Squirrel Stone Caching & Dust-bathing

As with almost any species, there are some facets of behaviour that are more difficult to explain than others; squirrels are no exception. Two behaviours in particular warrant further attention: caching of non-food objects, typically stones, and dust bathing.

Stone caching

A great many species of mammal are known to hide surplus food for later retrieval, a process known as caching (pronounced “cashing”, from the French cacher, ‘to hide’) and squirrels are among some of the most proficient cachers known to science, even going so far as organising their larders to help them remember what’s stored where. There are, however, a handful of reports of squirrels taking inedible objects and caching them.

The first time I came across this behaviour was in an account from New Year’s Day 2005, when Maria Salmon contacted veteran squirrel biologist John Gurnell about some strange behaviour she had seen from a Grey squirrel visiting her garden. The squirrel had been stealing pebbles, each about the size of a walnut (i.e. approx. 5 x 3.5cm / 2 x 1.5 in.), every day for weeks – to the point where Ms. Salmon needed to buy some more. This curious behaviour continued until at least the end of February and apparently the squirrel, one of several visiting the property, would come into the garden and make straight for the pebbles, ignoring the food put out, and carry one off somewhere. It was never established where the stones were being taken (the squirrel didn’t always leave in the same direction) or what was happening to them.

A Grey squirrel carrying a pebble to be cached on a lawn. We're unsure why squirrels sometimes bury non-food items, but it may be a way to help "dilute" their caches and reduce potential pilfering. - Credit: Roy & Marie Battell

In August 2010 a user posted to the online RSPB Community forum recounting similar behaviour; a squirrel was taking gravel and pebbles from their garden “usually after licking it all over”. When larger pebbles were added, the squirrel shifted its attention to these and it would typically pick up four or five before it settled on one to take, suggesting there is a specific trait the squirrel is seeking. In this individual’s case, however, while some stones were cached, others were left on fence posts, on the summer house roof or under the summer house. Subsequently, I’ve come across reports online of similar stone-burying behaviour from Greys both here, in the USA (Maine and California) and Canada (Peterborough, Ontario). In some cases the stones were licked first, but in all cases the stones were removed and buried.

In April 2011, a lady contacted me describing:

A cheeky grey squirrel runs up to the house from the bottom of the garden, sniffs the ground for a few seconds then carefully selects a rather large piece of gravel (c. 2cm in length, smooth, more of a small pebble), puts it in his mouth, finds another the same size and puts that in his mouth too, then runs off down to the conifers at the bottom of the garden!!

The squirrel was doing this most of the day and had removed quite a few stones. More recently, Vicky Harrison got in touch to tell me about Greys visiting her property that “consistently take small stones from my garden and bury them, usually next door”. Vicky first noticed this behaviour in the winter of 2017, and it appeared to be just a single individual that was collecting pebbles who was “very particular about the size (acorn) and shape (smooth) of the selected stones”. By the following autumn four of the five or six visiting squirrels were doing it and, in some cases, jagged stones were taken. The stones were taken throughout the day and mostly buried in a neighbouring garden, although Vicky found several in her flower pots.

A Grey squirrel (Sciurus carolinensis) caching a nut in the lawn. This behaviour is innate in both Reds and Greys. - Credit: Marc Baldwin

This is certainly intriguing behaviour and not something I, or many squirrel biologists based on my research, have ever observed personally. I have been unable to find any reference to this in the literature and Lucia Jacobs, a neuropsychology professor at the University of California, Berkeley, who has spent much of her academic career study caching behaviour in Greys, told me she has observed the caching of non-food items only very rarely – in one case a squirrel stole their experimental cue, which was shaped like an ear of corn, and cached it. It is perhaps also significant that all reports I have found to date involve Grey squirrels, which are less arboreal and tend to live at higher density than Reds, although this may equally reflect this species’ more cosmopolitan distribution and propensity for visiting gardens. Nonetheless, all accounts describe a patient deliberation about the selection of the stones and the regularity with which it is carried out, suggesting some purpose the squirrel was invested in.

