1. BIO2431 – The Biology of Mammals – Small mammal trapping March 28, 2014
Population analyses of a small mammal community
Introduction
In this study, populations of small mammals in South-WestEngland were analysed usinga capture-mark-recapturetechnique and
subsequent population estimates. 100 Longsworth traps were set in 5 transects of 20 traps each, and contained food and bedding to
keep the animals comfortablewhiletrapped. The majority of animalswere marked by clippingtheir fur and then the recapture
event took placetwo days after the firsttraps were set. Most animals caughtwere also sexed and weighed before release.Thi s
data, alongwith population estimates, areanalysed in this study.Population estimates used three different methods; minimum
number alive(simply thenumber of uniqueindividualsin thestudy), Petersen’s estimate and Seber’s estimate. Petersen’s estimate
is calculated by multiplyingthe number of individuals marked by the total number caughtin the second sampleand dividingby the
number of recaptures [1]. Seber’s estimate is a simplealteration in which 1 is added to each variableand 1 taken away from the
resultingvalue[1]. Population estimates for common shrews have not been calculated due to insufficientdata,discussed later. CI
refers to the 95% confidenceintervals.
Populationanalysis
(Figuresadaptedfrom Field Guide to The Animalsof Britain, [2],[3],[4],[5])
1
2
3
4
5
3
2
4
5
1
Figure 4 (above):Wood mouse (Apodemus
sylvaticus)
1. Sandy browncoatwith whiteunderparts
2. Yellow streak on chest
3. Lightly haired, long, slender tailoften used for
balance
4. Long hind legs usedfor bounding and jumpin2g
5. Larger, moreconspicuous ears and eyes than other
small rodents
3
2
4
5
1
Figure 3 (left):Bank vole (Myodes glareolus)
1. Slightly larger andmoreprominentears thanthe
field vole,but smaller than themouse
2. Longer, furrier tailthanthefield volebut shorter
than the mouse
3. Glossy,chestnut-brown coatthat fades to grey
underneath
4. More rounded snout and less prominenteyes
than mice
5. Plump, stocky body
2
53
1
4
6
7
Figure 1 (left):Common shrew (Sorex araneus)
1. Small, roundedears setclose tothehead. Less prominent than thewhite-toothedshrew
2. Slightly longer, morepointedsnoutthan pygmy or watershrews
3. Slightly smaller body size than a mouseor water shrew, larger thana pygmy shrew
4. Orange brown coatand paleundercoat ofdense,velvety fur,compared to thewater shrew’s
distinctive blackcoat
5. Shorter, less furry tail than eitherthewater shrew or white-toothed shrew
6. Very small eyes withpooreyesight
7. Red teeth
Figure 2 (left):Fieldvole (Microtus agrestis)
1. Smaller ears than thebank vole
2. Shorter, pinker tail than bankvole
3. More dense fur than thebank voleto provide
insulation during thewinter that is grey-brown
above, creamy grey below
4. More rounded snout and less prominenteyes
than mice
5. Plump, stocky body

2. BIO2431 – The Biology of Mammals – Small mammal trapping March 28, 2014
Commonshrew(Sorex araneus)
Average length = 48-80mm - Tail = 24-44mm - Average weight = 5-14g [3].
The common shrew is often firstnoted and identified by the loud, shrill vocalizationsmadewhen two individualsfightover territory,
as they areextremely territorial [2]. Shrews will only socializein the mating season,which occurs from April onwards [2]. A female
will havelitters of six or seven young, usually fromseveral different fathers, who may be seen followingher around in a ‘c aravan’-
likeprocession,each young holdingon to the next [2][3]. In summer, a female may be identified by white marks on the back of her
neck where she has been grasped by the male duringmating [2] Shrews are often killed by predators such as domestic cats and
owls,but then left and not eaten due to the foul-tastingexuberancereleased from their skin [2] [3].
Field vole (Microtusagrestis)
Average length = 90-115mm - Tail <40% of head and body - Average weight = 20-40g [4].
