ACCUMULATION RATE OF DENTINAL GROWTH
LAYERS IN THE MAXILLARY TOOTH OF THE
SPERM WHALE*SEIJI OHSUMI, TOSHIO KASUYA AND
MASAHARU NISHIWAKIINTRODUCTION

The growth layers in the dentine of tooth as an age characteristic for the delphinids
was first described by Nishiwaki and Yagi (1953). Yablokov (1958) reported thelayered growth in the section of teeth of sperm whale. Then, Nishiwaki, Hibiya andOhsumi (1958) reported that the buried teeth in the maxillary gum are most usefulfor age determination of the sperm whale. Nishiwaki, Ohsumi and Kasuya (1961)
re-examined Laws's report (1960) on the lamination in the mandibular boneof the sperm whale and showed that the accumulation rate of the lamination isequal to that of the growth layers in the dentine of maxillary tooth.
The maxillary teeth have been collected in the coastal waters to Japan andnorthern part of the North Pacific on many sperm whales, and they have beenused to get the age distribution of the sperm whales caught in these waters. Nevertheless, it is necessary to make sure of the interpretation of the dentine growth(accumulation rate of growth layers) for the calculation of mortality rate or fluctuation of the population size from the age distribution of the sperm whale.
Relating with the subject, Nishiwaki and Yagi (1953) tried to make a time
mark in the teeth of the blue-white dolphin (Stenella caeruleo-alba) by the intra vitum
staining method consisting of the injection of the lead acetate into the dorsal muscle,
but they could not get a expected result, obstructed by the difficulty of the breedingof the dolphin. Sergeant (1959) studied the teeth of the bottlenosed dolphin(Tursiops truncatus) which was born at an aquarium and died there, and he got
the result that the growth layers are accumulated one every year in the dentine ofthe dolphin. Recently, Sergeant (1962) reported that the same accumulation rate
could be used in the case of the pilot whale (Globicephala melaena) studying the stageof dentine deposition.
The teeth of the sperm whale differ from those of delphinid whales in theirstructure. For example, the pulp cavity is closed in the teeth of old delphinid
whales, whereas in the sperm whale it is not closed all over the life. Then, there
are some questions in application of the accumulation rate of the growth layerof delphinid whales to the sperm whale. And it is needed to study the accumulation
rate of growth layers in the teeth of the sperm whale.
Following methods may be used to investigate on the subject;
1. Breeding experiment which was used by Sergeant (1959).

* Dedicated to Proffessor T. Ogawa for his sixtieth birthday
15

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

16

2.

Time marking in the teeth of living animal which was used by Nishiwaki
and Yagi (1953).
3. Investigation of teeth from the recaptured whales.
4. Study of the seasonal change of the stage of growth layers in the dentine.
5. Comparison of dentine deposition stage with the physiological conditionof the whale.
6. Comparison with other age characters and the ecological knowledges.
7. Population dynamical examination on the age distribution based on
the number of growth layers.
Among these methods, the first and the second can not be used for the sperm whale
because of the difficulty of the breeding at present.
We studied, in this report, chiefly on the seasonal change of the width ofgrowth layer, and examined the result by the teeth of recaptured whales and by
the age distribution based on number of growth layers. After then we have got
a conclusion on the accumulation rate of the growth layers in the dentine of the
sperm whale.
We are very much indebted to the staffs of our Institute, namely Dr. Kazuo
Fujino and Messrs. Takahisa Nemoto, Tadayoshi Ichihara and Keiji Nasu, whohelped us in collection of tooth samples and the biological investigations of sperm
whales. Many thanks are also given to assistant proffessor Dr. Takashi Hibiya of
the Faculty of Agriculture, Tokyo University and to Dr. Fukuzo Nagasaki of the
Takai Regional Fisheries Research Laboratory who kindly discussed on our draft.
MATERIAL AND METHOD

Sperm whales possess about 13 pairs of teeth in their upper jaw. There are somewhales whose maxillary teeth are exposed out of the gum in various numbers. In
practice, one tooth was collected from each whales, and the tooth was demanded
to have been hurried in the gum and to have been placed at the nearly central partof the teeth raw.
The collected teeth were boiled for about half an hour, removed from thegum, then they were dried. In the laboratory the teeth were cut longitudinallywith a saw and a grinder into a plane which containes the long axis of tooth, andthen the surface of the longitudinal halves was polished with a whetstone. The
polished surface, which we observe, is not necessarily a flat plane, but need along
the longitudinal axis. After the number of the dark band of the growth layers werecounted under the binocular dissecting microscope ( X 8), the feature of growthlayers were projected on the screen by means of reflecting surface projector ( x 20)
and their width were measured. As shown in Fig. 1 the dark band of the growthlayers were measured at two points on the dentine, namely the width from themargin of the pulp cavity to the starting margin of the latest dark band and thatfrom the latest dark band to the dark band formed in the previous season. And
on the ten specimens, we measured the width of all growth layers of the teeth.
Some teeth were ground to translucency for microscopic sections and stained with

DENTINAL GROWTH LAYER OF SPERM WHALE

17

carbol-fucsin or hematoxillin and their fine structure was observed.
The teeth of 448 sperm whales were used for the study of seasonal accumulationof growth layers in the dentine, and they were collected between the first decade of
May and that of November mainly in 1961 and partly in 1960 in the coastal waters
to Japan and the northern part of the North Pacific.
The age distribution of the 1226 female sperm whales caught in the coastal
waters to Japan in 1960 and 1961 were calculated based on the number of thegrowth layers in the teeth. We judged the sexual maturity and pregnancy of thefemale sperm whales from the ovaries, mammary glands and uterine cornu whichwere examined by the staffs of The Whales Research Institute, and we calculated
the ratios of pregnant whales to sexually mature females. The pregnancy wasdecided by the existance of corpus luteum in the ovary, because the whales are opened
their belly before towed to the land station and fetus is often lost.

