Actually one of the things I really liked about Shen's jungle was that I could gank at level 2, whereas with Amumu taking bandage toss before level 4 could put him back if the gank didn't succeed. I also seemed to be able to keep my health up much more with Shen. Not sure why Amumu is tiered higher for jungling.
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"What if you just eat vegetarians?"-neonreaper
"^ You are what you eat. ^_~"-Koiji