Hedgehog Spines - Potential Bio-mimicry Inspiration

Hair is a product of evolution that can only be found in mammals. Hairs can have various functions to mammals, which include signaling, sensing the environment, camouflage, to retain body warmth as wells as a signal to ward of predators, or to find a mate. Quills and spines are a form of modified hair which are common in four groups of mammals. These groups are:

1.            Hedgehogs (Erinaceomorpha: Erinaceidae, Erinaceinae)

2.            Tenrecs (Afrosoricida: Tenrecidae, Tenrecinae)

3.            Echidnas (Monotremata: Tachyglossidae)

4.            Rodents (Rodentia)

Quills and spines both serve different purposes although they have a hard and thick outer layer of keratin. Quills act as a defensive armor for porcupines, as they are long, highly irregular in length and they break and separate from the host’s body quite easily. Spines on the other hand serve as a shock absorber for hedgehogs as they are all of the same length all over hedgehog, embedded well in the skin, curved slightly and firm.

The spines of the hedgehog consists of many contained air pockets that are separated by a regular spacing of septum coupled with longitudinal stringers. The cross section of the structure of hedgehog spines can be viewed from Figure 1. The existence of the network of air pockets in the spines give them a high strength to weight ratio. Besides that, the air pockets helps prevent breakage of spines as well as buckling.

Figure 1: Cross section of the hedgehog spine.

The mechanical properties of spines across different species may vary depending on the microstructure of the spine. Figure 2 shows the microstructure of the Erinaceus europaeus spine, whereas figure 3 shows the microstructure of the Hemiechinus sp spine. The septa of the former is much more widely spaced, and has longitudinal stringers that are much deeper as compared to the latter. This piece knowledge is crucial, as the internal structure of the hedgehog is essential in order to successfully design knee-pads which can absorb any excess shock. By varying the depth of the longitudinal spine, as well as the distribution and thickness of the septum, the amount of shock that can be absorbed by the aforementioned knee-pads can change tremendously.

Figure 3: Transverse (a) and longitudinal (b) section of Hemiechinus sp spine. Scale is 100 µm

Figure 2: Transverse (a) and longitudinal (b) section of Erinaceus europaeus spine. Scale is 100 µm

By using the concept of bio-mimicry, the hedgehog’s spines can be model as a potential for impact resistant materials and structures. There were many experimental studies that were conducted in the past on the mechanical properties for the spines of the hedgehog. From those studies, the critical Euler bulking force for a single hedgehog spine was identified to be about 6N and this was found by conducting bending and compression test on the spines of the hedgehog. Moreover, tests for the consistency of the durability performance of the hedgehog’s spine were also studied upon as it is important for various applications such as concussion mitigation for the football helmets [4]. Among the tests that were conducted was the dynamic impact test by using a dual-arm crash pendulum. Figure 4 shows the schematic of the pendulum apparatus during the trial of collision on the hedgehog’s spine that has been retrieved from a naturally deceased hedgehog.  
 

Figure 4: Impact test on hedgehog spine using pendulum

After the test was conducted, it was found that the spines in the humid sample has minor damage and remains intact after a consecutive of 5 collisions of each being 8 Joules. However, the spine which was of the dry sample shows a significant amount of breakage after just 2 collisions of each being 8 Joules.

Figure 5 shows the results of the hedgehog spine after impact testing [5].

Hence, from this literature study, this concept of using the hedgehog’s spines or quills that acts as the animal’s safety mechanism can provide some promising applications in future materials. In this study, the design hedgehog’s spines are used to redesign elbow pads and knee pads for motorcyclist as improve the mechanical properties and reduce impact of motorcyclist when acted upon.

References

[1]      http://animaldiversity.org/collections/spinesquills/

[2]      http://dlx.booksc.org/15100000/lib....pdf

[3]      http://www.hedgehogcentral.com/quills.shtml

[4]      http://sci-hub.cc/http://journals.sagepub.com/doi/pdf/10.1177/1045389X9000100109

[5]      http://sci-hub.cc/http://www.sciencedirect.com/science/article/pii/S175161611630039X

Introducing Hedgehog

African Pygmy Hedgehog (Atelerix Albiventris)

Hedgehogs have been around for the last 15 million years share a distant similarity to shrews (family Soricidae) and does not experience any significant changes since then. It belonged in the Erinaceinae subfamily in the eulipothyplan family Erinaceidae. Seventeen species of hedgehog are discovered through parts of Europe, Asia and Africa. New Zealand however, is an area where hedgehogs does not originally belong to and can only be found in the region after introduction. Hedgehogs are not native to Australia and other than the already extinct Amphechinus genus, no other hedgehogs are species native to the Americas.  

Their spiny protruding spines or quills are technically hairs made stiff with keratin. Contrary to general belief, hedgehog spines are not poisonous or barbed and unlike porcupines, do not detached easily from their bodies. Quills detachment only occurs during “quilling”. Like shedding, quilling is a process where the immature spines fall out as they are replaced with stronger, adult spines. It can also occur when a hedgehog contract disease or under extreme stress. 

European Hedgehog (Erinaceus Europaeus)

Skeletal System of a Hedgehog

All species of hedgehogs possess ability to roll into a tight ball as a defensive manoeuvre. This caused their spine to roll outwards. This is enabled by two large muscles that spans along the back of a hedgehog. When the creature rolls into a ball, its body, feet and belly is tucked inside the outside layer of quills. This helps hedgehogs to protect themselves from imminent danger from predators; forest hedgehogs are prey to carnivorous birds and ferrets while smaller species are often hunted by foxes and wolves. Since hedgehogs often spend most of their time on trees away from the ground, it rolls into a ball and jump towards the ground in the event of an attack. Their quills absorb the impact of the fall as the body touches the ground. This is the aspect that is deemed unique for a hedgehog and will be the subject for this research. 

Other unique characteristics that hedgehogs have include: 

• PRIMARILY NOCTURNAL. Some species can be found sleeping throughout large portion of the day under bushes, grass or dens dug in ground. Depending on the temperature, species and food abundance, wild hedgehogs can enter themselves into hibernation. 
• FAIRLY VOCAL. They communicate through grunts and squeals depending on species. 
• ANNOINTING RITUAL. As it encounters a new scent, it will lick and bite the source. A scented froth the formed in its mouth and is pasted on its spine using its tongue. This is believe to camouflage the hedgehog from the predators in the area and to some cases, generating a poison that can be used to infect predators that are poked by it spines.  
• NATURAL IMMUNITY TO VENOMS. Hedgehogs, among opossums, mice and moles, have some degree of immunity against some snake venom. This is made possible due to the protein erinacin in the animal's muscular system. However, it is only available in small amounts and viper crushing bite might still be fatal. In addition, hedgehogs contain mutations in the acetylcholine receptor that prevents binding against another snake venom, α-neurotoxin.