Fig. I. Semi-diagram of longitudinal section of a maxillary tooth of the sperm whale.
B1, B2 : Dark bands in the dentine, C : Cement, D : Dentine, E : Enamel, 0 : Osteodentine, M: Margine (predentine), 11 : Lastly formed growth layer, m: Forming
growth layer.

We used also the teeth of the eight marked whales which were marked by
Japan in the North Pacific and recaptured until 1962.
STRUCTURE OF THE MAXILLARY TOOTH

Sperm whale tooth is composed of dentine, enamel and cement, and enamel isformed in the fetul stage and later it is covered with cement. Laminations areformed comparatively regularly in both dentine and cement, which are observed
macroscopically on the ground surface of the tooth with reflex light as a mutuallyarranged dark and light bands.
When the laminations are observed on the ground transparent section, as thedark band has higher translucency than light band, it changes into clear band, andthe light band appears opaque band by the defused reflection. The photographs

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

18

of the maxillary teeth of two males and two females are shown in Plates I and II.
They are photographed with transparent light from the ground sections. The clear
band shows anisotrapic character under the polarized light, which is a evidence ofthe existance of developed crystalline structure. And the opaque band is stained
better than the clear band with carbol-fuchsin. So we concluded that the clear
band has well developed crystalline structure and is probably better calcified. The
width of clear band is smaller than that of opaque band. We used a clear bandas the base line of a growth layer and defined one growth cycle as the layer between
the beginning of a clear band and the beginning of the next clear band.
TABLE I. BIOLOGICAL DATA OF THE SPERM WHALES USED TO
MEASURE THE EACH WIDTHS OF THE GROWTH
LAYERS ACCORDING TO FIG. 2 a, b
SampleNo.

S

exBody length(feet)

Weight of testes, or no.
of corpora in ovaries

1
2
3
4
5
6
7
8
9
JO

Male
Male
Male
Male
FemaleFemaleFemaleFemaleFemaleFemale

52
50
47
35
36
36
44
34
34
28

8.0, 6.5 kg
7 .5, 7.0 kg
3.3, 3.5 kg
0.5, 0.8 kg
0-8, 0-7
lost.
0-6, 0-9
1-4, 0-6
0-1, 0-5
0-0, 0-0

No. of growth
Mean width ofgrowth layer (mm)layers
55
38
30
18
51
43
40
29
25
6

0.81
0.83
1.03
0.94
1.08
0.98
0.75
0.94
0.89
0.79

Range of width ot
growth layer (mm)0.40-1.30
0 .55-1. 25
0.45-1.30
0.35-1.20
0.30-1.30
0.35-1. 20
0.30-1.20
0.30-1.15
0. 55-1.60
0.60-0.92

Usually one clear band is composed of two parallel and closely situated clear
bands, and one or two clear lines are sometimes observed in a opaque band but they
are usually very narrow and indistinct. The neonatal line of the sperm whale tooth
was already reported by Nishiwaki, Hibiya and Ohsumi (1958). When observedwith reflex light, the dentine formed in the fetul stage is one light band and neonatalline is the first dark band in the tooth of which tip is not worn out. Though thewidth of the growth layer varies in sex and age, the structural difference is notobserved. Osteodentine is often found in the various part of dentine, but it is not
mentioned here. The microscopical observation of the dentine of the sperm whales
reveals, as is shown in Plates V and VI, the many dentinal tubules running parallelly
through the dentine and the space among these tubules is filled with matrix. The
branch of dentinal tubules seems not to develope well in sperm whale maxillary
tooth. Though the dentinal tubules are observed well in the opaque band namely
light band, some of them are obscure in the clear band (dark band) and appears as
if they are discontinued. But there are some tubules which are observed running
through the clear line.
The matrix seems to occupy a considerably more space in clear band thanopaque band. The well developed interglobular spaces are observed in the dentine
formed soon after the birth and some are formed in the fetul stage.

DENTINAL GROWTH LAYER OF SPERM WHALE

19

CHANGE OF THE WIDTH OF GROWTH LAYERS ACCOMPANIEDWITH AGE, AND ITS FLUCTUATION IN EACH YEARSEach width of the every growth layers were measured on the 4 males and 6 females.
As is shown in Fig. 2 a and b, the width of the growth layers shows a very wide
individual and age variation. The mean width of 15 growth layers from 2nd layer
to 16th layer fall in the range between 0.75 mm and 1.08 mm (Table I). In someteeth the fluctuation of the width of the layers is very large and does not show aconstant value or tendency to change. Though the width has a tendency to decrease gradually as refered later, there is such a specimen as No. 6 in which widthof layers vary from 0.30 mm to 1.30 mm.
1.3
1.2
1.1

J.O
0.9
0.8
0.7

\)_

No.1

____

0.6

1.0
No. 2

0.9

s
5
...

