Spine (zoology)

In zoology, spines are hard, needle-like anatomical structures. They appear in both vertebrates, which have backbones, and invertebrates, which do not. These structures serve many roles in the natural world. They act as defensive tools against predators. They can also help with movement or communication. Spines are found in a wide variety of species. This includes mammals, fish, and even tiny invertebrates.

In mammals, most spines are actually modified hairs. These structures have a spongy center. This soft core is covered by a thick, hard layer of keratin. Keratin is the same tough material found in human fingernails. Some mammalian spines, like those of the North American porcupine, have microscopic barbs on their tips. These barbs help the spine anchor into a subject. This design reduces the force needed to penetrate skin. It also increases the damage caused during a puncture.

Spines appear in many different forms across the animal kingdom. In bony fishes, specifically ray-finned fishes, fins feature bony spines called lepidotrichia. These are often called rays and are covered by thin skin. In contrast, lobe-finned fishes have much shorter spines. Their fins are instead dominated by a muscular lobe. Some fish, such as the lionfish, possess prominent venomous spines for defense.

Stingrays possess a barbed spine on their tail modified from dermal denticles.

Invertebrates also utilize spines. Sea urchins have hard spines, and some mollusks like the venus clam have them on their shells. Even caterpillars have tiny, detachable spines called setae. These can cause severe irritation upon contact.

Evolutionary history shows how these structures change over time. An ancient synapsid called Dimetrodon had very long spines on its backbone. These spines were joined by a web of skin to form a sail. In the Mesozoic era, the mammal Spinolestes had spines similar to modern spiny mice. Many defensive spines evolved due to the need for protection in exposed environments. There is a known correlation between body armor and certain lifestyles. Intermediate-sized mammals, roughly 800g to 9kg, often develop spines. This happens because they are too large to hide easily but too small to fight back.

Spines are often linked to a lower basal metabolic rate, or BMR. BMR is the rate at which an animal's body uses energy. Many mammals with spines are myrmecophagous, meaning they primarily eat ants and termites. These animals tend to move more slowly. There is also a correlation between spine development and a reduction in brain size. This may be because the body devotes so much energy to forming complex spines. However, this trend is not seen in all species, such as porcupines. Some animals also use aposematism, which means using bright colors to signal danger.

Many animals use spines as a passive defense mechanism. Unlike teeth or claws, which are active tools, spines wait to be used. They rely on the predator making contact. Some species use them aggressively by rolling into spiked balls to lunge at enemies. While most venomous spines are found in fish and invertebrates, some evidence suggests hedgehogs might coat their spines in toxins from toads. For humans, injuries from venomous spines should be treated seriously. In contrast, a porcupine quill is not poisonous and can be removed by pulling it out firmly.

Humans have used spines and quills for many different purposes. Historically, people have used them for jewelry like necklaces and bracelets. Indigenous peoples of North America practiced quillwork to decorate textiles with porcupine quills. In the past, quills were even used as early writing pens. Today, scientists study spines for biomimetics. This means they look at nature to design new technology. They study the porosity of sea urchin spines to design new ceramics. They also study them to help design better medical syringe needles.