Maybe the marine rats in your ship were as tough as you navy's
The mices I saw were medium sized ( not that I study mices / rats ) and the poison was there for some time.- a few days.
I cannot prove 100% that it is thirst, but they do run away from their hiding place, just before dying.
Something does mess their head up. That's for sure.
And , the 10 rupee wala poison does wonders !
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