1. The tumors of all animals studied showed identical chromosome rearrangements (not too surprising considering their physical signs of disease were identical and cancer is a genetic disease...).
2. The animals often fight and bite each other in the face.
To me the evidence seems quite indirect at best. An alternative interpretation of the findings, for example, could be that the facial tumors are caused by an initiating mutation that leads to a defect in chromsome segregation during mitosis. Such a defect could result in the observed loss of the certain chromosomes and the observerd rearrangements in daughter cells. But we should probably all stop biting each other just as a precautionary measure.
PS - You can now get the paper.