Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.
Volume 7
Issue 2
Wereszczyńska A. , Nowakowski W. 2004. WHAT FOOD ALLOWS BANK VOLES TO STAY FIT IN SPRING?, EJPAU 7(2), #01.
Available Online: http://www.ejpau.media.pl/volume7/issue2/biology/art-01.html


Anna M. Wereszczyńska, Wojciech K. Nowakowski



Rapid growth of rodent densities in forests of the Central Europe is associated with mast seasons of deciduous trees, mainly oak and beech. This increase is explained by more abundant food base, mainly acorns, also in spring and summer.

Key words: bank vole, spring, natural food quality, running wheel activity, detoxification enzymes..


According to many authors multi-year cycles of numbers of small forest rodents in the Central Europe are related to mast years of deciduous trees, in particular – of oaks [13,16,29]. High crop of seeds is often considered a natural experiment with supply of additional food to an ecosystem [15]. In the Central Europe oaks produce seeds every 6-9 years and acorns are considered high-quality food [29]. In the year following the mast one usually rapid growth of forest rodent numbers is observed, lasting till early autumn. This increase is explained by more abundant food base, mainly due to acorns, also in spring and summer [29]. However, sometimes the rapid increase of forest rodent populations does not correlate with mast years of oaks or other deciduous trees [13]. Thus, it may be assumed that acorns are not necessary for the cyclic (multi-year) spring and summer increase of the number of rodents.

Besides reproduction, and thus – number dynamics and animals’ fitness, the level of liver detoxification enzymes can be a good indicator of the food quality. The presence of the detoxification enzymes is an evidence for the presence of xenobiotics in food, such as alkaloids, terpenes, phenols and coumarine. They may disturb food digestion and absorption processes [21,28].

Thus, what is the role of acorns in the diet in spring, at the moment when herbs preferred by the rodents are already abundant in the forest ground layer [6,32] and seeds are practically absent [9]?

The aim of the present study was to compare the response of the bank vole to two types of their natural food in spring, at the beginning of reproduction season.


For the experiment we have chosen the bank vole, a generalist herbivory species. Two months old animals, which have not been breeding yet, originated from captivity where they were provided with standard food (standard mouse pellets, carrot, apple, sunflower seeds, oats). To adapt them to a new diet, they were habituated to the natural food for 10 days before the experiment, i.e. apart from mouse pellets, they were provided with acorns and herbs found at that time in the forest.

During the experiment the bank voles were kept single in glass terraria of dimensions 20x35x40 cm. The animals were provided with ball-valve drinkers (water ad libitum) and hides with a lining made of paper towel stripes. In terraria running wheels for rodents of 12-cm diameter and equipped with electronic counters of rotations were fixed. The experiment was carried out under laboratory conditions (natural fotoperiod, temperature 18-20 °C) in two replicates: the first in March 2002 (21-30.02 – 9 days) and the second in April 2002 (9-19.04 –10 days).

During the experiment the bank voles were provided with natural food collected the same day from 100 ha of a 80-90 years old forest dominated by Scotch pine (Pinus silvestris) with admixture of oak (Quercus robur). Three types of diet were distinguished:

  1. H-diet – plants most abundant in the forest ground level during the experiment and preferred by wild bank voles [5,6]: Anemone nemorosa, Oxalis acetosella, Viola sp., Fragaria vesca, Geum urbanum, Ajuga reptans, Luzula pilosa, Vaccinium myrtillus. Every second day 20 g of the fresh mass of each herb species was provided to the animals (140g in total every second day).
  2. A-diet – only acorns, randomly collected under various trees (20 g of acorns every second day).
  3. F-diet – (full diet) all herbs and acorns ad libitum, mouse pellets, oats, sunflower seeds, apple and carrot (ca 100g every second day).

The bank voles were divided into two groups, 6 adult individuals in each (3 females and 3 males), kept under the same conditions. Group I in March was fed exclusively H-diet and group II – only A-diet. In April, the diets were shifted and the animals from group I were given A-diet and those from group II –H-diet. Between the two turns, we assigned the compensation period to balance differences in fitness and body mass of the animals, and to observe their reaction to a high-quality diet (full diet). In this period all animals were provided with F-diet.

