FOSSIL POPULATION STRUCTURE AND MORTALITy OF THE CAVE BEAR FROM THE MOKRICA CAVE (NORTH SLOVENIA) STRUKTURA FOSILNE POPULACIJE IN UMRLJIVOST JAMSKEGA
MEDVEDA IZ MOKRIŠKE JAME (SEVERNA SLOVENIJA)
Irena DEBELJAK
1Izvleček UDK 902.035:569.74(497.4)
Irena Debeljak: Struktura fosilne populacije in umrljivost jam
skega medveda iz Mokriške jame (severna Slovenija):
Struktura fosilne populacije jamskega medveda iz Mokriške jame je bila proučena z namenom, da bi dobili nove podatke o vedenju in umrljivosti te izumrle vrste. Starost ob smrti je bila ocenjena za 128 različni� osebkov, in sicer na podlagi analize cementni� prirastnic, formiranosti korenine in obrabe krone levi� M1 zob. Po frekvenčni distribuciji primerkov v enolet- ni� intervali� la�ko ocenimo trend umrljivosti v različni�
življenjski� obdobji� in ji� interpretiramo glede na podatke o današnji� medvedi�. V prvotni mrtvi združbi so po vsej ver- jetnosti prevladovali mladiči. Izjemno kr�ki molarji manj kot 6 mesecev stari� živali se niso fosilno o�ranili. Enoletniki so najbolj številčno zastopana starostna skupina v fosilni popu- laciji iz Mokriške jame. Umrljivost je drastično upadla, potem ko so jamski medvedi preživeli svojo prvo �ibernacijo v drugi zimi. Najnižja stopnja umrljivosti je bila ugotovljena za sta- rostno obdobje 9-15 let, ko so bili jamski medvedi očitno na vr�uncu moči. Z analizo cementa ugotovljena najstarejša sta- rost znaša okoli 30 let, kar kaže, da je bila najdaljša življenjska doba podobna kot pri današnji� medvedi�. Analiza zobni�
tkiv je pokazala, da je bila umrljivost v jami sezonsko omejena – večina živali je poginila pozimi in zgodaj spomladi. Spolna struktura je bila raziskana na vzorcu 750 podočnikov. Izrazito večji delež samcev v skupini starejši� mladičev in napol odras- li� živali bi la�ko razložili z dejstvom, da je tudi pri današnji�
medvedi� obdobje osamosvajanja bolj kritično za samce. Pri mlajši� odrasli� in zreli� odrasli� živali� je bila umrljivost ver- jetno večja pri samica�. Spolna struktura odrasli� medvedov, še posebej starejši� živali, kaže, da so Mokriško jamo kot zimski brlog večinoma zasedali samotarski samci.
Ključne besede: jamski medved, umrljivost, starostna struk- tura, spolna struktura, cement.
1 Ivan Rakovec Institute of Palaeontology, ZRC SAZU, Novi trg 2, Ljubljana, Slovenia, e-mail: irenade@zrc-sazu.si Received/Prejeto: 25.09.2007
Abstract UDC 902.035:569.74(497.4)
Irena Debeljak: Fossil population structure and mortality of the cave bear from the Mokrica cave (North Slovenia) The fossil population structure of t�e cave bear from t�e Mokri- ca cave was evaluated to provide new data concerning t�e be-
�aviour and mortality of t�is extinct species. Age at deat� was estimated for 128 different individuals by analysing cementum increments, root formation and crown wear of left M1 teet�.
After t�e frequency distribution of specimens t�roug� one- year intervals, t�e mortality trends can be estimated for various lifetime periods, and interpreted in accordance wit� data for present-day bears. The original deat� assemblage was presum- ably juvenile-dominated. Extremely fragile molars of less t�an 6 mont� old cubs did not get preserved. yearlings are t�e most numerous age class in t�e fossil population from t�e Mokrica cave. Mortality drastically dropped after cave bears survived t�eir first �ibernation in t�e second winter. The lowest mortal- ity rate was observed in t�e 9-15 years age group, w�en cave bears would be expected to be in t�eir prime. The oldest age re- corded by cementum analysis is approximately 30 years, w�ic�
indicates t�at t�e maximum life span was similar to present-day bears. Study of dental tissues s�ows t�at t�e mortality in t�e cave was seasonally restricted – t�e majority of deat�s in t�e cave occured during winter and in early spring. Sex structure of t�e fossil population �as been studied on t�e sample of 750 ca- nines. The significantly �ig�er proportion of males in t�e group of older juveniles and subadults could be explained by t�e fact t�at t�e weaning period is more critical for males also in pres- ent-day bears. In young adults and prime adults t�e mortality was presumably �ig�er in females. The sex structure of adult bears, especially in t�e sample of older individuals, indicates t�at t�e Mokrica cave was used as winter den mostly by solitary males.
