"Rats came up from the canal, fed on the plentiful corpses, and multiplied exceedingly. While I stayed here with the Welch. a new officer joined the company and, in token of welcome, was given a dug-out containing a spring-bed. When he turned in that night he heard a scuffling, shone his torch on the bed, and found two rats on his blanket tussling for the possession of a severed hand." - - Robert Graves.
Multiplying EXCEEDINGLY and with vigor!
From that era of the Great War [WW1] we have:
Trench raid, trench foot, trench fever, trench knife, trench club, trench shotgun, AND TRENCH RAT!
From the Spartacus web site courtesy of John Simkins we have vivid descriptions of the trench rat, populating the same living space as the soldiers of WW1, a disease carrying pest parasitic on the human species, a scavenger living on the refuse of mankind, an omnivore and during a time of war CONSUMING WITH RELISH THE FLESH OF THE BATTLEFIELD DEAD!!
The filthy conditions of the trenches and the lack of a "proper system of waste disposal" allowing for rat populations to explode, "exceedingly", tins as mentioned being the empty container left over from canned rations, the cans merely tossed out into "no mans" land providing a feast for the vast multitude of RATS!
"There was no proper system of waste disposal in trench life. Empty tins of all kinds were flung away over the top on both sides of the trench. Millions of tins were thus available for all the rats in France and Belgium in hundreds of miles of trenches. During brief moments of quiet at night, one could hear a continuous rattle of tins moving against each other. The rats were turning them over." - - George Coppard.
These were brown rats? A species that has become so accustomed to living as a parasite dependent on man that the creature would become extinct if humans were to disappear from the planet?
Rats too when given an abundance of food breed at an exponential rate, a single mated pair producing a larger brood more often with a higher percentage of their offspring reaching maturity the young doing so at an earlier age. Exponential population growth virtually limitless in scope!
Efforts to contain that rat population futile at best, the rat so vigorous and adaptive, flexible and responsive in a way few other animals are! RAT!