Voles not living in holes

OK, so that was more irregular than I would have liked (at the start of the last post, the ‘Hello World’ welcome to the blog one, I stated that Tetrapod Teeth & Tales would be updated on a semi-regular basis, but maybe I should aim for semi-irregular and see how it goes). Life, work, and a great conference on ‘Tooth Morphology and Differentiation’ in France followed by an excellent lab visit to Professor Marcelo Sanchez at the Paläontologisches Institut, Zurich University (go there for awesome images of developing living and fossil vertebrates) got in the way. But here’s the post I was drafting before all that…

(Oh and the weather in Finland has been almost the warmest anywhere in Europe recently, over 30 degrees C inside the Arctic Circle in Lapland, so the snow pics below are even more incongruous. We pretty much transitioned straight from winter to summer, with the happenings below, er, happening in the couple of springlike weeks. But other than the timing, the rest of the post still stands!)


A couple of weeks ago in Helsinki, right around the time I was writing the first post of this blog (on how ridiculously insanely huge vole incisors are), the snow in the garden was melting pretty quickly. This winter saw another huge snowfall in southern Finland, much above long term averages, so to get from the road to our front door we had to shovel snow. A lot of snow. Right next to the path the shovelled snow went on top of the already fallen snow, which was then covered in fresh snow, and more shovelled snow, and so on and on for about 5 months (it was still snowing at easter), making for some pretty tall snow piles. But come spring time it tends to melt fairly quickly, and so after a couple of weeks of melting all that was left in the front garden was the remains of a few of the piles of shovelled snow, close to the path.

Strangely, one of the piles of snow had a circle in the middle where it had melted through to the lawn first. I walked past that a couple of times a day for a few days before stopping to investigate. And this was what could be seen:


Some moss and grass, oak leaves, and the remains of acorns. The circular shape of the mossy structure had clearly been made, but by what? My mum was staying with us at the time, and while I was at work she did some quick research and reckoned it could be either voles or lemmings. We looked up the distribution of lemmings – basically in Finland the Arctic circle very roughly marks the southern limits of their southern distribution, and although Helsinki is a fair way north, that’s still 700km or so north of here. So lemmings were out, but a little more research suggested based on the acorn remains that a few of the Finnish rodents could have been the culprits in addition to voles.

As the snow continued to melt, more details of the nest became clear. With the surrounding snow gone, a number of slightly dug out trackways leading in various directions from the nest emerged:


Nest with trackways and my size 11 shoe for scale. Nest is just up and left of my foot…

A few of these led to holes in the ground, and a few to small piles of either whole acorns or the shelled remains of acorns. Additional piles of acorns or acorn shells were scattered about the rest of the garden, though the nest makers were very neat as the remaining meals and acorn rubbish were pretty much never both found in the same cache.

This neatness was continued in a second surprising find. As the snow melted and I wanted to be able to ride my bike in to work, I did a little tidying in the garage. Going through a pile of bike, ski and hiking shoes in a corner, one of them rattled. I upended it and about 12 intact acorns came spilling out. Cool I thought, the nest maker has been coming into the garage and using my shoes as a fallback stash of acorns. But even more surprising was when I started tipping out all my shoes to see if any others had acorns in them – none did, but a shoe from a different pair had a neat pile of acorn shells in! The neatness of separating eaten meals from eaten shells extended to my boots! (The acorns were in a right boot and the shells in a left, whether that has any biological significance I have no idea…).


As you can see, the nest is a reasonable height – taller than my shoe – and about as long as my shoe across. It looks pretty comfortable, with a nice mossy structure and dry oak leaves woven into roughly the centre. We’d still not been able to determine what exactly has built it, though the tracks or runs from the nest suggested more strongly a vole of some description. Luckily, I work with people whose research is on rodents, so I knew exactly the folks to ask.

Professor Heikki Henttonen of the Finnish Forest Research Institute is a world expert on small mammals, especially rodents, and in particular those living in sub-arctic to arctic regions. I’ve previously loaned some samples of shrews from him for a research project on their teeth, so he seemed like the person to ask. After a couple of emails, and a couple of photos showing the pellets (poop) found in and around the nest, he had an answer for me: The runways around the nest suggested the vole Microtus agrestis, the field or short-tailed vole, and the pellets were consistent with that.


Microtus agrestis. The tall face hides the ridiculously insanely huge incisors… Image by Bruce McAdam

It looks like all this winter we’ve had a number of these delightful little guys running around under the snow, making nests, storing acorns and recycling the shells in neat piles, and occasionally visiting the garage to do the same in my shoes. The tunnels and runways they make under the snow are partly excavated from the ground (my lawn!) and partly tunnelled in the bottom layer of snow, forming a complete tunnel from the two halves. Once the snow has melted, they abandon the nests and trackways and retreat underground, so they are possibly still in the garden but in the tunnels leading underground from the holes on the edge of the lawn.

