Why do pigs crawl to sleep?

When wild boars first appeared, there were also wild boars that were not crawling and sleeping. Wild boars that were crawling and sleeping could detect enemies attacking them earlier (sounds travel faster and farther underground than in the air) At the same time, crawling to sleep is more in line with the physiological characteristics of animals (it can relax the muscles of the whole body). The wild boars that did not crawl to sleep died due to natural enemies and diseases, and the remaining wild boars were crawling to sleep. Wild boars were domesticated into domestic pigs. This feature is retained!