Category: Rodents

All teeth all the time (for one issue only anyway)

A quick post hopefully as a prelude to a return to more regular offerings.

Last year a short symposium in honour of the Finnish palaeontologist Professor Mikael Fortelius‘ 60th birthday was held in Helsinki. As part of the celebrations, a large number of researchers were approached with the view to publishing a Festschrift volume of the Finnish journal Annales Zoologici Fennica. The expectation was a decent number would say yes and a volume could be produced. In the end, practically everyone asked produced a paper, so the volume now totals 25 papers (including the preface and a history of Finnish palaeontology) and a gargantuan 284 pages!

Because of Mikael’s great interest in teeth, alongside macroevolution and ecology (and a soft spot for rhinos), a large number of the papers feature research heavily focused on dentitions of various types. Indeed, the largest of the three subsections of the volume is titled ‘Teeth, Diet and Ecology’! I count 14 papers directly focused on some aspect of dental evolution, morphology, function and development; more than half of the research papers in the volume, with at least a few of the others more indirectly using tooth data. There should be something there for anyone with an interest in tetrapod teeth.

MikaelFest

Mikael’s Festschrift cover – bonus Tetrapod Teeth & Tales tokens for guessing the tooth depicted!

Anyway, the volume (Annales Zoologici Fennica 51: 1-2) is now out, and thanks to a generous donation from the Helsinki EvoDevo community, the entire thing is available as open access papers! That’s right, 25 top notch articles, at least 14 on teeth, available free for everyone. I was a coauthor on one of the papers, entitled ‘Gastrointestinal and dental morphology of herbivorous mammals: where does the Laotian rock rat fit?‘, and hopefully will post a little about that paper and some of the concepts involved in the near future. But please, head over to the journal now (clicking here takes you to the index, unfortunately there is no direct link to the volume which, as above, is Annales Zoologici Fennica 51: 1-2) and read as many free papers on teeth as you fancy!

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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!)

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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:

VoleHoleSnow

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:

VoleFoot

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…).

VoleHoleHeight

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.

MicrotusVoleWhiskers

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:

VoleOcclusal

(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).

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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!

Refs

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.

Vole incisors are ridiculously insanely huge

Mike Taylor of the always excellent Sauropod Vertebra Picture of the Week blog recently acquired the skeleton of a squirrel, and commented on the ridiculousness of the size of the single incisor in the lower jaw. In squirrels these run for about three quarters of the length of the entire jaw in addition to the crown of the incisor visible at the front end, and are heavily curved and so at some points lie just inside the bone margins of the jaw. Mike also commented that ‘the tooth literally could not be any bigger’. Here’s the blog post, and the killer image from it is reproduced below:

img_0800-squirrel-right-mandible-medial-tooth-juxtaposed

It just so happens that my colleagues at the University of Helsinki, Aki Kallonen of the Laboratory of Microtomography, Department of Physics, and Elodie Renvoise, my fellow postdoctoral researcher in the Jernvall EvoDevo lab, Institute of Biotechnology, have been X-Ray Computer Tomography (CT) scanning vole jaws in 3D. Mike Taylor was able to remove his squirrel incisor without damage to the jaw, but CT scanning allows a look inside the jaw with the teeth still in place. Aki and Elodie kindly allowed me to use their data to make a picture that shows the single incisor inside the vole jaw:

VoleJawLingualmirror

The image shows differing density values, in the same way a traditional X-ray image shows the denser bone as white with the less dense soft tissue, muscles, skin, fat etc, as transparent. In this case, it’s coloured from transparent = lowest density (in the vole jaw this is thin bone, as there was no soft tissue on the specimen scanned), through orange representing intermediate density, with yellow the highest density.

As can be seen, the incisor curves below the molar teeth and reaches all the way to the rear of the jaw, excluding the sticky out bits at the top and bottom (the condyle and angular process respectively). In comparison, in the squirrel it only reaches about three quarters of the way back. The last section of the incisor in the vole appears as a dark oval in the CT scan, indicating that although there is space at the back of the jaw for the end of the incisor, it isn’t mineralised and so shows as a region of very low density (and that the bone surrounding it here must also be thin). You can then see the developing tooth increasing in density as you go forwards, becoming orange where the tooth dentine has mineralised.

The lower part of the front half of the incisor ,which is the only portion covered in enamel, the hardest and densest tooth part, shows up as an orangey yellow grading into a bright yellow, indicating a very high density in the CT scan. Similarly, the outlines of the vertical ridges of the three molar teeth, also enamel covered, are a bright orange or yellow, and so high density. Their unusual shape is because, like a horse molar tooth, the vole molar teeth grow continuously for all, or nearly all, of the animals lifespan. The most rearward part of the incisor space is probably full of the stem cells that allow the incisor to continuously grow.

In addition to extending further rearwards than in the squirrel, I’d say that the vole incisor sticks out a little further than the squirrel incisor at the front of the jaw, and occupies more of the jaw in the diastema (the gap in front of the molar teeth) and below the molars. The incisor is actually longer than the entire lower jaw. As other parts of the vole jaw are also smaller than in the squirrel, I’d say that the vole incisor is larger than the squirrel incisor relative to the jaws containing them. This means that for the squirrel incisor, the tooth literally could not be any bigger the tooth could be considerably bigger! It also means that the vole jaw is reaching the absolute limits of how big the incisor can be. The entire jaw consists pretty much of just the incisor, with the molars and the three processes for muscle attachment (coronoid and angular) and articulation with the skull (condyle) basically hanging off the incisor’s edges. In fact, if you look closely, you can see that the molar tooth roots have to bend around the incisor to fit in the dentary bone…