More wolves released at Yellowstone reintroduction site

These wolves were part of a larger group captured in British Columbia earlier this year--the second year of a controversial plan to repopulate the United States' northern Rocky Mountains with the endangered predator species.

Ranchers, who are challenging the federal government program in court, argue that wolves attack and kill livestock.

They are particularly concerned about this week's release because calves are born this time of year. Ranchers said they had not been told wolves were arriving in their area.

Five wolves brought from Canada last year have been killed, including one shot to death Saturday in Meeteetse, Wyoming, not far from the park. It is a federal offense to kill wolves because they are an endangered species. Officials are investigating the earlier shootings.

So far, it is not known if any of the female wolves are pregnant. Wolves generally mate from late February through early March and pups are born about 60 days later. If any of the wolves are pregnant, they will need time to pick den sites, park officials said.

Officials also said they will have to monitor the activity of grizzly bears that are emerging from their dens and are attracted to the roadkill left for the wolves in the acclimatization pens.

This story written by Reuters correspondents