Yes, Ofm, they do close up. Other semps can also do the same thing. I've observed this on calcareums too.
I have photos of a couple of jovis on a really sunny (maybe bordering on scorching) kind of day. Both photos taken on the same day and you can see the difference of their reaction to where they were situated.
The first photo's jovi is sitting in morning sun location where the sun has passed while the second jovi is sitting in afternoon sun. Both photos (taken in April) were taken one right after the other and both jovi's were in same kind of soil mix.
Note the amount of rocks in soil to help improve drainage. I buy cactus soil and add large grain sand (not playground sand which is too fine) and small rocks to form my semp planting mixture. Some specialty semp nurseries offer their own mixture for sale. Sometimes I buy theirs so that I don't have to mix but there's always cactus mix available to start with to add some sort of grit or rock to help drainage.
