A federal judge for the first time in US history heard arguments in a case that could determine whether animals enjoy the same constitutional protection against slavery as human beings.

US District Judge Jeffrey Miller called the hearing in San Diego after Sea World asked the court to dismiss a lawsuit filed by People for the Ethical Treatment of Animals that names five orcas as plaintiffs in the case.

'Hell unleashed' ...An orca performas at Sea World Orlando. Credit:Reuters

PETA claims the captured killer whales are treated like slaves for being forced to live in tanks and perform daily at its parks in San Diego and Orlando, Fla.

"This case is on the next frontier of civil rights," said PETA's attorney Jeffrey Kerr, representing the five orcas.