Comparing simple methods for ageing roe deer Capreolus capreolus: Are any of them useful for management?

Abstract

The dynamics of ungulate populations depend not only on the size, but also on the sex- and age-structure of the population. Successful management therefore depends on obtaining estimates of the age composition. Variation in performance due to age can be fairly well described by stages, and simple, rough methods for ageing cervids can therefore be useful to management. We assessed the performance of three relatively simple and objective methods based on tooth wear (height of molar), weight of eye lenses and diameter of pedicles (males only) on a sample of 77 female and 81 male European roe deer Capreolus capreolus from Lier, Norway. The relationship between tooth wear and age was linear, whereas the relationship between weight of eye lenses and diameter of pedicles was curvilinear with age, likely making them unreliable for old age classes. However, as only three males and six females ≥ 6 years old were included, we were unable to assess the uncertainty in age estimation for older age classes precisely. No simple method could precisely age roe deer, even up to five years of age. Our results do suggest that tooth wear, i.e. height of molar, can serve as a very simple and objective measure of age in roe deer, given that moderate precision (an error rate of ± 1 year and a success rate of 70% up to four years of age) is sufficient to reach management aims. As residuals between age estimates based on tooth wear and diameter of pedicle were not correlated, combining these methods improved the fit slightly. Since tooth wear may differ between areas, the scales presented here may perform less well in other areas, and a calibration for each area is clearly recommended.

Full text

101
© WILDLIFE BIOLOGY · 12:1 (2006)
SHORT
COMMUNICATION
Comparing simple methods for ageing roe deer Capreolus
capreolus: are any of them useful for management?
Atle Mysterud & Eivind Østbye
Mysterud, A. & Østbye, E. 2006: Comparing simple methods for ageing roe
deer Capreolus capreolus: are any of them useful for management? - Wildl.
Biol. 12: 101-107.
The dynamics of ungulate populations depend not only on the size, but also on
the sex- and age-structure of the population. Successful management therefore
depends on obtaining estimates of the age composition. Variation in perform-
ance due to age can be fairly well described by stages, and simple, rough meth-
ods for ageing cervids can therefore be useful to management. We assessed the
performance of three relatively simple and objective methods based on tooth
wear (height of molar), weight of eye lenses and diameter of pedicles (males
only) on a sample of 77 female and 81 male European roe deer Capreolus capre-
olus from Lier, Norway. The relationship between tooth wear and age was linear,
whereas the relationship between weight of eye lenses and diameter of pedicles
was curvilinear with age, likely making them unreliable for old age classes.
However, as only three males and six females ≥ 6 years old were included, we
were unable to assess the uncertainty in age estimation for older age classes pre-
cisely. No simple method could precisely age roe deer, even up to five years of
age. Our results do suggest that tooth wear, i.e. height of molar, can serve as a
very simple and objective measure of age in roe deer, given that moderate pre-
cision (an error rate of ± 1 year and a success rate of 70% up to four years of
age) is sufficient to reach management aims. As residuals between age esti-
mates based on tooth wear and diameter of pedicle were not correlated, com-
bining these methods improved the fit slightly. Since tooth wear may differ
between areas, the scales presented here may perform less well in other areas,
and a calibration for each area is clearly recommended.
Key words: eye lense weights, pedicle diameter, tooth sectioning, tooth wear
Atle Mysterud, Centre for Ecological and Evolutionary Synthesis (CEES),
Department of Biology, University of Oslo, P.O. Box 1066 Blindern, N-0316
Oslo, Norway - e-mail: atle.mysterud@bio.uio.no
Eivind Østbye, Department of Biology, University of Oslo, P.O. Box 1066
Blindern, N-0316 Oslo, Norway - e-mail: eivind.ostbye@bio.uio.no
Corresponding author: Atle Mysterud
Received 10 September 2004, Accepted 7 November 2004
Associate Editor: Jean-Michel Gaillard
Short communication articles are short scientific entities often dealing with
methodological problems or with byproducts of larger research projects. The
style is the same as in original articles
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102 © WILDLIFE BIOLOGY · 12:1 (2006)
Ungulate populations are strongly structured according
to age and sex (reviews in Tuljapurkar & Caswell 1997,
Gaillard et al. 1998, 2000), so there are large individual
differences in performance within populations that are
related to sex and age. The dynamics of ungulate popu-
lations of similar sizes may subsequently vary accord-
ing to the age and sex composition of the herd (Coulson
et al. 2001). Therefore, a manager needs not only to
obtain indices of abundance or density (as well as oth-
er relevant information regarding condition of the ani-
mals and habitat quality), but preferably also of the sex
and age composition of the population in order to pre-
dict future dynamics, and simple methods to age ani-
mals remain a central challenge in wildlife management.
