Assuming it was a female dog, how did he know it was dead?

http://www.mlive.com/news/bctimes/index.ssf?/base/news-8/116170304311310.xml&coll=4
A 44-year-old Saginaw man remains jailed today on charges of bestiality after he was seen engaged in sexual acts with a dead dog, Michigan State Police troopers said.