SKIN/COAT/PELAGE - Editorial Comment

Adult: Hedgehogs are basically brown in colour with spines over the dorsal surface and hair on the face, legs and underside. The hair coat on the face and underside is uniform in colour in Erinaceus europaeus, which is one of the features distinguishing this species from the closely related Erinaceus concolor. The spines, modified hairs about 20 mm long and 2 mm thick, are not uniform in colour but banded brown and white; there may be a total of about 7000 spines per hedgehog. The spines have a complex internal structure which allows them to act as an efficient shock-absorber, as well as being effective as a deterrent against many predators. Except for the juvenile moults spines are usually lost and replaced individually. The hedgehog has specialised cutaneous muscles which allow it to roll into a tight defensive ball.

Adult colour variations: include individuals with occasional white spines, "blond" hedgehogs with paler than normal colouration, true albinos with pink eyes and nose and white individuals with black eyes and nose. Melanistic (black/dark brown) individuals have not been recorded.

Newborn/Juvenile: Newborn hoglets are pink and naked with a taut, bloated skin. The first white spines protrude through the skin soon after birth. The second, pigmented, set of spines starts to emerge after just a couple of days and by 2-3 weeks they obscure the first set. The third set of spines (adult) begin to emerge at about six weeks old.

(References are available in detailed literature reports below)

Hair / Moult

• Erinaceus species (Erinaceus - (Genus)) have a number of anatomical characteristics which include absence of papillae on smooth spines and a central parting of the spines over the crown of the head.(B228.1.w1)
• "Ventral pelage unicoloured." This is one of the distinguishing features of Erinaceus europaeus as distinct from Erinaceus concolor - East European hedgehog in which the ventral pelage is bicoloured, being white on the breast. (J82.18.w1)

Hair:

• A small number of long whiskers are present over the snout area. (B258.w2)
• The face, underside and limbs are covered in coarse fibrous grey-brown coloured hair. (B142, B260.1.w1, B262.2.w2) This hair coat is quite sparse and may not provide much insulation to the hedgehog. (B254.6.w6)
• A skirt of hair is present along the flanks. (B262.2.w2)
• Thin hair-like spines are found at the junction between the hair and spine areas along the flanks and forehead. (B228.2.w2, B258.w2)

Spines:

