Authorities in Mexico say the monarch butterflies that winter in the mountains west of Mexico City have survived a severe cold snap that hit the area this week.

Mexico's environmental protection agency released photos of patches of frost, snow and ice in areas of the butterfly reserve. Activists had expressed concern about the unusual cold snap, because driving rain and bitter cold in 2001 killed millions of monarchs at the reserve.

But the agency also released photos Friday showing clumps of butterflies still hanging from trees. They tend to drop off trees when they're frozen.

The agency says its employees observed "branches and trunks covered with butterflies in good condition."