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Aspects of the biology of bank voles.

Discovery.

The European bank vole (Clethrionomys glareolus) was described for the first time in 1780 by the German naturalist Johann Christian Daniel von Schreber (1739-1810) which, of all places, discovered the type speciment on the Danish island Lolland. As I understand from this website, he originally named the specie Mus glareolus.

Picture borrowed from Wikipedia - presumably with their blessing.

Common names.

Clethrionomys glareolus (Latin); Rødmus (Danish), Långsvansad Skogssork or Ängsork (Swedish), Klatremus (Norwegian), Metsämyyrä (Finnish), Bank vole (English), Rötelmaus or Waldwühlmaus (German), Nornica ruda (Polish) and Campagnol roussâtre (French).

NB: Some researchers believe the correct name for the genus Clethrionomys ought to be Myodes (See"Chronologi")

Placement in the phylogeny.

Both more recent references and their predecessors place the bank voles in the Regium: Animalia - Phylum: Chordata - Classis: Mammalia - Ordo: Rodentia.

The older references continued from there dividing Rodentia into the sub-order Myomorpha (the "mouse-like" rodents)- super-family: Muroidea (the "mouse-group") - family: Cricetidae (the "Hamster-family") - sub-family: Microtinae (the "voles"? - unsure if that's the popular name in English) and finally the tribus Microtini. In this tribus they placed the bank voles next to a group consisting of the two species of field voles and the European water vole. A sister-tribus (Lemmini) to Microtini contained the lemmings.

The more recent references invented new names and divided Rodentia further up into the sub-order: Sciurognathi (the "Squirrel-like order") - family: Muridae (the "mouse-family") - sub-family: Arvicolinae (bank voles; lemmings; field voles).

The difference between the more recent references and their predecessors is that the voles originally were placed as a sub-family to the hamster-family (which again was a sister-family to the mouse family Muridae).

The more recent references has "degraded" the hamster-family to now become a sub-family to the Mouse-family and in addition squeezed the two sister tribus (Lemmini and Microtini) together as one which now is part of the sub-family Arvicolinae, together with the muskrat (Ondatra zibethica).

Other recent references do alternatively divide Rodentia into four families: Gliridae or Myoxidae (Dormice), Zapodidae (Jumping mice), Microtidae (voles and muskrats) and finally Muridae ("Genuine mice": various members of the genus' Apodemus, Micromys, Mus and Rattus) and to make the confusion total, yet other recent references place the voles (Microtidae) together with the "genuine mice" in the same family (Muridae) or operate with alternative sub-families to the sub-order Myomorpha. Here you can see the what Wikipedia believes to be the correct phylogeny for the moment, and this private homepage mentions some names of researchers working with vole phylogeny but in short, I will suggest special interested in this subject to use real book from the nearby University library. If not doing anything else, then this little sketch demonstrate that phylogeny is basically a matter of arbitrary borders and more or less qualified judgement and to give this issue its finishing touch, even the biological notion of how to define a specie is no longer what is used to be.

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Sister- and sub-species :

According to Viro & Niethammer (1982) there is eight (maybe nine) sister-species to Clethrionomys glareolus and within this genus there is 29 sub-species (see table 1). The free online encyclopedia Wikipedia do currently operate with 7 species in total and the reason to this discrepancy is (as mentioned above) that species, as last and most basic unit in the taxonomy, is more or less arbitrary in nature, just as all the other categories of units (genus, families, orders, classes, phyla - even Regium)

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Tabel 1:

Note: Wikipedia has recently started to use the name Myodes istedet for Clethrionomys. See here.

Note X: According to Claassens & O'Gorman (Letter to Nature, 1965) a Clethrionomys glareolus Schreber was actually captured near Listowel, County Kerry in Ireland in 1964. I have yet to see this letter.

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Appearence.

The bank vole in Denmark (C. glareolus) has as an adult a head-body length between 8-12 cm and a tail, which typically is half as long. In my experience, the tail is typically between four and seven cm and usually two-coloured (light ventrally; dark brown/black dorsally). The fur is typically light grey/white ventrally and in young voles (age 1-3 months) brownish on the flanks and back. The adult colouring, a beautiful chestnut, can be seen from the age of two months, but usually comes grow out after three. Adult bank voles typically weigh between 20 and 25 gram in nature where pregnant females can top the scale at 46 grams. The heaviest male I have ever caught weighed 34 gram, but in captivity a few males reached around 40 grams. I have uploaded some pictures of Bank voles from Danish forests which can be seen here.

