You can candle tortoise eggs and there are certainly signs like "chalking", color changes from whitish or yellowish to orange-ish or red-ish, and sometime blood vessels can be seen later in the process. Later in the term, the whole egg looks solid, but is that a baby tortoise, or is it rotten inside? The problem is that it is just not always clear with tortoises. Certainly not to me, and others have said the same. Sometimes you candle and think there is nothing, and then a baby hatches out of it a month or two later. Or vice versa. I think most of us tortoise people just incubate them all and leave them alone until they hatch or rot.
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I recall reading that the incubation period varies widely. The numbers 3-6 months come to memory for leopards. Whatever the numbers are, the question is: why such a wide range?
Thanks