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From Identification to Dissertation

In 2005, a mother and two baby rats were discovered in Salt Lake City onboard a truckload of sterile medical supplies shipped from China. The mother was destroyed, but the babies were kept. The rats were sent to Dr. Michael Mares, the Sam Noble Museum’s Director and a noted mammalogist, for identification. However, a rather puzzling DNA analysis was conducted at Oklahoma State University. Although the mother looked like a common ship rat, DNA suggested she was not a pure ship rat. Dr. Mares teamed up with genetics graduate student Dr. Justin Lack of Oklahoma State University in a pursuit that would eventually yield novel discoveries for invasion biology.

“In our efforts to learn more about Rattus in the United States, the project eventually grew into Lack’s dissertation, which is a fine piece of work involving both the ship rat and the Norway rat, two of the most destructive invaders in the world,” recalls Dr. Mares.

 Along with Dr. Ron Van Den Bussche and Dr. Meredith Hamilton of OSU and Dr. Janet Braun of the Sam Noble Museum, they began researching the invasion history of various Rattus species using population genetics, a method of research in which the geographic patterns of genetic diversity are analyzed to understand gene flow and evolutionary history. During the study, the group encountered a third species, Rattus tanezumi, capable of hybridizing with the ship rat (Rattus rattus).  For photographs of these species, click here.


Rattus tanezumi

Although a population of Rattus tanezumi had been described from California, Dr. Lack and his colleagues were the first to report the second and only other known population in Florida. This discovery suggests Rattus tanezumi likely invaded the United States twice, once on each coast. Globalization in means of transport (ships, trucks and planes) enabled the hybridization. Unlike the hybridization of various dog breeds, which are considered to be the same species, Rattus tanezumi contains genomic material from Rattus rattus (a separate species) and may become genetically swamped by the more widespread and aggressive species.

“Genomic swamping like this is very rare in mammals, making this a really cool result,” said Dr. Lack. “However, a major consequence has been the potential loss of the R. tanezumi nuclear genome.”

To better understand the invasion origins of the Norway and ship rat, Dr. Lack and his colleagues conducted a second study that provided insight into dispersal patterns. They found that the Norway rat invaded from at least four sources, most likely two from Asia and two from Europe and/or Africa, whereas the ship rat appears to have entered the United States in a single wave of invasion, with subsequent invasions only occurring in southeastern Florida but never moving inland.


Their work showed that the Norway rat, Rattus norvegicus, continues to move throughout the United States, whereas the ship rat, Rattus rattus, no longer continues to disperse or enter. Analyses suggest that competition with the more aggressive Norway rat may limit the success of the ship rat. Through this study, researchers gained insight into how these species disperse and repopulate outside species lines. They are two of the principal pest species in the United States and can have great adverse effects on human health by acting as reservoirs and dispersers for a variety of diseases, from bubonic plague to typhus and leptospirosis. This work on dispersal patterns can be useful in limiting the spread of the rat-borne pathogens.