Cancer does indeed occur in wild animals. Studying diseases of
wild animals is a specialty field in biology, and it has
at least one journal. Here's an abstract re: marine mammals.

1. Gauthier, J. M.; Dubeau, H.; Rassart, E.; Jarman, W. M.; Wells, R. S.
     Biomarkers of DNA damage in marine mammals.
     Mutation Research. Aug. 18, 1999. 444(2):427-439.

Abstract:
     Certain environmental contaminants found in marine mammals have been shown
     to cause DNA damage and cancer. The micronuclei (MN), sister chromatid
     exchange (SCE) and/or chromosome aberration (CA) assays were used to assess
     baseline (spontaneous) levels of DNA damage in blood lymphocytes of
     individuals of the relatively healthy and lightly contaminated Arctic
     beluga whale (Delphinapterus leucas), Sarasota Bay, FL, bottlenose dolphin
     (Tursiops truncatus) and Northwestern Atlantic grey (Halichoerus grypus)
     and harp (Phoca groenlandicus) seal populations. MN cell (MNC) frequencies
     ranged between 2 and 14/1000 binucleated (BN) cells and were statistically
     similar between species. In bottlenose dolphins, MNC frequency was
     correlated with age and was significantly higher in females than in males.
     No intraspecific variation in MNC frequency was found in beluga whales.
     Intraspecific variation was riot tested in seals due to the small sample
     size. Frequencies of SCEs and total CAs, excluding gaps, ranged,
     respectively, between 1 and 15 SCE(s)/per cell and 4-6 CAs/100 cells in
     beluga whales. SCE and CA frequencies did not vary with age or sex in
     beluga whales. The MN, SCE and CA assays were found to be practical tools
     for the detection of DNA damage in marine mammals and could be used in the
     future to compare DNA damage between relatively lightly and highly
     contaminated populations.


Tom Billings