In a zoological context, spines are hard, needle-like anatomical structures found in both vertebrate and invertebrate species. The spines of most spiny mammals are modified hairs, with a spongy center covered in a thick, hard layer of keratin and a sharp, sometimes barbed tip.
Occurrence
Mammals
Spines in mammals include the prickles of hedgehogs, and among rodents, the quills of porcupines (of both the New World and the Old), as well as the prickly fur of spiny mice, spiny pocket mice, and of species of spiny rat. They are also found on afrotherian tenrecs of the family Tenrecinae (hedgehog and streaked tenrecs), marsupial spiny bandicoots, and on echidnas (a monotreme). An ancient synapsid, Dimetrodon, had extremely long spines on its backbone that were joined together with a web of skin that formed a sail-like structure. Many mammalian species, like cats and fossas, also have penile spines. The Mesozoic eutriconodont mammal Spinolestes already displayed spines similar to those of modern spiny mice.
Fish
Spines are found in the fins of most bony fishes, particularly actinopterygians (ray-finned fishes), who have folding fan-like fin made of spreading bony spines called lepidotrichia or "rays" covered by thin stretches of skin. In the other bony fish clade, the sarcopterygians (lobe-finned fish), the fin spines (if any at all) are significantly shorter and each fin is instead dominated by a muscular stalk ("lobe") with a jointed internal appendicular skeleton. The limbs of tetrapods, who descended from sarcopterygian ancestors, are homologous to the paired pectoral and pelvic fins. Some fish, such as scorpion fish and lionfish, has prominent sharp, venomous spines for anti-predator defense. The tail stinger on a stingray is also a type of barbed spine modified from dermal denticles. The acanthodians, an extinct class of ancient fish that are paraphyletic to the cartilaginous fishes, have prominent bony spines in the front (rostral) edges of all fins except the tail. The primary function of these rigid spines are generally presumed to be defensive against predators, but other proposed roles are as cutwaters to reduce drag or as holdfasts against subsurface currents.
Invertebrates
Defensive spines are also found in invertebrate animals, such as sea urchins. They are a feature of the shell of several different species of gastropod and bivalve mollusks, including the venus clam Pitar lupanaria. Many species of arthropods also have spine-like protrusions on their bodies for defensive purposes. For example, the rostra on many shrimp species form a sharp spine that can be used against predators. The urticating bristles or setae on many caterpillars and New World tarantulas are essentially tiny detachable spines that can cause severe irritation upon contact. Those on the Lonomia caterpillars are venomous and can cause lethal coagulopathy, hemolysis and kidney failure. Spines are also found in internal organs in invertebrates, such as the copulatory spines in the male or female organs of certain flatworms.
Function In many cases, spines are a defense mechanism that help protect the animal against potential predators. Because spines are sharp, they can puncture skin and inflict pain and damage which may cause the predator to avoid that species from that point on. The spine of some animals are capable of injecting venom. In the case of some large species of stingray, a puncture with the barbed spine and the accompanying venom has occasionally been fatal to humans. Animals such as porcupines are considered aposematic, because their spines warn predators that they are dangerous, and in some cases, potentially toxic. Porcupines rattle their quills as a warning to predators, much like rattlesnakes use their rattles.
Spine evolution in mammals
Evolution
Predation defense Defensive spines in mammals may have evolved due to the need for defense in exposed environments where they are vulnerable to predation. This includes permanent spines like hedgehog prickles and detachable spines like porcupine quills. Because selection acts on phenotype, defensive morphologies are modified from existing forms. Mammalian defensive spines are modified hairs.
Trade-off with speed and camouflage Body armor (spines, quills, and dermal plates, etc.) has been linked to lower basal metabolic rates (BMRs). This is consistent with the hypothesis that intermediate-sized mammals who struggle to conceal themselves either develop body armor as defense or evolve to move quickly (necessitating higher BMRs). Mammalian body armor has also been linked to dietary habits, specifically those of insectivorous mammals. Myrmecophagous mammals, mammals which primarily eat ants and termites, typically have lower BMRs and are not as fast-moving. These traits potentially explain why myrmecophagous mammals tend to have body armor. More generally, it has been suggested that because insectivorous mammals are often rooting in the soil with their heads down, they have evolved body armor such as spines that compensates for decreased awareness of predators. There is a correlation between intermediate-sized mammals (~800g to 9 kg), open habitat, insectivorous diets, and defensive body armor such as spines. Data suggests that intermediate-sized mammals with increased environmental exposure are selected to evolve morphological defenses due to larger size and open habitats. These mammals develop body armor as they're too large to be able to hide easily, but too small to fight effectively. While there is a correlation between open habitats and high morphological defense, it has been found that some species of defended rodents, like spiny rats and porcupines, are more associated with closed, arboreal habitats. It is unclear as to whether the correlation between body armor and insectivorous mammals' lower BMRs is due to lower BMRs necessitating body armor, or body armor allowing an insectivorous lifestyle that reduces BMR.
Aposematism Some species' defensive spines have evolved with aposematism, signaling their danger through color. It has been observed that species that have evolved both defensive spines and aposematism have fewer spines, suggesting that aposematism may allow species to reduce the costs associated with producing more appendages.
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