The close relationship between mice and humans seems to have begun with the earliest settled people around 15,000 years ago – even before the advent of farming that made our stored crops a draw for the rodents.
“The question that interests us is: do house mice become associated with humans due to farming or before farming?” says Lior Weissbrod at the University of Haifa, Israel.
To find out, Weissbrod collected 272 mouse molars from 14 archaeological sites in Israel dating from 200,000 to 10,000 years ago, working with Thomas Cucchi of the French National Center for Scientific Research and colleagues.
They identified two mice species from these teeth – the house mouse (Mus musculus domesticus) and its wild, short-tailed relative, the Macedonian mouse (Mus macedonicus) – giving an indication of how their distribution changed over time.
At that time, people who had previously been mobile hunter-gatherers started to settle in fixed locations in the eastern Mediterranean. These people, called the Natufians, built stone houses with hearths and buried their dead.
“This suggests the Natufian people were quite deliberately ‘putting down roots’, placing their ancestors in a particular location,” says Terry O’Connor, an archaeologist at the University of York, UK. But they continued to hunt and didn’t farm.
Modern mouse model
To help interpret their archaeological data, the researchers also studied Masai settlements in Kenya, which served as a living model of the Natufians. Unlike the Natufians, the Masai are not hunter-gatherers, but, importantly, they don’t cultivate crops either.
“The Masai have livestock and are mobile to an extent, but are not farming, so they gave us a good model,” says Weissbrod.
In the Masai settlements, a different but analogous pair of mouse species predominate: the short-tailed Wilson’s spiny mouse (Acomys wilsoni) and the fiery spiny mouse (Acomys ignites), which is more suited to living around humans. The fiery spiny mouse made up 87 per cent of mice trapped in the settlements, but only 45 per cent of those caught outside the settlements.
That suggests this long-tailed species has a competitive advantage over the Wilson’s spiny mouse in the settlements. But since the Masai only live in the settlements for part of the year, the fiery spiny mouse doesn’t completely exclude its relative.
In the archaeological record, the house mouse first appears in the earliest Natufian settlements about 15,000 years ago, immediately displacing the short-tailed Macedonian mouse. But 3000 years later, when the Natufians returned to a seasonally mobile lifestyle similar to the Masai, the Macedonian mouse made a comeback. However, house mice still comprised about 80 per cent of the mice living in the settlements.
After that, from about 11,500 years ago, people began farming and became totally sedentary, and the short-tailed mice were displaced once again.
This shows a strong link between levels of human mobility and the proportions of different mice, says Weissbrod.
“When people are sedentary, they’re giving a competitive edge to the house mouse. Once they’re moving back to a mobile way of life, the competitive edge shifts again to the wild one,” he says. “If it’s in between, not quite high mobility or sedentary, you get this unique pattern of sharing the habitat, mainly house mice with some wild ones.”
Two aspects of sedentary life probably gave house mice the edge: people storing food and dumping waste.
“They’re not providing food at the scale that farmers that grow crops and store them in large quantities [do], but they certainly collect wild plants, grasses and cereals and store them,” says Weissbrod. “If people are more and more staying permanently in one place, the accumulation of trash is also growing. Organic material is going to be available year round and provide a constant source of food for other species.”
Coping with stress
We don’t know when the lineage of domestic mice split from that of wild mice, says Weissbrod. It is also unclear what allows house mice to thrive in human settlements and outcompete wild mice. Perhaps their longer tails increase their agility, helping them escape danger, he says. And they probably have a more flexible diet and can tolerate the higher stress levels associated with living in close proximity to humans, he adds.
O’Connor says the prevalence of the bones of animals in prehistoric settlements suggests early settlers tolerated these freeloaders.
“At the Neolithic site of Çatal Hüyük, for example, middens of refuse are common across the site and the deposits have yielded a lot of house mouse bones,” he says. “It is quite difficult to project from bones in sediments to mice running around a settlement, but it looks as if mice were a common part of the everyday scene, with little evidence of any efforts to deter or extinguish them.”
Brian Boyd of Columbia University, New York, says we should avoid reading too much into the similarities between the Masai and the Natufians. “They inhabit their own ecologies and their own histories, and therefore their complex social relationships with animals within these ecologies and histories will not be the same,” he says.
While life among early farmers may have had plenty to offer mice, they had to wait until 7000 years ago for their first taste of cheese.
But for at least the first few millennia, it would have been quite a care-free existence – cats were only domesticated much later, some 9500 to 4000 years ago.