">~
..c:

~0

...

00

No. 3

'0

..c:

.:s
~

0.9
0.8
0.6
0.5
0.4
0.3
0.2
0.1
5

10

15

20

25

30

35

40

45

50

Number of growth layers

Fig. 2 a Change of width of growth layer with the increasement of number of growthlayers in the dentine of maxillary teeth. Male sperm whales.

55

20

S. OHSUMI, T. KASUYA AND M. NISHIWAKIThe mean width is lesser in female than in male. It is 0.85 mm in males
and 0. 75 mm in females. The mean width of each number of growth layers areshown in Fig. 3. Generally speaking, the width of the growth layers shows thetendency to decrease as the age goes by. In the males it shows the maximum
width of 1.08 mm at the 5th layer and at the age of 40th layer it decrease to
0.7 mm. In the female shows the maximum width 1.01 mm at 3rd layer and
0.5-0.6 mm at 40th layer.

0.9 \ [ \~:~ -v---=---~0-

10

0.6
5

10

15

20

25

30

35

40

45

50

Number of growth layers

Fig. 2 b Change of width of growth layer with the increasement of number of growthlayers in the dentine of maxillary teeth. Female sperm whales.

55

DENTINAL GROWTH LAYER OF SPERM WHALE

'E

21

1.0

..§
00

.;:"'" 0.8
.c

~

\

E

"'

"
.c

\/

0.6

A

\

\

I

b.,..,.. .c-<>--v_,,...4

"C

,0

·~

c

~

;;;
"

0.4

JO

20

15

25

35

30

40

Number of growth layers

Fig. 3. Change of mean width of growth layers with the increasement of number ofgrowth layers in the dentine. Open circle and broken line: Females, Closed circle
and solid line : Males.
Standard cleYiation
Mean" ' ' ' ' ' ': : : : : : ' ' ' :' 'i' ' ' ' ' ' ' 'j' ' ' ': : : : :~':':'i' ' :':':' ' ' f4tl

20

I

I
I
I
I

15

I
I
I
I

~
~

I
I
I

c

"g
10
'"
~

I

""

I

I
I
I
I

I
I

5

40

80

120

140

Ratio of the width of growth ]ay~r to the former layer

180
(%)

Fig. 4. Frequency distribution of the ratio of widfo of a growth layer to the former
layer, and standard deviation of the ratio.

Though in the tooth of the male, the width of the layers are rather broad,between l 9th and 24th layer which differs from that offemales, we can not conclude
whether it is the general tendency of the male teeth or not, for only three males
are measured its all layers in the teeth. In any case, there are fluctuations in the

22

S. OHSUMI, T. KASUYA Al"'\JD M. NISHIWAKI

width of each growth layers and it will show that the accumulation of the growthlayers in the teeth vary from year to year and by individuals.
Table 1 will show that the shrinkage of the dentine of old layers does not occur.
For example the mean width of a young sample No. 10 of which tooth containes
6 growth layers is 0. 79 mm and that of an old sample No. 5 of which tooth containes
51 layers is 1.08 mm. And we calculated the ratio of the width of one growth layer
to the width of the former layer, which frequency distribution is shown in Fig. 4.
These ratios distribute in the range from 46 % to 173 % and have a large deviation.
The distribution of these ratios nearly fit to the normal distribution, though it has
some bias to the side of larger value. Their mean value is 99.9 %, which can bethought to be nearly 100%. This shows that as the average the width of a growthlayeris nearly equal to that of the former layer. The standard variation is 22.5 %.

160

..

~

}

140

..
0

0

00

o'

• 0

0

0
0

o,

0

'lo

00
0

0
0

0

0

0

60

0

0

~
I

10

20

30

40

50

(iO

70

Number of growth layers

Fig. 5 Change in ratio of width of growth layer to the former layer in the dentine ·of
maxillary teeth. Open circle : Female, Closed Circle : Male.

Fig. 5 is a graph of the relationship between the number of the growth layers
and the ratio of each two layers. There seems to be no difference in both sexes.
At the age of from 2 to 5 growth layers the mean values of the ratio are more than
100%. This shows that in this stage the width of the growth layer increases withage. After that stage, though there are some fructuations, it shows the tendency
to decrease. In the other words, it can be said that the width of the growth layer
decrease gradually, after the age more than 5 growth layers.

DENTINAL GROWTH LAYER OF SPERM WHALE

23

Though the decrease of the ratio becomes larger in old ages as stated before,
when the ratios are not grouped by the age, the mean ratio is nearly 100%, thismeans that the width of the growth layers have no bias, concidering in average.
FORMATION OF THE DARK BAND, ITS SEASONAND PERIOD

Fig. 1. is a diagrammatic figure of an upper tooth of a sperm whale. In this figure
Mis the margin of the dentine at the pulp cavity. B1 is the last dark band of growthlayer and B2 is the former dark band. Then, m is the length between B1 and M,
and 11 is the breadth between B1 and B2 • We measured m and g 1 on 447 sperm
whales with a reflecting projector of 20 magnification.
When we measured them the base line of the dark band was settled at theborder of a dark band and the former light band, which is shown in Plates.

