Believe it or not, that is actually a female lion. (Their male is a heterozygous brown lion). According to a docent I spoke with at Big Cat Falls, some older female lions after they go through menopause can develop a sparse mane. I found this fascinating, since this is the only lioness I have ever seen that on.
This closeup shot of just the front part of the face makes it look fuller than it is. (Unless, perhaps, it has grown thicker since I was there a few years ago). Here is a full body shot of the two white lionesses that shows how it looked, at least when I saw them. http://www.zoochat.com/837/white-lions-philadelphia-zoo-64250/
