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Background  The Asian house rat ( Rattus tanezumi ) and the brown rat ( Rattus norvegicus ) are closely related species and are partially sympatric in southern China .  Over the past 20 years,  R. tanezumi  has significantly expanded northward in China and partially replaced the native brown rat subspecies,  R. n. humiliatus . Although invasive species are often more aggressive than native species, we did not observe interspecific physical aggression between  R. tanezumi  and  R. n. humiliatus . Here, we focused on whether or not  R. tanezumi  was superior to  R. n. humiliatus  in terms of nonphysical competition, which is primarily mediated by chemical signals.    Results  We performed two laboratory experiments to test different paradigms in domesticated  R. tanezumi  and  R. n. humiliatus . In Experiment 1, we caged adult male rats of each species for 2 months in heterospecific or conspecific pairs, partitioned by perforated galvanized iron sheets, allowing exchange of chemical stimuli and ultrasonic vocalization. The sexual attractiveness of male urine odor showed a tendency (marginal significance) to increase in  R. tanezumi  caged with  R. n. humiliatus , compared with those in conspecific pairs. Hippocampal glucocorticoid receptor ( GR ) and brain-derived nutrition factor ( BDNF ) mRNA were upregulated in  R. n. humiliatus  and  R. tanezumi , respectively, when the rats were caged in heterospecific pairs. In Experiment 2, we kept juvenile male rats in individual cages in rooms with either the same or the different species for 2 months, allowing chemical interaction. The sexual attractiveness of male urine was significantly enhanced in  R. tanezumi , but reduced in  R. n. humiliatus  by heterospecific cues and mRNA expression of hippocampal  GR  and  BDNF  were upregulated by heterospecific cues in  R. n. humiliatus  and  R. tanezumi , respectively. Although not identical, the results from Experiments 1 and 2 were generally consistent.    Conclusions  The results of both experiments indicate that nonphysical/chronic interspecific stimuli, particularly scent signals, between  R. n. humiliatus  and  R. tanezumi  may negatively affect  R. n. humiliatus  and positively affect  R. tanezumi . We infer that chronic interspecific interactions may have contributed to the invasion of  R. tanezumi  into the range of  R. n. humiliatus  in natural habitats.

Asian house rats may facilitate their invasive success through suppressing brown rats in chronic interaction