To give you an idea of the type of teeth that the field vole has, here’s the same CT scan of the vole lower jaw we looked at in the first post, but from a different angle so that we can see the top of the molar teeth. This surface, where the food is chewed up, is known as the occlusal surface, since it is this part of the teeth that meet or occlude with the corresponding upper molars in the skull:


(The CT scan incidentally is of a different vole genus and species to the likely maker of the nest, in this case Myodes glareolus. As with the last vole image, credit to Elodie Renvoise and Aki Kallonen for the specimen, expertise, and CT scanning).

As well as having ridiculously insanely huge incisors, vole molars are actually relatively large compared to those of other rodents, especially the first molar of the three, found towards the front of the jaw, the m1. (That’s on the left of the jaw above, after the gap – diastema – following that ridiculously insanely huge incisor).

The evolutionary history of vole teeth has seen the front part of this first molar increase in size, adding cusps (or triangles of enamel and cementum) to enlarge the tooth. It’s been said that this has allowed them to more efficiently chew and process food, in this case all sorts of plant matter, and so take over from other rodents (mice etc.) right across temperate northern hemisphere regions (Renvoise et al. 2009).


So – voles, not living in holes, in my front garden. This post fits into the ‘tales’ category of the blogs title, being only indirectly connected with teeth but, to my mind, pretty cool nonetheless!


Renvoisé, E., Evans, A. R., Jebrane, A., Labruère, C., Laffont, R., & Montuire, S. (2009). Evolution of mammal tooth patterns: new insights from a developmental prediction model. Evolution63(5), 1327-1340.



  1. Allen Hazen

    Welcome back!
    … Shoveling snow, again, and again, and again…was part of life in Edmonton (Alberta, Canada) this past winter as well as in Helsinki!
    I’m sure we have Microtus voles here too, but I haven’t seen any. The most noticeable local (I live in the city of Edmonton, but in an area that is mostly single-family houses with small yards around them) acorn (etc) stashing rodent here is the red squirrel (Tamiasciurus hudsonicus), an animal with much less extremely derived molars. (And one which readily climbs– and builds nest in– trees: maybe this allows it to co-exist with voles by niche partitioning?)

  2. Ian Corfe

    Thanks Allen!

    There are 62 species of voles in the genus Microtus, split into 8 or 9 sub-genera, and they range across Eurasia and N. America, so I imagine at least some of them are found in Edmonton! I’ve yet to see any near Helsinki, but did see a lot of voles of unidentified species scurrying around my cabin and on the race course of a mountain bike race up inside the Arctic Circle a couple of years ago…

    I had to look Tamiasciurus hudsonicus up – the Eurasian red squirrel is Sciurus vulgaris. Sadly it isn’t doing as well as Tamiasciurus hudsonicus which appears to be expanding in range – the Eurasian red squirrel is being displaced by the non native, North American grey squirrel (Sciurus carolinensis) in the UK. There are still plenty across most of Eurasia though – I saw one in the garden yesterday. We have a few ‘regulars’ with fairly predictable routines including running along the length of the house in the bushes, jumping on the top of a fence and running along that, jumping off an oak brank onto the roof with a loud bang etc! There has also been what we think are a pair of brothers who chase each other through the snow and around tree trunks and up and down trees in the garden. They do this frequently, often many days in a row, and without properly fighting or mating, so I don’t think they are rivals or mates…

    Those lot do gather and bury some of the acorns, though not as many as the voles did this winter. It was a bumper crop for all sorts of trees in Finland last summer, everyone had way too many plums and apples, and the acorns were certainly not in short supply. I think so many were stashed for later that there has been a huge number of oak saplings growing round the house, often in unusual places, well away from the two oaks…

    We also have other neighbourhood squirrels hopping across the road to collect acorns and take them back to their patches! There’s one living in the roof of a neighbours house that we can see go in and out through the living room window, I haven’t yet mentioned it as I’m not sure they would let it stay…

    And finally, the teeth – I have to confess, I’m not even certain what the molars of the (Eurasian!) red squirrel are like, but as you say they are certainly less derived than those of the vole. I’m not even sure which species Mike Taylor had in his jar that he photographed and blogged to provide the impetus for this blog, I’ll have to ask!

  3. whateversongularities

    I just joined a tracking club here in Guelph (Ontario, Canada, Traditional Attawandaronk-Neutral territory) and we found a bunch of those vole trackways and explored them. Lots of snail shells to be found, and in seeing the size of those incisors helps me understand how they are crunching their way through those shells.

    Thank you for all the details and explainations! I have learned so much from this post.

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