Keywords: cave bear, mortality, age structure, sex structure, cementum.
Age and sex structure of t�e fossil population of t�e cave bear (Ursus spelaeus) from t�e Mokrica cave (Mokriška jama) was studied to generate new data and insig�ts con- cerning t�e be�aviour and mortality of t�is extinct spe- cies.
Mokrica cave is located in t�e Kamnik-Savinja Alps (N Slovenia) between Mokrica and Košutna, at an alti- tude of 1500 metres, about 1000 metres above Kamniška Bistrica river valley. It is �orizontal, about 45 metres long and 7 to 20 metres wide �orizontal cave, formed in Tri- assic limestone. Its entrance faces in Nort�east. Mokrica cave, also called ‘Bear cave’ (Medvedja jama), �as been well known since t�e early 1800’s for its ric� accumula- tion of cave bear remains. Systematic excavations were carried out by Mitja Brodar during 1954-1956 and 1960 in t�e entrance part of t�e cave (Brodar, 1959). Ten strata (5-14) of Pleistocene sediments containing cave bear re- mains �ave been recognized t�ere, approximately 6 me-
tres t�ick. The majority of cave bear remains (isolated teet� and fragmented bones) were recovered from strata 6 and 7. Systematic palaeontological analysis of t�e mate- rial was carried out by Rakovec (1967). Cave bear domi- nates t�e assemblage; fossil remains of ot�er animals (wolf, cave lion and ibex) are scarce and �ave been found only in stratum 7 (Rakovec, 1967). Artefacts from strata 6 and 7 �ave been attributed to t�e Aurignacian cultural level (Brodar, 1959). Findings of M. Brodar suggest t�at cave bear remains mostly derive from t�e Middle Würm- ian period (OIS 3).
Data on t�e cave bear population structure from t�e Mokrica cave can be compared to data (Debeljak, 2004) obtained from a similar study of cave bear mate- rial from anot�er well-known Aurignacian station and Middle Würmian �ig� Alpine cave bear site Potočka zi- jalka, w�ic� is situated just 17 km away from t�e Mokrica cave.
MATERIAL AND METHODS INTRODUCTION
Fossil population structure of t�e cave bear from t�e Mokrica cave �as been derived from teet� t�at were col- lected along wit� ot�er fossil material during Brodar’s excavations more t�an 50 years ago. The majority of t�e material is stored in t�e Natural History Museum in Ljubljana, but some specimens are kept in t�e palaento- logical collection of t�e Department of Geology, Univer- sity in Ljubljana. The sediment �as not been water-sieved during t�e excavations in t�e Mokrica cave. For t�is rea- son, milk teet� were not recovered and it is t�erefore not possible to study t�e mortality of cubs younger t�an 6 mont�s.