Fortunately, as performance can be fairly well described
by age stages (i.e. juvenile, subadult, prime-aged and
senescent stages; Gaillard et al. 1998), relatively coarse
methods may be suitable for management.
Tooth sectioning is clearly the most widely used age-
ing method in ungulate research in Scandinavia and
North America, as it has proven a highly reliable age-
ing technique in a number of ungulates inhabiting the
strongly seasonal environments at northern latitudes (see
e.g. Reimers & Nordby (1968) for reindeer Rangifer ta-
randus, Mitchell & Youngson (1969) and Hamlin et al.
(2000) for red deer and elk Cervus elaphus, and Aitken
(1975) for roe deer Capreolus capreolus). Tooth sec-
tioning is based on counting annuli in the tooth cemen-
tum, which arise due to seasonally retarded growth dur-
ing winter and increased growth during summer. How-
ever, to be able to read these lines it is necessary to have
quite sophisticated laboratory equipment, or to send the
samples to laboratories charging ~10€ per individual.
In contrast, alternative methods either utilise the con-
tinuous wear with age (e.g. molar wear; Hewison et al.
1999), growth in the size of the eye lenses (Maringgele
1979, Ashby & Henry 1979, Angibault et al. 1993) or
growth of the pedicles of male antlers (Stubbe et al.
1987, Stubbe 1997). However, the relative performance
of these methods for ageing roe deer is largely unknown.
Further, although the use of tooth wear has been subject
to much research, several studies have assessed tooth wear
based on a subjective scale (Szabik 1973, Hrabe & Kou-
bek 1987, Cederlund et al. 1991). Studies that used an
objective criteria (e.g. height of the molar) differ in their
conclusions, reporting either limited (Cederlund et al.
1991), high (Ashby & Henry 1979) or variable (Hewi-
son et al. 1999) success in predicting age.
In this paper, we compare three fairly simple methods
for ageing roe deer using data on age obtained by tooth
sectioning (as known-age material is unavailable). Our
aim is to determine the success of these methods as
viewed from a management perspective. However, such
methods may also be useful for capture-mark-recapture
studies, because a method for ageing live animals with-
out the extraction of teeth would be valuable (Festa-
Bianchet et al. 2002).
Material and methods
Study area
The study area is located in the Lier valley near Sylling
in the municipality of Lier in the county of Buskerud in
southern Norway (between 59°45'-60°00'N and 10°05'-
10°20'E). Most of the area is forested and situated with-
in the boreonemoral region (Abrahamsen et al. 1977).
Vegetation is varied and dominated by Norway spruce
Picea abies mixed with Scots pine Pinus sylvestris on
the drier and poorer locations. Along the valley bottom
on richer soil, deciduous forest is dominant, fragment-
ed by small, cultivated fields (Kjøstvedt et al. 1998). The
topography is extremely hilly (see Mysterud & Østbye
1995, Mysterud 1999, Mysterud et al. 1999 for a further
description of the study area).
Data
Jaw bones, eye lenses and skulls from 77 female and 81
male roe deer aged ≥ 1 year old were collected and pro-
cessed by Eivind Østbye during 1985-2001. We exclud-
ed fawns, as these can be aged by their pattern of tooth
eruption (only 4-5 cheek teeth in their first autumn;
Cederlund & Liberg 1995). As tooth sectioning is regard-
ed as a highly reliable method for age determination in
strongly seasonal environments such as Norway (for roe
deer, see Aitken 1975), we use 'tooth section age' as
'known age'. The Matson Laboratory in the USA did all
the age determination by tooth sectioning. Any error in
the ageing will most likely increase the error rates report-
ed, and thus if the methods are successful, this would
likely lead to a conservative result. The following three
methods were compared:
Method 1: molar height
Tooth wear is a well-known method frequently used for
ageing cervids including roe deer (e.g. Hewison et al.
1999). We used an objective measure, namely the height
of the second molar (M2) measured to the nearest 0.1
mm using a calibre.
Method 2: eye lens weight
The size of the eye lenses has also been used previous-
ly for ageing roe deer (e.g. Maringgele 1979, Ashby &
Henry 1979). We extracted both eye lenses from fresh
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heads, and used the average weight (to the nearest 0.001
g) after fixation in 10% formalin for 10 days and subse-
quent drying at 80°C.
Method 3: diameter of the pedicles
For males, a method for ageing individuals based on the
diameter of the pedicle has been described (Stubbe et al.
1987, Stubbe 1997). We used average length and width
on both sides of the head as a measure of diameter.
