Wolves

In a map I am making, obviously involving wolves, I am trying to make it so that your wolf grows over time. As you reach a certain level, which will vary between the different types of wolves, your present wolf will be automatically changed into the wolf following it. Anyway, I'm having some trouble thinking of original or practical ideas for wolf species. Anyway, heres what I've got so far:

Level 5 Wolf Cub-> White Wolf ( Premature hair grown in, acts as a slight layer of protection, hence the improvement of armor )

Level 10 White Wolf -> Grey Wolf ( Premature hair is about half gone, claws have begun to grow, premature hair replaced by permanent hair which is much stiffer and tougher than its predecessor)

Level 10 Grey Wolf -> Brown Wolf ( Full wolf camo in effect, can move faster than previous versions, claws full grown, 'wolf senses' [invis detector])

Level 20 Brown Wolf -> No idea.....

Level 25 ??? --> alpha male

what I would like an opinion on is whether i should use the level system described above, or create a variable that counts kills, and uses them to determine when a wolf moves on? Larger prey = more points of course.

And, again, I'd like any ideas on new kinds of wolves, because quite frankly im having trouble thinking of any.

I think that actually these two different types of leveling up are exactly the same. If better guys give more kills, then its the exact same thing. So, I say go with the easy way, go with the way they already set up for you, go with the expierance. (easy is always better ^_^ )

oh and by the way

lvl 30: Expieranced wolf
lvl 35: Pack leader (are wolves in packs??...)
lvl 40: Expieranced pack leader
lvl 45: Skillfull pack leader
lvl 50: Ultimate wolf (had to be on level 50 ^_^ )