I analysed fossil population age structure using first lower permanent molars (M1), bot� isolated and still at- tac�ed in mandibles. Only left specimens were analysed to prevent t�e doubling of data for t�e same individu- als. For 107 isolated M1 specimens age at deat� was de- termined by t�e analysis of dental tissues (dentine and cementum) in cross sections t�roug� t�e root, about 7.5 mm below t�e crown. Thin sections and etc�ed/coloured ground sections were prepared for all samples (for labo- ratory tec�niques see Debeljak, 1996; 2000). In juvenile specimens I measured t�e t�ickness of t�e root wall. The relative widt� of pulp canals and dentine increments can be used as criteria to assess individual age in juveniles and subadults (up to t�e 4t� year). The individual age of all teet� belonging to animals older t�an 1 year was de-
termined by counting cementum increments. This is t�e most reliable and objective met�od for determining in- dividual age of many mammals and �as long been used routinely in present-day bears as well (see Debeljak, 2000 and references t�erein). Cementum is a bone-like tissue t�at is continuously deposited on t�e surface of toot�
roots t�roug�out life. Dental cementum is composed of so-called ‘winter’ and ‘summer’ layers, similar to growt�
rings in trees (Fig. 1). After t�e nature of t�e outermost cementum increment, it is sometimes possible to deter- mine even t�e season of deat�. Dental tissues of 21 left M1 teet� still attac�ed in mandibles �ave not been micro- scopically examined, because it would be impossible to extract t�em wit�out visible damage to t�e jawbone. For t�ese specimens individual age was roug�ly estimated by comparing t�eir crown wear patterns to t�e teet� of known individual age. Altoget�er, age at deat� �as been estimated for 128 different individuals.
In bears, sex can be determined after dimensions of canines. Several studies indicate t�at t�ere is no signifi- cant difference between measurements of lower and upper canines (Koby, 1949; Kurtén, 1955; Rabeder, 2001). Bot�
upper and lower specimens were included in t�e anal- ysed sample, because it is often impossible to distinguis�
between t�em w�en t�ey are damaged or �eavily worn, especially in t�e group of older individuals. Measure- ments of crown widt� of 377 undamaged specimens were
taken at t�e crown base. The �istogram of crown widt�
s�ows clear bimodal distribution (Fig. 2); specimens
wit� crown widt� larger t�an 18.5 mm were attributed to males and specimens wit� crown widt� smaller t�an 18
mm as belonging to females.
Mean value of crown widt� is 21.3 mm for males and 15.8 mm for females. Also Kur- tén (1955) determined mean size difference of more t�an 5 mm and joint overlap of less t�an 1 per cent in t�e Mixnitz sample. Sexual dimorp�ism is so well pronounced in ca- nines t�at usually even t�ose specimens t�at are too dam- aged or worn to be measured accurately can be sexed based on t�eir overall dimensions.
Altoget�er, population sex structure (males to females ratio) �as been establis�ed on t�e sample of 750 canines (isolated and t�ose still attac�ed in jawbones) of indi- viduals older t�an 18 mont�s. Canines of yearlings were omitted from t�e study because t�ey are composed only of a �ollow crown cone w�ic� is usually damaged in t�e lowermost part and t�erefore difficult to measure.
All t�e canines �ad been previously separated into 3 different age classes. Canines wit� t�e root widely open at t�e apex are classified into t�e group of older juveniles and subadults. Our studies of canines, w�ose individual age �as been determined by cementum analysis, s�ow t�at t�eir root closed at t�e age of 5-6 years. Canines wit�
a closed root can be t�erefore all assigned adult status.
Adult specimens were furt�er divided into two groups:
specimens wit� slig�tly to moderately worn crowns, and specimens wit� markedly to �eavily worn crowns. There is no clear distinction between t�e two groups. In ca- nines, crown wear was quite erratic and �eavy wear does not necessarily indicate old age. Canines could �ave been broken and pat�ologically worn even by prime adults.
However, preliminary study of dental cementum in ca- nines from Divje babe cave indicates t�at t�e majority of specimens from t�e first group belonged to 5-15-year- old individuals (young- and prime adults) and specimens from t�e second group to 15-25-year-old individuals (older adults).
Fig. 1: Dental cementum in the cave bear m1 (inv. number m. 4228) from the mokrica cave.
‘Summer’ and ‘winter’ increments can be seen, divided by lines of arrested growth. Dark dots are cementocyte lacunae. An arrow indicates the direction of cementum deposition. Thin section;
plane-polarized light (left image) and cross-polarized light (right image). The field is about 0.8 mm wide.
Fig. 2: histogram for crown width (in mm) of upper and lower canines from the mokrica cave. Bimodal distribution shows marked sexual dimorphism.
