Tourists gather in throngs at Pier 39 in San Francisco, where hundreds of sea lions bask in the sunlight on the dock. The creatures can certainly be an entertaining sight, but among the boisterous barking and the cute antics of the young are animals suffering in silence.

A distressing number of sea lions here are crippled by genital tumors, but they mask their pain to avoid the attention of predators. Many of the sick sea lions noticed by onlookers and brought to the nearby Marine Mammal Center in Sausalito die of renal failure or paralysis when the tumors travel up their genital tracts and push against the kidney and spine.

Recently, researchers discovered that sea lions that died of genital carcinoma had an 85 percent higher concentration of toxic polychlorinated biphenyls (PCBs) in their bodies than the other sea lions, as well as high concentrations of the pesticide DDT. Many of the sea lions are born near the Channel Islands, where 1,700 tons of DDT were dumped before it was banned in 1972.

This isn’t an isolated case. From Tasmanian devils in Australia to Beluga whales in the frigid waters of Canada’s St. Lawrence estuary, animals are dying of cancer, and a new report by the Wildlife Conservation Society (WCS) has found a common cause: pollution created by humans.

Denise McAloose, the study’s lead author and chief pathologist for WCS’s Global Health Program, says she’s very worried about the impact of humans on the environment.

“As the human population continues to grow and utilize resources and damage the environment, I do believe that we will continue to see the emergence of disease, including cancer in wildlife."

Experts say we can still turn things around. Some species have shown significant drops in carcinogenesis once contaminants are taken out of their environments, such as catfish in Ohio’s Black River. Cancer nearly killed all of the fish before a nearby steel plant’s coking facility closed in 1983. Cancer rates dropped 75 percent.

“Now we need collaboration and cooperation across conservation organizations, public-health communities, as well as governments to make changes that have positive outcomes for animals and the planet,” says McAloose, "Because that will have positive impacts on the human population."

Thumbnail photo: marc smith/Flickr