120~

0

•
0

l O O r - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --~o----- -- -- ----

Though the samples were desirable to be of a same age, we could not collecta sufficient number of samples, thus the ages of samples distribute from 5 to 35 layers
old (mean 18.7) in the males, and from 4 to 27 layers old (mean 14.4) in thefemales respectively.
Nearly all of the sample (92%) were collected in 1961, in order to eliminate
the influence of the fluctuation by year. The samples were also collected extend-

S. OHSUMI, T. KASl)YA AND M. NISHIWAKI

24

ing over 6 months from the first decade of May to the first decade of November.
We grouped the samples to the first, middle and last decade of each month.
Each group contains from 20 to 30 samples.
If the dark band is formed seasonally the value of m will increase with theelapse of time. As shown in the anterior chapter, the width of dentine shows wide
individual variation, but the ratio of m to 11 will show the time passed since thebeginning of the accumulation of the recent dark band.
Fig. 6 shows the relation between m/1 1 and the season. It can be easily
known from the figure that the ratio (m/1 1 ) have the tendency to increase as the
progress of time. The growth rate of the growth layer does not seem to show thedifference between male and female, for there seems not to be efficient difference
between the ratio of m/1 1 in both sexes. But the ratio shows wide variation in
every decade (the difference of the minimum and the maximum is about 55%).

12

10

2

-20

20

40

Deviation of m/1 1 from the mean value in each decade(%)

Fig. 7.

Frequency distribution of deviation of m/1 1 from the mean value in each decade.

The variation from the mean value of each group is shown in Fig. 7 which
nearly fits to the normal curve. This means that the season when dark band isformed is one season, and a next growth layer can not be accumulated on the wayof accumulation of a growth layer.
After the first decade of October there happens such a sample which ratio
m/1 1 shows more than 100 %, and after the last decade of October there is the
samples which seems to be soon after the beginning of the formation of a new dark
band. These phenomena suggest that in some specimens the formation of the darkband already occurs in the first or second decade of October. When the lower
border of Fig. 5 is deduced to April, it is presumed that there is such individuals

25

DENTINAL GROWTH LAYER OF SPERM WHALE

of which dark band begins to be formed in April. Accordingly it is supposed thatthe dark band begins its formation during the 7 months between the first decadeof October and the last decade of April with the height of prosperity in January.
Fig. 8 shows the mean value of m/1 1 in each decade. Though the sample
covers only 6 months and does not cover the season when dark band is formed inmost individuals and we can not definitely conclude, it seems that growth layer
does not grow linearly but conversed sigmoidally.
Figs. 7 and 8 show that the cycle of the formation of the dark band is not 6
months as considered in the previous paper. And the growth layer seems to growmore slowly in summer or from ] uly to September than the other seasons. So it
is inadequite to apply a straight line to the data which were not covered in allseasons.

100 -------- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ----------------==:::- -/,,......,,,,.,,,.,.

80," / /

~

----

Nov.

Dec.

Jan.

. _.___,.,...-..

/,.,...,..,..~-.-::·~,....

~ 60

:::::
s

40

20

Dec.

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jui.

Aug.

Sept.

Oct.

Fig. 8. Three accumulation curves of the growth layer in the dentine of maxillary
tooth of the Northern sperm whales. Closed circle: Mean value, of m/1 1 Solid line:
Estinated curve, Chain line: Regression line calculated with the mean values, Broken
line : Fitted straight line when the accumulation cycle is expected to be one year.

But for a trial, we calculated the regression line from the mean values of each
decade groups, which is shown by following formula,y = 1.772x+21.64
y: 100 m/11
x: No. of the decade from the first decade of January when x is 0.
and this is shown by the chain line in Fig. 8. This does not seem to fit thegiven data. When the ratio y is 0, the time x is minus 12.2 and this means the first
decade of last September. And in the middle decade of March in the next year
(x is 44.2) y becomes 100%. By this formula the cycle of the accumulation of
growth layer is shown to be 56.4 decade, namely 1.5 7 year long and it does not
fit to the data because if the cycle is 1.5 year long, the samples should separate
into two groups.

S. OHSUMI, T. KASUYA AND M. NISHIWAKI26

The broken line in Fig. 8 is made to fit the each mean values best, on theassumption that the cycle of the accumulation of growth layer is one year long.
This line less deviates from the data than the chain line which is already explained.
On this line the time wheny shows 0% is the first decade of January, which fits asa matter of course, with the result already got by us.
EXAMINATION OF GROWTH RATE WITH THE MAXILLARYTEETH OF THE MARKED WHALES RECAPTUREDAs shown in Table 2, the teeth of the eight marked whales were collectedby 1962 in Japan. In order to examine the accumulation rate of the growthlayer by the tooth of marked sperm whales, it is desirable that the age when thewhale was marked is as young as possible and that the time elapsed since the time ofmarking is relatively long, for the elapsed time since marking is desirable to benearly equal to the whole life of the recaptured whale.
TABLE 2.

