Ancient silk paintings dating to China’s Ming dynasty have yielded new clues about the domestication of brown rats, which according to researchers, could have spread to the rest of the world while the treasure ship fleets of explorer Zheng He ruled the oceans.
The artwork could be the earliest evidence of such domestication, shedding light on the taming of an animal that is vital to biological and clinical research.
“The imperial paintings, The Silk Scroll of Three Rats, depicted domesticated brown rats with coat colour variation in China during AD1425–1435,” the team wrote in a paper published in the peer-reviewed journal npj Heritage Science on April 12.
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“It was more than two centuries older than the known record in Japan, presenting the earliest evidence of brown rat domestication.”
The researchers said that based on the inferred pattern of the introduction of wild brown rats across the world, domesticated brown rats may have spread by stowing away on ships used by admiral Zheng He during the Ming dynasty (1368-1644). The expeditions involved tens of thousands of men and hundreds of ships known as the Treasure Fleet.
Brown rats, also known as common or street rats, are a widespread species of rat found around the world, but are believed to have originated in northern China and Mongolia.
The domesticated forms of the brown rat include fancy rats, which are kept as pets, as well as laboratory rats. Laboratory rats are one of the most common animals used in biological research, but tracing the early domestication of these creatures has proven challenging.
While there was some documentation of rats used in traditional medicine in ancient China, evidence of early domestication of the wild brown rat in its homeland was scarce, the researchers said.
The earliest known domestication of brown rats as pets was recorded in Japan in 1654 during the Edo period, with breeding varieties such as an albino coat colour documented in guidebooks.
The researchers, from the Chinese Academy of Sciences Kunming Institute of Zoology, and Yunnan Agricultural University, studied The Silk Scroll of Three Rats painted by Zhu Zhanji, the fifth emperor of the Ming dynasty, from the collection of the Palace Museum in Beijing.
“The scroll consists of three paintings of three rats showing the morphological characters of the brown rat,” the researchers said.
One of the rats depicted was an albino with white hair and red eyes, resembling domesticated rats in Japan from long ago, as well as modern Wistar rats, a standardised albino rat strain used widely in research. A light brown rat was also pictured, wearing a gold collar and chain.
These two rats, painted in colour, were both eating lychees, which the team said were an expensive fruit at the time that was produced only in southern China, suggesting the rats may have been “carefully raised in the imperial palace gardens in Beijing”.
“Given substantial behaviour – such as tameness – shifts based on genetic changes in the domestication of rats, it was unlikely that the emperor painted wild or tentatively captive brown rats,” the researchers said.
Demographic history suggests that human migration may be responsible for the introduction of wild brown rats into Southeast Asia, which were eventually transported into West Asia and Europe via maritime trade routes.
“This scenario raises the possibility that early domesticated rats may have spread, potentially alongside wild brown rats, during the Ming treasure voyages and subsequent maritime trades,” the researchers said.
The Ming expeditions were a major undertaking of seven voyages around the Indian Ocean that took place from 1405 to 1433.
“Improved demographic inference methods and the growing accumulation of genomic sequencing data, particularly from Japan’s rat resources or even qualified ancient DNA, hold promise for elucidating the fate of these early domesticated rats,” the team said.
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