Vision:
Most mammalogists with an opinion on the subject seem to agree that vision is probably not of the utmost importance to hedgehogs, pointing out that their niche (moving around in the, sometimes tall, undergrowth at night) makes vision somewhat inutile. In an environment of twigs and brambles the large eyes sported by other nocturnal mammals (e.g. bush babies, Galagidae) would probably be a liability. Consequently, it follows that they probably do not have exceptional eyesight and, as is the case for most nocturnal animals, colour vision is probably of little consequence to them.
A retinographic study published in the journal Vision Research during 1973 reports that hedgehogs have a rod-dominated monochromatic retina containing rhodopsin (the visual pigment in rod cells) with a peak sensitivity of about 500 nanometres – this lies within the narrow range of between 493 and 502 nm (i.e. blue-shifted) reported in surface-living terrestrial animals, including humans. However, while the aforementioned paper makes no mention of cone (colour-sensing) cells on the retina of Erinaceus europaeus, in his 1965 Hedgehogs, A Comprehensive Study (cited in Nigel Reeve’s, Hedgehogs), Konrad Herter notes that about 4% of the hog’s retina are cone cells -- humans and other primate are closer to 25% -- and writes of how he was able to train his subjects to distinguish yellow from shades of blue and grey. Ultimately, it seems that in good light conditions, hedgehogs may have the potential for limited colour vision.
The rod cells in their moderately-sized eyes probably impart sufficient scotopic (low light) vision, allowing the animal to distinguish shapes and moving objects in moonlight. A further testament to the unimportance of vision to hedgehogs comes from Pat Morris’ study on the impacts of supplemental feeding on this species published in Mammal Review during 1985. During his study, Dr. Morris tracked a virtually-blind male hedgehog that, in spite of the occasional collision with objects in his path, lived an otherwise normal life.
Olfaction (Smell) / Tactility (Touch):
Observations on hedgehog olfactory sensitivity have suggested that smell probably plays an important role in their reproduction and sociality – I say “probably” because, although we know hedgehogs have a keen sense of smell and sniff the air constantly, there have been very few rigorous studies into the role scent plays in the lives of any hedgehog species. Perhaps the most telling sign that scent is of importance to hedgehogs can be seen in their morphology; they have a long snout with a large moist tip (called a rhinarium). In conjunction with olfaction, it seems that much of the hedgehog’s sense of tactility (touch) is concentrated around the face and in the long guard hairs that fringe its spines.
Internally, Nigel Reeve notes (in Hedgehogs) that the brain of these animals possesses well-developed olfactory lobes. Additionally, hedgehogs have a well-developed vomeronasal organ (VON, sometimes referred to as Jacobson’s organ). The VON comprises a pair of blind-ending sacs, connected via ducts to the mouth and nasal cavity – most vertebrates have a VON, which feeds its sensory input into the accessory olfactory bulbs. While the function of the VON in humans -- found in the vomer bone, between the nose and mouth -- remains something of a mystery, in other animals it seems to be involved in the sampling of scents – the flicking of a snake’s tongue and the rapid tongue movements and drawing back of the lips by ungulates and big cats (referred to as ‘flehmen’) are some of the best examples. Among the molecules capable of being detected by this organ, are steroid (i.e. sex) hormones. The function of the VON has not been conclusively demonstrated in hedgehogs, but some biologists consider it probable that it plays a role in analysing reproductive pheromones and it has been implicated in self anointing
Despite the current lack of information regarding the importance and use of scent by hedgehogs, the presence of sexual accessory glands (males mark the ground, and perhaps the female with odiferous secretions exuded from the penis tip) and proctodeal glands -- sebaceous glands, about 7mm long and 5mm wide (1/3 by 1/5 in.), located just inside the anus, which may potentially add scent to the faeces -- suggest that scent probably has an important role in reproductive (and perhaps also intraspecific recognition) behaviour. Dr. Reeve also notes that, to the exclusion of the large multi-lobed sebaceous glands in the corners of the mouth and the meibomian (eyelid) glands, hedgehogs have very few specially developed skin glands. This is not necessarily surprising; after all, many of the scent glands found on mammals are involved in marking territory, as much as providing information as to the fitness of the individual who secretes it, so it is perhaps not surprising that fewer skin glands are to be found in a non-territorial species like the hedgehog
Audition (Hearing):
In common with olfaction, there are no studies looking at the importance of sound to hedgehogs. Nigel Reeve notes (in Hedgehogs) how hedgehogs flinch when keys are jangled or tongues are ‘clicked’ near them, which implies that they probably hear in the ultrasonic (20kHz and above) – anyone who owns a bat detector can testify to the horrible metallic grating that keys make when jangled near it. On my Bat MKII detector, tongue clicking seems sharpest and loudest at around 42 to 45 kHz and barely discernable above 50 kHz; this fits well with the hearing frequency figure of 45 kHz quoted by Les Stocker in The Complete Hedgehog.