MARKED SPERM WHALES WHICH WERE RECAPTURED AND
COLLECTED THEIR MAXILLARY TOOTH

Body
Body
Weight Growth
Elapsedlength at length at No. of PregMark Date
Datetime (A)Sex marking recovery ovulation nancy of testes layers B/A
no. marked recaptured (year-month)(B)(kg)(feet)(feet)
18
0-2, 0-1 None
I. 78
35
J2871 12.ix.52 24·X·62
10- I! Female
None
39
3.85
lost
10- l! Female
35
J2878 12· IX·52
24·X·62
Male
J2883} 12.1x.52
32}
20
1.98
38
1-2, 0-3 144cm
10- 1! Female
24·X·622812984
9- I Female
0-3, lost None
30
3.30
35
36
J3166 13·IX·52
20·X·6135
0-0, 0-2 None
14
I. 71
J3237 17.v1.53 30·VIII·61 8- 2! Female
37
10
1.48
lost
7- 3! Female
25
35J6381 7.v1n.55 13.x1.62
J6658} 7 ·VIII .55 18· VIII ·62
17338
J8278 22· VII·60 ll VI·6l
0! Male30

36

0.8, lost

12

I. 71

0-10% Male

50

47

5.2, 5.3

44

49.5

7-

A sperm whale, No. 3237 was marked in Japanese coastal waters m 1953,
and recaptured in 1961 also in the almost same region. This whale is a female
and the time elapsed since it was marked till it was recaptured is 8 years and 2.5
months. Though whales are apt to be estimated their body length larger thantheir true length especially in small whales at the time of marking, its estimatedbody length when it was marked was 37 feet long. Therefore it can not havebeen so young whale then. When it was recaptured, it was already adult (number of ovulations of this whale was 2), and the growth layers in upper tooth was
numbered to be 14. The ratio of the number of the growth layers to the years elapsed
since the time of marking till recapture is 1. 71. And considering that the age at thetime of marking of the individual must have been not so young, the accumulation
rate of the growth layers formed after the marking should be less than 1. 71 per year.
Similar results were obtained in 1962 from four recaptured whales, namely
Nos. 2871, 2883, 6381 and 6658 whales. Elapsed times of them were 10.1, 10.1,
7.3 and 7.0 years respectively, and the number of growth layers of them were also

DENTINAL GROWTH LAYER OF SPERM WHALE

27

18, 20, 10, and 12 respectively. Therefore, the number of growth layers devided
by elapsed years were 1. 78, 1.98, 1.48 and 1. 71 respectively for four whales.
Especially the number of growth layers of No. 6381 whale was only 10, when
it was recaptured after 7 years and 3 months. Then, the accumulation rate of thegrowth layer is considered as one per year, the whale would have been about 3 years
old (10-7.3 = 2.7) at the time of marking. Now, according to the growth curveof the sperm whale by Nishiwaki, Hibiya and Ohsumi (1958), the body length at
the age of three growth layers is considered to be 22 feet. On the contrally, thebody length of the whale No. 6381 at the time of marking was 25 feet. Considering
the error of the estimated length, this fairly agrees with the calculated body lengthfrom the growth curve.
Above five materials are considered to suggest strongly that the accumulation
rate of growth layers must be less than two per year, and considering the age at thetime of marking, the rate may be one per year.
A sperm whale No. 8278 had lived less than only 11 months after it wasmarked and had been estimated its body length to be 50 feet when marked, so it will
have been fairly old when it was marked. For these two reasons this whale is not
so useful to examine the accumulation rate of the growth layers (44 growth layers
is numbered in the upper teeth).
The whale No. 3166 was recaptured after 9 years and 1 month after themarking. Although the time passed from marking till recapture is longer thanNo. 3237 by 11 months, the dentine of the upper teeth contained 30 growthlayers, and 3.30 is the quotient of the number of layers divided by the year elapsed
from the time of marking to recapture. This quotient is larger than 2. Though
one ovary had been lost, the other ovary contained 3 corpora albicantia. So thiswhale may have been older than No. 3237 when it was recaptured. If it is assumed
that No. 3166 was much older than No. 3237 at the time of marking, the ratio 3.30
of No. 3166 is not inconsistent with the assumed accumulation rate of the growthlayers. No. 2878 whale was recaptured after 10.l years from the time of marking,and the number of growth layers was 39, then the devided value is 3.85. Thisvalue is thought to be obtained jn the similar case of No. 3166 whale.
EXAMINATION WITH AGE DISTRIBUTION BASED ON
THE NUMBER OF GROWTH LAYERS

Fig. 9 shows the age distribution of 1,226 female sperm whales caught in the Japanese coastal waters in 1960 and 1961 based on the number of the growth layers in thedentine of the upper tooth. The age distributions are converted on both assumption that one growth layer is accumulated in a year and that two growth layers ina year.
As shown in Fig. 10, the catch of the sperm whales in the coastal waters toJapan has been increasing gradually and recently it attained more than 2,000
whales and since 1959 the maximum catch was restricted to 2,100 whales, which
is about 4 times more than the catch in 1920. But the rate of the catch to the

28

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

stock of sperm whales in the coastal waters to Japan calculated from the recovery
rate of the marked whales is 0.5% in recent years (Ohsumi, unpublished data).
In spite of the sudden increase of the catch since 1957, the age distributionof the catch, which is shown on the semi-logarithmic graph in Fig. 9, shows to decrease with nearly a straight line in the age groups of I 3 to 4 7 growth layers. This
will mean that the rate of catch to the stock of sperm whales is very small in the
coastal waters to Japan. So, we consider that the mortality rate calculated fromthe age distribution of the catch will show natural mortality in the most parts.
Even if the natural mortality is affected by fishing, it does not influence to thelatter calculations.
200,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~---,

,,,,x
I \

I

100

~

80

I

X

\~
\

:

*><

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Age (years)

Fig. 9. Age distribution of female sperm whale caught in the adjacent waters to Japan
in 1960 and 1961. Ages are based on the number ot growth layer. Open circle andSolid line : One growth layer is accumulated in a year, Cross and broken line : Twogrowth layers are accumulated in a year.

