Daily layers and hibernation marks in incisor dentin of ...

Acta Theriologica 35 (3 - 4): 345 - 356, 1990. PL ISSN 0 0 0 1 -7 0 5 1

Daily layers and hibernation marks in incisor dentin of Sicista pseudonapaea and some biological remarks

Galina A. KLEVEZAL and Michail V. M IN A

IClevezal G . A. an d M ina M . V. 1990. D aily layers and hib ern atio n m arks in incisor dentin o f Sicista pseudonapaea an d som e biological rem arks. A cta theriol. 35: 3 4 5 -3 5 6 . T he stru c tu re o f the d entin o f A ltai gray birch m ouse Sicista pseudonapaea S trau tm an , 1949 incisors w as studied. T he birch mice were cau g h t in July 1976 (3 individuals) and in June 1988 (33 individuals) in southw est A ltai near M a rk ak o l L ake (a b o u t 1800 m a.s.l.). The labelling o f anim als w ith tetracycline w as carried o u t in 1988. A nim als’ age w as d eterm ined by the n u m b e r o f layers o f periosteal bone in the m andible. T he d entin o f incisors was studied in lo n g itu d in al g ro u n d sections and in longitudinal and cross sections stained with E hrlich hem atoxylin. D istinct daily layers in the d entin were revealed. M oreover, strongly c ontrasting b a n d s were found which are believed to be form ed du rin g hibern atio n . T he presence o f these h ib ern a tio n m ark s m akes it possible to estim ate the tim e o f w aking up o f birch mice from h ib ern a tio n . T his is possible as long as the m ark does n o t d isap p ear in the course o f grow th a n d the w earing o u t o f an incisor. It was speculated th at m ales o f birch mice w ake up ap p ro x im ate ly tw o weeks before females. M eth o d s o f estim ating eru p tio n rate an d the tim e o f com plete replacem ent o f incisors are offered. T he tim e o f com plete renew al o f up p er and lower incisors o f the sam e individuals proved to be a p proxim ately the sam e. In one year for old birch m ice it is significantly sh o rte r th an in older individuals (7", = 4 9 .6 + 1.17; T2= 57 .6 + 1.72 respectively). It is supposed th at hib ern atio n m ark s are form ed not only in birch mice but also in o th e r h ib ern a tin g rodents. N . K. K oltzov Institu te o f D evelopm ental Biology, U S SR A cad. Sci., 26 V avilov St., 117334 M oscov, U SSR K ey w ords: dentin, daily layers, h ibernation m arks, Sicista pseudonapaea

Introduction

An interest in the m icrostructure of mam malian teeth increases drastically after it was shown that the age of animals could be determined by the num ber of growth layers in tooth tissues. However the incisors of rodents did not draw much attention in this respect since it was clear that annual layers could not be accum ulated in them because of their rapid renewal thus making them unfit for age determ ination. However, it has been shown that one can judge retrospectively about various events in an anim al’s life using structural characteristics of layers in tooth tissues (Klevezal 1988). Therefore the incisors of rodents as recording structures (for the definition of the term see: M ina and Klevezal 1970, Klevezal 1988) should be reconsidered. It has been shown by oral biologists that as an incisor wears out a new dentin formed in the basal part of the incisor shifts to the apex of the incisor and then disappears (wears out). The rate of shifting ( = rate of eruption) determines the rate of dentin form ation. Since the rate of eruption can be rather fast, the incisor dentin in

