Norway rats (Rattus norvegicus) and roof rats (Rattus rattus) are different species. Occasionally, mating occurs if the two species are kept together in captivity. The resulting pregnancies are not successful and the embryos die. This is an example of A) mechanical isolation. B) hybrid sterility. C) zygote mortality. D) lack of F₂ fitness.

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The unsuccessful pregnancies resulting in the death of the embryos suggest the occurrence of zygote mortality, limiting successful hybridization between Norway rats and roof rats.

Norway Rats (Rattus Norvegicus) and Roof Rats (Rattus Rattus) Are