Perhaps one of the most bizarre facets of hedgehog behaviour is the curious act of self-anointing. The term was first coined by Maurice Burton in an account of the behaviour published in New Scientist in October 1958, but was first recorded by German zoologist Ludwig Heck in 1912; he called it selbstbespuchen, or ‘self-spitting’. Self-anointing involves the hedgehog covering its spines with a frothy saliva-stimulant mixture; the behaviour itself may take a few minutes or several hours, during which the hedgehog is totally absorbed and almost oblivious to the activity in its surroundings. This behaviour is reported frequently, and most people who have experience with hedgehogs have witnessed it at some point. Nigel Reeve describes the process in Hedgehogs. When a hog encounters a stimulating object or substance, it is sniffed, licked and then chewed – the trigger object may be taken into the mouth, but sometimes the odour alone is sufficient to trigger anointing. The resulting saliva is applied to the spines and fur with the underside of the tongue as the hedgehog contorts its body and licks itself.
There have been numerous theories proposed to explain this fascinating behaviour, but none fits all cases. Typically, the most oft-cited observation is that trigger objects have smells or tastes that can be considered pungent or acrid, or be novel to the hog. Nonetheless, in his 1969 book The Hedgehog, Maurice Burton notes how hedgehogs have been known to self-anoint when presented with distilled water, which has no smell or taste. Also, while some substances that are known to elicit anointing are pungent (e.g. polish, glues, dog urine and faeces. etc.), others are, to us at least, much less so (e.g. glazed surfaces, cotton etc.) and some pungent and/or acrid substances (e.g. petrol, vinegar, whisky etc.) don’t appear to induce self-anointing. Similarly, the stimulus doesn’t need to be novel and hedgehogs can be stimulated by the same substance on successive occasions, while objects that stimulate self-anointing on one occasion may not do so on another.
The list of objects/substances recorded as inducing self-anointing is quite remarkable, not to mention extensive. In Hedgehogs, Nigel Reeve lists 34, including fox fur, human sweat, carpet, varnish, creosote, tobacco, newsprint and even tortoises! In his 1976 paper to Animal Behaviour, Robert Brockie presented his observations on the self-anointing of wild hedgehogs in New Zealand. Brockie suggested that, because he only found saliva-covered spines on individuals during the courtship season, self-anointing may be a way of presenting sexual odour. Reeve, however, found evidence of anointing outside of the breeding season in his golf course subjects and this phenomenon has since been reported in both sexes, when solitary, when meeting and in all ages (even still-blind nestlings). Indeed, in a short paper to the International Zoo Yearbook in 1966, B. Morris described how a nine-day-old hoglet was observed self-anointing, apparently stimulated by the resin on the sides of the heated box they were being kept in at the University of Nottingham’s zoology department.
Despite the foregoing, some studies have suggested patterns in the behaviour. In a 2005 paper to the journal Acta Theriologica, a team of biologists at the University of Antwerp led by Helga D'Have reported that self-anointing was “clearly dependent on gender, age and season.” During 2002, the scientists recorded the occurrence of self-anointing among nearly 200 tagged hedgehogs from several populations at five study sites in Antwerp, Belgium. Incidence among the hedgehogs was relatively low, with only 46 (11%) showing signs of self-anointing. (This figure may appear higher than most other studies, which observed the behaviour in less than 2% of animals, but these researchers recorded both recent and 'trace' self-anointing, while other authors only monitored recent activity – when the trace evidence is removed, the results from this study recorded recent self-anointing in seven animals, about 2%).
The data from the Belgian study show that males tended to self-anoint more than females, with subadult males tending to self-anoint most; young males accounted for 35% of observations, young females 18%, adult males 8%, and adult females 4%. There was a peak of self-anointing observed during the summer, with almost 40% of the adults and about 15% of the juveniles found with evidence of self-anointing in July. Observed self-anointing declined in August (about 12% adults and 8% juveniles) and then increased among adults during September (around 35%).
Overall, but there are five main schools of thought on the reason for self-anointing. The first theory (the so-called “camouflage hypothesis”) suggests that covering the spines with a foreign substance may serve to mask the hedgehog’s odour and thereby impart some anti-predator quality. Indeed, from their results, D'Have and her team suggest that the camouflage hypothesis may explain the self-anointing patterns that they observed. The biologists note that juveniles self-anointing more than adults could be a bid to provide extra protection from predators at their most vulnerable life stage. Similarly, as we have seen, adult males range further than adult females and this may explain their higher self-anointing frequencies – ranging further means you're more likely to encounter both predators and conspecific males. To the best of my knowledge, there no studies looking at whether self-anointing does make hedgehogs less prone to being detected by predators (or conspecifics), but many of the substances that induce this behaviour would not be efficient in repelling most potential predators.
