I have two female RES that are about 4 inches, they have been with me for about 2 1/2 years. They spend all their time together ( where else would they go ? ) bask on top of the other, swim together, etc.
I also have two hatchling painted who do the same exact thing, even the basking on top of one another. If they were solitary , wouldn't they spend time in opposite corners of the tank and pretty much ignore each other ?
This is not to advocate that everyone run out to get a companion turtle or even that they wouldn't be perfectly happy as an "only child ", but just to make a point that yes, they are social animals that enjoy each other's company ( unless of course, they don't ! ).
painted patience
