Estimating age through tooth wear. A pilot study on tooth abrasion in Apodemus

(Rodentia, Mammalia)

by M. FREUDENTHAL'*2, E. MARTIN-SUAREZ' and N. BENDALA'

'Departamento de Estratigrafia y Paleontologia. Universidad de Granada, 18071 Granada, Spain

E-mail : elvira@ ugr.es 2~aturalis (National Museum of Natural History). Postbus 9517.

Danvinweg 2.2333 CR Leiden. The Netherlands E-mail : fieudenthal @naturalis.nnm.nl

Correspondence should be addressed to E. Martin-Sulirez

Summary. - Age at death may be estimated by the degree of wear of the dentition, expres- sed as the percentage of the occlusal surface that is occupied by dentine ; this method is applied to the Mg of a population of Late Miocene Apodemus from Gargano (Italy), and of a population of extant Apodemus from Doiiana National Park (Spain). The values obtained represent the rela- tive age of each individual within its population. Some authors consider condylobasal length as a measure for age in extant populations. We measured this parameter in the Doiiana population, and checked its correlation with the tooth-wear parameter.

Risumt - L'8ge au moment de la mort peut Ctre estim6 par le degr6 d'abrasion de la denti- tion, dCfini c o m e le pourcentage de la surface occlusale occup6e par la dentine ; cette mCthode est appliquee aux M, d'une population d'Apodemus du MiocBne r6cent du Gargano (Italie), et d'une population d'Apodemus actuels du Parc national Doiiana (Espagne). Les valeurs obtenues representent 1'8ge relatif de chaque individu, ?i l'intkrieur de sa population. Plusieurs auteurs considBrent la longueur condylo-basale c o m e une mesure permettant d'estimer l'ige des popu- lations actuelles. Nous avons mesurC ce paramktre pour la population de Doiiana, et test6 sa cor- rClation avec le parambtre du degr6 d'usure de la dentition.

KEY WORDS : Apodemus, tooth abrasion, estimating age.

INTRODUCTION

Methods used in the study of fossil mammal material frequently differ from those used in the study of recent animals. When the fossils studied do have extant relatives, it would be desirable to apply the same criteria for both fossil and recent material, in

Mammalia, t. 66, no 2, 2002 : 275-284.

order to allow comparison of the results. In this paper we propose a method for estima- ting the age at death in two populations (one fossil and one recent) of rodents of the genus Apodemus.

Methods for estimating the age at death, that are based on soft tissue data, are not applicable in paleontology, nor are they in the study of material from owl pellets : Para- meters like body weight, or dry weight of the eye lens (Adamczewska-Andrzejewska 1971, 1973 ; Nabaglo and Pachinger 1979 ; Gurnell and Knee 1984 ; Qu6rB and Vincent 1989), are simply not available in fossil material or from owl pellets. Other parameters, like the condylobasal length of the skull, as defined by de Southern (1964), are not applicable because of the fragmentary preservation of fossil and owl-pellet material.

The dentition is a valuable criterion for estimating age, since teeth are worn down during the life of the individual. Several authors (Saint Girons 1966 ; Adamczewska- Andrzejewska 1973 ; Gurnell and Knee 1984 ; Lin and Shiraishi 1992) have proposed methods to estimate age in species of Apodemus, based on the wear pattern of the teeth. However, these are based on complete tooth rows that are present in specimens from owl pellets, and frequently available in fossils from karst fissure fillings, but hardly ever present in fossil material from stratiform deposits. In the latter case only isolated teeth may be expected to be available in statistically useful numbers. We tested a method based on isolated teeth of the dentition of Murinae. Our test material consists of a population of fossil Apodemus teeth from the Miocene of Italy and one of recent Apodemus material from Spain. Among the 6 different elements of the murid dentition we chose the lower third molar for our analysis, because of its simple dental pattern, that permits to work with relatively few parameters. In view of the positive results obtained in this pilot study, a next step might be to extend this test to other elements of the dentition ; it may be assumed that the method is useful for other species of Muri- nae, and may even be applicable in any other animal with bunodont teeth.

