Not getting into the helmet debate, but last night I was cycling in the moonlight through Richmond Park (2000 acres and 800 deer) - without a helmet - and a bellow came from a group of red deer I was passing, presumably aimed at me. I'd never heard a noise from them before. Having seen this video, I kept a weather eye on them as I accelerated away.

A friend was once rammed by one in her Mercedes. The park police told her they didn't seem to like German cars.