1.   As it happens, mother rats give birth to very different ratios of sons and daughters depending on how they are faring.

2.   All were at a vulnerable stage in development, preparing to walk and leave the security of the nursing mother rats that accompanied them into space.

3.   The space agency believes the rats grew listless because the mother rats became dehydrated and were unable to nurse.

4.   Though the space agency plans an investigation, it believes that the mother rats became dehydrated and unable to nourish the youngest of the animals.