The issue arises from a lawsuit filed by rights group People for the Ethical Treatment of Animals (PETA) in a San Diego court on behalf of five orcas named Tilikum, Katina, Corky, Kasatka and Ulises.

The whales perform water acrobatics at the SeaWorld amusement parks in San Diego and in Orlando, Florida.

PETA argues that continuing the whales' "employment" at SeaWorld violates the 13th Amendment to the US Constitution, which prohibits slavery.

District Judge Jeffrey Miller heard arguments in the complaint Monday and reviewed the response from SeaWorld, which asked that the lawsuit be dismissed. His ruling is expected to come later.

The suit, filed in October 2011, asked that the court declare that the orcas are "held in slavery and/or involuntary servitude by defendants in violation of the Thirteenth Amendment to the United States Constitution."

"It's a new frontier in civil rights," said Jeff Kerr, PETA general counsel, who described the hearing as a "historic day."