Spines

Spines are hard, pointed projections that appear on a range of organisms, serving a variety of ecological and physiological roles. They can deter herbivores, reduce water loss, provide camouflage, or aid in movement and protection. Spines may be part of the external surface, such as epidermal outgrowths, or arise from deeper tissues, including skeletal structures in animals. In human anatomy, the word spine is also used for the vertebral column, the central support of the axial skeleton.

Botanical spines are most familiar on cacti and other arid-adapted plants. In these species spines are typically modified leaves or stipules and are often organized into areoles in cacti. Spines differ from thorns, which are modified stems, and from prickles, which are outgrowths from the epidermis. Spines can help shade the plant, reduce transpiration, and deter herbivory.

Animal spines can be internal or external. Internally, many vertebrates bear spinous processes on their vertebrae—bony projections that anchor muscles and ligaments. External spines include porcupine quills, hedgehog spines, and the calcareous spines of sea urchins, which function mainly in defense. The material of spines varies by lineage, from keratin in some mammals to bone and mineralized tissue in echinoderms.

Spines show considerable diversity in form and function across taxa and reflect adaptations to ecological niches. The term spine is often used broadly, but its precise meaning depends on whether the projection is a plant organ, a vertebral process, or an external defensive structure.