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1·A cow's estrus cycle averages 21 days.

母牛的发情周期平均为21天。

2·A cow 's estrus cycle averages 21 days.

母牛的发情周期平均为21天。

3·Male chinchillas tend to be easier to handle because they are not subject to temperament fluctuations due to the estrus cycle.

雄性龙猫往往更容易相处，因为它们没有波动的发情周期。