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.
Meanwhile, Dr Forrest's group has constructed an experimental set-up, described in a paper recently submitted to the Evolutionary Computation Journal, that mimics the way real immune systems identify invading pathogens.
Readers of a paper (including the peer reviewers) have to trust that the researchers correctly converted raw data and experimental conditions to abbreviated formats.