London — Animal rights activists said on Tuesday they were outraged by news that a Scottish zoo had allowed its African lions to commit incest.

Advocates for Animals said it was "irresponsible, unethical and totally unacceptable" that Glasgow Zoo had let its brother and sister lions, Leo and Cleo, breed and produce three cubs.

"We were a bit suspicious why they had not been parading them around more and then they admitted that they had allowed brother and sister lions to breed," campaigns director Ross Minett told Reuters.

The zoo admitted its two lions had committed incest but denied it had been keeping the three-month-old cubs under wraps, saying the mother had kept them hidden and had only brought them out when the animal park was closed.

"We have had cubs, and the parents are brother and sister. It was a failure of the contraceptive that the vets had put in place," Roger Edwards, the zoo's chief executive officer said.

"We did what we could to prevent it," he told Reuters. "Our major concern is to check that the cubs are healthy and are thriving."

Mr. Edwards, who said Advocates for Animals was spearheading a campaign against the zoo after publishing a damning report earlier in the year, admitted it was the second time Leo had got Cleo pregnant. The cubs had been stillbirths the first time round.

Trisha Holford, of wildlife charity the Born Free Foundation, said lions only rarely committed incest in the wild and said the zoo had acted irresponsibly.

"Nature has all sorts of systems to stop inbreeding. In lion society young males are kicked out when they reach adulthood so they can't breed with their sisters," she told Reuters. She said that, since the lions would be living together, Leo should have had a vasectomy. She also questioned what reputable animal park would now take in the inbred cubs.

Glasgow Zoo has kept lions since it opened at its current location in 1947.