Last updated: September 23, 2019
Topic: AnimalsBirds
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The gnawers rhythm is one of the of import topics in the surveies of little mammals. It is necessary to analyze the population copiousness each twelvemonth for understands it. However, it is indispensable to analyze besides different parametric quantities, as the influence of the season and the twelvemonth, the sex and age, and what is the ecology of these gnawers. We studied these parametric quantities by utilizing snap pin downing during four Sessionss in June and September 2011, 2012, and we concentrated on dominant coinage: The bank field mouse ( Microtus agrestis ) . We expected a lower proportion in 2012 but we found in September an of import proportion of female as in 2011, whereas there was a big lower of gnawer ‘s proportion in June 2012. These observations might be explained by several factors as the predator-prey interaction, the snow screen, the quality and measure of nutrient, female behaviour and diseases or does the rhythm of the gnawers becomes irregular?


Study country:

The survey country for the catch caparison is a portion of Oyer, a municipality in Oppland state ( Figure 1 ) , with a entire size of 640A km2 kmA? . This country is located in the south-east portion of Norway.

The traps were filed in this Oyer country and in 23 transects that were of different sort. Indeed, they were in different heights: 300 to 1100 metres, in different type of landscape: unfastened, forest or shrub. And different trees and bush beds: deciduous, retem, pine, spruce and besides Field bed: bare land or grass, berries, herbs, lichens or mousses.

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Figure 1 a, B: Oppland Position in Norway ( a ) and the traps place in Oyer ( B )

Snap caparison:

The catch caparison lasted for four Sessionss for three yearss in June, September 2011, and June, September 2012. We have as a first measure set out traps in the first twenty-four hours. These traps were made with carrots with peanut butter for enticed the gnawers and attached to the releaser ; except in June 2011 because we used a little piece of rope soaked in soya oil, so possibly this may be affected the consequences. We added a roof for protected the birds and we have fixed the traps in the dirt. These in each country defined antecedently and incorporating one transect of 10 traps separated of several metres. Each place of traps that filed was registrated in the GPS for uploaded co-ordinates.

The following twenty-four hours, we have checked each trap and recovered the at bay gnawers. At the same clip we noticed if the traps were released with the carrot losing or non, if we caught carnal or the trapped was moved, untouched or absent. The enticement was besides remitted in topographic point. Thereby the following and last twenty-four hours we able to look into once more the traps and wrote of the same manner the notes refering those. Each animate being caught was retained in a fictile bag with the day of the month, Numberss of the transect and traps, and at the twenty-four hours ‘s terminal in the deep-freeze.

As a 2nd measure, we did the designation of the at bay gnawers. We detailed the coinage, sex, age ( juvenile or grownup ) , the weight and tallness of the organic structure and the tail. For the grownup female we have specified if they had a litter and how many babes. As for the grownup male we specified the testicles length and the presence or non of the tubili, epididymis.

For the sex designation we were aid with the sexual features: phallus, anus, scrotal testicles, anus, pierced vagina, button, mammillas, and so we made a dissection for confirm or continue the analysis. And we identified the coinage with books mentions.


Eleven different species of little gnawers were trapped: Myodes glareolus, Microtus agrestis, Microtus peconomus, Myodes rufocanus, Arvicola terrestris, Apodemus sylvaticus, Apodemus flavixollis, Sorex spp. , Neomys fodiens, Myopus schisticolor, Lemmus Lemmus, Sicista betulina. The bank field mouse ( Microtus agrestis ) is the dominant coinage. We find it in general in grasslands and forest glades. It is an herbivore and it constructs a web of tracks among the grass bunch on the surface of the land. Its eating is composed normally green veggies, in bulk grass and a high proportion of dicotyledonous workss, forb, and others nutrient points like berries, seeds, lichens, Fungis and sometimes insects ( Hansson 1971 ; Viro & A ; Sulkava 1985 ; Ferns 1976 ; Myllymaki 1977 ) .The bulk of engendering season is in the early summer months and subsequently in the fall ( Myllymaki 1977 ) .

Datas analysis:

We defined the figure of animate beings that were caught in the four periods and for which species and the gimmick per unit attempt. Then, we studied the difference in figure of gimmicks for the different seasons and old ages with a logistic arrested development. We used the p-value for known if the variables are important or non. We concentrated the remainder of our survey on the bank field mouse, the difference of sex and age in the caught and still for the seasons and twelvemonth. We did a logistic arrested development with the sex and age as response. Finally, the habitat choice of the bank field mouse, we used besides a logistic arrested development, with as first measure a stepwise backwards choice with the different home grounds variables and we excluded variables with P & gt ; 0.05.


