Why hair becomes bubbles?

[pomme]

http://photos.yahoo.com/bc/hapzunglam/lst?...rc=ph%26.view=t

I haven't put any metrial nodes to the hair. I did some setup. But i have no idea why it becomes bubbles. It happens to me sometimes, even I do a group of hair like fur. I am sure that the anti alias turned way up. Please give idea.