"A dirty marine joke" joke

It's 5 in the morning. The marine recruits are lined up outside their barracks. Nude. It's mid-January. In Alaska. The sergeant walks up to the first marine and whacks him across the... [fill in the blank].
The sergeant barks: "Did you feel that, soldier?"
The recruit responds: "No, sir!"
The sergeant: "Why not, soldier?"
The recruit: "Because I'm a rough tough marine, sir!"
The sergeant goes to the next marine and whacks him across his... [fill in the blank]. The sergeant bellows: "Did you feel that, soldier?"
The recruit screams: "No, sir!"
"Why not, soldier?" "Because I'm a rough tough marine. Sir!"
The sergeant goes to the third marine and etc. etc. "Did you feel that?"
"No, sir!"
"Why not?"
"Because it belongs to the guy behind me!"