Studies looking at hedgehog species other than E. europaeus suggest hearing between 8 and 85 kHz, with a peak at 20 kHz. In their 1969 paper to Journal of Auditory Research, Richard Ravizza and his team from Vanderbilt University in Tennessee report that Hemiechinus auritus can hear tones from 250 Hz to 45 kHz, although the peak was at 8 kHz and at 42 kHz the discrimination was only 2.5%; extrapolating from the audiogram, the biologists conclude that this species can probably hear up to about 60 kHz. Ravizza and his colleagues write: “In comparison to the opossum, the hedgehog is measurably more sensitive throughout most of its [auditory] range but is, nevertheless, less sensitive than most other mammals.” For comparison, humans can hear within a range of about 20 Hz – 20 kHz (depending on age), dogs 30 Hz – 45 kHz, cats 45 Hz – 65 kHz, and bats 2 kHz – 110 kHz.
In his Frequency Hearing Ranges in Dogs and Other Species article, Louisiana State University neuroscience professor George Strain lists “hedgehog” as hearing in the range of 250 Hz to 45 kHz, although no species or specific reference is given for the value (judging by the range, this is probably the Raviza study). Finally, in his paper on the middle ear anatomy of fossorial (burrowing) and non-fossorial mammals, Dr. Matthew Mason at St. Catherine’s College in Cambridge, reports that E. europaeus has a smaller pars tensa (tympanic membrane) and an otic cavity volume that’s less than half that of their H. auritus specimen, despite their specimen of E. europaeus being larger. The larger tympanic membrane (the membrane that closes the middle ear off from the outside world) in H. auritus may allow this species to hear lower frequencies than E. europaeus.
While we have no definitive auditory ranges for the hedgehog, this species has been reported to emit several different sounds. These sounds include the “twittering” of hoglets and the “shrill piping whistles” (at around 8 kHz) of nestlings. Other sounds include “clucking”, “quacking”, various snorts, spits, huffs, and ultrasonic “clicks”. Interestingly, Walter Poduschka has suggested in several papers that hedgehogs may use the ultrasonic clicks to echolocate. However, the evidence is unconvincing and despite the data presented by Dr. Edwin Gould (Tulane University, Louisiana) in his Ph.D thesis in 1962, and subsequently in a co-authored paper to the Journal of Experimental Zoology, suggesting that shrews may be capable of echolocation, no such ability has ever been conclusively demonstrated in hedgehogs.
Perhaps the sound every hedgehog keeper fears is “the scream” – in an online article about hedgehog vocal repertoires, hog keeper Mike McGary writes: “My young male (Adam) has been known to scream when frightened. This isn't a small squeak, but a full-fledged rabbit-caught-in-a-trap scream.” The experiences of hedgehog owners suggest that screams aren’t necessarily related to fear or danger. On the Hedgehog Central discussion boards, one member writes: “The hedgie scream of death will make your blood curdle. … Emma did it one time when HORRORS, we rearranged her cage on her and she was royally pee'd off about it. Pebbles got a nail caught in her blankie as she was trying to leave her igloo and of course she couldn't go too far. Their room is on the second level of the house and everyone in all parts of the house heard her and came running wondering what was wrong. We all thought for sure she was being killed.” This suggests that frustration may be a cause of the scream, at least in captive individuals. In Hedgehogs, Dr. Reeve mentions that he’s never heard “the scream” in wild hedgehogs, although it is fairly well documented in the literature.
Sun Jan 08, 2012 1:50 pm