Population estimates;
2013 – MNA = 3 – Petersen = 0 (CI = 0/0.54) – Seber = 5 (CI = 0.28/5)
2014 – MNA = 1 - Petersen = 0 (CI = 0/0) – Seber = 1 (CI = -0.57(0)/1)
Field voles,aptly, areusually found in fields amongthe damp, tussocky grass which they prefer as food and habitat[2]. They
particularly feed on bents, fescues and hair grasses and usethe shredded grass to make their nests, which ar e often made atthe
baseof grassy tussocks,under logs or a blanket of snow [2][4]. Males are very territorial and aggressive,vocalizingloudly to defend
their territory [2] [4]. They usually breed from April to December, with females havingfive or six litters of four or five offspringin a
year [4]. Though they are prolificbreeders,they have an average lifespan of up to a year, and are the mainstay dietof many avian
and mammal predators, such as weasels,foxes,kestrels and owls [4]. They are thought to make up up to 90% of the diet of barn
owls,and thus are a very important prey species for barn owls and other charismatic predators [2] [4].The populations of field voles
have been shown to fluctuategreatly in 1-4 year cycles in responseto predation,but are still thought to be Britain’s mostabundant
mammal, with population estimates of approximately 75 million individuals[4].
Bank vole (Myodesglareolus –formerly Clethrionomysglareolus)
Average length = 90mm - Tail = 60mm - Average weight = 28g [2].
Population estimates;
2013 – MNA = 6 – Petersen = 0 (CI = 0/2.44) – Seber = 15 (CI = 3.34/15)
2014 – MNA = 2 – Petersen = 2 (CI = 0.28/8.26) – Seber = 2 (CI = -0.27(0) / 3.83)
The bank voleis known for creatingtunnels, either underground or in dense undergrowth, alongwhich itforages on seeds, berries,
fungi and other plantmaterial,which may be stored underground in larders [2].The bank vole also usually raises its youngi n a nest
underground, with a litter of four or five, four or five times a year between April and September [2]. Like field voles,though they
breed often and in abundance,fewer than half born survivethe firstfew months [2]. Each volehas a home range, which is usu ally
no more than about 50m, and males tend to range further than females [2].
Woodmouse(Apodemussylvaticus)
Average length = 81-103mm – Tail = 71-95mm - Average weight = 13-27g [5].
Population estimates;
2013 – MNA = 7 – Petersen = 7.5 (CI = 2.92/27.53) – Seber = 7.4 (CI = 2.74/19.1)
2014 – MNA = 12 - Petersen = 15 (CI = 7.61/34.74) – Seber = 14.88(15) (CI = 7.53/31.11)
The wood mouse, also known as the long-tailed field mouse, arefound in a variety of habitats and have largehome ranges;
travellingup to 400m each night, with males generally havinga larger territory than females [2]. Females may overlap in their home
ranges, and many may nest together in the winter in the largeunderground burrows where the mice spend the majority of their
day, storingfood and raisingyoungin nest chambers [2] [5].The mice usually breed from March to October and have litters of four
to seven young approximately four times a year [2] [5]. They feed mainly on nuts, seeds and berries and their population sizes may
exhibitcircannual cycles in response to plantproductivity [5].

3. BIO2431 – The Biology of Mammals – Small mammal trapping March 28, 2014
Analysisof variance in body size
Methods
Within species,differences in weight between years and between sexes were analysed usingWilcoxon rank signed tests,as near ly
all data were non-normal. Between species,differences in weights were analysed usinga Kruskal-Wallischi-squared test, as data
were non-normal. To investigate sex ratio within species,a Yates’ chi-squared testwas used as the degree of freedom for the
analysiswas one. When investigatingdifferences in weight between sexes and species and sex ratio,the populations from2013 and
2014 were treated as independent samples and analysed together. This is a relatively safeassumption asitis unlikely thata ny
individualsfrom2013 would have survived to have been recaptured in the 2014 sample.
Common shrews and water shrews were omitted from these analyses as therewere insufficientdata to complete the calculations.
This was due to the shrews being handled for very brief periods of time, and no parameters outside species beingnoted. The shrews
were handled for a minimal amountof time as they can become stressed very easily and often die in trappingstudies [6]. As such,
only the calculationsfor bank voles,field voles and wood mice arepresented here.