Fresh food was given to the animals every second day. All left food remains were collected and dried at 50°C (remains of H-diet for 2 days, of A-diet for 4 days) and then identified as to the species and weighted with 0.01 g accuracy. The amount of the dry mass of each type of food consumed by the voles was calculated by comparing the dry mass in the remains with the dry mass of weighted samples of each herb species and acorns collected at the same time.

The animals were weighted every 5 days; records from the counter in running wheels, including the distance run, duration of activity and the mean speed, were recorded daily. The activity of detoxification enzymes was measured only once, at the end of the second turn, i.e. on April 19. Levels of both enzymes, of cytochrome P-450 [25,26] and glutathione transferases [19,20] were measured in the microsome fraction of livers of animals fed in the second turn with herbs (4 individuals) and with acorns (4 individuals). The control (4 individuals) were animals from the captive group starved for 48 hours. The animals were subjected to euthanasia by a dislocation of neck vertebrae (according to prescriptions concerning laboratory animals’ euthanasia in Laboratory Animals 1997, 31: 1-32)


The level of consumption

During the experiment consumption of the dry mass of both types of food increased (Fig. 1.). In March, the voles fed on the H-diet consumed on an average 5.89g/24 hours (SD=4.90, N=24) of the dry mass of herbs; in April the consumption increased to 8g/24 hours (SD=3.10, N=30); t-test– t=4.6, p<0.001. In animals kept on the A-diet the consumption of the dry mass also increased from 4.55g/24hours (SD=1.67, N=24) in March to 5.81g/24 hours in April (SD=0.48, N=30), t=2.721, p<0.05).

Fig. 1. Changes in average food consumption of six individuals during period of varied feeding

However, the intake level of various herb species differed. Consumption of the bugle Ajuga reptans increased (from 2.17 g /24 h in March to 3.34g in April, t=4.73, p<0.001), as well as of the wood avens Geum urbanum (from 1.25g/24 h in March to 1.83g in April, t= 2.55, p<0.05) and the wood anemone Anemone nemorosa (from 2.42g/24 h in March to 2.7g in April, t=2.478, p<0.05). On the contrary, consumption of the wild strawberry Fragaria vesca decreased (1.11g/24 h in March and 0.4g in April, t=3.384, p<0.001), as well as of rhizomes of the wood anemone Anemone nemorosa (from 0.37g/24 h in March to 0.21g in April, t=3.308, p<0.002) and the hairy wood rush Luzula pilosa (from 1.1g/ in March to 0.41g in April, t=3.363, p<0.002).

The share of the bilberry Vaccinium myrtillus in the diet in April also decreased in comparison with March, but in April all animals were eating small amounts while in March one time they took a little, another time – a lot (difference in variance F= 7.839, p<0.001). We did not observe any differences between months in consumption of the dry mass of the wood sorrel Oxalis acetosella.

During the experiment, percentage of each species in the diet changed. Among the offered herbs, Oxalis acetosella, Fragaria vesca, Vaccinium myrtillus, Anemone nemorosa showed the same shares in the diet in both turn of the experiment (changes – n.s.) (Fig. 2). ). The percentage of Ajuga reptans increased from 19.3% in March to 26.5% in April (t=-4.406, p<0.001), of Geum urbanum from 9.3% in March to 14.4% in April (t=-3.5 p<0.001). The shares of Anemone nemorosa rhizomes decreased from 3.8% in March to 1.6% in April (t=3.5, p<0.001), and Luzula pilosa from 7.7% in March to 3.2% in April (t= 3.79, p<0.001) (Fig. 2).

Fig. 2. Percentage of each herb species in the H-diet consumed by bank voles in March and April. Anemone nemorosa (A.n.), Anemone nemorosa rhizomes (A.rh.), Oxalis acetosella (O.a.), Ajuga reptans (A.r.), Geum urbanum (G.u.), Luzula pilosa (L.p.), Fragaria vesca (F.v.), Vaccinium myrtillus (V.m.)