The first surveies refering the gnawer ‘s population rhythm started in Finland and it has been demonstrated after many statistical analyses informations ( Henttonen et al. 1995, Hanski et Al. 1999 ) . During the last century, there was a batch of sentiments refering this rhythm, but the being of a regular and cosmopolitan rhythm appears as an grounds particularly at high latitudes ( Krebs and Myers, 1974 )

In Northern latitudes, the gnawer ‘s population is characterizes by a rhythm periodic of 4-5 old ages intervals ( Hansson & A ; Hanttonen 1985 ) and fluctuates in higher and lower proportion. A batch of survey have been conducted for attempt to explicate this rhythm and many hypothesis were been suggested. That ‘s why it is necessary to reap informations each twelvemonth to go on the survey of this rhythm.

A late surveies revealed at first in Finland a alteration in the rhythm, more stable and irregular, in peculiar a prolongation of the period, and besides in others portion of Fenniscandia ( Lindstrom & A ; Hornfeldt 1994 ; Steen, Ims & A ; Sonerud 1996 ; Laaksonen, Korpimaki & A ; Hakkarainen 2002 ) . For illustration in cardinal and southern Fennoscandia the rhythms ‘ cyclicities change with 7 old ages periods whereas no survey of this sort of long rhythms exists for the minute ( Sundell et all. 2004 ) , but it was excessively late for concluding of a existent alteration in the rhythm.

The bank field mouse is the dominant species in our country, that ‘s why we interested in peculiar at this coinage, its copiousness and besides its ecology, in peculiar its favourite home ground. In 2012 with the regular rhythm of the gnawers, we expected a low period of copiousness population than in 2011.This support by one recent survey did antecedently of ruddy fox diet, which suggests in their scats a diminishing proportion of little gnawers. But our consequences did n’t demo that. This abnormality of the rhythm is the chief topic of our survey.


We caught 154 animate beings, in the proportion shown: 80 % were a bank field mouse, 8 % a termagant, 6 % a Grey red-backed field mouse, 2 % a Yellow-necked mouse and 1 % a Field field mouse, Wood lemming, Root field mouse, Lemming and Wood mouse ( Table 1 ) . And ( Table 2 )

Number of caught


per centum

Bank field mouse



Field field mouse



Grey red-backed field mouse






Root field mouse






Wood lemming



Wood mouse



Yellow-necked mouse



Entire general




June 2011

5,51 %

September 2011

6,67 %

June 2012

2,17 %

September 2012

7,97 %

Table 1: figure of animate beings caught and for which species

Table 2: the gimmick per unit attempt per species

We analyzed the difference in figure of gimmicks for the different seasons and old ages.

The season is a important factor for the figure of caught ( p & lt ; 0, 01, chiA? = 16,11 ) and besides the interaction between the season and the twelvemonth ( p = 0,001, chiA? = 10,2283 ) , but the twelvemonth was n’t a factor important ( p & gt ; 0,005, chiA?= 1,36 ) ( table 3 ) .We might see the consequence between these two factors with the Figure 2. There was n’t a big difference between June and September in 2011, but in 2012 there was a great difference between these two seasons. In fact, in June 2012 the caught was really lower than September 2012. Finally in September 2012 the caught was approximately in the same proportion of 2011, rather high, even higher than 2011.





& lt ; 0,001




& gt ; 0,05

Season: fYear




Table 3: logistic arrested development for the figure of caught response with the season and twelvemonth

Figure 2: The consequence between the season and the twelvemonth for the caught

The bank field mouse was the dominant species, so, we continue our survey with this species. We studied the proportion of female and male, juvenile and grownup ( Figure 3 ) . As sing the genteelness, in general in summer months our consequences showed those expected. In fact, in June for the two old ages, the grownups were more of import and in September it was the juveniles, because of the coevals rhythm. But in September 2012 there was a surprisingly high proportion of big female. We besides noticed in June 2011 a inclination of high proportion of females than male. Furthermore, as position antecedently we might see in June 2012 an of import lower of gimmicks proportion and a wholly absence of male juvenile.