Results
There were no significantdifferences in weights between sexes for any of the species studied (bank vole – W = 8.5, p = 0.49, field
vole – W = 0, p = 0.5, wood mouse – W = 46.5, p = 0.34.) There were also no significantdifferences in weights between years for any
species studied (bank vole – W = 11, p = 0.12, field vole – W = 1, p-value= 1, wood mouse – W = 42, p-value= 0.09).
Between species,however, there was a significantdifferencein body sizeas illustrated in figure5 (Kruskal-Wallis chi-squared =
7.7503, df = 2, p-value = 0.02).
Despite havingwhat appeared to be bias in the sexes of each sample,there was no significantdifferences between sex ratio
compared to the expected values (wood mouse – χ 2 = 0.45,p=0.50, field vole – χ 2 = 0.25, p= 0.62, bank vole – χ 2 = 1.13, p= 0.29).
This was possibly dueto havingsuch small samplesizes,and a more intensive samplingeffortmay have provided a better estimate
of sex ratio in the populations. Thesex ratios for each species are represented in figure 6.
Discussion of the methods used in this report
Capture-Mark-Recapture (CMR)
When makingdecisions abouthowto estimate population sizes,itis often the casethat, due to limited resources, the most cost-
effective method is used rather than the most accurate.CMR is one such relatively cost-effective method, with small mammal traps
availablefromjust£4 [7]. The cost, however, increases with the experimental effort, and to acquirean accuratepopulation
estimate, many traps areusually required,which also need to be checked frequently, possibly up to four times a day [6]. Thus,one
of the main disadvantages of CMR is the largeamount of work needed [6]. It has been estimated than over 50% of the total
population need to be captured and marked in order to obtain narrowconfidence intervals [6]. The frequency of capture required
for accurateestimates also conflictswith ethical considerations[6]. For example, shrews are protected under Schedule 6 of the
Wildlifeand CountrysideAct of 1981 [3] as they become easily stressed when captured and thus must not be held for more than a
few hours at a time.
Another issuewith directly handling wild animals isthepossibility of diseasetransfer,as many small mammals often carry
zoonotic diseases such asplague,hantavirus,rabies, leptospirosisand cowpox [6]. Thankfully,the majority of these diseases areno
longer common in the UK, [8] however there is always thepossibility of an undiscovered diseaseor simply the spread of disease
within the population,thereby negatively affectingthe animalsinvolved,thatwe should always consider.Onemajor advantageof
usingCMR over other methods is thatit allows thecollection of other parameters, such as weight and sex as we have considered in
this study [6].
Figure 5 –Variation in
weight between species
of small mammal
This illustrates the
significantvariation in
weight betweenbank
voles, field voles and
wood mice inour study.
Bankvole Fieldvole Woodmouse
15202530
Species
Weight(g)
Figure 6 –Sex ratio estimates
in speciesofsmall mammal
This illustrates thenon-
significantsex ratios ofbank
voles, field voles and wood
mice thatwerestudied. The
relative proportions ofcolour in
the bars represents therelative
proportions offemales to males
in the study populations.
75%
25%
40%
25%
75%
60%
B A N K V O L E F I E L D V O L E W O O D M O U S E
Females Males
n = 8 n = 4 n = 20

4. BIO2431 – The Biology of Mammals – Small mammal trapping March 28, 2014
There are several assumptions associated with CMR, of which many are often violated.The firstis thatthe population is clos ed,
that is,there are no migrations,births or deaths [6]. Itis usually impossibleto be certain if this holds true, however there are ways
of minimizingthe probability of violatingit.The firstis to have a well-defined study area, for example by usingan isolated
population (e.g. island) or havingthedates of captures closetogether [6]. In this study, the recapture event was two days after the
traps were firstset, which ensured a more closed population as the likelihood of increases or decreases in the population wa s
reduced. The timing of the capture event also aided in this;early march is beforemost breeding seasons of sma ll mammals have
started, and thus has a reduced likelihood of births [2]. A problem with havingthe events so closetogether, however, is that is
causes possibleviolation of another assumption;that animals mix freely within the population [6].Two days may not be enough
time for animalsto re-disperse.However, given the generally small home ranges of the animals (as discussed above) it is likely that
they would not need much more time than this to fully re-disperse.Unfortunately, havinga home range and being territorial,as
most of the animalsstudied were, also means that the animals arelesslikely to be freely mixed within the population [6]. This
highlights oneof the difficulties with CMR as a method; means to increasevalidity often decrease accuracy,and viceversa [6].
