Last week in the Journal of Experimental Biology, Stanford University biology professor Mark Denny concluded male sprinters could get the 100m record down to 9.48 and the 200m down to 18.63.

BBC: Bolt eyes world record domination

The most likely answer, as he explains in a paper in the Journal of Experimental Biology, is that urine, though cheap to produce when water is plentiful, is a physiologically costly product when water is scarce.

ECONOMIST: Softshell turtles urinate through their mouths