Guinea pigs can'tclimb over. They may stand on their hind legs and peep up but there should still be another 2-4 grid squares above their heads. Guinea pigs are not as flexible as other rodents. Their skeletons and spines were not built to climb, they were built to run fast. Where guinea pigs are native, there are less trees and more brush/flat land. They need to be able to run faster but not need to climb. Lids are not needed unless you have a cat or something. The only way a guinea pig would climb over is if there is something next to the grid sides and they could stand on top and climb over. Adult guinea pigs can't really do that anywho but if you had a baby, they can climb out because they still have a wide range of flexibility because they are young. And besides, most guinea pigs don't want to escape because they have a nice big cage, unlimited hay and water, pellets, fruit, and veggies. They also have play companions and toys! Who wouldn't want to leave that?