‘Hairy” medieval book covers previously thought to be made from deer or boar skin are in fact made of sealskin, researchers have found.

The covers of the 12th- and 13th-century books from French monasteries were made using seals believed to be from Scandinavia, Scotland and potentially Iceland or Greenland, hinting at extensive medieval trade networks.

Researchers analysed volumes from a Cistercian monastery, Clairvaux Abbey, and its daughter monasteries, identifying 43 sealskin books.

“Contrary to the prevailing assumption that books were crafted from locally sourced materials, it appears that the Cistercians were deeply embedded in a global trading network,” reads the research article, published today in Royal Society Open Science.

The books are protected by a cover, or “chemise”, on which hairs remain. While historical library catalogues attribute the skin to either boar or deer, the distribution of the hair follicles does not match either animal. Researchers therefore conducted protein and DNA analysis on the covers, which revealed that harbour seals, a harp seal and a bearded seal were used to make them.

The use of sealskin for manuscript covers was common in Scandinavian countries and in Ireland. However, its use in France was “surprising” to researchers because of the inland location of the monasteries and the lack of evidence of seal populations on the French coast in the middle ages.

“I was like, ‘that’s not possible. There must be a mistake,’” said Élodie Lévêque, of Panthéon-Sorbonne University in Paris, on finding out that the samples had come back from testing as sealskin. “I sent it again, and it of course came back as sealskin again.”

There is no written record of the purchase of sealskin at Clairvaux Abbey. However, the monasteries may have had access to sealskin because they were located near Norse trading routes, said researchers.

The findings support the “notion of a robust medieval trade network that went well beyond local sourcing, linking the Cistercians to wider economic circuits that included fur trade with the Norse.”

Norse settlers arrived in Greenland in the 10th century, remaining until the 13th century, and traded walrus ivory and fur with northern Europe.

Today, the sealskins seem brown in colour, but “it is highly unlikely that Cistercians would have covered their books with brown skins,” wrote the researchers, because brown was characteristic of the Benedictine order – which the Cistercian order branched off from in 1098, seeking a stricter interpretation of the Rule of St Benedict.

Cistercians are, instead, known for white clothing and objects, and therefore, sealskin was probably chosen because of its light grey or white colour. The brown colouration seen today is likely due to degradation over time. The monks may not have known that they were using sealskin at the time, as seals were rarely depicted in medieval art.

“In the middle ages, seal exploitation was common in northern Europe and persists in all Arctic cultures today,” said researchers. They were hunted for their meat as a food source, their blubber for heating, and their skin to make clothing including gloves, boots and coats.