The mortality rate calculated from Fig. 9 is shown below.
Accumulation rate of growth layer
One growth layer in a year (one layer age)
Two growth layer in a year (two layers age)

Mortality co-efficient
0. 056
0 .121

Mortality rate
5. 3%
11. 4%The mortality rate calculated by one layer age largely differs from that of two

29

DENTINAL GROWTH LAYER OF SPERM WHALE

layers age.
The reproductive rates are calculated on the assumption that the mortality
rate lasts in the same value throughout the life span. Now, the female sperm
whales attain the sexual maturity at the age of 9 growth layers according to
Nishiwaki, Hibiya and Ohsumi (1959).
And the pregnant rate is given in Table 3, which is based on the biological
data collected in the coastal waters to Japan in 1960 and 1961.
Clarke (1957) reported that the gestation period of the sperm whales in thenorthern waters is 16 months and the maximum of the pairing and parturition occurs in April and in August respectively. Then, from April to August, there shouldbe two groups of pregnant females, one group is the females which become pregnant
in the former year and the other group have become pregnant in the next year.
And the half of the apparent pregnancy rate must be the true pregnancy rateduring the seasons from April to August. In September we used the 2/3 of apparent pregnancy rate as a true rate. Because in September there are still females
to be delivered of a calf.

2,500

2,000

~ l,500~.c•"~ 1.000
:E
500

'20

'25

'30

'35

'40

'45

'50

'55

'60

·Year

Fi biJ 10. Variation of catch of sperm whales in the adjacent waters to Japan (after theInternational Whaling Statistics and Japanese Whaling Statistics).

By these mathods we got the couclusion that the true pregnancy rate willbe 25"'33%, and mean pregnancy rate in the season from May to November
is calculated as 28.7%. Clarke (1957) reported the low pregnancy rate of 27%
on the North Atlantic sperm whales. This is almost the same value as ours.
In the next place, we calculated the recruitment value " R " corresponding
to the varying mortality rate from 0.03 to 0.13, on the two assumptions that thefemale sperm whales become reproductive from the age of 9 growth layers, namely
9 years old in the case of one layer age and 5 years old in the case of 2 layers age,

30

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

and corresponding to the varying mean pregnancy rate of the matured female
sperm whales from 0.25 to 0.35 through the reproductive life span.
The R-values calculated from these figures are shown in Table 4. Naturally,
when the R-value is lower than I, the stock of the sperm whales will decrease, andif higher than 1, the stock increases.
And here next two problems come into question.
( 1) The error caused by the assumption that the mortality rate is constantthroughout all the life span.
(2) The ignorance of the high mortality rate soon after the birth, which isusually observed in the polulation of animals.
TABLE 3. PREGNANT RATE OF SPERM WHALES CAUGHT IN THE
ADJACENT WATERS TO JAPAN IN 1960 AND 1961 SEASONS

Pregnant whales
Resting and lactating whales
Total whales% of pregnant whales
Rate
Possible pregnant
ratio (%)
TABLE 4.

May &
Jun.
24
15
39
61.5

Jui.

Aug.

Sept.

Oct.

Nov.

Total

28
14
42
66.7

90
83
173
52.0

205
291
496
41.3

64
154
218
29.4

14
40
54
25.9

425
597
1,022
41.5

l/2

l/2

l/2

2/a

lfi

l/1

30.8

33.3

26.0

26.8

29.4

25.9

28.7

R-VALUES OF THE SPERM WHALE

Pregnant rate
0.35,.....--"-.
Survivalrate
9 years
5 years
0.97
5.02
4.44
0.96
3.57
3.03
0.95
2.71
2.21
0.94
2.14
1.67
0.93
I. 74
1.30
0.92
1.44
1.03
0.91
1.21
0.83
0.90
0.68
1.03
0.89
0.89
0.56
0.88
0.77
0.46
0.87
0.67
0.38

0.25

0.30
~

9 years
3.80
2.60
1.89
1.43
1.11
0.89
0.71
0.58
. 0.480.40
0.33

5 years
4.30
3.06
2.33
1.83
1.49
1.25
1.04
0.89
o. 76
0.66
0.58

___.._______
9 years
3.17
2.16
1.58
1.19
0.93
0.74
0.59
0.48
0.40
0.33
0.27

5 years
3.58
2.54
1.93
1.53
1.24
1.03
0.89
o. 74
0.62
0.55
0.48

When the mortality rate is presumed from the mature and stabilized ages
considering the above mentioned two points, R-valu·e needs to be fairly higher value
than 1 in order to keep the stock constant.
If 2 growth layers are accumulated in a year, the R-value can be more than
1, only when the mortality rate is lower than 9%.
But the mortality rate calculated from two layers age is 11.4% and therefore
R is 0.75, which prohibits the maintenance of the stock. Even when the pregnancy rate is 35%, R-value is 1.01 which can not be suitable value, for the mortality
rate soon after birth is thought to be higher than that of the later stage.
On the other hand, if it is assumed that one growth layer is accumulated inDENTINAL GROWTH LAYER OF SPERM WHALE

