For the third year in a row, the population of endangered Mexican gray wolves has grown in the U.S. Southwest, according to the U.S. Fish and Wildlife Service.

There are at least 75 Mexican gray wolves, also known as lobos, roaming the wilds of Arizona and New Mexico, by the agency's count. That's up from 58 animals in 2011.

However, the survey found there were only three breeding pairs among the 13 known wolf packs, down from six pairs in 2011. The small number of breeding pairs, and slow growth of the population, has led some conservationists to urge the FWS to release more lobos into the wild. There are currently only about 300 of the animals kept in captivity, according to Defenders of Wildlife, an environmental group.

The Mexican gray wolf — a subspecies of the gray wolf — is one of North America's rarest mammals, according to the FWS. They once ranged throughout Mexico and the Southwest, but due to widespread hunting and trapping were driven to extinction in the United States by the mid-1900s. A small number remained in Mexico, and now, in captivity.

The latest survey of wild Mexican wolf numbers was conducted by biologists using a fixed-wing aircraft and a helicopter. Pups born in the summer must survive through the end of the year to be counted. The latest count includes 20 wild-born pups that met that criterion, the FWS reported.

For the first time in four years, one male wolf was released by the FWS on Jan. 7, to replace an alpha male lost to a criminal shooting in July 2012, according to the Center for Biological Diversity, a conservation group. However, that male wandered outside the established territory of his intended mate, an alpha female, and was recaptured on Jan. 29, the group reported.

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