Another assumption of CMR that is somewhat easier to overcome is thatthe marks do not fall off or become less visibleover time
[6]. In this study, as fur clippingswere used, it is highly unlikely thatthese would become less visibleover the spaceof two days,and
thus we can be fairly surethatany individualsrecaptured would have been correctly identified as so. One must also ensurethat the
mark or act of trappingthe animal does not affect the probability of its surviva l or subsequentrecapture, and that all individuals
have equal ‘catchability’ [6].Individuals arerarely equally likely to be captured, regardless of the type of trap used [6]. For example,
juveniles and subordinateindividuals areless likely to be captured or may never be captured whereas territorial males thatare
more wide-ranging are more likely to be captured [6]. Particularly with the voles in this study [2] where the males aremore
territorial and havelarger ranges,this could be an issue. The method of markingused in this study was fortunately unlikely to affect
behaviour or survival rate. The assumption of the act of trappingthe animalsnotaffecting the probability of recaptures is also often
violated as animalscan become ‘trap-happy’ or ‘trap-shy’ [6]. This can be mediated by placing‘locked’traps thatcannot be set off,
but have bait,that will attractthe animalsbutnot capture them. This can help make all animals‘trap-happy’before the startof the
experiment, as longas it is conducted soon after [6]. This could be a possibleaddition in a futurerepeat of this study.
One other complication with this method that was evident duringthe study was the factthat many traps were ‘sprung’ but did not
capture an animal.The resultof this was fewer traps,decreased samplingeffort, and the possibility of animals beingexcluded from
traps that had already been sprung, which would reduce the accuracy of the population estimates. The already relatively low
samplingeffort meant that the population estimation formulaeproduced biased results (the number of recaptures has to be greater
than eight in order for the results to be unbiased [1]).
Population estimate methods
The three methods used in this study had different advantages and disadvantages.Minimumnumber alive(MNA) is a very simple
calculation butusually grossly underestimates the actual population sizeand so is rarely used. The Peterson method also produces a
biased estimate; it usually overestimates the population size [1]. The Seber method can be unbiased if the number of recaptures is
greater than seven, but in this study that was never accomplished [1]. All of these methods assumesamplingwithoutreplacement,
and one mark/recapture method [1].
Other methods, such as the Schnabel method would have involved multiplerecaptures. This method has all the sameassumptions
as the others, but allows for easier identification of violations[1]. One would plot a regression of the proportion of marked
individualsand this should belinear unless a violation occurred in which caseitwould be curved [1]. However, this does not give
any information as to which assumption has been violated [1].
References
[1] Krebs, C. J. (1999). Ecological methodology (Vol. 620). Menlo Park, California:Benjamin/Cummings
[2] Morris,P. (1987) Field guide to the animals of Britain, Reader's Digest Association,(122-151)
[3] The Mammal Society, Common shrew - Sorex araneus, [Online] Available:
http://www.mammal.org.uk/species-factsheets/Common%20shrew (23/03/2014)
[4] Swann, S., the Mammal Society, (2012) Field vole, [Online] Available:http://www.mammal.org.uk/fieldvole(23/03/2014)
[5] Swann, S., the Mammal Society, (2012) Wood mouse, [Online] Availablehttp://www.mammal.org.uk/woodmouse(23/03/2014)
[6] Sutherland, W. J. (Ed.). (2006). Ecological census techniques: a handbook.Cambridge University Press
[7] NHBS, Economy Mammal Trip-Trap, [Online] Available:
http://www.nhbs.com/economy_mammal_trip_trap_tefno_175667.html?ad_id=1509 (23/03/2014)
[8] DEFRA, (2013) Zoonose report UK 2012 [Online] Available:https://www.gov.uk/government/publications/zoonoses-report-uk-
2012 (23/03/2014)