Age at death in itself is not an important issue. It may help, however, in evalua- ting population structure in a fossil population, compare it with extant faunas, and make assessments about wheter a fossil population may be representative for the living fauna it was derived from.

Our fossil material consists of 214 M3 (110 sin. and 104 dext.) of Apodemus gorafensis Ruiz Bustos et al., 1984 from the Late Miocene karst fissure Biancone in Gargano, Italy (Freudenthal 1976). This collection is kept in the National Museum of Natural History, Leiden, The Netherlands. It is often difficult to determine a species on the basis of its M3 ; in this case, however, there can be no doubt about the classifica- tion, because only one species of Apodemus is present in the locality Biancone.

We compared ths fossil material with a collection of 49 skulls (98 M3, 32 males and 17 females) of extant Apodemus sylvaticus (L.), captured in Doiiana National Park (Huelva, Spain), and kept in the collections of the Biological Station Doiiana (Sevilla), with catalogue numbers EBD. It has been chosen at random from the animals captured in the years 1973, 1978, 1980 and 1981, at various sites withn the park (covering an area of some 25 square km). Fossil populations are supposed to be based on the pre- dating activity of birds of prey, and their source area is supposed to be fairly large too. Of course our recent material cannot cover the thousands of years time span probably involved in the accumulation of the fossil material in a karst fissure.

TOOTH ABRASION n\T APODEMUS 277

METHODS

The wear surface of the M, was drawn in occlusal view, under a binocular micro- scope with camera lucida at 50 x enlargement, and the drawings were digitalized at 300 DPI. Direct digitalization of the specimens, without the intermediate step of drawing, is unpractical, due to shines, color stains and other irregularities on the tooth surface.

The occlusal surface is composed of protoconid, metaconid, hypoconid+entoconid, anterolabial cusp, bordelines of the dentine fields, enamel, and the exterior bordeline (Fig. 1). The surfaces of all these fields were measured in pixels, using the histogram tool of Adobe Photoshop 3.0 and 5.5.

Fig. 1. - Example of the occlusal surface of M3 in a juvenile (nr. EBD 924, WI = 12.0), adult (nr. EBD 1028, WI = 33.0) and senile (N. EBD 2621, WI = 62.0) individual of Apodemus sylvaticus from Doiiana. Pd = protoconid, Md = metaconid, HEd = hypoconyd + entoconid, En = enamel, D = dentine.

Theoretically the bordelines should not occupy space, but the intermediate step of drawing the specimens makes it inevitable that they occupy some 7 % of the tooth sur- face, so a correction should be made. We divided the number of pixels occupied by each borderline equally between the two bordered fields. The pixels occupied by the exterior border were attributed to the enamel field to an amount of 50 %, and the remaining 50 % was discarded.

Other possible correction methods are : attributing the surface of the borderlines to the corresponding dentine field, attributing it to the enamel field, or discarding it. We tried these out, and found, that the results are not substantially influenced by the cor- rection method chosen, and the differences are certainly smaller than the error caused by the imprecision of drawing. This also means that the method is very robust, since an imprecision of 7 % is not important for the final result.

The wear index, WI, defined as percentage of dentine in a molar surface, is calcu- lated as WI = 100D/(E+D), where D is the number of dentine pixels and E is the num-

, ber of enamel pixels, both corrected as described above. WI may be considered to be a measure for the relative age of the individual : WI = 0 in the unworn molars of very young individuals, and may come close to WI = 100 in the completely worn teeth of very old individuals (Fig. 1).

Following Gurnell and Knee (1984), we defined 10 classes of WI with 10 % inter- vals, and the number of individuals in each class represents the number of deaths during its interval (Fig. 2), so these classes are, to some extent, age classes.

