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hedgehog is any of the spiny mammals of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera, found through parts of Europe, Asia, and Africa, and in New Zealand by introduction. There are no hedgehogs native to Australia, and no living species native to the Americas (the extinct genus Amphechinus was once present in North America). Hedgehogs share distant ancestry with shrews (family Soricidae), with gymnures possibly being the intermediate link, and have changed little over the last 15 million years. Like many of the first mammals, they have adapted to a nocturnal way of life. Hedgehogs' spiny protection resembles that of the unrelated porcupines, which are rodents, and echidnas, a type of monotreme.

The name hedgehog came into use around the year 1450, derived from the Middle English heyghoge, from heyghegge ("hedge"), because it frequents hedgerows, and hogehogge ("hog"), from its piglike snout. Other names include urchinhedgepig and furze-pig. The collective noun for a group of hedgehogs is array.

Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin. Their spines are not poisonous or barbed and unlike the quills of a porcupine, do not easily detach from their bodies. However, the immature animal's spines normally fall out as they are replaced with adult spines. This is called "quilling". Spines can also shed when the animal is diseased or under extreme stress.

A defense that all species of hedgehogs possess is the ability to roll into a tight ball, causing all of the spines to point outwards. The hedgehog's back contains two large muscles that control the position of the quills. When the creature is rolled into a ball, the quills on the back protect the tucked face, feet, and belly, which are not quilled. Since the effectiveness of this strategy depends on the number of spines, some desert hedgehogs that evolved to carry less weight are more likely to flee or even attack, ramming an intruder with the spines; rolling into a spiny ball for those species is a last resort. The various species are prey to different predators: while forest hedgehogs are prey primarily to birds (especially owls) and ferrets, smaller species like the long-eared hedgehog are prey to foxes, wolves, and mongooses.

Hedgehogs are primarily nocturnal, though some species can also be active during the day. Hedgehogs sleep for a large portion of the day under bushes, grasses, rocks, or most commonly in dens dug in the ground, with varying habits among the species. All wild hedgehogs can hibernate, though not all do, depending on temperature, species, and abundance of food.

Hedgehogs are fairly vocal and communicate through a combination of grunts, snuffles and/or squeals, depending on species.

SELF-ANOINTING

Hedgehogs occasionally perform a ritual called anointing. When the animal encounters a new scent, it will lick and bite the source, then form a scented froth in its mouth and paste it on its spines with its tongue. The purpose of this habit is unknown, but some experts believe anointing camouflages the hedgehog with the new scent of the area and provides a possible poison or source of infection to predators poked by their spines. Anointing is sometimes also called anting because of a similar behavior in birds.

Like opossums, mice, and moles, hedgehogs have some natural immunity against some snake venom through the protein erinacin in the animal's muscular system, although it is only available in small amounts and a viper bite may still be fatal. In addition, hedgehogs are one of four known mammalian groups with mutations that protect against another snake venom, α-neurotoxin. Pigs, honey badgers, mongooses, and hedgehogs all have mutations in the nicotinic acetylcholine receptor that prevent the snake venom α-neurotoxin from binding, though those mutations developed separately and independently.

OLFACTORY SENSE

The olfactory regions have not been thoroughly studied in the hedgehog. In mammals, the olfactory part of the brain is covered by neopallium, making it difficult to expose. This difficulty is not impossible, as it varies from one species to another. Tests have suggested that hedgehogs share the same electrical activity as cats.

DIET

Although traditionally classified in the now abandoned order Insectivora, hedgehogs are omnivorous. They feed on insects, snails, frogs and toads, snakes, bird eggs, carrion, mushrooms, grass roots, berries, melons and watermelons. Berries constitute a major part of an Afghan hedgehog's diet in early spring after hibernation.

HIBERNATION

During hibernation, the body temperature of a hedgehog can decrease to about 2 °C (36 °F). When the animal awakes from hibernation, the body temperature rises from 2–5 °C (36–41 °F) back to its normal 30–35 °C (86–95 °F) body temperature.[10]

REPRODUCTION & LIFESPAN

Depending on the species, the gestation period is 35–58 days. The average litter is 3–4 newborns for larger species and 5–6 for smaller ones. As with many animals, it is not unusual for an adult male hedgehog to kill newborn males.

Hedgehogs have a relatively long lifespan for their size. Larger species of hedgehogs live 4–7 years in the wild (some have been recorded up to 16 years), and smaller species live 2–4 years (4–7 in captivity), compared to a mouse at 2 years and a large rat at 3–5 years. Lack of predators and controlled diet contribute to a longer lifespan in captivity (8–10 years depending on size).

Hedgehogs are born blind with a protective membrane covering their quills, which dries and shrinks over the next several hours. The quills emerge through the skin after they have been cleaned, or it falls off.

DISEASES

Hedgehogs suffer many diseases common to humans. These include cancer, fatty liver disease and cardiovascular disease.

Cancer is very common in hedgehogs. The most common is squamous cell carcinoma. Squamous cell spreads quickly from the bone to the organs in hedgehogs, unlike in humans. Surgery to remove the tumors is rare because it would result in removing too much bone structure.

Fatty liver disease is believed by many to be caused by bad diet. Hedgehogs will eagerly eat foods that are high in fat and sugar. Having a metabolism adapted for low-fat, protein-rich insects, this leads to common problems of obesity. Fatty liver disease is one sign, heart disease is another.

Hedgehogs uncommonly transmit a characteristic fungal skin infection to human handlers as well as other hedgehogs. This ringworm or dermatophytosis infection is caused by Trichophyton erinacei, which forms a distinct mating group within the Arthroderma benhamiae species complex.

Hedgehogs can suffer from balloon syndrome, a rare condition in which gas is trapped under the skin as a result of injury or infection and which causes the animal to inflate; in 2017 the BBC reported a case in which a male hedgehog was "almost twice its natural size, literally blown up like a beach ball with incredibly taut skin". The head vet at Stapeley's Wildlife Hospital, Bev Panto, said, "In my career I have seen three or four of these cases and they are very strange every time and quite shocking […] When you first see them they appear to be very big hedgehogs but when you pick them up they feel so light because they are mostly air. She added that the condition was unique to hedgehogs because they have significant space under their skin as a result of their ability to curl up. A similar case had been reported in 2013. The British Hedgehog Preservation Society advises that:

There is no single cause for this condition. The air can be removed by incising or aspirating through the skin over the back. Antibiotic cover should be given. This may be associated with lung/chest wall damage or a small external wound acting like a valve or a clostridium type infection.

- Information courtesy of Wikipedia

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