Hair is a defining feature of mammals: it is only found in mammals, and every mammal has hair at some point in its life. Over the course of mammalian evolution, hairs have been modified and specialized to perform many functions, including insulation, camouflage, signaling (to potential mates or potential predators), sensing (via whiskers or vibrissae), and defense.
Some mammals have modified hairs that are enlarged, stiffened, and strengthened to form spikes of different shapes and sizes. These structures, called spines or sometimes quills, always have a thick, hard, outer tube of keratin (the fibrous protein that makes up all hair as well as claws, nails, and horn sheaths) and tapered, pointed tips.
Beyond that basic design, however, the spines of different species are highly variable in size, shape, and function. Porcupine quills, for example, are different lengths over different parts of the body, fall out relatively easily, and break readily when bent, whereas hedgehog spines are the same length all over the body, are well embedded into the skin (“you can pick up a hedgehog by a single spine!” Vincent 2002, p.30), and are stout and slightly curved. Functionally, most spines serve as defense from predators, but spines are also important tools for communication, shock absorption, and rain protection.
Although the two terms are often used interchangeably, “spines” and “quills” have slightly different definitions. “Spine” is a more general term for any hair that has been modified into a hardened, spiked structure, whereas quills are a specific type of spine. “True” quills have a spongy core and are found in echidnas as well as some rodents.
Spines and quills evolved independently at multiple places and times in the mammal tree of life; thus, they are an example of convergence. The oldest mammal known to have enlarged, hardened hairs for defense is Pholidocerus, an extinct ancestor of the hedgehog that lived over 40 million years ago. However, hairs do not fossilize as readily as other structures like teeth and bones, so spines and quills may have evolved much earlier.
Today, spines or quills are found in four major groups of living mammals: hedgehogs (Erinaceomorpha: Erinaceidae, Erinaceinae), tenrecs (Afrosoricida: Tenrecidae, Tenrecinae), echidnas (Monotremata: Tachyglossidae), and rodents (Rodentia). The latter group includes Old World and New World porcupines (Hystricidae and Erethizontidae, respectively), the spiny rats (Echimyidae), and the Old World rats and mice (Muridae), all of which have species with spiny pelage.
Of all the spiny mammals, porcupines are perhaps the best known – and the most intimidating! There are two families of porcupine, the Old World porcupines (Hystricidae), which live in Europe, Africa, and Asia, and the New World porcupines (Erethizontidae), which live in North and South America. In both families the backs and sides of the porcupine are covered with true quills, which they erect and flare if predators approach too closely. Old World porcupines have short, flattened spines or bristles on the head, neck, feet, and belly, while the undersides of New World porcupines are covered in much softer fur. All baby porcupines (or porcupettes) are born with their quills, but the quills are too small and flexible to be very effective against predators until the porcupine is several months old. Porcupine quills are loosely rooted, so they detach easily when they are embedded in an attacker or when the porcupine shakes its body (although the popular myth that porcupines shoot quills at their enemies is not true).
New World porcupine quills are such formidable and effective weapons that they have even been known to kill would-be predators like dogs and foxes. The reason that these quills are so dangerous becomes obvious at the microscopic level (Fig 1). The tips of each quill are incredibly sharp and able to pierce through skin more easily than an 18-gauge hypodermic needle (about the same size of the quill). Once it has entered the skin, its microscopic backward-facing barbs prevent the quill from being removed without causing significant pain and damage to surrounding tissues. To make matters worse, the quill tip breaks off easily so that it remains in the attacker, and it can work its way deeper and deeper into the skin over time with each muscle contraction or movement. In his book The North American Porcupine, researcher Uldis Roze describes a New World porcupine quill breaking off in his bicep, only to emerge two days later in his forearm (p. 24)!
Predators often keep their distance, however, because porcupines use a variety of aposematic signals, or warnings to enemies to stay away. The quills, especially those on the crest and rump, can be erected into an intimidating, spiny display with “whirring” or fanning motions. The high-contrast black-and-white coloration of the quills, which is fully developed in New World porcupines at three months of age, may also provide a visual warning sign to their nocturnal, and often colorblind, predators.
Old World porcupines may use acoustic aposematism as well. The terminal ends of their tails are covered in hollow, capsule-shaped quills 5 to 8 cm in length called “rattle quills” which, as the name suggests, produce a rattling noise when vibrated. Tail rattling alone appears to deter most solitary predators, who have likely learned to match the rattling sound with pain. Unlike New World porcupines, the quills of Old World porcupines are unbarbed, although they are still quite sharp and capable of penetrating flesh.
But how do porcupines protect themselves from being injured by their own quills? Indeed, many examples from scientific and anecdotal literature show that New World porcupines fall relatively frequently from trees, presumably forcing their own quills into their skin. One way that porcupines appear to limit self-injury is through natural antibiotics in their quills. Roze et al. (1989) found that the quills of New World porcupines are coated in fatty acids from exocrine secretions that prevent bacterial growth, likely to prevent infection from self-inflicted wounds.
