Why are Wu Zetian's "grave keepers" headless?

Wu Zetian was buried in Ganling after her death, and an ancient statue of 6 1 was built for her "guarding the mausoleum". These stone statues originally had heads, which were preserved until the Ming Dynasty, as can be seen from Liu Bowen's Travel Notes on Youganling in the early Ming Dynasty. But in the middle and late Ming Dynasty, the ancient statue of 6 1 became a headless "grave keeper" for four reasons.

First, it shows that at the end of that year, there was a big earthquake in Ganling area, and the stone statue of Shouling was destroyed in the earthquake. According to Ming History, a strong earthquake of magnitude 8 or above occurred in Huaxian County, Shaanxi Province on January 23rd, the thirty-fourth year of Ming Jiajing. The head of the stone statue was broken in this earthquake, only the head was broken, because the neck of the stone statue was very fragile.

Second, at the end of the Ming Dynasty, a terrible plague occurred in Ganxian County, killing countless people. At that time, the feudal people thought that these stone statues of Ganling were at work. Only by moving their heads can we stop the plague and save the whole county. So, everyone cut down all the stone statues.

Third, at the end of the Qing Dynasty, a large number of westerners came to China to explore, collect cultural relics and resell them abroad to make money. Thieves saw the Ganling 6 1 stone statue, thought it was priceless and wanted to resell it. However, these stone statues weigh thousands of kilograms, which are not only difficult to carry, but also easy to expose their targets. So they had to knock off the head of the stone statue with a hammer and take it away, and then buy it out.

Fourthly, the head of the stone statue was lost in the hands of Eight-Nation Alliance in the late Qing Dynasty. At that time, Eight-Nation Alliance saw so many foreign envoys standing respectfully in front of Emperor China. He felt it was a shameful thing, so he was angry and smashed the head of the stone statue with a hammer.

Finally, no matter which of the above four statements is true, Wu Zetian's "grave keeper" has no head, which is obviously caused by acquired factors.