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I feel like I should have gotten my lil Leo a buddy to hang out with! I know this may sound dumb but- do turtles get lonely? is it better to have more than one?

http://www.redearslider.com/forum/viewtopic.php?t=3176&highlight=social

This thread might help you with your questions.

I've had both situations. My old Fric lived by himself for the 23 years I had him. In fact, he didn't want other turtles in the pond with him male or even female. He was very territorial and immediately attacked without warning.
Now I have two males. Scoota is 6 months older than Pepe and they live very harmoniously in the same pond together. Even at meal time they wait their turns with little or no aggression.
The funniest thing they do is what I call the "wave" contest. They have this from time to time and as you all know, this is a mating ritual. In their case though, it's a nose to nose frenzy as to who can do it the best ... or at least it appears that way. The results also appear to be boredom after awhile ... maybe even a little frustration. Then they'll go their separate ways.
I'm lucky to have the room for two of them too. They do need a lot of space

Also, two turtles equals twice the cost, twice the tank size, bigger/double everything not no mention double vet bills if they get sick or hurt. Plus if they do end up being aggressive towards each other, you'll have to get another complete set up or have to give one up.