Preferences of the bank voles for various herb species also changed (Fig. 3). During the experiment bank voles were consuming gradually less Vaccinium myrtillus – in March 14.7% and in April 9.1% (t= 1.98, p=0.05), Fragaria vesca - decrease from 23.0% of the dry mass in March to 14.6% in April (t=2.06, p<0.05), Luzula pilosa – decrease from 28.6% of the dry mass in March to 9.0% in April (t=4.26, p<0.001), and Anemone nemorosa rhizomes - decrease from 28.6 % of the dry mass in March to 17.3% in April (t=2.98, p<0.05). Changes in amount of the consumed dry mass of the remaining species: Anemone nemorosa, Oxalis acetosella, Ajuga reptans, and Geum urbanum are not significant (Fig.3).

Fig. 3. Preferences of bank voles to various species of herbs during different months. Anemone nemorosa (A.n.), Anemone nemorosa rhizomes (A.rh.), Oxalis acetosella (O.a.), Ajuga reptans (A.r.), Geum urbanum (G.u.), Luzula pilosa (L.p.), Fragaria vesca (F.v.), Vaccinium myrtillus (V.m.)

The body mass of bank voles

We did not note any significant changes in the body mass of bank voles kept on the H-diet as well as on the full diet. However, in both turns of the experiment animals fed with acorns were loosing weight (Tab. 1). Differences in changes of the body mass between both groups of animals were significant – in March the body mass of bank voles kept on A-diet decreased for 3.46g on an average, and on H-diet increased for 0.45g (t=4.25, p<0.001, df=10). Similarly, in April the body mass of bank voles on A-diet decreased for 3.33g and on H-diet increased 2.02g (difference in body mass change was significant at p<0.001, t=4.92). The bank voles body mass kept in April on H-diet increased from 19.33 g to 21.35 g (Tab. 1) but this change was statistically non-significant, however the sign test proved the direction of changes (the increase of this parameter) significant (Tab. 2).

Table 1. Changes in body mass of bank voles during the experiment. Bold - statistically significant (A-diet – acorns, H-diet – herbs, SD – standard deviation, N – number of animals)

First day of the turn
mean ± SD (N)

Last day of the turn
mean ± SD (N)

Test t
t, p.

1st turn (21-30 March)            H-diet


17.38 ±1.41 (6)

21.08 ±1.82 (6)

17.83 ± 0.82(6)

17.63 ± 2.56 (6)

0.7; ns

2.7; <0.05

Compensation period (F-diet)

After H-diet

After A-diet


17.83 ± 0.82(6)

17.63 ± 2.56 (6)


18.00 ±1.05 (6)

19.33 ±1.72 (6)


0.67; ns

1.78; ns


2nd turn (09-19 April)             A-diet



18,00 ±1.05 (6)

19.33 ±1.72 (6)


14.67 ±0,75 (6)

21.35 ±2.17 (6)


6.3; <0.001

1.8; ns

Table 2. Significance levels of directions of body mass changes of bank voles. Arrows show the direction of change. Bold – statistically significant. (A-diet – acorns, H-diet – herbs, F-diet – full diet, Z – value of the sign test, N – number of animals)

Group I
Z; p (N=6)

Group II
Z; p (N=6)

1st turn (21-30 March)


1.15; ns

(A-diet) ¯

2.04; <0.05

Compensation period


1.15; ns


1.79; ns

2nd turn (09-19 April)

(A-diet) ¯

2.04; <0.05


2.04; <0.05


Bank voles on A-diet were more mobile in both turns of the experiment than individuals kept on F-diet (Tab. 3). Moreover, the mobility of the bank voles on A-diet in April was distinctly higher than of those on H-diet. The distance covered within 24 hours by bank voles on A-diet increased from 4.7km in March to 10.1km in April (t=3.47, p<0.001, df=113) and the time of activity respectively from 3928s in March to 7088s in April (t=3.48, p<0.001, df=109). However, no significant differences in average speeds occurred between both groups. The mobility of the bank voles on the full diet was lower than that of animals kept on A-diet and H-diet in March and on A-diet in April and was at a comparable level to mobility of bank voles on H-diet (Tab.3).