However, they are defined as equal intervals of WI, and wear progress is not linear through time (Adamczewska-Andrzejewska 1973), or, in other words, our age classes do not necessarily represent equal time intervals. The amount of 10 classes is

MAMMALIA

a l l rn3

50 rn3 sin.

20

10

rn3 dext.

10

- r'o :o :o :o L 20 jo do $0 WI

Bioncone

rn3 sin. - r'o 2'0 i o lo do 20 jo 810 do WI

N

rn3 dext.

t::wl Doiiona

Fig. 2. - Histograms of WI of M3 of Apodemus sylvaticus for the Biancone and the Doiiana sample.

arbitrary. One might choose another amount, but there is no reason to assume that this would change the results. Gurnell and Knee (1984) also adopted the amount of 10 classes.

Furthermore, wear progress is not necessarily equal in all individuals (Gurnell and Knee 1984), so molars of equally old individuals may have a different WI, and fall in different age classes. This is clearly demonstrated in the histogram of Fig. 5, where the asymmetry of wear is plotted : left-hand and right-hand teeth of the same individual (in our Doiiana sample) frequently have a different degree of wear, and may belong to dif- ferent classes.

Reliability of the method

The best way to test the reliability of this method would be comparison with spec- imens whose age at death is known. For a population living in the wild this is normally not the case ; in laboratory populations, the age at death is a controllable factor, but wear progress is not necessarily the same in laboratory conditions and in natural sur- roundings (Gurnell and Knee 1984).

TOOTH ABRASION IN APODEMUS 279

The best correlation with age is found for the dry weight of the eye lens (e.g. Gur- nell and Knee 1984, R = 0.74), but for a comparison with our fossil material we need a method based on hard tissue.

Adamczewska-Andrzejewska (1973) proposed a method based on the definition of six stages of wear of the upper right molar row. Evidently the results are biased by

. subjective class attributions. Gurnell and Knee (1984) extended this method, and defined a wear index based

on 10 wear classes. In laboratory specimens they found a correlation coefficient of 0.62 ' between their wear index and age. The condylobasal length (CBL), measured from the occipital condyle to the anteroventral border of the premaxillary (see definition in de Southern 1964), resulted in a slightly lower correlation of 0.59. Logarithmic transfor- mation of the data gives a higher correlation coefficient (0.76).

None of these methods can be applied to the Biancone material because it consists of isolated teeth ; on the other hand, both the wear index and CBL may be measured in the Dofiana population. The CBL values have been obtained with vernier calipers with an error of ca 0.05 mm. The values obtained for CBL in the 49 specimens from Dofiana cover the range from 21.25 to 26.45 mm.

Table 1 gives the coefficients of correlation (linear and logarithmic) between WI and CBL for the entire Dofiana sample, and for several selections. The best correlations are found for CBLAogWI.

CBL does not measure age in natural conditions, but simply the size of the individ- uals. The paper by Gurnell and Knee (1984) is based on animals living in captivity with abundant water and food. Under natural conditions, the availability of food is not

, constant, and may influence growth substantially (Gurnell and Rennolls 1983), and CBL is just a measure of growth. Consequently, WI may be expected to correlate bet-

, ter to a true age parameter. Table 1 shows important differences between the two sexes ; R is much higher in

the females than in the males. This is difficult do explain, the more so since Gurnell and Knee (1984) found the opposite tendency : a higher correlation in males than in females.

An argument against using the parameter WI may be that wear does not only depend on age, but also on the hardness or softness of the food (Saint Girons 1966 ; Andrzejewski and Liro 1977 ; Nabaglo and Pachinger 1979) and on the resistance to abrasion of the enamel, that may vary per individual (Adamczewska-Andrzejewska 1966). Nevertheless, the data given by Gurnell and Knee (1984) for specimens of A. syl- vaticus in captivity, and Adamczewska-Andrezjewska (1971, 1973) for individuals of A. agrarius (Pallas, 1778) captured in the field, show that the degree of wear of the molars is still a good criterion to estimate age in these animals. The latter author indicates two characteristics of the wear process that are highly interesting : 1) it does not depend on the size of the animal, and 2) there are little differences between the sexes.