Spines offer a clear advantage to species like porcupines, whose quills are known to deter, and sometimes even kill, predators. However, the spines from the remaining rodent families (e.g., Muridae and Echimyidae) are comparatively ineffective at warding off attackers, so their function is not fully understood.
Structurally, the spines of echimyid rodents are not round but flattened and easily bent, and they are rarely aposematically colored. Many spinous species are preyed upon by snakes, birds, and other mammals, who do not seem to be deterred by the spines. Still, the spines likely provide some, albeit minimal, protection by serving as a passive suit of armor.
Another hypothesis (Lekagul & McNeely 1977) is that the spines of many rodents function to protect against rain, which keeps insulating and guard hairs dry. Indeed, the armored rat (Hoplomys gymnurus), which has the largest and best-developed spines among the echimyid rodents, is abundant in very wet lowland forests in the Neotropics. Thermoregulation is very important for small mammals, which can lose body heat quickly, so spines may be critical for keeping these species dry and warm.
Hedgehogs (Erinaceidae: Erinaceinae; Fig. 2) are well known throughout Europe, Asia, and Africa for their distinctive coat of spines, which are sometimes called quills or “prickles.” Compared with New World porcupines, whose quills are barbed and vary in size and shape across the body, the spines of the hedgehog are unbarbed and more or less uniform. The spine tapers to a sharp point on the distal end, but it is bulbous where it grows out of the skin, which keeps the spine firmly embedded in the skin and not easily removed.
Baby hedgehogs are born with their spines, but they are covered with a fluid-filled membrane to protect the mother during birth. Within a day, this covering shrinks, dries, and disappears to reveal about 150 white, flexible prickles. These spines are relatively soft at first, but they harden and darken over the first weeks of life and are eventually replaced with hard, adult spines via “quilling,” a process that occurs within the first six months.
Because their spines are unbarbed and undetachable, hedgehogs do not actively defend themselves from predators like the porcupine does. The spines – over 5,000 on average – instead serve as passive body armor, and the hedgehog’s best defense is to roll into an unappetizing ball. Rolling causes all of the spines to point outwards, and a powerful muscle called the orbicularis panniculi, located at the margin of the quill-fur interface, acts as a drawstring to keep the head, legs, and belly of the hedgehog tightly hidden within the impenetrable ball of spines and inaccessible to predators.
On the microscopic level, hedgehog spines have a rather different appearance than rodent spines. The outer walls are relatively smooth and the core is composed of air pockets separated by regularly spaced septa and “longitudinal stringers” for extra strength. Indeed, hedgehog spines are incredibly strong and resistant to breaking and buckling – Vincent and Owers (1986, p. 68) noted that they “have never seen a hedgehog spine with a broken tip.”
This observation led Vincent and Owers to hypothesize that hedgehog spines provide more than just passive defense from predators. In an experiment that tested the strength and mechanical properties of quills and spines, these scientists found that hedgehog spines are highly elastic to strong impact forces. There are many reports of hedgehogs climbing trees and walls then descending by simply rolling and dropping to the ground. It is therefore possible that hedgehog spines evolved as shock absorbers from falls rather than defense from predators.
Hedgehog spines are also adept at wicking fluids (they can absorb up to 11% of the spine’s weight in water), which is useful for a behavior called “anointing.” Anointing is when the hedgehog encounters a new scent, licks or chews the source, then produces a foam in its mouth that it spreads onto its spines. The purpose of anointing is poorly understood, but Burton (1969) listed seven possible explanations:
Follow-up studies helped to pinpoint which of the seven possible functions of anointing were most likely used by hedgehogs. Robert Brockie (1976) found that anointing was heavily correlated with mating and suggested that the spines retain scents that are important in sexual behavior and intra-species communication. The following year (1977), Edmund Brodie reported that hedgehogs use toxic secretions from toads to deter predators. The toxins are taken into the hedgehogs’ mouths and transferred to their spines through licking, likely to increase pain in a would-be predator. This shows that spine anointing has multiple important functions for hedgehogs.
Tenrecs (Tenrecidae) are placental mammals found in tropical Africa and Madagascar. These small- to medium-sized animals (2-2000 grams) have a stunning variety of behaviors, physiologies, and appearances. One subfamily of tenrec has evolved spines and is therefore aptly named the “spiny tenrecs” (Tenrecinae). There are five species of spiny tenrec, all of which are endemic to (only found on) Madagascar: the hedgehog tenrecs (Setifer setosus and Echinops telfairi), the streaked tenrecs (Hemicentetes nigriceps and Hemicentetes semispinosus), and the common or tailless tenrec (Tenrec ecaudatus).