• Hedgehog spines are modified hairs made of a protein called keratin. (B228.2.w2, B285.w1)
• Spines are present over the back and crown of the head whilst hair is absent in these areas. (B228.2.w2)
• The total number of spines present in the coat of a hedgehog varies with its age and body size. (B228.2.w2)
• Total spine numbers are believed to vary between approximately 3500 spines on a newly independent youngster to up to, or in excess of, 7000 in a large adult hedgehog. (B228.2.w2 B260.1.w1)
  • Historical estimates of total spine number in the range of 16000 are believed to be mistakes owing to a faulty measuring technique. (B254.6.w6)
  • Adult hedgehogs have approximately 5000 spines in their coat in total. (B262.2.w2)
• Spine length is approximately 2 cm (B262.2.w2); 22 mm (B142); 2-3 cm.(B285.w1)
• Typical hedgehog spine dimensions are 2-3 cm long with a 2 mm diameter. (B254.6.w6)
• The internal anatomical structure of hedgehog spines makes them both light, strong and resistant to buckling. (B228.2.w2)
• The profile of the hedgehog spine narrows at its point of emergence from the skin and bends through an angle of approximately 60 degrees (B260.1.w1). This anatomical adaptation represents a point where the spine will flex if it receives a physical blow, absorbing the force like a shock absorber system (B228.2.w2, B254.6.w6, B285.w1)
• Spines are erected under the control of erector muscles made of smooth muscle. Erection of the spines produces an interlocking array of sharp bristles. (B228.2.w2, B285.w1)
• The base of spines are anatomically specialised with an expanded hemispherical basal bulb which fits tightly into the spine follicle. This strong junction means that significant force is required to pull out a spine which is more likely to break at its base when under pressure. (B228.2.w2)
• Hedgehog spines are short with a smooth tip and their design is very resistant to breakage. (B228.2.w2)
• Spines have a hollow architecture with a network of transverse septa and longitudinal struts to resist lateral compression and add to its rigidity respectively. (B228.2.w2)
  • This design maximises spine strength whilst maintaining light weight. (B285.w1)
• Spines consist of an outer cortex and inner medulla. Longitudinal stripes visible along the surface of the spine correspond to the internal longitudinal struts which project into the medullary space. (B258.w2)
• A detailed study of the structure and property of spines showed that spines of Erinaceus hedgehogs, which average about 20 mm long, are basically cylindrical, tapering at the tip while at the base they taper to a narrow neck before expanding into a mushroom-shaped bulb within the skin. Within each spine are evenly-spaced longitudinal "stringers" and the spine is divided by horizontal septa into a series of separate spaces. The insertion of each septum into the wall is complex, with fibres changing their orientation by 90° to run into the wall. The structure appears designed to maximise the ability of the spines to absorb impact in the event of a fall, absorbing energy, with the bulbous base both anchoring the spine and preventing it being pushed into the body of the animal when the tip of the spine is under a load. A considerable force is required to bend a hedgehog spine while structural failure occurs only under a force 200 times that required to buckle the spine initially. The tips are smooth; they do not easily break and having penetrated an object they easily slide back out again, suggesting that, unlike the spines of porcupines, their role in agonistic behaviour is secondary to their shock-absorbing ability. (J46.210.w2)
• The spines, which are about 20 mm long, narrow to a neck near the base then expand again into a ball or bulb seated in the skin. The spines "give" at the neck, cushioning any fall. The bulb is anchored very firmly in the skin (it is possible to lift a hedgehog up by one spine; this is likely to be painful for the hedgehog). The tip is solid, with only outer cortex while the remainder of the spine has two layers, an outer cortex and inner medulla which consists of air-filled spaces separated by horizontal plates. The spines are colour banded white, brown and black, being white at the base and tip. They are marked with longitudinal lines. There is no seasonal moult although spines are certainly shed. Muscles attach to the bottom of each spine. (B255.1.w1, B255.2.w2)
• "Hedgehog spines do not bend easily, but when they do, they buckle elastically and structural failure occurs only at a force 200 times greater than the force required to buckle the spine". (B228.2.w2)
• The design of the hedgehog spine is thought to maximise its function as a shock absorber and not its ability to impale an aggressor as is this case with the spines of the porcupine. (B228.2.w2)
• Spines "cushion" the blow if hedgehogs drop or fall from a height.(B147) Spines also act to protect the hedgehog against predators.
• The spines act as a shock-absorber, cushioning any fall. The narrow neck near the base of each spine acts as a flexible joint; below this, in the skin, the spine expands again into a bulb. Each spine is a modified hair, produced from a papilla in a follicle in the skin. The tip is solid, consisting only of cortex (the outer layer) while most of the spine is hollow, the thin cortex (in which there are about 24 longitudinal grooves) enclosing a thicker medulla which contains many small horizontal compartments separated from one another by thin struts of material. (B289.3.w3)
• No sweat glands are present in the spined area. (B289.3.w3)
• Hedgehog spines are sufficiently sharp to penetrate soft human skin. The spines are often contaminated with dirt and faecal material which can be inoculated on handling with the risk of introducing bacteria leading to infection. (B228.2.w2)
• N.B. The similarity in appearance of spines between the hedgehog and a number of other species is an example of convergent evolution.. These species include porcupines (Hystricidae - Old-world porcupines (Family), Erethizontidae - New-world porcupines (Family)), the spiny mice from Africa and Asia (Muridae - Rats, mice, voles, gerbils etc. (Family)), the spiny anteaters (Tachyglossidae - Spiny anteaters (Family)), the spiny dormice (Platacanthomyidae), cane rats from Africa (Thyronomyidae), New World spiny rats (Family Echimyidae) and mice (Cricetidae) and Tenrecs (Tenrecidae - Tenrecs, Otter-shrews (Family)) . The hedgehogs (Erinaceidae - Hedgehogs, moonrats (Family)) and the species listed above are not closely related. (B254.2.w2, B228.1.w1)

Moults and spine loss:

• Hedgehogs have three generations of spines in their lifetime. The first set of spines are unpigmented and present at birth. The second generation of spines are pigmented, begin to emerge at 2 days of age, and obscure the first generation spines by two to three weeks of age. The third generation, or adult, spines gradually replace the smaller second set spines from the time of independence at approximately six weeks old. (B228.2.w2)
• Adult spines are moulted individually and replaced continuously so that the defence mechanism which they provide remains intact throughout the year. (B228.2.w2)
• Periods of intense spine loss instead of a continual slow turnover have been described in several species within the Family Erinaceidae. (B228.2.w2)
• Spineless hedgehogs are vulnerable to predation because they lack their defence mechanism and are unlikely to survive long in the wild. (B260.1.w1)
• Spine moult is not seasonal. (B142, B262.2.w2)
• Spines last over a year before they are shed and replaced. (B262.2.w2)
• Spines may remain in the coat for up to 18 months before they are shed and replaced. (B142, B254.6.w6, B260.1.w1); spines have been observed to remain in place for two years. (B255.2.w2)

Colouration:

• Spines are pale creamy brown in colour with a dark brown band near the tip. (B142, B262.2.w2); hedgehog spines have distinct colour variation along their shaft. The spine is brown at its base, cream/ white at its mid-shaft with a dark coloured band positioned before the white spine tip. The dark band is black in young hedgehogs and chocolate brown in older individuals. (B254.6.w6)
• Pale coloured or white spines occur in some individuals. (B262.2.w2)
• Some individuals have patches of white spines within their coat (B142, B254.6.w6) which may represent an inherited characteristic. (B254.6.w6)
• Occasional reports of hedgehogs with unusual general appearance occur. These include true albino, blond and spineless individuals. (B262.2.w2)
• True albino hedgehogs are seen with pink nose and feet. White hedgehogs with black nose and eyes are also observed. (B142)
• In some individuals the general colouration is lighter than in most. (B255.2.w2)
• The introduced population of hedgehogs on Alderney comprise a high proportion of blond individuals, though to be due to high degree of inbreeding in the descendants from the small founder population. (B262.5.w5)
• Blond hedgehogs are also known as leucistic in appearance. (B262.5.w5)
• Hedgehog spines are a cream colour with a broad dark brown band positioned close to the spine tip.(B260.1.w1)
• Hedgehog spines may change in appearance with increase in age. Young individuals typically have spines with colour marks in distinct black and white whilst older animals have spines with cream and brown colouration. (B260.1.w1)
• Melanic colour variant hedgehogs with a completely black coat have never been reported. (B142, B228.1.w1, B254.6.w6, B260.1.w1)
• Individual hedgehogs with sparse or absent spines have been observed. It is unclear whether this may be related to trauma or be genetically determined. (B142)