Using the degree of wear of the third molars implies a small limitation : it excludes from the studied population the very youngest age class, since the M3 break through in

- the third or fourth life week (Variavsky and Krylova 1948 ; Lin and Shiraishi 1992). For practical reasons this cannot be considered a problem, since at that age the animals have not yet been weaned (Lin and Shiraishi 1992), they remain in the nest, and there is hardly any chance thant they be caught, neither by bids of prey, nor by man.

The most important flaw in using wear as an age indicator is pointed out by Adamczewska-Andrzejewska (1973) : its discriminating power diminishes with grow- ing age. Anyway, this problem of decreasing resolution appears to be inherent to all methods of age estimation based on growth processes (Gurnell and Knee 1984).

TABLE 1. - Coefficients of correlation between WI and CBL in the Dofiana population. The separation of young and old individuals is arbitrarily chosen at WI = 28 (see Fig. 3).

Correlation R N CBUWl all specimens 0.5130 90 LogCBLNVl all specimens 0.5087 90 CBUlogW l all specimens 0.5792 90 IogCBUlogW l all specimens 0.5764 90

CBUWl male specimens 0.4414 58 LogCBUWl male specimens 0.4399 58 CBUlogWl male specimens 0.4821 58 IogCBUlogW l male specimens 0.4798 58

CBLNVl female specimens 0.6277 32 LogCBUWl female specimens 0.6210 32 CBUlogWl female specimens 0.6841 32 IogCBUlogWl female specimens 0.6790 32

CBLNVI left specimens 0.5221 45 LogCBLNVl left specimens 0.5176 45 CBUlogW l left specimens 0.6048 45 IogCBUlogW l left specimens 0.6019 45

CBUWl right specimens 0.5051 45 LogCBLNVl right specimens 0.5011 45 CBUlogWl right specimens 0.5564 45 IogCBUlogWl right specimens 0.5537 45

CBUWl young animals 0.4233 72 LogCBUWl young animals 0.4220 72 CBUlogWl young animals 0.4283 72 IogCBUIogW l young animals 0.4297 72

CBUWl old animals 0.0082 18 LogCBUWl old animals 0.0022 18 CBUlogW l old animals 0.0354 18 IogCBUlogWl old animals 0.0294 18

Demographic results

The histograms in Fig. 2 represent the structure of the populations from Dofiana and Biancone respectively, based on the value of WI. The most important difference is found in the first wear class, the least worn (youngest) individuals being more frequent in the Biancone material.

TOOTH ABRASION IN APODEMUS 281

In Fig. 3 CBL is plotted against WI for each individual of the Doiiana population. In this figure two groups of points may be distinguished : in the little-worn individuals ( M < 28) a small increase in wear coincides with a strong increase in size (CBL) ; or, in other words, the animals grow rapidly, and wear progress is slow. In the group with WI > 28 the opposite is observed : the skull doesn't grow anymore, and wear progresses

, steadily. The aspect of our Fig. 3 is very much like fig. 1 in Gurnell and Knee (1984), where CBL is plotted against age in days. The main difference is, that in their figure (based on a population in captivity) the number of old individuals is much hlgher, and the animals may live up to two years. In natural populations life expectancy is assumed to be not over one year (Niethammer 1978). The separation point between young and old individuals, that we place arbitrarily at about WI = 28, lies between 150-200 days of age in the laboratory population studied by Gurnell and Knee (1984).

+ female 0 mole

Fig. 3. - Correlation of WI and CBL in the Doiiana sample of Apodemus sylvaticus.

The survivorship curves (Fig. 4a with linear scale and Fig. 4b with logarithmic scale) for Dofiana and Biancone are very similar. Differences are observed in the last

; two WI classes, with percentages of 5 % or less for Dofiana, where one specimen more I or one specimen less would substantially change the shape of the curve.

