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Wolves in Yellowstone National Park are 3.6 times more likely to attack individual male elk without antlers , or groups of elk in which at least one male is without antlers .
 Yellowstone National Park
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Yellowstone National Park