The common names of tenrecs, like hedgehog tenrec, shrew tenrec, and mole tenrec, refer to their superficial resemblances to other mammals (although in reality they are more closely related to elephants and aardvarks than to their namesakes). This is especially true for the two species of hedgehog tenrec, Setifer setosus and Echinops telfairi, who look strikingly similar to “true” hedgehogs (Erinaceidae). Like hedgehogs, hedgehog tenrecs are dorsally covered in a coat of hardened spines and can roll into a nearly impenetrable ball when threatened, although the muscles for rolling are quite different (non-homologous) between the two (Fig. 3). In fact, hedgehogs and hedgehog tenrecs look so similar that the U.S. Department of Agriculture has banned the import of hedgehog tenrecs – popular in the exotic pet trade – in an attempt to control the spread of foot-and-mouth disease in livestock, not because tenrecs are known to transmit the disease but because hedgehogs are, and infected hedgehogs may be mistaken for hedgehog tenrecs.
Hedgehog tenrec spines are rather short and stout compared to species like the porcupine, are uniformly sized across the dorsum, and are not barbed. At birth, the spines are already visible and protruding slightly from the skin, although they are initially soft and hair-like. The spines do not fall out easily, as the fibers of the subcutaneous muscle (m. cutaneous maximus) are inserted into the spine bases. On the microscopic level, the core of each spine consists of a series of septa, similar to true hedgehogs. However, unlike hedgehogs, the septa of hedgehog tenrecs are more closely spaced, the external walls of the spines are thinner, and there are no longitudinal stringers.
While hedgehog tenrec spines are certainly used in defense and are also hypothesized to function both in anointing (Eisenberg & Gould 1970) and shock absorption (Vincent 1986), they may have another function: communication. During courtship and marking behaviors, Echinops often moves its body muscles rhythmically, causing its dorsolateral quills to rub against each other and produce a low sound.
Tenrecs are the only mammals in the world that are known to communicate with one another through sound produced by spines. This behavior is most extensively developed in relatives of the hedgehog tenrecs called the streaked tenrecs (genus Hemicentetes). Streaked tenrecs have evolved a special “stridulating organ,” which is a muscle that controls the movement of a patch of modified spines near its rump (Fig. 4). These spines are enlarged, do not have barbs, and are more difficult to remove than other spines on their bodies, because they are deeply connected to the muscles that control the spines’ movements. Stridulation, the rubbing together of spines, produces pulses of sound between 2 and 200 KHz (humans generally hear within a range of 12 to 20 KHz). While streaked tenrecs also produce noise from their mouths, stridulation appears to be an important form of communication because it occurs while feeding, during social contact, during courtship, during exploration, and when fighting or fleeing.
The common or tailless tenrec (Tenrec ecaudatus) has a structure similar to the stridulating organ of Hemicentetes, although it is only present in juveniles. In the common tenrec, spines (Fig. 5) are molted by juveniles and are replaced by softer fur in adults, with the spines on the stridulating organ replaced last. The stridulating organ is not as active in the common tenrec as it is in streaked tenrecs; the common tenrec vibrates its spines only when it is frightened, often in conjunction with the erection of quills on its neck and back, and the sound it produces occurs at a reduced frequency range (12 to 15 KHz). In adult common tenrecs, even after the stridulating organ disappears, the fur on that area of the back vibrates rapidly when individuals are threatened or scared.
The stridulation sounds produced by tenrecs appear to be used solely for intra-species communication and not for warning predators, but the bright yellow and black stripes of spines in streaked tenrecs and juvenile common tenrecs may be aposematic -- a message to enemies to stay away! Indeed, streaked tenrecs are the only tenrecs with barbed, detachable spines for active defense. When threatened, streaked tenrecs erect a circle of menacing spines around their heads and, if necessary, buck at attackers to drive spines deep into their skin.
Echidnas or “spiny anteaters” (Tachyglossidae) are obscure, medium-sized mammals (up to 16 kg) from Australia and New Guinea. Although they superficially resemble a cross between the anteaters of South America and the hedgehogs of Eurasia and Africa, they are not closely related to either. In reality, they are in the order Monotremata, an ancient lineage of egg-laying mammals that contains only five living species: four species of echidna and the platypus.
Of all of the spiny mammals, the quills of the echidna are perhaps the least studied. While they resemble hedgehog quills in shape – they are short, thick, and unbarbed — their structure is more akin to porcupines – the core of the quill is filled with a foam-like material. The quills have very robust walls relative to their diameters (2 mm in diameter with 0.5 mm thick walls) so they are extremely resistant to bending. This and the relatively blunt tips on each quill suggest that they function as tough body armor, but future research may reveal whether echidna quills are also used in anointing, shock absorption, or one of the many other functions associated with the spiny mammals.
Kathryn M. Everson (author); Link Olson (instructor)
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