RESULTS AND DISCUSSION
AGE STRUCTURE:
The results of t�e individual age analysis of 107 isolated left M1 teet� are s�own in t�e frequency �istogram of individuals in one-year age classes (Fig. 3). Frequencies
in some of t�em are not integers because, for example, in t�e case of a specimen wit� age estimation of 14-15 years, one �alf was attributed to t�e class of 14-year-olds and one �alf to 15-year-olds. From t�is profile, t�e course
of mortality in t�e cave can be estimated for various life periods or age groups. Fig. 4 s�ows t�e proportions of different age classes in t�e sample of 128 left M1 speci- mens, including bot� isolated and t�ose still attac�ed in t�e jawbones. yearlings and 2-year-olds are considered to be juveniles, 3-4-year-olds are subadults, 5-8-year-old individuals young adults, 9-20-year-olds prime adults and t�ose older t�an 20 years old adults (Craig�ead et al., 1995). The mortality in t�e main age groups will be discussed briefly:
Like in present-day bears, cave bear cubs were born in a winter den, most probably in December or January.
Still unerupted M1 crowns of 2-4-mont�-old cubs t�at died in t�e first winter, before t�e onset of spring, were
�ollow and fragile as an egg s�ell. They were completely destroyed post mortem and are t�erefore not represented in t�e analysed sample (Fig. 3). In Divje babe cave, w�ere all t�e sediment �as been water-sieved during t�e excava- tions, a very large number of milk teet� �ave been recov- ered. The analysis of fossil population structure from Divje babe I site (Debeljak, 2002) indicated t�at t�e mortality
was �ig�est for t�is youngest age group. Most likely, t�is was t�e case in t�e Mokrica cave as well, w�ic� is indicat- ed by an arrow in Fig. 3.
yearlings (9-15-mont�- old individuals) are t�e most numerous age group in t�e fossil population wit� a 21.1% s�are (Figs. 3, 4), sug- gesting t�at t�e mortality in t�e second winter was very
�ig�, alt�oug� presumably lower t�an t�e mortality in t�e first winter, as explained above. Very �ig� mortality in yearlings may be at least partly attributable to differ- ent predators, suc� as wolves and male bears. Cannibalistic be�aviour of adult males and t�eir aggression towards cubs is well known in present-day bears (McCulloug�, 1981;
Tietje et al., 1986; LeCount, 1987; Miller, 1990; Craig-
�ead et al., 1995; Deroc�er &
Wiig, 1999). Bears �ibernate in t�eir second winter for t�e first time (Craig�ead et al., 1995; McNamee, 1997).
Difficulties in establis�- ing and maintaining special metabolic processes t�at are c�aracteristic of �ibernation (Nelson et al., 1983; Hellgren, 1998) could be important factor for t�e observed �ig� mortality during t�is period.
W�en t�eir fat reserves are not adequate, bears are un- able to start �ibernating, �owever, only �ibernation en- ables t�em to survive more t�an 6 mont�s wit�out food.
Cases of yearlings t�at died in t�eir den during especially long winters �ave been reported for present-day bears (Sc�oen et al., 1987; Rogers, 1981; 1987).
The frequency of 2-year-old individuals is also �ig�, but significantly lower t�an in yearlings (Fig. 3), w�ic�
indicates a s�arp decline in mortality after t�e second winter. The proportion of yearlings in comparison to 2- year-olds was presumably even larger originally, because t�eir teet� were muc� more prone to destruction. M1 teet� of yearlings �ave only about 1.25 mm t�ick root wall and are t�erefore more easily fragmented t�an M1 teet� of 2-year-old individuals wit� mec�anically more resistant, approximately 2.5 mm t�ick root wall.
The measurements of t�ickness of root wall by juve- Fig. 3: ‘mortality profile’ – frequency distribution of isolated left lower m1 teeth through one-year
intervals (N=107). Extremely fragile molars of less than 6 month old cubs were not preserved. The supposed highest mortality in this age group is indicated by an arrow.
Fig. 4: pie chart with proportions of left m1 teeth; isolated and still attached in mandibles (N=128) from different age groups.
nile M1 teet� (Fig. 5) yielded anot�er conclusion, t�at t�e mortality in t�e cave was not distributed evenly t�roug�- out t�e year. The first peak in t�e distribution s�own in Fig.