• At birth the pink skin of the hoglet is taut with a "bloated appearance" and is covered with "pimples" which correspond to the points of emergence of the first set of approximately one hundred unpigmented white spines. (B228.8.w8)
  • These are unpigmented and white in appearance and number approximately one hundred in total.(B142, B228.8.w8, B262.10.w10, B254.15.w15, B260.6.w6, B289.3.w3)
• The skin appearance changes rapidly from the first hour of life and gradually "deflates" to a soft and wrinkly state over 24 hours, during which time the first set of spines emerge. (B228.8.w8)
• Whilst the first set of spines do not usually emerge from the skin prior to birth, some reports describe that spines may protrude slightly, dependent on the duration of parturition and maturity of the neonate (B228.8.w8)
• A 'parting' line devoid of spines is present in the hoglet and extends from the midline of the brow over the dorsum of the spine to the rump area. This dividing line corresponds to the embryological development of the orbicularis musculature. (B228.8.w8)
• Hedgehogs have three generations of spines in their lifetime. The first set of spines are unpigmented and present at birth. The second generation of spines are pigmented, begin to emerge at 2 days of age, and obscure the first generation spines by two to three weeks of age. The third generation, or adult, spines gradually replace the smaller second set spines from the time of independence at approximately six weeks old. (B228.2.w2)

Dermis, Subdermis and Epidermis

• The skin with spines over the back and crown of the head is grey in appearance with no sweat or sebaceous glands present. (B228.2.w2)
• Sweat and sebaceous glands are abundant over the haired skin and soles of the feet. (B228.2.w2)
• The epidermis is thin over the areas of the body covered with spines. A thick corrugated fibrous layer is present with very few blood vessels which are restricted to the deeper tissue layers. The sparse number of blood vessels close to the skin surface may act to reduce heat loss from the spined areas. (B228.2.w2)
• The panniculus carnosus is a layer of striated muscle below the spine follicles. (B228.2.w2)
• A layer of fatty tissue is located below the panniculus carnosus which increases in depth as a result of lipid deposition before hibernation (B228.2.w2)
• Complex cutaneous musculature permits spine erection and allows the hedgehog to roll into a tight defensive ball. (B142, B228.2.w2, B255.1.w1, B258.w2, B262.2.w2)
  • The fronto-dorsalis (fronto-cuticularis, fronto-orbicularis or preorbitalis dorsalis) muscle, which extends over the hedgehog's forehead, is used to draw the spiny skin of the head down to protect the face (B258.w2, B228.2.w2)
  • The caudo-dorsalis and caudo-abdominalis muscles, located over the base of the back, function to retract the rump and tail respectively when the hedgehog curls into a ball. (B228.2.w2)
  • The panniculus carnosus muscle is developed into a thick circular band of orbicularis muscle around its margins. Co-ordinated action of a number of muscles acts to pull the spined skin over the head, limbs and tail in a similar fashion to a 'hood'. Contraction of the orbicularis muscle then acts to tighten the opening to the undercarriage. (B258.w2, B228.2.w2)
• The skin and orbicularis muscle layer are only loosely connected to the underlying structures: this may be seen when a very relaxed or anaesthetised hedgehog is picked up by the scruff. (B258.w2, B228.2.w2)
• White fat is accumulated under skin prior to hibernation. Thermogenic brown fat is accumulated over shoulders [and other sites] prior to hibernation. (B142)

• At birth the pink skin of the hoglet is taut with a "bloated appearance" and is covered with "pimples" which correspond to the points of emergence of the first set of approximately one hundred unpigmented white spines. (B228.8.w8)
• The skin appearance changes rapidly from the first hour of life and gradually "deflates" to a soft and wrinkly state over 24 hours, during which time the first set of spines emerge. (B228.8.w8)

Glandular Structures

• A lobulated proctodeal gland (sebaceous) is present within the entrance to the anal opening (6-7 mm long by 4.5 mm wide). The gland's function may involve adding odour cues to faeces, although behaviour patterns associated with anal scent gland marking have not been observed in the hedgehog. (B228.2.w2)
• Specialised glands in hedgehog species (Erinaceus - (Genus)) include Meibomian glands located in the eyelids and lobulated sebaceous glands in the commissures of the mouth. (B228.2.w2)
• Sweat glands are located over the haired areas of the hedgehog pelage, the foot soles and circumanal area. (B228.2.w2)

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