We also tried to combine tooth wear and diameter of
the pedicles (Stubbe et al. 1987, Stubbe 1997).
Statistical analyses
We used linear models to determine the relationship
between our response variables (M2 height, eye lense
weight and pedicle size) and the predictor variables. We
used model selection and assessed fit with the Akaike
Information Criterion (AIC; Johnson & Omland 2004)
or by comparing r2, as the latter was used in previous
studies. We tried adding sex, a sex*age interaction term
and a second-order term for age (in addition to only age).
We also tried an ln-transformation of the response vari-
ables. The most parsimonious model was subsequently
used to derive predictions. We then established intervals
for each age by interpolation from the predicted values
of the best model, and then reclassified the data into age
by using these intervals to check the success rate. All mod-
els were run in S-Plus version 6.2 (Crawley 2003).
Results
Height of the second molar was the only measure lin-
early related to age (Fig. 1A, Table 1). A second-order
term for age entered both the model for the weight of
the eye lenses and for the diameter of the pedicles (see
Table 1). The weight of the eye lenses did not increase
after seven years of age. The diameter of pedicles
seemed not to increase after 4-6 years of age, but this
was only based on two data points, and the relationship
was linear up to five years of age (testing for age2 below
six years of age: T = 0.128, P = 0.898; see Fig. 1). The
weight of the eye lenses was larger for females than for
males. The fit was improved slightly by ln-transform-
ing the height of the molar (r2 = 0.547 vs 0.590), but this
did not apply for the weight of the eye lenses (r2 = 0.713
vs 0.698) or the diameter of the pedicles (r2 = 0.391 vs
0.375). Excluding ages > 7 years decreased the perform-
ance of molar height slightly (r2 = 0.484).
Except for 1-year-old roe deer, no method was able
to determine 50% to the correct age. However, if allow-
ing for ± one year, the success rate of the tooth-wear
method was > 70% up to four years of age, but with a
tendency to overestimate age (Table 2). Similarly, the
success rate (± 1 year) of the eye-lense method was > 60%
up to five years of age in both sexes, and up to four years
for the size of the pedicle.
It may be possible to combine methods to improve
accuracy in ageing if the same individuals are not over-
or underestimated by both methods, i.e. if the residuals
are not correlated. There was no correlation between
Table 1. Results from model selection and parameter values from the most parsimonious models. Model selection was done starting with
age only, then adding (age)2, sex and sex*age, including or excluding factors depending on whether fit was improved or not. AIC = Akaike
Information Criterion, ΔAIC = difference in AIC value between the AIC for the model given in that row and the most parsimonious model
(lowest AIC), l.s. mean = least square mean, SE = standard error, T = Student’s T test statistic, P = significance probability.
Parameter AIC ΔAIC l.s. mean SE T P
A. ln(molar height)
Intercept 2.1362 0.0135 157.944 0.000
Age -237.151 0.000 -0.0641 0.0046 -13.950 0.000
(Age)2-236.053 1.099
Sex -235.466 1.686
Age*sex -234.097 3.054
B. Eye lense weight
Intercept 0.2184 0.0050 43.914 0.000
Age -681.193 42.292 0.0379 0.0029 12.999 0.000
(Age)2-716.754 6.731 -0.0021 0.0003 -6.890 0.000
Sex -723.485 0.000 -0.0119 0.0040 -2.958 0.004
Age*sex -721.629 1.856
C. Pedicle diameter
Intercept 15.3328 0.7397 20.729 0.000
Age 315.399 2.051 1.9410 0.4918 3.946 0.000
(Age)2313.348 0.000 -0.1255 0.0628 -1.997 0.050
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104 © WILDLIFE BIOLOGY · 12:1 (2006)
residuals estimated from tooth wear and those estimated
based on the diameter of the pedicle (rPe = 0.127, df =
66, P = 0.302), very weak correlation between residuals
based on tooth wear and those from size of the eye lenses
(rPe = 0.217, df = 117, P = 0.018), whereas the residuals
from using the size of eye lenses and the diameter of
pedicles to age roe deer were correlated (rPe = 0.522, df =
57, P < 0.001). This indicates that combining methods
using measures of body size will not add much more
predictability, whereas combining tooth wear and meth-
ods using size will improve success. Indeed, slightly
improved precision was obtained when combining tooth
wear with the diameter of the pedicles (see Table 2).
Table 2. Predicted values for the relationship between age and height of the second molar (in mm), weight of the eye lenses (in g) and diameter
of the pedicles (in mm). The intervals used to estimate age in roe deer were based on the most parsimonious models given in Table 1, and
the success rate (proportion correct and proportion within ± 1 year) is given relative to age estimated based on tooth sectioning.