5 represents yearlings t�at died during t�eir second winter or in t�e following early spring. The second peak consists of 2-year-olds t�at died in t�eir t�ird winter. The interven- ing �iatus indicates t�at bears were absent from t�e cave during t�e second summer period. Specimens t�at could be attributed to about 6-8-mont�-old cubs are also missing in t�e sample. Bot� gaps in t�e mortality profile suggest t�at cave bears were not returning to t�e cave after t�ey
�ad left it in t�e spring (from April to early June). Per�aps t�ey visited it occasionally, but t�ey stayed away at least during mont�s of intensive feeding. Microscopic analysis of growt� layers in dental tissues (dentine and cementum)
�as also s�own t�at great majority of deat�s in t�e Mokrica cave occured during t�e �ibernation and in a s�ort post-
�ibernation period. The same results from Potočka zijalka (Debeljak, 2004) contradict some older assumptions t�at
�ig� Alpine caves were continuously occupied by bears in all seasons, all t�e year round (E�renberg & Sickenberg, 1929; E�renberg, 1967).
It seems t�at mortality of 3-year-olds was not muc�
lower t�an in 2-year-olds (Fig. 3). Alt�oug� 3-year-olds were p�isically stronger, t�is was probably quite a critical age. Brown bear females usually abandon t�eir young at t�e age of about 2.5 years, sometimes one year earlier or later (Bunnel & Tait, 1981; Craig�ead et al., 1995). Promi- nent decline of mortality by 4-year-olds (Fig. 3) indicates t�at t�e weaning period presumably took place already in t�e t�ird year, like in present-day bears.
Frequencies of M1 teet� belonging to young adults are more or less evenly distributed t�roug� one-year in- tervals (Fig. 3). At t�e age of 5-8 years brown bear females w�elp t�eir young for t�e first time (Bunnel & Tait, 1981;
Craig�ead et al., 1995). The mortality could be somew�at
�ig�er in young, inexperienced cave bear females in comparison to older females. Fig. 3 s�ows t�at t�ere was a furt�er decline of mortality by prime adults, after t�e age of 8, and became more prominent again after t�e age of 15. As in present-day bears, t�e mortality caused by factors ot�er t�an man is extremely low in t�is age group (McNamee, 1997). Rare cases of 9-15-year-old bears t�at died in t�e Mokrica cave could be explained by natural causes (injuries, disease) and per�aps even by occasional
�unting by pre�istoric man. Rakovec (1967) assumed t�at man came to t�e cave from time to time only, above all during t�e summer mont�s w�en �e �unted cave bears. However, t�ere is no direct evidence for cave bear
�unting in t�e Mokrica cave. Furt�ermore, t�e supposed
�unting would probably take place during �ibernation, w�en bears were in t�e most vulnerable position.
It seems t�at in t�e group of older adults, mortality
started to rise already at t�e age of 16 (Fig. 3). In Potočka zijalka moderate mortality increase was observed not earlier t�an around t�e age of 20. This difference could be due to a small sample size. The proportion of individuals older t�an 20 years was most likely �ig�er in t�e origi- nal deat� assemblage t�an it could be inferred from t�e analysed sample (Fig. 3), because remnants of extremely worn teet� were easily broken and �ave not been recov- ered during excavations. There is a somew�at lower pro- portion of older adults in t�e sample of isolated M1 teet�
(Fig. 3) t�an in t�e sample of all M1 teet� (Figs. 4 and 5), because 12 out of 21 M1 specimens still firmly attac�ed in t�e mandibles were attributed to individuals older t�an 15 years, based on t�eir crown wear. However, wit�out sectioning t�e root and microscopic examination of den- tal cementum it was not possible to accurately determine t�eir individual age.
Mortality around t�e age of 20 can be attributed to senescence-related malnutrition and disease. M1 teet�
were �eavily worn by t�is age, sometimes to suc� a de- gree t�at pulp canal in t�e root became exposed. Dental pat�ologies are very frequent on jawbones of older cave bears, t�at could �inder feeding to suc� degree t�at ani- mal could not gain enoug� weig�t before winter. It �as been suggested in t�e past t�at t�e longevity in cave bear was severely restricted by t�e �eavy wear of teet� and t�at it was somew�at s�orter t�an in t�e brown bear, because cave bear teet� wore out more quickly. Kurtén (1958; 1976) expressed doubt t�at any cave bear lived beyond about twenty years of age. The mortality profile from t�e Mokrica cave does not support t�is �ypot�esis.
