That is NOT Japanese culture. That is ancient Chinese culture. Almost Japanese don't know the reason. I expect that who knows it will write the answer.

Thank you for your answer, so in first place those kanjis where just for naming the parts of the sky. (:

Oops. *Ancient Chinese people divided the sky into 12 parts to record time,

子 isn't pronounced as "nezumi". It could just means a mouse and with this meaning it is read as "ne". Ancient Chinese people divided the sky into 12 parts record time, and named them 子, 丑, 寅, 卯, 辰, 巳, 午, 未, 申, 酉, 戌, and 亥 respectively. Chinese people also combined each of them with animals. 子 is combined with a mouse.