Viking cat skeletons reveal a surprising growth in the size of felines over time

Many animals shrink when they become domesticated—the average dog is about 25% smaller than its wild cousin the gray wolf, for example—but a curious thing appears to have happened to cats during the Viking era: They got bigger. More research is needed to confirm the new finding, but there’s a good chance it had to do with being better fed.

“Such a shift has never been documented elsewhere, as far as I know,” says archaeozoologist Wim Van Neer of the Royal Belgian Institute of Natural Sciences in Brussels, who was not involved in the study.

When Julie Bitz-Thorsen was an undergraduate at the University of Copenhagen, her adviser, archaeozoologist Anne Birgitte Gotfredsen, gave her an unusual task: Sift through dozens of bags of material from archaeological sites all over Denmark, and carefully pick out all the cat bones. Gotfredsen wanted to find out how much Iron Age, Viking, and medieval cats differed from modern house cats.

All domesticated cats are descendants of the Near Eastern wildcat (Felis silvestris lybica), a diminutive, tawny feline that still stalks Middle Eastern deserts. Although the oldest evidence of domesticated cats comes from a 7500-B.C.E. grave in Cyprus—early Egyptians likely did the slow, patient work of cultivating house cats’ lovable personalities. As early as 1700 B.C.E., cats started to sail across the Mediterranean, carried aboard ships as gifts and to eradicate pests.

By 200 C.E., the people of Iron Age Denmark were keeping cats. Among charred human bones in a cremation grave from that period, researchers discovered a cat ankle bone with a drill hole, suggesting it was worn as an amulet. The Vikings—who were farmers as well as seafaring marauders—apparently raised cats for their warm fur and to control pests. By 850–1050 C.E., cat pelts started to bring a high price in Denmark.

In the new study, Bitz-Thorsen painstakingly removed hundreds of cat skulls, femurs, tibias, and other bones from bags of mixed dog, horse, and cow bones stored at the Zoological Museum in Copenhagen. The remains encompassed more than 2000 years, beginning in the late Bronze Age and ending in the 1600s. Many came from pits where Vikings disposed of cat carcasses after removing their fur. From the marks on the bones, “You can tell the cats were skinned—they have cut marks, or the neck has been broken,” Bitz-Thorsen says.

Cat bones are less common than the remains of other domesticated animals in most archaeological sites, so the new cache of bones is scientifically valuable, Van Neer says. “I do not know of any other series of cat bones that cover such a long period, with so many individuals.”

After carefully measuring the bones with an electronic caliper, Bitz-Thorsen and Gotfredsen compared them with those of modern Danish cats dating from 1870 to the present.

On average, domesticated cats grew by about 16% between the Viking Age and today, they report this month in the Danish Journal of Archaeology .

The study only focused on Danish cats, so the findings may not be generalizable to other parts of the world. However, a 1987 study of a collection of cat bones from Germany bolsters the idea that domestic cats of the medieval age were smaller than modern-day pets.

One reason may be more access to food. During the medieval period, mounting waste from expanding towns attracted more pests and provided cats with better nourishment, boosting their numbers and potentially their size. Between the late Middle Ages and today, cats became treasured and well-fed, reducing the energy they expend on finding food, Bitz-Thorsen says.

But it’s not clear whether cats got bigger simply because they were eating more or whether something changed in their genes to make them larger, says University of Oslo postdoc Claudio Ottoni, who studies cat domestication. To answer this question, scientists will need to analyze DNA in ancient cat bones, he says, and look for chemical signatures of a changing diet.