Archaeologist takes part in rat study

They are dirty, disease-ridden and repulse most people, but rats are a fascinating species and now more is known about the vermin during the early days of the region’s European settlers.

Historic St. Mary’s City Maryland Heritage Scholar and archaeologist Henry Miller recently helped collaborate on a study titled “The ratting of North America: A 350-year retrospective on Rattus species compositions and competition” that appeared in a recent edition of ScienceAdvances magazine.

“We would like to better understand the changes and the evolution of species related to America,” Miller said in a May 24 interview. “But, when you’re in an urban setting, how does the change in species affect health issues and that sort of thing. And the more we understand about these species in the future we will be better able to control processes and create a healthy environment for people in urban settings.”

Miller supplied the bones of seven individual rats — dated between 1675 and 1710 — most of which were discovered while excavating the lower layers of the cellar of the St. John’s House Plantation in Historic St. Mary’s City.

The study lead was Eric Guiry, and one of the questions asked was whether the black rat (rattus rattus) or brown or Norwegian rat (rattus norvegicus) arrived in North America first.

Evidence from shipwrecks in the 1550s confirms the evidence of black rats, though evidence at 13 sites in North America from Canada down to Texas suggests they did not not arrive in North America until around 1730.

The brown rat reached North American shores around the time of the American Revolution in the 1770s.

The black rat favors above ground and higher elevations like attics and ceiling joists, whole the brown rat prefers to burrow in cellars and underground.

They didn’t appear to need to fight for burrows or even food — evidence suggests the rats, which are omnivorous, most likely ate corn and grains, and maybe fish or oysters or even scraps on animal bones.

But one thing is clear — they were unable to coexist and after a span of about 70 years, brown rats had all but wiped out their cousins.

“By 1800 we’re seeing a very substantial decline and very few black rats are even identified throughout the 1800s,” said Miller, who added it is unknown if the brown rats attacked the black rats or why it happened. “To see how dramatic the transition was between these two species, that was surprising. We expected we’d find the black rat co-existing much longer. They weren’t directly competing for the same things, so why would one so remove the other.”

Strangely enough, both black and brown rats co-exist in Australia and New Zealand.

“Was the black rat incapable of responding to the pressures of the brown rat population?” Miller asked, adding that the brown rat is slightly larger and more aggressive. “This is one of the mysteries we have not yet resolved.”

“While the isotopic compositions of black and brown rat diets do show meaningful average differences, there is still considerable overlap,” the study said, “likely reflecting a situation where brown rats had come to occupy part of the niche space previously used by black rats.”

Regardless, Miller said he was pleased to take part in the study, which expects to be ongoing, about one of the area’s invasive species.

“What I was particularly pleased with was we took information I gathered in the 1970s, and here we are almost half a century later we’re reapplying it to a new set of questions, and solving and answering some of those questions,” Miller said. “I find that really nice because we often don’t think a pool of information can tell us about X, Y and Z, and then also tell us about questions we didn’t even think of at the time.”