A legal bid to free five orcas from captivity at SeaWorld on the grounds that their "enslavement" is in violation of the US constitution is a "strategic error", a conservationist has warned.

The case, brought by People for the Ethical Treatment of Animals (Peta) against SeaWorld in the US, to be heard this week, has triggered controversy over applying the 13th amendment – which abolished "slavery or involuntary servitude" in America – to non-humans such as killer whales.

In the UK, the Whale and Dolphin Conservation Society senior biologist Philippa Brakes said that taking a large-brained mammal, which would naturally range in groups over a huge area, against its will and putting it alone in a situation where it was harming itself because of its misery "amounts to slavery".

But she said most of the public would not necessarily see it that way, and they enjoyed seeing orcas and dolphins in marine zoos without being aware of the "hideous lives they're leading".

In the US, Peta has faced criticism over its bid to pursue the freedom of an animal under the 13th amendment.

While the case would bring publicity to the issue of the rights or interests of "non-human persons", something for which some people have been arguing for a long time, if the case fails and there is then case law history against recognising those rights, that would not be helpful for the cause, Brakes warned.

"I would love to be wrong, and that they find for the orcas in this case, but I doubt very much that's going to happen, and I think it's a strategic error," she said.

She said those concerned for the welfare of mammals, in captivity and in the wild, should use the increasing body of science that showed that they were intelligent creatures capable of suffering in order to argue for their interests to be recognised – not as equal to humans, but still "persons" with rights.

The science showed that cetaceans are big-brained marine mammals which form complex societies and even have different cultures within species in different parts of the world, she said. And it was important to take people along with the movement towards recognising the legal rights of non-humans.

"It's more than court cases, it's really about changing people's attitudes and understanding," she said.

While Peta's court case may not deliver freedom for killer whales, Brakes said: "I do think a tipping point is coming. We have to use the science to argue to that tipping point. All we're trying to do is have the genuine interests of these animals recognised."