Ravens are known to cache non-food items when they know they're being watched, which scientists think helps them learn how to be pilfered from and make better caches. - Credit: Marc Baldwin

Unfortunately, there are still a great many unknowns about this behaviour. We don’t know if it is sex- or age-specific, for example. We also don’t know whether the stones are ever retrieved or relocated, as regular caches are. We do know that squirrels are very acute when it comes to selecting seeds for caching and have a good sense of smell, suggesting that it is unlikely they have mistaken the pebbles for food. In Vicky’s description, squirrels visiting across years exhibited the behaviour, implying they were presumably feeding themselves in between these bouts of pebble caching.

Thus far, there appear to be three theories offering an explanation for this behaviour. One suggests, based on finding stones in the lining of a squirrel’s drey, that they may be used as ballast or even ‘radiators’, although in my view this is an unlikely explanation and it seems probable that the stones were incidental inclusions. A second hypothesis, provided by Natural Heritage Information Centre zoologist Don Sutherland, considers that it may be an attempt to disguise food caches and reduce cache-pilfering by other squirrels. Off-hand, it seems this could work in two ways: it may serve to ‘dilute’ the caches such that any potential thief would potentially uncover a stone for their trouble rather than a nut; or a stone could be placed on top of a food item to fool a potential pilferer. I’ve not come across evidence to support either of these in the current observations, but both seem plausible.

Finally, Professor Jacobs pointed to research by scientists at the Konrad Lorenz Forschungsstelle, part of the University of Vienna’s Faculty of Life Sciences, which found ravens openly cache small toys they’re given while other birds look on, but hide when they’re caching food. The researchers suspect that the ravens do this to practice being pilfered from and it’s possible squirrels do something similar with the stones they take. It must be recognised, however, that, again, there’s no evidence to support this in the current observations; very few, if any, observers have seen what the squirrel does with the stone and no caches have been methodically studied.

I would be very interested to hear from readers who have observed this behaviour or have theories as to its purpose.

Dust bathing

Animals, particularly birds, rolling around in dirt or sand is not an uncommon sight. This is a phenomenon we refer to as a maintenance behaviour, because we think it helps rid the bather of parasites, while also probably helping soak up excess oils in fur and feathers to keep them and the skin in good condition. For mammals, this is largely speculative, but in the case of some birds, chickens in particular, we know that they develop skin conditions if denied access to a dust bath, suggesting it is essential for their wellbeing.

A Grey squirrel dust bathing in the fragments inside a decaying tree trunk in the UK. - Credit: Zack Porter

Interestingly, despite this being a behaviour often recorded in ground squirrels, dust bathing appears less commonly witnessed among tree squirrels and I can find no reference to it in the literature. I have, nonetheless, come across a handful of reports online and from readers describing Greys rubbing themselves on the ground, in wood fragments of decomposing logs and even in flower pots, engaging in what appears to be dust bathing. (I would be interested to hear from anyone who has observed similar behaviour in Red squirrels.) These accounts all have a couple of elements in common: they always occur in dry dirt/wood – I’m not aware of any examples of wallowing behaviour in squirrels – and are typically punctuated by bouts of grooming and scratching.

It is also worth noting that some of the same body and chin rubbing that is associated with scent marking is often observed during dust bathing; this may be coincidental, or it may imply that dust baths have a role in territorial maintenance as well as pelage maintenance. Certainly, this seems to be the case in some rodents and in his Ph.D. on Belding's ground squirrel (Urocitellus beldingi) ecology, issued in 1972, University of Arizona biologist Larry Turner described how these rodents dust bathed as part of their grooming regime as well as during aggressive encounters with intruders. Turner noted that squirrels often rubbed their upper backs while bathing, apparently depositing scent from the dorsal skin glands that lie in the shoulder region:

Secretions from this gland leave an odor in dust bathing areas, and at the same time, the dust absorbs excessive secretions which might mat the fur. Thus dust baths may serve the dual function of scent marking an area and personal cleanliness.

A Grey squirrel dust bathing in soil at the base of a tree in the US. - Credit: Dancing Llama

As far as I know, no studies have thus far been conducted on bathing sites of grey squirrels.