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rodents is the most sensitive recording structure. The use of recording structure in ecological studies of rodents was began by tetracycline labelling o f nonhibernating rodents (Klevezal and M ina 1980, 1984). The purpose of this presentation was to dem onstrate possibilities of using dentin in ecological studies of hibernating rodents taking Altai gray birch mouse as an example. Material and methods All the A ltai gray birch m ice Sicista pseudonapaea S tra u tm a n , 1949 under study were caught by snap trap s n ear M a rk ak o l L ake (southw est A ltai, a b o u t 1800 m a.s.l.) on m o u n ta in slopes covered w ith shrubs and forest. T hree anim als were caught in July 1976 an d 33 were c au g h t in Ju n e 1988. In 1988 b ait with tetracycline w as p u t a t tw o p lo ts on a so u th e rn exposed slope covered m ostly w ith shrubs. Steam ed oats flakes w ith sunflow er oil were used as the bait (Klevezal and M ina 1980, 1984). T he bait on one p lot was arra n g e d in 5 row s w ith 20 bait po in ts in a row. T he distance betw een po in ts in a row and betw een row s was 5 m and 10 m, respectively. T he b a it was left a t plots for tw o days, then all the u n eaten b ait w as rem oved and 4 - 9 days later snap trap s were set on every second b ait points and in lines 10 m, 30 m and 50 m a ro u n d the plots. O ne up p er an d one low er incisor were taken from each anim als an d lateral an d m edial sides o f each o f the incisors were gro u n d till the pulp cavity w as exposed. T hese lo n gitudinal g ro u n d sections were stored in glycerine. T he rem aining upper an d low er incisors were decalcified a n d cross sections th ro u g h basal, m iddle an d apical p a rts o f the teeth were p rep ared using freezing m icrotom e. T he cross sections w ere stained with E hrlich hem atoxylin and m o u n ted in glycerine. T he up p er incisors o f fo u r individuals were used to m ake longitudinal stained sections th ro u g h the basal and the apical halves o f the to o th . All the gro u n d sections were studied in UV light (the source O U - 18). In sections w here tetracycline lines were found we m easured un d er the m icroscope, using an o cular m icrom eter, the distance from the basal m argin o f a section up to the p o in t w here the dentin ban d labelled w ith tetracycline co n ta cte d the enam el and also the w idth o f the dentin betw een the tetracycline line and the pulp cavity in different p a rts o f the section. The direct line distance from an incisor base up to its apex (AB, Fig. 1) w as m easured w ith a caliper rule or w ith an o c u la r m icrom eter before grinding the to o th . M easurem ents o f distance O K , K M (sagittal w idth o f an incisor) an d K N (w idth o f the labial wall) (Fig. 1) were m ade w ith an o c u la r m icrom eter on ground sections. G ro u n d sections o f six up p er and six low er incisors were p h o to g rap h e d . T h eir pictures were enlarged ( x 37.7) and using them the w idth o f the to o th wall w as analysed as a function o f d istance m easured along the o u ter arc o f an incisor outline from the basal m argin o f the section.

Fig. 1. Schem e o f m easurem ents o f birch m ouse incisors.

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All the stained sections were studied un d er the m icroscope, the m easurem ents being m ade w ith an ocu lar m icrom eter. T h e follow ing m ain ch aracteristics o f the incisors w ere used: incisor length (L) m easured along the o u ter arc o u tlin in g the labial m argin o f the longitudinal sections o f the incisor; rate o f e ru p tio n (R ) estim ated by the speed o f shifting o f a fixed po in t in d entin along the incisor; tim e o f com plete renew al o f an incisor (T) estim ated as the tim e (days) from the m o m en t w hen a d e n tin u nit is form ed a t the base o f the incisor, in its labial wall, up to the tim e w hen the den tin u n it disappears a t the apex due to incisor w ear; rate o f d entin ap p o sitio n estim ated as the rate o f w idth increase o f the incisor wall. Incisor length (L) w as calculated as:

L = nrn°/m °; r = AB2IW K +O K /2 n° w as estim ated while tak in g into a cc o u n t th a t sin n°/2 = A B /2 r (Fig. 1). R a te o f e ru p tio n was estim ated as R = d/t, w here d - distance from the basal m argin o f a n incisor up to the p o in t w here the tetracycline line c o n ta cts enam el on the labial wall o f an incisor (Fig. 1), t - period (days) from con su m in g tetracycline b a it by an anim al up to the tim e w hen the anim al w as cau g h t. Since the b ait w as offered to a nim als during tw o days we got m inim al an d m axim al possible values o f t w hich gave us m axim al an d m inim al, respectively, possible values o f R. R a te o f d en tin ap p o sitio n in the m iddle p a rt o f an incisor w as estim ated in a sim ilar way, as = L N /t, w here L N distance from tetracycline line to the edge o f the pulp cavity (Fig. 1). Age w as determ ined by the n u m b er o f grow th layers in the m andibles (Iv an te r 1973) o f 13 birch mice cau g h t in June a n d for 3 individuals c au g h t in July. In d u b io u s cases the c o u n ts o f layers in den tin and cem entum o f m o la r were used. These individuals served as sta n d a rd s w hen the age o f o th er anim als in the sam ple w as estim ated by the upper m o lars wear.

