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

U.S. 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.

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, Calif., and Orlando, Fla.

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

Sea World's attorney Theodore Shaw called the lawsuit a waste of the court's time and resources. He said it defies common sense and goes against 125 years of case law applied to the U.S. Constitution's 13th amendment that prohibits slavery between humans.

"With all due respect, the court does not have the authority to even consider this question," Shaw said, adding later: "Neither orcas nor any other animal were included in the 'We the people' ... when the constitution was adopted."

Miller listened to both sides for an hour before announcing that he would take the case under advisement and issue his ruling at a later date. The judge raised doubts a court can allow animals to be plaintiffs in a lawsuit, and he questioned how far the implications of a favourable ruling could reach, pointing out the military's use of dolphins and scientists' experiments on whales in the wild.

Kerr acknowledged PETA faces an uphill battle but he said he was hopeful after Monday's hearing.

"This is an historic day," Kerr said. "For the first time in our nation's history, a federal court heard arguments as to whether living, breathing, feeling beings have rights and can be enslaved simply because they happen to not have been born human. By any definition these orcas have been enslaved here."

The issue is not about whether the animals have been subjected to abuse, the defence said. If the court were to grant orcas constitutional rights, Shaw warned the ruling would have profound implications that could impact everything from the way the U.S. government uses dogs to sniff out bombs and drugs to how zoos and aquariums operate.

"We're talking about hell unleashed," he said.

PETA said a ruling in its favour would only help to protect the orcas in the entertainment industry and other cases involving animals would have to be decided on their own merits.

Kerr said Sea World employees are in violation of the 13th amendment because their conduct is enslaving an intelligent, highly social species that suffers from its confinements in ways similar to what humans would experience.

Brushing animals off as property is the same argument that was used against African-Americans and women before their constitutional rights were protected, PETA says.

Shaw pointed out that argument does not translate because both women and African-Americans are people for which the Constitution was written to protect.

Miller did not specify when he would issue his ruling.