Figure 3: The bank vole proportion of male grownup ( M- A ) and juvenile ( M- J ) , and female grownup ( F-A ) or juvenile ( F-J ) for the four Sessionss.

To foster the analysis of the difference in sex and age, we did a logistic arrested development. The twelvemonth and the season were n’t important for the sex ( twelvemonth: p=0,21, chiA? =1,53 and season: p=0,13, chiA? =2,27 ) , besides for the year/season interaction ( p=0,16, chiA?=1,98 ) ( Table 4 ) ; but when we observed the consequence secret plan we notice one difference in September between 2011 and 2012 with a positive incline ( Figure 4 ) .

Then, we studied the consequence of the twelvemonth on the sex merely for September. We found a correlativity non important ( p = 0,07, chiA? = 3,23 ) , but even so we might state there was a tend to a low proportion of female in 2011.





& gt ; 0,05



& gt ; 0,05



& gt ; 0,05

Table 4: logistic arrested development of the season and twelvemonth consequence on the sex

Figure 4: The interaction consequence secret plan of twelvemonth and season on the sex ( with sex01 = female )

As respects the Age, the season was important with a negatively incline ( p= 1,24e-10, chiA? =41,404 ) ( Table and Figure 5 ) , which confirms the coevals rhythm June/September noticed before with the figure 2. And twelvemonth and year*season were n’t important ( p & gt ; 0.05 ) ( Table 5 ) .





& gt ; 0,05



& lt ; 0,001


fYear: Season


& gt ; 0,05

Table 5: logistic arrested development of the twelvemonth consequence on the sex in September

Figure 5: consequence secret plan of the season on the age

Finally we studied the home ground penchant. We described for each home ground the figure of trap had been set and the per centum of those holding trapped a bank field mouse ( Figure 6 a, B, degree Celsius ) . We have did a first decision on where they most likely to be caught and thereby what are their favourite home ground and their diet. In first measure we may state they prefer an unfastened and bush home ground. Then, if we interested for the tree bed for Begin, the gaining controls were more of import in tree bed with spruce or none tree. For the field bed, the gimmicks was in great bulk in grass. Finally the shrub bed, the caught was more divided, but the dominants beds were the spruce and the deciduous shrub.

After these consequences we did the statistics. We tested for each home ground if there was a relation with the caught of the bank field mouse, ( table 6 ) . The height, the dominant home ground and the tree bed were important for gaining control the bank field mouse ( p & lt ; 0,001 ) .

There was another factor in the dominant home ground: the height. For the higher height ( 1000 and 1100 metres ) none bank field mouse was caught, so we might reason they does n’t populate in this type of home ground. The most height was in 500 metre and we noticed in 400m a lower proportion than the others altitude ( Figure 7 ) .

Figure 6A a, B, degree Celsius, vitamin D: the per centum of bank field mouse trapped for each home ground, dominant home ground ( a ) , tree bed ( B ) , field bed ( degree Celsius ) , bush bed ( vitamin D )





& lt ; 0,001




& lt ; 0,001




& lt ; 0,001


Table 6: logistic arrested development of the relation between the home ground and the caught of bank field mouse

Figure 7: the caught of bank field mouse harmonizing to the height


The foundation of our survey was the abnormality of the gnawers ‘ rhythm and the favourite home ground of the bank field mouse.

In fact, our instance the consequences showed an abnormality. It is of import to detect an uncertainly of the consequences obtained caused by the different enticement used in June 2011. It had possibly affected the caught, but we do n’t cognize how.

The gnawers rhythm of 3 to 5 twelvemonth periods, expected in 2012 a lower proportion of gnawers ‘ population than 2011 but in general we obtained a figure of caught non truly affected by the twelvemonth which it was non a important factor, although instead by the season and the interaction of year/season, even if we have noticed in June 2012 a great lower of the proportion of caught than 2011. But in this same twelvemonth, in September the caught was higher and about in the same proportion that the caught in 2011. If we use the observation of the bank field mouses proportion which the dominant coinage in our instance, we reach the same consequences. In fact, in June 2012 the caught of bank field mouse was really low. In add-on, one time more the consequences revealed in September 2012 a surprisingly proportion of grownup female bank field mouse ( Figure 3 ) .

Our consequences showed that the rhythm seem non as expected, it is earlier to pull decisions and it is necessary to go on the survey and roll up more informations. But we might get down to do some premises.