Results S ex and age composition o f the sample

In 1988 the birch mice were sampled on June 8 - 9 beyond the labelling plots and on June 1 3 -1 6 and June 1 8 - 19 at both the plots and beyond them. There was only 1 female am ong 21 birch mice caught before June 18, but there were 5 females among 12 individuals caught on June 1 8 -1 9 . Age of birch mice in the sample can be conveniently expressed as the number of winters survived by the animals. Studying m andible cross sections o f animals caught in June we found one cement line on some of them very near to the m argin of the bone. It was clear that those animals survived one winter. There were also specimens where no cement lines were formed but because they were adult we included them into the same age group as overwintered. The group of birch mice which survived one winter was thus represented in the sample of 1988 by 15 individuals. We also distinguished 15 animals which survived two winters, 2 animals which survived three and 1 animal which survived four winters. All three birch mice sampled in 1976 had survived two winters. There were not only prim ary but also additional lines in the bone tissue of birch mice which survived two and m ore winters. Distinct layers could sometimes be seen in cementum of lateral parts of cheek teeth roots and in secondary dentin of their crowns. These layers were used as a subsidiary criterium of age in dubious cases.

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Size o f the incisors

M ean upper incisor length (mm) was estimated as Lu= 8.9 ± 0 .96 (n = 23), mean lower incisor length L, = 11.7 ±0.111 (n = 11), mean sagittal width estimates of the incisors were Wu= 0.88 ± 1.225 (n = 6) and W {= 0.99 ±2.449 (n = 6), respectively. Mean width of labial wall (mm) in the middle of incisor was m a—0.257 ± 0.029 (n = 22) and ra, =0.210 ±0.031 (n = 16). W idth of the labial wall in the middle of the upper and the lower incisors makes up 67 and 52 per cent, respectively, of their maximal width. M easurements of the labial wall in different parts of an incisor from its base to the apex show that an increase of the wall width proceeds at a fairly constant rate until a little more than 1/2 of the interval in the upper incisor and about 2/3 of the interval in the lower one is reached (Fig. 2).

Fig. 2. Increase o f m axim al w idth o f the labial wall in a low er incisor o f a tw o-year old birch m ouse from the base to the apex.

Tetracycline labelling

Five birch mice were caught which had tetracycline lines in the dentin of their incisors. We could measure shifting of the label from basal margin (Fig. 1, d) of both the upper and the lower incisors in one individual only, which was one year old. For five or six days from consuming the tetracycline bait by the animal up to its death the basal point of the tetracycline line shifted for 900 mkm in the upper incisor and for 1239 mkm in the lower one. Thus the minimal and maximal possible rate of tooth eruption can be estimated for the upper incisor as Ru= 150- 180 m km /day and for the lower incisor as /?, = 207-248 mkm/day. As for this birch mouse Lu= 9.4 mm and L, = 12.2 mm, we estimate minimal and maximal time of the complete renovation as Tu= 52 - 63 days and T ,= 4 9 - 5 9 days. In all five labelled individuals the distance from the tetracycline line up to the pulp cavity in the basal part of the tooth was somewhat bigger than in the middle one. The rate of dentin apposition in the middle of the upper and lower incisors was estimated as 7 .0 -8 .9 m km /day and 6 .3 -8 .3 m km/day, respectively.

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Fig. 3. C ro ss section th ro u g h the m iddle p a rt o f the up p er incisor o f a tw o-year old birch m ouse (m ale, caught o n June 14). 1 — daily layers, 2 — strongly co n tra stin g b and (h ib ern atio n m ark). T able 1. T h e nu m b er o f d e n tin layers in the labial wall o f the m iddle o f the incisors o f birch m ic e .1n um ber o f survived w inters, 2/the value w as determ ined by dividing the w idth o f the wall by m ean w idth o f one layer, 3,for the higher values o f the T, 4/for the lesser values o f the T, u —upper, 1—lower.

Coll. No.

Sex

Age'

L ength o f the incisor (L), mm u

38 39 62 77 80

m m m m m

1 1 1 1 1

9.76

82 501 567 568 41

m m m m f

2 2 2 2 3

8.31 8.30

8.63 8.83 8.50

1

12.21 11.08

Tim e o f com plete renovation o f the incisor ( T), days

N u m b er o f dentin layers in the incisor u

1

u

1

24 23 25-26 25 26

23-24 24 25 24 24

48 46 50-52 50 52

46-48 48 50 48 48

28 31 302 29 26-27

27

56 62 60 58 52-54

54

m ean (u + 1)

49.6 ± 1.173/

/d = 3.84

9.58

11.24

12.00