Koalas have a built-in seat made of thick cartilage at the base of their spine that lets them sit comfortably on hard branches for hours.

The koala (Phascolarctos cinereus) has a specialized cartilaginous pad at the base of its spine, firmly fused to the pelvic girdle—specifically the ischium bones. This structure creates a stable, cushioned platform that supports the animal's weight while resting in eucalyptus trees.Koalas spend 18 to 22 hours daily sleeping or resting to conserve energy, a necessity driven by their diet of eucalyptus leaves, which are nutrient-poor and toxic. This sedentary lifestyle requires anatomical adaptations documented in journals like the Australian Journal of Zoology, where researchers have highlighted the cartilage pad's crucial role in enabling such extended periods of immobility.The cartilage pad works with the koala's curved spine and strong limb muscles to create a "locking" posture, allowing them to wedge securely into branch forks without risk of falling. Their thick, woolly rump fur provides additional padding and insulation.Evolutionary biologists recognize this pelvic modification as rare among arboreal marsupials—a highly specialized solution to the unique demands of canopy life. Without this internal "seat," the constant pressure on soft tissues would cause significant physical damage over time.