31

a year, the mortality rate is calculated to be 5.3 %. And when the pregnancy rateis assumed to be 25% and 35%, the R-value is 1.5 and 2.1 respectively. These
values surely permit the maintenance of the stock even when other various factors
influenced on the stock.
In fact, the stock of the sperm whales in the coastal waters to Japan does notshow evidence of decrease and the catch rate is considered to be low. Thus theexamination of the growth layer in the maxillary tooth based on the age distribution leads to the conclusion that it does not explain the actual conditions of the stock
to assume that two growth layers are accumulated in every one year and it seems tobe suitable to consider that less than two probably one growth layer is accumulated
ma year.
DISCUSSION

All the results obtained in the preceding chapters seems to show that one growthlayer is accumulated in every year in the dentine of maxillary tooth of the sperm
whale.
On the accumulation rate of the growth layer in the tooth of the toothed whale,
Sergeant (1959) already reported that one layer accumulates in one year on thetooth of the bottlenosed dolphin. This result was obtained from the captive
dolphins which lived in an environment largely differed from the natural conditions.
But we (Nishiwaki and Ohsumi, unpublished data) got the fact that the number ofgrowth layers in the teeth of the bottlenosed dolphin at the attainment of sexual
maturity in natural condition nearly coincides to the age of same stage for thecaptive bottlenosed dolphin in the report by Tavolga and Essapian (1956). Then
Sergeant's result will be considered to be applied for the dolphin in natural environment.
Sergeant (1962) also obtained the same result on the accumulation rate of
growth layer for the pilot whale in natural environment.
On the accumulation rate of the lamination in ear plug of baleen whales,
Ohsumi (1962) suggested on the biological data of the recaptured fin whales thatthe ear plug accumulates one lamination in a year.
Although Pinnipedia animals are classified differently from Cetacean animals,
one growth layer of tooth is accumulated in a year (Scheffer; 1950, Laws; 1953,~etc.).
And the feature of growth layers in the dentine of some Pinnipedia looks very simmilar to that of dolphins.
According to Omura and Kawakami (1956), based on the marking investigation in the North Pacific, the growth of sperm whales may be much slower thanthat· is generally believed. Further biological materials which had been collected
later from recovered marked sperm whales also support the result.
Chuzhakina ( 1961) reported a paper on the ovaries of the sperm whale, andshowed a table on the relation between the age and the total ovulation, although
we do not know the method of age determination in this study. Then, the above
relation coincides fairly well to the figure and formula by Nishiwaki, Hibiya and·32

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

Ohsumi (1958) on the relation between the number of growth layers in the maxillary tooth and the total ovulations, when we assumed that one growth layer accumulates in a year.
We think that the formation mechanism of the growth layers in the dentine will
be related with the metabolic cicles, and its final solution will depend on the physiological study on the periodicity of the metabolic and reproductive mechanismsof the whale. However, the already known ecological informations on the whale
seems to support the assumption that the whales have one year periodicity in themetabolic cycles.
In this report, we examined the seasonal change in the ratio of the width of thelast growth layer to that of the former growth layer. But the ratio does not necessarily show a stage of the growth layer period of the animal, for the width of thegrowth layer has fairly large fluctuations between samples or each layers in one
sample. In order to prevent this error, it will be better to divide a growth layer
into many stages and to decide the stage to which the samples belong. But, toour regret, this method was not used.
In this report we presumed that the dark (clear or translucent) bands in thedentine begin to form in winter and its prosperous season falls in the first decade ofJanuary. The dark band in the dentine is thought as a well calcified tissue andseems to be accumulated as the result of a good nutritious condition of the animal
and Mc-Laren (1958) reported on the teeth of ringed seal Phoca hispida that theclear band is formed in the season of moult and opaque band is formed in summer.
Sergeant (1959) reported on the teeth of Tursiops truncatus that the dark bandis formed in the pairing season of early spring.
And he (1962) also reported on Globicephala melaena that its pairing season
extend over 6 months with its flourishing season in April and May, and the darkband is formed in this season.
The sperm whales in the northern phemisphere have their maximum pairing
season in April (Clarke, 1957). We estimate that light (opaque) band is formed insummer and that dark band is formed in about three months proceeding thepairing season.
This result does not coincide with the already published data, but coincides
with the report that in the fur seals, the dark band is formed in April proceeding
2 or 3 months to the pairing season (Kubota et al., 1962). Though the change ofthe seasonal abundance of the food of the sperm whales is not yet known sufficiently,
there seems no change between June and November (Clarke 1957).
Omura (1950) reported that the thickness of the blubber begins to decreasein January, and attains the minimum in June and July and then increase rapidly
from September to November in the North Pacific sperm whale. If the accumulation of nutrition occur proceding to the pairing season and the nutrition affect theformation of the dark band, it will be probable that the dark band is formed in thetooth of sperm whales in January. But we think that the season when the darkband is formed whould not be concluded now, for our ecological knowledge on the
sperm whales in winter is very insufficient and our materials used in this report isDENTINAL GROWTH LAYER OF SPERM WHALE

