Animal anointers

I was recently asked to provide a comment for a BBC Wildlife reader’s observation of two rooks tearing open used cigarettes and rubbing their chests in the tobacco. I did so, and found that there was far more of interest than I could possibly fit into the required couple of sentences, so I provided a concise response for the magazine and channeled everything else here.

Anointing behaviour is a well-known phenomenon in the animal kingdom, involving an animal applying some plant, soil, insect or excretory product to itself or another individual. For example, capuchin monkeys rub millipedes into their fur, and orangutans will rub themselves with herbs. Hedgehogs are regular anointers: their natural response to encountering a new, noxious or smelly substance is to lick and chew it, and then coat its spines with the salivary mush. They will even chew up toad poison glands and, after frothing at the mouth for a bit, lick themselves with the saliva-poison. It’s such a part of being a hedgehog that young hoglets start self-anointing before they have even opened their eyes!

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Self-anointing hedgehog: by Gisrenist [CC BY-SA 3.0] via Wikimedia Commons: https://commons.wikimedia.org/wiki/File%3AHedgehog_self-anointing.jpg

I could reel off many more examples, but the more interesting question is why do animals go to the trouble of applying these –often unpleasant- substances to themselves or others? Is it beneficial? Here’s an overview of what science has to say on the matter:

Sexy scents

Scent-marking is a common behaviour throughout the animal kingdom, usually involving application of some substance to something in the environment to signal territory. Perhaps lesser known is that some animals also mark others with scent, normally urine, and that this can serve a sexual function. For example, many rodents, including rabbits, guinea pigs, chinchillas, porcupines and maras (the fourth largest rodents in the world), pee on intended mates. The proper name for this is enurination, and unlike regular urination it involves shooting a directed jet of urine at the recipient. Male rabbits and hares will often leap in the air and over their object of desire while spraying urine at her. In the maras both sexes do it: the male stands on his hind legs and sprays towards his mate, and the female responds by bending over and spraying it backwards. Definitely bizarre but – unlike in humans – it doesn’t seem to be kinky. Males may spray females for mate-guarding purposes, basically advertising to other males that ‘this one’s taken’. Females, on the hand, may spray at males to provide them with information on reproductive status. It may also be an outlet for sexual frustration: male rodents may spray urine at the female if she has not adopted the mating position, and the female may spray at the male if he is pestering her and she is not interested. Who knew that urine could be such a powerful communication tool?

There is little evidence that animals anoint themselves for sexual functions, other than humans of course – the perfume industry can attest to our love affair with scent. One study’s authors tentatively suggested that the two spider monkeys they observed applying chewed up plant mush to their armpits and breastbone may have been doing so to increase sexual attractiveness, but more evidence is needed to support this idea.

Influencing behaviour

In group-living animals chemicals can be remarkably powerful tools for modulating behaviour. A good example is behaviour suppression in worker bees: the queen bee releases a substance called Queen Mandibular Pheromone, and this actually prevents other females (i.e. the workers) from being able to reproduce. Directly marking other individuals with odorous chemicals (i.e. scent) may therefore serve social, as well as sexual, purposes. Back we go to urine, and to wild rabbits, where the dominant male frequently urinates on other rabbits in his group to signal his social status. The scent also allows recognition of group members. If you take a rabbit out of its group, mark it with scent from a different group, then place it back in its original group the strangely smelling animal will be attacked.

The hedgehog anointing its spines with toad poison may do so because predators who come too close experience more pain when they are pricked with a poison-coated spine than a regular spine. This was demonstrated in the 1970s by a biologist called Butch Brodie and six intrepid graduate students who volunteered their arms for science. The results were clear: 1/7 pricked with a plain hedgehog spine experienced redness and irritation, compared with 6/7 after being pricked with a toad poison-coated spine. The hedgehog appears to be using the substance to influence the behaviour of potential predators, who may be less keen to predate hedgehogs after suffering this experience.

Medicinal/personal hygiene

The previous examples showed how anointing behaviour can function to influence the behaviour of others, either sexually or socially. It is also thought to occur for reasons relating to personal hygiene or medication, and the majority of self-anointing behaviour fits into this category. Capuchins, for example, anoint themselves with all sorts of things, including millipedes, onions and limes, and they seem to apply these substances to body areas which are harder to see and therefore harder to groom. Bridging over to the social sphere they have also been observed rubbing up against other monkeys in their group after anointing, a phenomenon described as ‘mutual medication’.

Many bird species engage in a behaviour called ‘anting’, in which they rub ants over their feathers and skin, or squat down among an ant colony and allow the ants to crawl over them. Some ants secrete formic acid when agitated to deter predators and colony intruders. Formic acid is also an effective insecticide and a popular theory suggests that birds engage in the behaviour to annoy the ants into secreting acid over their feathers and kill feather parasites. It’s by no means conclusive, though – an alternative theory is that the birds are emptying the ants’ formic acid reserves so that they can eat them, and another theory suggests the birds are simply getting ‘high’ from the acid vapours…

black_drongo_i2_img_5683
Black drongo showing anting behaviour. By JM Garg [CC BY-SA 3.0], via Wikimedia Commons: https://commons.wikimedia.org/wiki/File%3ABlack_Drongo_I2_IMG_5683.jpg

The letter to which I was responding concerned the potential use of tobacco as an anti-parasite strategy: two rooks were observed at a motorway service station ripping open discarded cigarettes and rubbing their chests on the tobacco. Now, it’s not often that I pair the words ‘cigarette’ and ‘benefit’ together, but there are a couple of tantalising examples of animals potentially deriving benefits from tobacco. Tobacco leaves naturally contain high concentrations of nicotine, a potent insect neurotoxin produced by the plant to ward off insect predators. One study found that parasite-infected bumblebees preferred to feed on nicotine-laced sugar solution than those without, and the progress of their infections was slowed as a result. In 2012 a report was published on house sparrows and house finches incorporating fluff from used cigarette butts into their nests. Researchers found that chicks from nests with the most, ahem, butt fluff had fewest parasites.

It’s possible that the rooks in question were deriving similar anti-parasite benefits, and while this is a tempting conclusion it’s impossible to do anything other than hypothesise about a single observation. Perhaps it was also a social or sexual display between the individuals, or maybe it had no discernible function but provided some intrinsic value to the individuals. It would certainly be an interesting topic to investigate further.

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