The 'lines' are open

[Dandelion]

I'm so sorry for this vague title. You know the 'lines' on the tortoises' belly? Does that have a name?

Anyway, this picture explains what I'm trying to say better:



Where I have circled red, I have two tortoises that the lines in those places are a little open/separated. I am guessing there must be something in the soil, because their top shell is not affected. I imagined that the big one I have is pregnant with eggs and the shell may have separated a little to make way for them. But the other tortoise is young. Could it be quick growth?

Any help is much appreciated!