Prevue Pet Products offers its Grass Tunnel as a natural, woven grass activity center for guinea pigs, dwarf rabbits and other small animals, the company reports. Designed to satisfy a small mammal's natural instinct to burrow, explore and nest, the product is made of couch grass, plant cane, burlap and sisal rope. It measures 13 1/3-in. long by 5 1/2- to 6-in. in diameter.