The oldest age recorded is approximately 30 years, w�ic�
indicates t�at t�e maximum life span was similar to pres- ent-day bears; in t�e wild t�eir life span is 25-30 years (Bunnel & Tait, 1981; Deroc�er, 2000).
Fig. 5: Number of m1 specimens in classes with particular thickness of root wall, measured approximately 7.5 mm under the crown.
Specimens (N=42) belonged to yearlings and 2-year-olds.
The comparison of proportions of different age groups for t�e Mokrica cave and Potočka zijalka is given in Fig. 6. The most striking difference between t�e two sites is in t�e proportion of individuals younger t�an 18 mont�s, w�ic� is muc� �ig�er in Potočka zijalka (64%
vs. 21.1% in t�e Mokrica cave). Suc� a discrepancy could be at least t�eoretically explained if Potočka zijalka cave would be used primarily by females and t�eir offspring and t�e Mokrica cave on t�e ot�er �and by adult males.
However, t�is was obviously not t�e case. On t�e con- trary, males predominate in Potočka zijalka in even
�ig�er proportion t�an in t�e Mokrica cave. It is also not likely t�at different ecological conditions would influ- ence t�e survival rate of juveniles to suc� a great degree.
As it �as been already pointed out, teet� of individuals younger t�an 18 mont�s are �ollow and were t�erefore muc� more prone to destruction t�an teet� of older in- dividuals. In t�e analysed sample of 89 M1 teet� from Potočka zijalka, t�ere are even 10 specimens (11.2%) of 2-4-mont�-old individuals, w�ic� indicates t�e excep- tionally good state of preservation of t�e fossil material.
This assumption is furt�er supported by t�e very �ig�
ratio of M1 teet� of yearlings to mec�anically more resis- tant M1 teet� of 2-year-olds; t�e ratio of 8.5 to 1 �as been establis�ed in t�e Potočka zijalka sample, in comparison to less t�an 1.5 to 1 ratio for t�e Mokrica cave. Teet� and mandibules from Potočka zijalka are generally in a muc�
better state of preservation t�an t�ose from t�e Mokrica cave. Discrepancy in t�e proportion of less t�an 18 mont�
old individuals between t�e two sites could be t�erefore explained by different tap�onomic conditions. Sediment
protects skeletal material from destructive tap�onomic factors, suc� as scavenging, trampling and weat�ering (Lyman, 1994). The condi- tions were more favourable for preservation of juvenile teet� w�en t�e material be- came quickly buried in t�e sediment, w�ic� was prob- ably t�e case in Potočka zi- jalka. Furt�ermore, t�e pro- portion of �ollow and for t�is reason often fragmented juvenile teet� is also related to t�e accuracy in collecting fossil material during excava- tions. In Potočka zijalka also fragmented teet� were recov- ered during t�e new 1997- 2000 excavation campaign (Pac�er et al., 2004). On t�e ot�er �and, only more or less complete teet� were kept in t�e more t�an 50 year old collection of cave bear material from t�e Mokrica cave.
This could additionally contribute to t�e underrepresen- tation of juvenile specimens in t�e analysed sample. If we exclude M1 specimens of less t�an 18 mont� old in- dividuals, t�at are most sensitive to different tap�onomic conditions and recovery tec�niques, we get very similar proportions of t�e remaining age groups for bot� sites:
t�ere are 40.1% of 2-4-year-olds, 39.6% of young- and prime adults, and 20.3% of more t�an 20 years old adults in t�e Mokrica sample (n = 101), and 37.5% of 2-4-year- olds, 35.9% of young- and prime adults, and 26.6% of old adults in t�e Potočka zijalka sample (n = 32).