Estimated
age Pre-
dicted Lower
limit Upper
limit
Age - based on tooth sectioning
NProp.
correct Prop.
± 1 year
1 2 3 4 5 6 7 8 9 10 12
A. Molar height 137
1 7.941 >7.695 47 9 3 4 63 0.75 0.89
2 7.448 7.217 7.695 14 9 2 2 27 0.33 0.93
3 6.986 6.769 7.217 4 6 3 3 16 0.19 0.75
4 6.552 6.349 6.769 2 1 7 1 2 1 14 0.07 0.71
5 6.145 5.955 6.349 1 2 1 2 6 0.33 0.50
6 5.764 5.585 5.955 2 2 4
7 5.406 5.238 5.585 1 1 1 3
8 5.070 4.913 5.238 0
9 4.755 4.608 4.913 0
10 4.460 4.322 4.608 1 1 2
11 4.183 4.053 4.322 1 1 2
12 3.924 <4.053 0
B. Eye lense - females 77
1 0.263 <0.277 37 5 42 0.88 1.00
2 0.291 0.277 0.303 2 6 2 1 11 0.55 0.91
3 0.315 0.303 0.325 2 3 3 2 10 0.30 0.80
4 0.335 0.325 0.344 3 2 5 0.40 1.00
5 0.352 0.344 0.359 1 2 1 1 5 0.20 0.80
6 0.366 0.359 0.371 1 1 1 3
≥7 0.376 >0.371 1 1
B. Eye lense - males 81
1 0.251 <0.265 36 5 1 42 0.86 0.98
2 0.278 0.265 0.290 3 4 2 1 10 0.40 0.90
3 0.302 0.290 0.313 1 3 3 3 10 0.30 0.90
4 0.323 0.313 0.331 4 2 3 9 0.22 1.00
5 0.340 0.331 0.347 1 4 1 6 0.00 0.67
6 0.354 0.347 0.359
≥7 0.364 >0.359 1 2 1 4
C. Pedicle diameter 68
1 17.148 <17.931 21 6 2 29 0.72 0.93
2 18.713 17.931 19.370 6 3 2 2 13 0.23 0.85
3 20.026 19.370 20.558 2 2 1 2 1 8 0.13 0.63
4 21.089 20.558 21.495 2 2 2 1 7 0.29 0.86
5 21.900 21.495 22.461 2 1 3 0.00 0.00
≥6 22.461 >22.461 1 4 2 1 8
D. Tooth wear and pedicle diameter 67
1 22 9 1 1 33 0.67 0.94
2 7 1 3 1 12 0.08 0.92
3 2 2 1 2 7 0.14 0.71
4 3 3 3 1 10 0.30 0.90
5 1 1 2 0.50 1.00
≥6 2 1 1 3
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Discussion
We compared the performance of three fairly simple
methods for ageing roe deer based on: the relationship
between age and tooth wear, the size of the eye lenses
and the diameter of the pedicles. The main result is that
none of these simple methods could age roe deer pre-
cisely. However, if a lower precision is sufficient, some
of them may still be useful for management.
The general body size in roe deer males reaches a pla-
teau at about 4-5 years of age (Andersen et al. 1998).
Although based on only two data points, we found a pat-
tern consistent with this. From around 4-6 years of age,
the diameter of the pedicles does not seem to increase
further (see Fig. 1). Any size measure will likely per-
form equally well, making this the least useful method.
It is difficult to see any advantages of this method over
for example tooth wear rates.
Size of the eye lenses is a simpler method than tooth
sectioning, and the method performs quite well. Indeed,
for ages up to five years it performs better than tooth
wear (see Table 2). Our results are roughly in accordance
with previous studies also reporting that a plateau for
eye-lense size was reached at about 6-7 years of age (Ma-
ringgele 1979, Ashby & Henry 1979). Ashby & Henry
(1979) concluded that eye lense weights were useless as
a method, since the relationship between age and eye-
lense size was not linear. However, this can be overcome
by fitting a curvilinear relationship for age (see Fig. 1),
and does not mean that the method is useless, but rath-
er that weight intervals for estimating age decrease with
increasing age (see Table 2). The method seems unsuit-
able for age determination above a threshold age of
around seven years. We also found a sex difference in
eye-lense weight, which was not tested for previously.
Sexing the animals can therefore increase performance
relative to previous accounts (Maringgele 1979, Ashby
& Henry 1979). However, drying and preparing the eye
lenses still requires some equipment not available to
most managers, and may therefore be less suited than
tooth wear if larger samples should be processed each
year in population monitoring.
