Darpa, the Pentagon's research arm, has already started to investigate ways for soldiers to fight without sleep or food. Now the agency wants to see if G.I.s can carry on without most of their blood.

"The vision for the Surviving Blood Loss (SBL) Program is to develop novel strategies that delay the onset of irreversible shock and allow an injured warfighter to survive with significantly reduced oxygen delivery for extended periods of time," a Darpa solicitation reads.

It's all part of a larger effort to shore up what the agency sees as the "weak link" in the military's chain: the fragile human being.

If all goes according to plan, phase one of the SBL effort will crescendo with a group of rats being able to live with 60% of their blood removed for three hours. Darpa wants to see a survival rate of 75%. Proposals are due by next February.