After t�e model proposed by Stiner (1990; 1994;
1998), t�e fossil population from t�e Mokrica cave ex�ib- its t�e so-called ‘normal non-violent attrition’ (NNVA) mortality pattern, w�ic� is in Stiner’s opinion consistent wit� �ibernation-related mortality, caused by non-vio- lent factors, principally starvation. However, in Potočka zijalka, w�ere fossil material is better preserved, ‘juve- nile-dominated’ mortality pattern �as been establis�ed (Fig. 7). Considering t�e presumed underrepresentation of juveniles in t�e analysed sample, t�e original deat�
assemblage was most likely distinctly juvenile-domi- nated in t�e Mokrica cave as well. The predominance of juveniles �as been reported from many cave bear sites (Bäc�ler, 1957; Kurtén, 1958; Rabeder, 1992; Andrews &
Turner, 1992; Grandal d’Anglade & Vidal Romaní, 1997;
Weinstock, 1999; 2000; 2001; Pac�er, 2000; Germonpré &
Sablin, 2001; Debeljak, 2002; Turner, 2002). In my opin- Fig. 6: Frequencies and percentages of left lower m1 teeth in different age classes from mokriška
jama (N= 128) and potočka zijalka (N=89).
Fig. 8: The proportion of female canines to male canines (upper and lower) in different age groups for the mokrica cave (N=750) and potočka zijalka (N=90). Number of specimens is shown inside the bars.
ion, t�e juvenile-dominated population structure is c�ar- acteristic of �ibernation-related mortality in typical cave bear sites, provided t�at fossil material, including fragile juvenile teet�, is well preserved and accurately collected.
Mortality patterns in t�e Mokrica cave and Potočka zi- jalka s�ow no evidence for intensive, non-selective cave bear �unting, w�ic� s�ould, according to Stiner (1990;
1994; 1998), affect all age groups randomly and result in
t�e so-called ‘living structure’ (LS) mortality pattern, re- sembling t�e age structure of t�e living population.
SEx STRUCTURE:
The results of sexing 750 canines of individuals older t�an 18 mont�s are presented in Fig. 8. In t�e group of older juveniles (2-year-olds) and subadults (3-4-year-olds), males predominate over females wit� 1.8 to 1 ratio (63.8%
males, 36.2% females). This could be explained wit�
�ig�er mortality rate of males in t�is age class, w�ic� was observed also in populations of present-day bears. The weaning period is obviously somew�at more critical for males t�an females. Subadult females are better tolerated by adult females and are often allowed to stay in t�e same
�ome range after weaning, w�ile subadult males usually travel larger distancies to establis� t�eir own territory (Elowe & Dodge, 1989; Swenson et al., 1998; McLellan
& Hovey, 2001). They can be outcompeted in t�e searc�
for food and can fall pray to adult males, w�o can be very aggressive toward subadult males. Intraspecific compe- tition is especially en�anced by �ig� population density and s�ortage of available food (McCulloug�, 1981; Bun- nel & Tait, 1981; Tietje et al., 1986; McNamee, 1997).
The proportion of males and females is nearly equal in t�e group of young and prime adults (~ 5-15-year-olds).
Hibernation period is considerably prolonged in females wit� cubs in comparison to solitary males in present-day bears (Judd et al., 1986; Sc�oen et al., 1987; Sc�wartz et al., 1987; Miller, 1990; Van Daele et al., 1990; McNamee, 1997; Manc�i & Swenson, 2005). Furt�ermore, females can get injured w�en protecting t�eir offspring. We as- sume t�at mortality was actually somew�at �ig�er in adult females t�an in adult males. However, if solitary males denned in t�e Mokrica cave more frequently, t�is could result in approximately equal proportion of bot�
sexes in t�e fossil population.
In present day bears, females first produce cubs by t�e age of 5-8 years (Bunnel & Tait, 1981; Craig�ead et al., 1995).
In t�e sample of 247 canines of prime and young adults, 42 specimens belong to t�e 5-8 years age class. Interestingly, 28 of t�em are of females and only 14 of males (2:1 ratio). It s�ould be taken into consid- eration t�at large, unworn ca- nines are t�e most attractive finds and t�at males could be artificially underrepresented in t�is age group. If not, t�is Fig. 7: Tripolar graph denoting proportions of three main age
categories in the samples of left m1 teeth from the mokrica cave and potočka zijalka. After Stiner (1990; 1994; 1998) – dark grey field represents NNVA (normal non-violent attrition) mortality pattern and light grey LS (living structure) pattern.
