality rate would fall into the 10-15% range.''
This disaster could not have happened at a worse time for the neonatals, who were particularly vulnerable and just learning to walk and seek out solid food. The poor state of some of them forced Linnehan to cancel one of the experiments, telling Mission Control that ''there is no meaningful data to be gained with these animals at this point''. In total, more than 55 of the mice were lost. ''It was an unforeseeable event,'' Linnehan said after the flight, ''and it's regrettable that it happened. However, we still got back most of the primary science.''
It later became apparent that the ill-fated neonatals were victims of a higher-than-normal rate of maternal neglect, who had themselves become dehydrated and unable to lactate adequately. Consequently, as the youngest mice grew anaemic, the mothers stopped feeding and grooming them. Linnehan drew praise from animal-rights groups on 1 May, after defying an instruction to destroy a rodent when ultra-fine electrodes implanted in its brain broke loose. ''Based on his expertise and professional opinion, he determined the animal was not in any danger and determined it was appropriate to return that animal to its housing,'' said mission scientist Jerry Homick.
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