Table 3. Mean daily distance covered by bank voles on different diets. Bold - statistically significant (A-diet – acorns, H-diet – herbs, F-diet – full diet, N – number of animals, arrows – compared values)

Group I
0 ± SD (N)
(km/24 hours)

Test t
t; p

Group II
0 ± SD (N)
(km/24 hours)

1st turn (21-30 March)


3.08 ± 4.6 (54)


1.9; ns


4.7 ± 4.9 (54)

Test t – t; p

1.98; < 0.05


3.7; < 0.001

Compensation period


1.7 ± 2.2 (60)


0.4; ns


1.9 ±2.6 (60)

Test t – t; p

5.85; < 0.001


0.91; ns

2nd turn (09-19 April)


10.1 ±1.08 (60)


5.4; <0.001


2.4 ±2.7 (60)

Activity of detoxification enzymes

Animals from both experimental groups in the second turn of the experiment presented increased levels of liver detoxification enzymes. Higher values of both enzymes (the level of cytochrome P-450 and the activity of glutathione transferases) were noted in the bank voles fed with H-diet (Tab. 4). In this group the level of cytochrome P-450 was 2.98-fold higher than in the control and the activity of glutathione transferases – 1.7-fold (Tab.4). In the bank voles kept on A-diet this increase was 1.2-fold for cytochrome P-450 and 1.4-fold for the glutathione transferases, respectively.

Table 4. The average level of detoxication enzymes in bank vole’s livers (H-diet – herbs, A-diet – acorns)


A-diet N=4

H-diet N=4

Cytochrome P-450,
(nM cyt P-450/mg of protein)




Glutathione transferases
(mM CDNB/mg of protein/ 1 min)





In our experiment the body mass, fitness and the level of the liver detoxification enzymes in the bank vole were related to the type of diet. Preferences of the bank voles for each herb species changed as well. The experiment design, i.e. the shift of diets between the groups and the observed change in animals’ behaviour eliminates the assumption that these differences could have been the effect of their individual habits. Both types of the natural diet caused an increase of the level of the liver detoxification enzymes (Tab. 4). Despite the fact that H-diet induced detoxification processes at a higher level than acorns, this has not visibly affected the condition of the bank voles.

Both in March and in April, the mobility of the bank voles fed with acorns were significantly higher than in the period when they were fed with the full diet or with herbs but only in April (Tab. 3). However, high mobility of the bank voles was not a result of their good condition. The bank voles, in spite of eating more and more acorns, were loosing weight (Tab. 1). The body mass decrease of the bank voles was not an effect of food amount limits as they were provided with a surplus of food – 12.5g of acorns per 24 hours. In captivity bank voles consumed 2-4g of seeds per 24 hours [5,8]. Thus, it may be assumed that the diet composed of acorns collected in spring in the forest is not suitable for bank voles and causes their discomfort to which they react with the increase of mobility. The animals kept on A-diet behaved completely opposite in autumn, 5 months earlier [31]. The higher mobility in runnig wheel can occur as a defensive reaction to dif ferent stress factors [24], including the one observed in food deprivation experiments [4,22] and such symptoms have been described as anorexia nervosa. The mobility of the bank voles clearly decreased after the shift of the diet to the full one (Tab. 3), thus – the cessation of the stressing factor. In addition, the decrease of the body mass was also stopped (Tab. 1) as well as activity.

