A war of the rats was raging in North America decades before the Declaration of Independence

Brown rats hitchhiked to North America in the mid-1700s, triggering a brutal war of the rats that saw a sharp transition from black to brown rat dominance on the continent, a new study finds.

The researchers made the discovery after studying centuries-old rodent bones from shipwrecks and North American settlements, which also revealed the brown rodents invaded decades earlier than previously believed.

North America wasn't always crawling with rats. Black rats (Rattus rattus) were the first to sail over with Europeans in the 1500s, possibly reaching the Caribbean shores with Christopher Columbus as early as 1492. Brown rats (Rattus norvegicus) came several centuries later, with their arrival date previously placed around the time of American Independence in 1776. Both species proliferated across the continent — but it turns out black rats weren't a match for their brown cousins. 

A new analysis suggests brown rats landed on North American soil around 1740 and rapidly pushed black rats out of burgeoning coastal cities. The new invaders may have established dominance by monopolizing food resources, reducing the black rats' reproductive success and gradually shrinking their population.

"By the mid-1700s, we see a significant decline in occurrence of black rats, matched by a sharp rise in the proportion of brown rats," researchers wrote in the study, published Wednesday (April 3) in the journal Science Advances. "The apparent shift from the black rats' complete dominance to near disappearance in our dataset occurred over a span of only a few decades."

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To retrace brown rats' journey across North America, the researchers examined rodent remains from 32 settlements and seven shipwrecks dating to between 1550 and the 1990s, stretching from the Gulf of Mexico to Canada. The team analyzed bone collagen to identify black and brown rats, as well as different isotopes, or versions, of carbon and nitrogen atoms to reconstruct the rodents' diets.