Well, they're not as intelligent as white rats, that's for sure.
However, my kids were once watching a programme about white rats and made a maze out of video boxes with a carrot at the end of it, and found that the guinea pigs did make it to the end of the maze. When my kids used to put them in a doll's pram, they made attempts to escape, which also shows signs of an incipient intelligence. Also when my kids made them go through a cardboard tunnel they always used to stay in the middle. They will climb over Tessa's tail, but avoid Scamp.
When you go past the shed they squeak, and it is arguable whether that is a sign of intelligence (they associate you with food) or whether they just have good hearing.
They are quite bovine creatures in that they spend much, indeed, all of the time eating, and during the summer when they are out at grass they expand to plump little rotund spheres. Or 'pot size' as my husband puts it. But he also calls them 'The Rodents'. They can reduce a large dandelion plant to a root in ten minutes, like cute , furry piranhas.
You can't toilet train guinea pigs but my kids used to carry them around the house in terry nappies.
They seem to have an appreciation for the finer things in life. For instance, in the morning they are to be found sun bathing in the hutch at the exact angle in which the sun comes through the shed window. They also like cherry tomatoes, but not turnip. They seem to have a rudimentary sense of aesthetics in that they like eating the nicest flowering plants in the garden, but avoid the weeds.
They also have personalities in that Misty is the bossy one. However, Daniel is the athlete as I find that when I try to put him back in the shed he can second guess me and swerves at the last minute, his little legs a-blur.
My kid's Uncle David has a natural affinity with the guinea pigs and we call him 'The Guinea Pig Whisperer'.