Ancient domestic pig genes may survive in Japan's wild boars

Associate Professor Michiko Niimi at the Nagoya University Museum and Graduate School of Informatics, along with Mr. Ritsuo Uchida, former member of the Shimane Prefectural Center for Archaeological Operations, analyzed Yayoi-period wild boar mandibles excavated from the Nishikawatsu Site (Matsue City, Shimane Prefecture) and revealed that domestic pigs were included among them. Analysis of mandible morphology and age composition revealed that pig husbandry and wild boar hunting coexisted at the site. In the San'in region, as in Oita and Aichi prefectures, pigs brought from the Chinese continent were raised and interbred. It is thought that genetic impacts from this remain in modern wild boars. The results were published in Volume 43 of the Journal of the Archaeological Society of Shimane and announced at the general meeting of the Society of on June 20.

Pigs are livestock created by humans over a long time through taming wild boars. This began in Japan when they were brought from the Chinese continent during the Yayoi Period. In the Honshu and Kyushu regions, wild boar hunting has been conducted since the Jomon Period, and it is known that in Oita Prefecture (Shimogori Kuwanae Site) and Aichi Prefecture (Asahi Site) during the Yayoi Period, pig husbandry and wild boar hunting were conducted in parallel.

In the process of domestication, where pigs split from wild boars, the shape of a number of bones changed. Representative examples include the head and mandible becoming shorter in the anteroposterior direction, which causes the anterior surface of the mandible to become concave and the rising angle of the anterior mandible to become larger. To confirm whether pig husbandry was also conducted in the San'in region during the Yayoi Period, the research group examined the "morphology" and "age composition of the population" of wild boar mandibles excavated from the Nishikawatsu Site (Matsue City, Shimane Prefecture).

When examining 26 mandibles which were excavated from the Nishikawatsu Site and whose anterior surface could be observed, domestication phenomena were confirmed in more than 80%. From this, it was revealed that most of the excavated wild boars were domesticated pigs or their descendants.

Next, the researchers estimated the ages of 45 pigs with preserved posterior molars (back teeth) based on tooth eruption patterns. Because pigs raised for meat are often slaughtered before reaching adulthood, domesticated populations are generally expected to contain a relatively high proportion of young individuals. The results showed an age distribution of 12 juveniles (27%), 19 young animals (42%), and 14 adults (31%).

Comparing this result with Jomon Period sites where wild boars were hunted and sites from the medieval and early modern periods, the proportion of young animals was considerably lower than at the Higashimura site (Okinawa Prefecture), where "almost all individuals were raised through husbandry," and slightly lower than at the Asahi Site, where "most individuals were raised through husbandry." From this, it can be inferred that both husbandry and hunting were conducted at the Nishikawatsu Site. Carbon and nitrogen stable isotope ratio measurements were also conducted on three specimens showing domestication phenomena.

As a result, a mixture of domestic and wild individuals was confirmed, indicating that re-wilding and interbreeding of domestic pigs were progressing. While pigs were raised at the Nishikawatsu Site during the Yayoi Period, they were not strictly managed. It is thought that many individuals escaped and became re-wilded.

During the Yayoi Period, pig genes originating from the Chinese continent spread widely throughout the population, and because wild boars living on the Japanese archipelago today are their descendants, they are thought to inherit genes from pigs originating from the Chinese continent.

Niimi stated: "Among the excavated mandibles, we select materials that retain morphological information well, extract the collagen contained within them, and use it to measure stable isotope ratios and age of origin, but because the materials were buried in soil for a long time, the collagen sometimes decomposes and does not remain. If collagen does not remain, isotope ratios and age cannot be measured. When we cannot measure a material that is clearly recognizable as a pig from its morphology, it is truly disappointing."