As sing the noteworthy low proportion in June 2012, it was an observation in conformity with the low twelvemonth of gnawer ‘s proportion expected in 2012 than 2011. But the high proportion found in September 2012 show it would look 2012 is non a low period than 2011 as expected. That might be explain by several factors.

In the first case we might propose several hypotheses. Either there is a low period in 2012 as expected and September 2012 is an stray instance or, there is non a low period and June 2012 is peculiar.

One of these factors is the predator-prey interaction. That suggest the marauders might impact the gnawer ‘s rhythm. It is a factor already analyze for explicate the multiannual rhythm of the gnawers, but this factor might besides impact the rhythm more in peculiar. A late survey showed that the specializer marauders might alter the rhythm multiannual rhythms to one-year fluctuation and the Renaissance man might altering the period and the amplitude of these fluctuations ( Korpimaki et al. 2002 ) . Furthermore, the Renaissance man marauders might stabilising the rodent population and besides turn the rhythm to a non-cycle ( Hanski, Hansson, Heikki & A ; Henttonen 1991 ) Then, we may be proposing that this factor would might impact the proportion of gnawers in our instance. Therefore it would be necessary to analyze the population of these marauders.

In add-on to this factor, the variable of snow screen might play an of import function. Indeed, the gnawers are more exposure to predation when there is an of import deepness of snow, as with the marauder ruddy fox ( Lindstrom & A ; Hornfeldt 1994 ) .We might hypothesis that in the winter 2011 the snow screen affected the gnawers and so in June 2012 the population was low.

We showed the season is a important factor for the proportion of gnawers. It is of import to analyze the difference in season and why this affected the population of gnawers. Other surveies have provided some replies. The consequence of nutrient on the denseness dependent construction of the gnawer. Indeed, there exists a multiannual oscillation in works measure and quality. The handiness of nutrient might a dynamic variable which might alter the belongingss of the rhythm ( Turchin & A ; Batzli 2001 ) .

In Sweden, Hanson ( 1971 ) studied the consequence nutrient and the fructification on the granivorous and herbivorous species. He suggested that the gnawers ‘ fluctuations might be affected by the works production. In add-on, another survey following this thought, Laine and Henttonen ( 1983 ) conducted a survey on the function of works production in microtine rhythms in nothern Fennoscandia. They studied the fluctuation in nutrient handiness of microtine in interaction with the clime and the works growing. And they suggest that the factors get downing the rhythm lessening are the qualitative and the quantitative alterations in the nutrient handiness of the gnawers. In add-on, they showed that in northern latitudes, workss needs several summers to finish the rhythm and accumulate modesty. With this clime and the workss rhythm factor we might do an premise, the demands nutrient for the gnawers ( berries, seeds ect ) was limited and deficient, that might be affected the proportion of gnawers in June 2012. The screen snow might play besides an of import function for the nutrient handiness ( Hansson,1971 ) .

Furthermore another factor has non been studied in our instance, the distribution of female depends of the nutrient handiness ( Jonsson, Hartikainen, Koskela, & A ; Mappes, 2002 ) .The female because of the energy necessary during the generative rhythm are needs an of import measure of nutrient ( Jonsson, Hartikainen, Koskela, & A ; Mappes, 2002 ) . The female bank field mouse normally territorially ( Koskela, Mappes & A ; Ylonen 1997 ) might travel more topographic point if the nutrients are non sufficient. In this instance, this might explicate the high proportion of big female in September 2012 by supplanting.

The diseases epidemic is besides a factor to take into history. The little gnawers have a big figure of macros parasites and micro parasites which are responsible in high mortality and diseases ( Hanski & A ; Henttonen ) . For illustration, a survey conducted in England showed a high mortality of gnawers after an epidemic in 1934, because of disease phrenitis caused by Toxoplasma ( Ferns 1976 ) .

Finally, as sing the home ground dominant of the bank field mouse, the consequence was those expected with their ecology and their penchant of nutrient. We might presume that we could n’t establish the bank field mouse in high height because of an inhospitable clime and the bulk of trees and others workss that provides their demands nutrient, are n’t turn in this sort of clime, but instead between 300 and 900 metres and more in 500m where we found the most bank field mouse. Likewise, the little gnawers are among the species where landscape constellation might hold an influence in the population kineticss because of its populating habitat spots with assorted size, location and quality. Furthermore the societal interactions and mortality during the dispersion may impact the population ( Hansson, 2001 ) .