Using either a subjective or objective assessment of
tooth wear has been a common approach to age roe deer
(Szabik 1973, Cederlund et al. 1991, Hewison et al. 1999).
When comparing a subjective assessment of wear with
an objective criteria (height of the molar), Cederlund et
al. (1991) reported that the subjective assessment of wear
seemed to perform slightly better. However, the persons
performing the assessment in Cederlund et al. (1991)
were skilled, possibly underestimating the error rates of
the subjective assessment. A more serious problem is
related to the fact that such subjective methods give biased
results which depend on the experience of the observer,
inexperienced persons typically being more likely to
overestimate age of younger animals and underestimate
age of older animals (Cederlund et al. 1991, Szabik 1973,
Hewison et al. 1999). The studies that have used molar
height to assess age differ in their conclusions, report-
ing either limited (r2 = 0.44; Cederlund et al. 1991), high
(r2 = 0.74; Ashby & Henry 1979) or variable success
Figure 1. Relationship between age (as determined by tooth section-
ing) and A) molar height, B) mean weight of the eye lenses and C)
mean diameter of the pedicles. The lines indicate 95% confidence
intervals.
0 4 10 12
AGE (years)
5
10
15
20
25
DIAMETER OF PEDICLE (mm)
C)
0 8 10 12
AGE (years)
1
3
5
7
9
MOLAR (M2) HEIGHT (mm)
A)
642
0 8 10 12
AGE (years)
0.1
0.2
0.3
0.4
MEAN WEIGHT (g)
Females
Males B)
246
2 6 8
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106 © WILDLIFE BIOLOGY · 12:1 (2006)
depending on population (r2 = 0.36-0.66; Hewison et al.
1999) in predicting age. Our study comes between this
(r2 = 0.59), while two other studies used molar height
without giving an exact fit (Stoddart 1974, Aitken 1975).
In our case, the fit was improved by an ln-transforma-
tion, which was not used in previous studies. Ashby &
Henry (1979) excluded ages of 8-10 years, and indeed,
the wear rate of very old age classes (in red deer) decreased
more than predicted even from a log-linear relationship
(Loe et al. 2003). Our fit was not improved by excluding
the few very old animals, while animals of old age were
not present in the study by Cederlund et al. (1991). At
least some of the within-age variation in tooth wear may
be due to reported individual variation in diet due to
local-scale habitat variation (Mysterud et al. 1999).
Since measuring the height of the molar is an objec-
tive method, it is possible to estimate the likelihood of
bias. Indeed, we seemed to overestimate age with the
tooth-wear method (see Table 2). Also, since error rates
are not much higher than for subjective criteria (Ceder-
lund et al. 1991), this makes it a more appropriate meth-
od for long-term monitoring when changes in the per-
sonnel performing the procedure are common. Some
caution is nevertheless required. Since the performance
of the methods in our study is scaled with the same mate-
rial, as was also done in previous studies (Ashby & Hen-
ry 1979, Cederlund et al. 1991), the intervals presented
may not perform equally well in other areas. Tooth wear
may vary somewhat between areas (Hewison et al.
1999), which may lead to biased estimates. This is not
overcome by using an objective assessment of wear.
Clearly, more sophisticated ways for measuring tooth
wear than molar height may increase precision (Pérez-
Barbería & Gordon 1998a, Pérez-Barbería & Gordon
1998b), but such measures cannot be easily obtained.
As residuals for age estimates based on tooth wear and
those based on the diameter of the size of the pedicle
were not correlated, an increased fit was obtained by
combining the two methods (Stubbe et al. 1987, Stubbe
1997), but this method can only be used with males.
Whether the precision reported for molar height in our
study is good enough or not, depends on the objectives.
For a manager, it is most important to be able to classi-
fy the proportion of juvenile, subadult, adult and senes-
cent stages (see Gaillard et al. 1998). Harvested popu-
lations also have a low average age (Langvatn & Loison
1999), so it may not be important to have a very high pre-
cision for old age classes. Using the height of the molar
provides a very easy method for ageing, and we argue
that it is suitable for most routine studies. However, we
clearly need more information to be able to predict spa-
tially variable molar wear rates in ruminants in general,
which is necessary in order to avoid biased estimates.
Even though these methods are mainly aimed at man-
agement, such methods may also be useful for science
when performing capture-mark-recapture studies, as the
ability to age live animals without having to extract teeth
would be valuable (Festa-Bianchet et al. 2002). This may
be feasible for some species, but error rates will likely be
higher due to measurement error, severely restricting the
applicability in studies of life history variation.
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