That is Doctor imitating an orange. He, apparently, made a wrong turn when he was sneaking into his stash of pepitas. Doc loves his pepitas.
He's my only rat with a passion for pepitas, and we've been keeping a little bag of pepitas on the counter over the couch where the rats play. Doc frequently finagles his way up to the bag, knocks it down, spills it. He's always making a mess with his pepitas. But, the other rats leave the pepitas alone.
By the way, orange rinds are another of the rare foods that you don't want to feed to rats, specifically male rats. Orange rind contains a carcinogen, d-limonene, which is a problem for male rats, but not female. Here's a post about that at RMCA.
And, if anyone is wondering, no, he did not nibble on the oranges.