Both curves are of Pearl's type I1 (Pearl 1928), and its interpretations is identical for both populations : the mortality rate is more or less constant, or, in other words, the probability of dying is independent of age, and not related to young individuals being less experienced, or old individuals being weaker than the rest of the population.

These is no significant difference in WI between left and right molars (see Table 2). The t-test gives a value of t = 0.100 at 96 degrees of freedom (p = 1.66 at 5 %

101

5c A Doiiono 4C 9 Blancone 3c

2c

1 c I

0 .? 5 : : : 2 +. c 0

E 1 L

.5

.4

.3

.2

Percentage dentine -0-0

Percentage dentine

Fig. 4. - Survivorship curves based on WI of M3 for the Biancone and the Doiiana sample.

TABLE 2. - Mean WI in the Doiiana population.

N Mean WI s.d. All M3 98 22.40 1 1.925 M3 sin. 49 22.53 12.173 M3 dext. 49 22.28 11.796

level). However in 21 specimens WI of M3 sin. is larger than WI of M3 dext., and in 28 specimens WI of M3sin. is smaller. We calculated t for the distribution of the dif- ferences between M3 sin. and M3 dext. per individual. The result, t = 0.734 at 48 degrees of freedom, is by no means significant. However, the histogram of Fig. 5 shows a bimodal distribution, with only a few cases in the class from - 1 to 0. This seems to indicate, that there are left-chewing and right-chewing individuals in more or less equal numbers, with a slight predominance of right-chewing.

DISCUSSION

The method presented here offers a way to quantify tooth wear and individual age, instead of estimating age in terms of very young, young, adult, senile, etc. It may be assumed to be useful in any tooth with more or less conical cusps, like many murids, artiodactyls, insectivores, carnivores, etc. In such teeth WTwill augment progressively

TOOTH ABRASION IN APODEMUS

DoRana WI dext. - WI s i n .

Fig. 5. - Asymmetric wear of M3 in the Dofiana sample of Apodemus sylvaticus.

(though not necessarily constantly) with age. It is, of course, not useful in teeth in which the walls of the crown are more or less parallel, like lagomorphs, microtids, many perissodactyls, etc.

The shapes of the survivorship curves and histograms for our two samples are remarkably similar. Assuming that the Dofiana samle represents a natural population, one might come to the conclusion, that the Biancone sample represents a natural popu- lation too, but we think such a conclusion is not warranted. The Biancone sample comes from a karst fissure filling that may have been accumulated over a long period of time, so it is -per definition- not a natural population. The resemblance with a natu- ral population may mean, that conditions have not changed fundamentally during the time of accumulation, and that our material is biologically homogeneous. Possibly a curve based upon material from a fissure filling with a mixture of material from several ages would show a different shape.

CONCLUSIONS

Estimating age at death on the basis of molar wear appears to be a valid method, but the calibration of our curves is problematic, due to the lack of a reliable parameter for estimating age in natural population. The method presented here offers a way to quantify tooth wear, and allow statistical treatment, in contrast to previously published methods, that rely on estimated wear classes.

The population structure of our fossil material from Biancone is remarkably simi- lar to the structure of our recent population from Doiiana.

ACKNOWLEDGEMENTS

We wish to thank the e Vicerrectorado de Investigati6n y Relaciones Internacionales de la Universidad de Granada >> for the grant that permitted one of us (N.B.) to carry out this investi- gation. We are grateful to Jose Cabot for the loan of the specimens of Apodemus sylvaticus from the collections of the << Estaci6n Biol6gica Dofiana >> (Sevilla), and to Daniel Garcia of the << Departamento de Biologia Animal y Ecologia >> of the University of Granada for his valuable comments. We are most grateful to the referees for their work, and we think they helped in improving our text considerably.

MAMMALIA

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