How much calcium and vitamin D₃ supplementation is beneficial to an adult tortoise who may be deficient?

Question

I have a 10 year old redfoot tortoise. Her natural origins are in South American, so she's not desperately dependent on UV-B radiation to survive like an African species is. She gets plenty of vegetables and has a 5.0 UVA/UVB light that I change about every 6 months (I know that it's supposed to be every quarter, but they are quite expensive and seem to retain some oomph for twice the prescribed time).

Nevertheless, when I take her outside (about once a month in the warm periods), she tends to pick with her beak at the white gravel that we have around the foundation of the house, leading me to believe that she may be calcium deficient. She had a cuttlebone when she was little, but she outgrew the need to chew at it, and I figured she was getting enough calcium through her diet and enough D₃ biosynthesis from the UV light.

In pursuit of giving her more calcium, I bought a Ca and D₃ sprinkle from the pet store, and I try to put it on her food about once a week. I'm wondering if this is enough, or if I'm fact overdoing it for an adult.

Answer 1

It's actually quite common with different species of tortoises to pick at, and even eat, pieces of white gravel. I don't think anyone has come up with a solid reason why, although I haven't really studied it too much to be honest. I can tell you the different reasons I've come across.

1. It could be an indication that they are lacking in something in their diet. It wouldn't hurt to review your tortoises diet, and make sure there's not something that he might be missing. Here is a decent resource to get you started Although it sounds like you've had yours for a while, so I don't think this is likely.

2. It could also aide in digestion. Kind of like how dogs will eat grass. It's not that hard to believe, as birds and several reptiles will do this. Especially since tortoises beaks don't allow for the the type of grinding that would be useful for a vegetarian diet.

3. One that I found interesting wast that shells and bones would be bleached white under the sun in the wild. So that color of white is instinctually recognized as a source of calcium.

As long as you keep an eye on her to make sure she doesn't swallow any overly large stones, I haven't heard anyone have trouble with impaction from it. Although I wouldn't say it's not a risk. I'm also not sure if there's any way to discourage it.