It has also been proposed that the substances applied to the spines may add an extra ‘irritation factor’ to the spines, causing a chemical trauma to any would-be predator. A rather famous experiment by Edmund Brodie Jr. published in Nature during 1977 demonstrated that African hedgehog (Atelerix) spines covered with cane toad (Bufo marinus) skin secretions (both manually applied and applied through self-anointing) caused immediate and intense local burning when jabbed into the arms of volunteers; “splotchy red areas” developed around the puncture site. Spines that had been washed in alcohol or coated with ordinary (i.e. non-anointed) hedgehog saliva caused no effect, suggesting it was the skin secretion that caused the irritation. While this may demonstrate a plausible explanation for self-anointing, any anti-predator effect is probably coincidental; it is difficult to imagine that spines anointed with saliva generated in response to more innocuous substances (e.g. carpets, tortoises, glass etc.) would prove to be an irritant, given that the saliva itself isn’t toxic (see below).
A third theory is that the saliva and its trigger compound(s) may serve a grooming purpose, conditioning the skin and spines or perhaps killing parasites, although the latter would presumably require some form of ‘toxic saliva’. According to a paper to Toxicon in 1999 by Dietrich Mebs, extracts from the Glandula submandibularis and Glandula parotis salivary glands of Erinaceus europaeus yielded no lethal, haemorrhagic or myonecrotic (heart damaging) effects when injected into mice – this certainly implies that hedgehogs (unlike other insectivorous mammals, such as shrews) do not have toxic saliva.
Perhaps the most insightful observation for anointing not being aimed at grooming is that Nigel Reeve found a hedgehog that had anointed its spines with dog faeces! Additionally, hedgehogs are impressively flexible and perfectly capable of grooming themselves, they just seem largely uninterested in doing so. Overall, there is no evidence to suggest that the spines need conditioning in order to remain supple, not least because they’re constantly replaced, nor that the self-anointing affects parasite burden.
The fourth theory suggests that self-anointing may be a non-adaptive behaviour. In other words, self-anointing doesn’t increase the likelihood of survival. This may be the case, although hedgehogs may spend a substantial period of time self-anointing and this is rare for a non-adaptive behaviour. Still, other non-adaptive suggestions, such as the saliva serving to cool the spines, don’t seem likely; were this the case we’d expect the saliva to be applied directly to the skin because there is no blood flow through the spines. In The Hedgehog, Maurice Burton suggests that licking may be a non-functional response to a stimulus (smell or taste) and the saliva is an annoyance that is wiped off onto the spines. This seems plausible, although it is curious that the saliva is always wiped onto the spines; never on the ground or on surrounding objects, and the hedgehog never uses its paws to wipe the saliva away, which would be quicker and easier.
The final theory, and the one that seems to have gained the most support, is that self-anointing plays a role in scent-marking, perhaps involving the vomeronasal organ. There is little doubt that the spines act as a large evaporative surface for any scent and this may be linked to the ‘mutual avoidance’ discussed above. It may be that the characteristic tongue flicking against the roof of the mouth helps clear the froth from the nasopalatine duct, which leads to the vomeronasal organ.
Whether any of the aforementioned theories do actually represent a true explanation of this peculiar behaviour, it seems clear that it must serve some adaptive function; hedgehogs are entirely absorbed during anointing, making them vulnerable to predators. Furthermore, when they’ve finished they are often dazed, tired and hungry, suggesting it can be an energy expensive behaviour.
It is worth mentioning quickly that self-anointing is not unique to hedgehogs, although it has been found in all hedgehog genera. In 1995, Zhongjian Xu, Hanbo Ding and Jian Zhang at the Fujian Normal University in China and Michael Stoddart at the University of Tasmania in Australia, published a paper in the Journal of Mammalogy describing self-anointing behaviour in the rice-field rat (Rattus rattoides) upon presentation with weasel scent. Additionally, a paper to the journal Primates by Matthias Laska, Verena Bauer and Laura Salazar reports how spider monkeys (Ateles geoffroyi) have been observed to self-anoint in the presence of three species of plant (Brongniartia alamosana, Cecropia obtusifolia and Apium graveolens) – the biologists concluded that the scent of the plants is used in a social context. Other species known to self-anoint include capuchin monkeys (Cebus spp.), owl monkeys (Aotus spp.) and Siberian chipmunks (Eutamias sibiricus).