Vet checks are a great way to begin the establishment of a new animal.

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Zane Neher.

The great and strong man is one who bears within his character many contradictions.
Crocodilian care.

if it's a baby that could be why the tail is thin or because well if it wasn't being looked after it probably wasn't being fed. I'd still get it checked by a vet though just because it's really dark looking. Skittles was really dark when she was a baby but you can only see 2 lines in her belly now.

I work at a vet hospital that works with a lot of reptiles, and it's possible that the dark area that you are seeing in the abdomen is a tumor. Tumors, both benign and malignant, are more common in reptiles, especially Leopard Geckos and bp's, than most people think. Personally, I think that they are relatively common in all reptile species, we just see more Leopard Geckos and bp's than other species. The thin tail is definitely not a good sign- a growing tumor takes a lot of nutrition away from the host. I would definitely take him to the vet for a physical and an x-ray. It is also possible that you are seeing the beginning of an impaction, not enough to be blocked yet, but enough to make him uncomfortable. Dehydration can damage the kidneys, resulting in a darker color as well. It could be a lot of things, so I think your best bet is to take him to the vet. Hopefully, you are only seeing the organs, but it's always best to have it checked out. Good luck to you!