Confusion with field voles (Microtus)

A bank vole which is specially young (hence with a brownish fur), and short-tailed, can sometimes be confused with a field vole (Microtus agrestis), which is roughly of the same size, but normally have a tail length that is 1/3 of the length of its body. The tail is not especially two-toned in colour (more evenly dark-brown); the fur on the belly is more "filthy/grey" and its stature is more robust with shorter legs and smaller ears/eyes. If you have caught enough voles you will seldom make a mistake in the field, but if it's a "bad vole-year" and you want to be on the safe side, just take the specimen inside and let it feast on the lab-chow for a month - then if it does not grow into a more "normal" bank vole, you just have to translocate it back to where you caught it in the first place. I think I have been in doubt maybe in around 1% of the times, and each time it turned out to be a field vole I had caught.

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Reproduction in captivity

The earliest pregnancies I have observed must have been founded around five weeks of age (sibling mating) and voles at the age of eight weeks mated with no problems, just as some voles, aged 1.5 - 2 years was still fertile. I have detailed the suitable age of weaning here.

Anna Buchalczyk (1970) wrote a very thorough paper based on 782 litters from 264 females, resulting in 2862 pups. She mainly used Polish outbred C. glareolus and found that the average littersize were 3.6 pups (range 1-10); that females at the age of 6-14 months gave birth to the largest litters and that the fertility (as measured by the average littersize) gradually decreased after the age of 15 months. Furthermore she found that the maximal lifespan in captivity was a little over 4 years and that females on average lived for 17.5 months (males a bit longer: 23 months).

Gustafsson et al., (1980) investigated C. glareolus too (Swedish, this time) and from a basis of eight females and eleven males their colony gradually expanded to 125 breeding females (i.e., somewhat inbred) which in total delivered 827 litters with 2844 pups. In effect a study comparable to Buchalczyk's study 10 years before, except that she used outbred voles. Gustafsson et al. found that littersize was dependent on the number of previous births for the female with an average littersize of 4.3 for primiparous, increasing to 5.3 pups/litter for multiparous females. Where Buchalczyk found a mortality of 15% from birth up to the age of 15 days, Gustafsson et al. found a mortality between 14% (average for the third litter; n= 85) and 60% (average for the 19 th litter; n= 4). Gustafsson et al. had three females in total which gave birth 20 times and the strength in this work is the rich data which shows the connexion between variables like littersize, mortality and degree of domestication (no. generations in captivity). Furthermore Gustafsson et al. found that the average length of pregnancy was 18.3 days for primiparous and if the specific pregnancy was a result of a post-partum mating, then the suckling pups from the first litter increased the average length of following pregnancy, so it lasted 22 days.

My own unpublished results, based on a completely outbred colony numbering 1307 bank voles is by the way quite comparable to the results from Buchalczyk and Gustafsson et al.

Post-partum matings (successful mating immediately after the female had given birth, or during the first 0ne or two days) is common and Clarke & Hellwing (1983) described this phenomenon very well using English C. glareolus with a distribution of data, which are very similar to my own. They found that 50% of the lactating females mated 1-2 days after the birth of their litter where Morrison et al., (1976) earlier had found that 57% of (presumably Alaskan) C. rutilus were mated post-partum (exactly during which time-interval was not specified).

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Breeding season.

I have not been able to pinpoint any specific time of the year where matings are particularly easy or hard to initiate in captivity - apparently Danish bank voles are capable to breed throughout the year . In the wild, breeding is restrained to spring, summer and autumn with the longest breeding season in the South of its distribution area and the shortest in the North. To give a couple of examples, then the breeding season in France lies between February and October (Viro & Niethammer (1982) and in Denmark typically between April to September (Danmarks pattedyr, 1993). I have a feeling we are approaching Southern conditions here in Denmark since the last decade has included one heat record after another - it is as if the autumn last one month more nowadays than it used to do.

Last updated the 19th of December, 2007