they don't look particularly fat to me, skinks are just built like that.

Has he been housing them together? If they have been co-existing in the same tank for 3 years then I'd be willing to bet they are not two males!

what kind of Blue tongues are they, btw. I love the coloring, very different from mine!
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3 fire skinks, 1 blue tongue skink, 2 plateds, 2 beardies, 1 mali uromastyx, and a wide assortment of geckos.