More than half of all pet deaths on airplanes have occurred on two airlines in the six months since airlines have had to reveal the number of animals that perish on their flights.

Twenty-one animals stowed in cargo areas of airplanes have died since the Transportation Department began requiring airlines in May to record the number of pets that die on flights. Twelve have died on Continental Airlines or American Airlines flights.

The deaths haven't caused alarm among animal rights activists because so few have occurred compared with the number of pets each airline carries. In addition, the reporting requirement is so new that it's too soon to compare deaths with historical data and chart an increase or decrease, they said.

"It's really too early to say which airline is doing the best job and which airline is being negligent," said Kelly Connolly, an issues specialist at the Humane Society of the United States.

The Agriculture Department, which enforces the law responsible for the new reporting requirement, is investigating some deaths on Continental and American flights.

Seven pets on Continental flights and five pets American Airlines flights have died.

Continental spokesman Martin DeLeon said the airline will transport about 90,000 animals this year and seven deaths - or one for every 12,857 animals carried - means most arrive unharmed.