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 (PETA) that names five orcas as plaintiffs in the case.
PETA claims the captured killer whales are treated like slaves, forced to live in tanks and perform daily at its parks in San Diego, California and Orlando, Florida.
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"This case is on the next frontier of civil rights," said PETA lawyer Jeffrey Kerr, representing the five orcas.
Sea World's lawyer, 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 American constitution's 13th amendment, which 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."
Judge 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 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 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 not just for zoos and aquariums but also for such things as the US government's use of dogs to sniff out bombs and drugs.
"We're talking about hell unleashed," he said.
PETA said a ruling in its favour would only help to protect orcas in the entertainment industry. 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.
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 counters that both women and African-Americans are human beings, and the constitution was written to protect prople.
The judge did not specify when he would issue his ruling.