CONCLUSIONS
Mortality pattern in t�e Mokrica cave s�ows no evidence for intensive, non-selective cave bear �unting. The age structure of t�e analysed sample of t�e cave bear fossil population ex�ibits t�e so-called ‘normal non-violent at- trition’ (NNVA) mortality pattern, wit� a 52.7% s�are of juveniles and subadults (1-4-year-olds), 31.3% of young- and prime adults (5-20-year olds) and 16% of old adults (>20 years old individuals). The Mokrica cave was used by cave bears as a winter and birt�ing den and was pre- sumably not visited regularly during summer mont�s.
The majority of deat�s occured during �ibernation or in s�ort post�ibernation period, mainly because of star- vation and predator attacks. Mortality was presumably
�ig�est in t�e first winter w�en cubs were born, �ow- ever, extremely fragile crowns of M1 teet� of individuals younger t�an 4 mont�s �ave not been preserved. year- lings (9-15- mont�-old individuals) are t�e largest age class wit� 21.1% s�are. Hollow teet� of yearlings presum- ably underwent muc� �ig�er tap�onomic losses t�an mec�anically more resistant teet� of older individuals. It can be inferred t�at t�e proportion of individuals young- er t�an 18 mont�s was muc� �ig�er originally and t�at t�e deat� assemblage was markedly ‘juvenile-dominated’.
The mortality rate dropped drastically after cave bears successfully survived t�e second winter. The next signifi-
cant decrease of mortality occured at t�e age of 4 years, w�ic� indicates t�at females left t�eir young in t�e t�ird year on avarage, like in present-day bears. The weaning age is especially critical for subadult males, w�ic� could explain �ig�er proportion of males (63.8% M, 36.2% F) in t�e group of older juveniles and subadults.
The mortality rate in young adults (5-8-year-olds) was still muc� �ig�er t�an in prime adults. From t�e age of 9 to 15 years, deat�s were very rare. Mortality was pre- sumably �ig�er in females, because t�ey �ibernated lon- ger t�an males. However, solitary males probably denned in t�e Mokrica cave more frequently, w�ic� resulted in approximately equal proportion of bot� sexes (51.8% M, 48.2% F) in t�e studied sample of canines of young- and prime adults (5-15-year-olds). In t�e sample of older adults, �owever, males clearly predominate (73% M, 27%
F), w�ic� is anot�er indication t�at t�e Mokrica cave was mostly occupied by solitary males. Old females c�ose t�e cave for t�eir denning place less often. Moderate increase in mortality can be observed already after t�e age of 16.
Mortality around t�e age of 20 can be attributed to se- nescence-related malnutrition and disease. The oldest age recorded by cementum analysis is approximately 30 years, w�ic� indicates t�at t�e maximum life span was similar as in present-day bears.
could be t�e evidence t�at maternity-related mortality was increased especially in t�e group of very young, in- experienced females.
In t�e group of older adults (~ 15-25-year-olds) males considerably outnumber females (73% males, 27%
females; 2.7:1 ratio), w�ic� is anot�er indication t�at t�e Mokrica cave was mostly occupied by solitary males. Old females obviously c�ose t�e cave for t�eir denning place
less often. In Potočka zijalka, even �ig�er proportion of males (83.3%) is observed in t�is age class. The predomi- nance of males cannot be explained by t�e assumption t�at males lived longer on average t�an females. This is not t�e case in present-day bears. On t�e contrary, some data even s�ow t�at females live longer on average t�an males (Craig�ead et al., 1995; McNamee, 1997; Wiig, 2000).
ACKNOWLEDGEMENTS
I t�ank Mitja Brodar and t�e Institute of Arc�aeology, ZRC SAZU, for t�e material provided by t�eir excava- tions, and Katarina Krivic for �er �elp wit� t�e cave bear collection in t�e Natural History Museum. I am also very grateful to Kata Cvetko-Barić for t�e laboratory prepa-
ration of t�in- and ground sections, to Vida Po�ar and Martina Pac�er for valuable comments and critical re- view of t�e text, and to Glenn Jaecks for language-edit- ing.
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