Acorns are regarded to be a high-quality food [5] in comparison with herbs, mainly due to their high calorific value and low content of fibre. Acorns are considered as the main factor promoting the rapid growth of the forest rodent numbers in spring and autumn after a mast year [12,29,23]. Our results indicate that this assumption rises some doubts. First, the bank voles form both groups were loosing weight in spite of eating gradually more acorns. Thus, acorns do not improve fitness of the bank voles in the studied period of a year. Second, in winter acorns disappear from the forest bottom and in spring all seeds produce seedlings. Thus, they do not increase the food base. As it was described by Gurnell [11], in his studies conducted in an oak forest in the southern Scotland, in 9 out of 13 years no acorns survived till the following spring and only in 3 years acorns kept productive as long as till the next summer. From year to year differences in the term of occurrence of seedlings can ta ke place, depending upon weather conditions, but in May all healthy seeds produce seedlings [17], even those stored in holes by rodents [18]. In germinating seeds, nutritive materials accumulated in cotyledons are decomposed and supplied to the growing young plant [10]. It is doubtful that the cotyledons of young oak seedling in spring have the same nutritive value as freshly dropped or wintering acorns. It is supported by the results of an identical experiment that we conducted 5 months earlier [31]. In autumn the bank voles fed with acorns showed better condition, their body mass was distinctly increasing and the level of mobility was lower than of herb-fed ones.

The bank voles fed with herbs were also more mobile than in the period when all experimental animals were kept on the full-diet but only in March, and the mobility did not differ from these fed on acorns in the same period (Tab.3). In April we did not observe any significant differences in mobility between animals fed with herbs and kept on the full diet (Tab. 3). Moreover, their body mass increased (Tab. 2), although this change was small and non-significant (Tab. 1). Such reaction of animals is an evidence for the high nutritional value of herbs in April, in comparison with March. The fact that the body mass of bank voles trapped in field was the highest in April [12] supports the assumption of higher quality of herbs in this month. In the diet of bank voles in this period, herbs of the forest ground layer distinctly prevail (up to 65%, [6,14,32]). Under natural conditions bank voles start breeding in April [2,27], when the abundance of the available food rapidly grows. This is also associated with the development of the forest ground layer [6,8,9]. All herbs selected for the experiment and preferred by bank voles [7] were perennial and, except for Luzula pilosa, dicotyledonous plant. As it was described by Batzli [3], dicotyledonous plants are more easily digested and more calorific than monocotyledonous ones. In March herbs started growing and had relatively small number of young organs, while in April the majority of plant species supplied to the animals during the experiment was flowering or producing flower buds, readily consumed by rodents [6,14]. Tast and Kalela [30] emphasise the high rank of generative plant organs in the diet of the bank vole in the first half of the reproductive season, thus – in spring.

During the experiment the bank voles were changing preferences for subsequent plant species. (Fig.3). So that, for example in April shares of Luzula pilosa and rhizomes of Anemone nemorosa in the diet decreased very clearly. In turn, the percentage amount of Ajuga reptans and Geum urbanum, which started formation of flower buds in this time, increased. Bank voles probably resign of less nutritive plants (Luzula pilosa has ceased flowering in April and storage materials of Anemone nemorosa rhizomes have been partly used for production of shoots).

Thus, bank voles react to changes of the quality of herbs of the forest ground layer and the composition of their diet shifts. Under natural conditions the diet of bank voles is very diverse. Gębczyńska [7] listed as much as almost 60 species of consumed herbs. So that even the increased food base (due to acorns) after winter population of bank voles does not have any problems with finding high-quality food and continuation of reproduction till autumn. During mild winters in years of a low crop of seeds, when herbs were not much destroyed by frost, bank voles initiated breeding a month earlier than in other years [15]. In winter acorns are considered an “extra food” [29] that allows for higher winter survival and reproduction. A similar effect was obtained by Andrzejewski [1] by supplemental feeding of bank voles with oat.

We have proved that acorn in spring have small nutritional value for rodents, while herbs (even in so low variety) are a full-value food, thus we conclude that the abundance of acorns only enables higher winter survival of bank voles and the increase of the pool of individuals that undertake breeding in spring. However, acorn availability is not crucial for keeping animals in good conditions in the period of the full development of herbs of the forest ground layer. Moreover, it seams that the rapid spring and summer increase of bank vole numbers after a mast year is based on herbs, similarly as in years with no abundance of seeds.


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Anna M. Wereszczyńska
Department of Genetics and Plant Physiology
University of Podlasie
B. Prusa 12, 08-110 Siedlce, Poland
e-mail: wereszka@ap.siedlce.pl

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