Have you heard the saying “In the land of the blind, the one-eyed man is king”? The Chinese expression for the same idea is 山中無老虎，猴子稱大王 (shān zhōng wú lǎo hǔ, hóu zi chēng dà wáng), literally mountain (山中) no (無) tiger (老虎), monkey (猴子) declares (稱) king (大王), meaning when the tiger is not in the mountains the monkey reigns as king.
I found another expression in English using animal imagery: When the cat’s away, the mice will play.
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