33

lacking in the samples during the seasons from November to April. We are rather
inclined to suppose the influence of some hormones on the formation mechanismof dentine layers.
As one of the means to give solution to this problems, the time marking on theteeth of a living sperm whale will be effective, but this method is now far from theutilization.
SUMMARY

Following results are obtained on the accumulation rate of growth layers appearedin the dentine of maxillary teeth of the sperm whale.
1. The light (opaque) and dark (translucent) bands are found accumulating
mutually on the surface of the longitudinally ground section of the dentine of the
sperm whale maxillary tooth. The dark band has affinity for carbol-fuchsin andshows a strong anisotropic character to the light. Though some narrow dark lines
are ordinary found in a light band, its difference in the sex and age is not found.
2. There are individual variations in the width of growth layers of the dentine, and their yearly fluctuations are also recognized in the same individual. But
generally speaking, the widths of layer are larger in male than those in female, andthey decrease with the age. However the decreasing tendency is mere, and therefore we may consider that the width of a layer is almost same as the width of thenext layer on an average.
3. As the result of the measuring the seasonal growth of latest layer, it isconsidered that one growth layer will accumulate in a year, and the dark band will
be formed in winter.
4. Biological investigation of a recaptured sperm whales suggests that theaccumulation rate of growth layer in the dentine must be under two per year andmay be probably one per year.
5. Pregnancy rate of the adult sperm female is considered to be 28%, judging
from the gestation period of the whale.
6. Age distributions of female sperm whale in the adjacent waters to Japanwas shown basing on the number of growth layers of maxillary teeth. And themortality rates were calculated in the two case of one layer age and two layers age.
They are 5 .3 % and 11.4 % respectively.
7. Calculating the recruitment-value with the mortality rates and pregnancy
rate, the stock of sperm whale in the adjacent waters to Japan is not able to explain
the case of two layer age, but able to explain in the case of one year age.
8. It will be concluded from the above discussion that the accumulation rateof growth layer is considered to be one layer per a year and the dark band may beformed in winter.
REFERENCES
CHIASSON, R. S. (1957). The dentition of the Alaskan fur seal. J. Mammal., 36: 310-319.
CHUZHAKINA, E. S. (1961). Morfologicheskaya Kharakteristika Yaichnikov samok Cachalota (Physeter

34

S. OHSUMI, T. KASUYA AND M. NISHIWAKI

catodon L. 1758) v Svyazi s Opredeleniem Vozraska. Trudi Inst. Mor. :(hivotnith A.N. S.S.S.R. 34: 33-55.
CLARKE, R. (1957). Sperm Whales of the Azores. Discovery &p., XXVIII: 237-298.
FUJITA, T. (1957). Histology of tooth (in Japanese). Ishiyaku Syuppan Co., Ltd. Tokyo, 241 pp.
KuBOTA, K., NAGASAKI, F., MATSUMOTO, K. and TsuBm, M. (1961). Histological studies on the growth layers
in the maxillary canines of fur seals as an indication of age. Part I. The teeth at the age of one to nine
year. Bull. Tokyo Med. Dent. Univ., 8(3) :,261-285.
LAWS, R. M. (1953). A new method of age determination in mammals with special reference to the elephantseal (Mirounga leonina Linn.) Sci. Rep. Falkland Is. Dependencies Survey, No. 2: 11 pp.
- - (1960). Laminated structure of bones from some marine mammals. Nature, 187 (4734): 338-9.
M1zuE, K. and JIMBO, H. (1950). Statistic study of foetuses of whales. Sci. Rep. Whales Res. Inst., No. 3:
119-131.
MuRAKI, Y. (1958). Comparative anatomical studies on the cementum of the mammalian teeth. Acta
Anatomica Ni,pponica, Vol. 33, No. 6: 583-611.
NrnHIWAKI, M. (1951). On the periodic mark on the baleen plates as the sign of annual growth. Sci. Rep.
Whales Res. Inst., No. 6: 133-152.
NISHIWAKI, M. and YAGI, T. (1953). On the age and the growth ofteech in a dolphin (Prodelphinus caeruleoalbus) (1). Sci. Reb. Whales Res. Inst., No. 8: 133- 146.
N1sHIWAKI, M., Hm1YA, T. and OmuMI (KIMURA), S. (1958). Age study of sperm whale based on readingof tooth laminations. Sci. Rep. Whales Res. Inst., No. 13: 135-153.
NrnHIWAKI, M., 0HSUMI, S. and KASUYA, T. (1961). Age characteristics in the sperm whale mandible.
Norsk Hvalfangst-Tid. 1961 (12) : 499-507. ·0HSUMI, S. (1962). Biological material obtained by Japanese expeditions from marked fin whales. Norsk
Hvalfangst-Tid., 1962 (5): 192-198.
OMURA, H. (1950). Whales in the adjacent waters of Japan. Sci. Rep. Whales Res. Inst., No. 4: 27-113.
OMURA, H., FUJINO, K. and KIMURA, S. (1955). Beaked whale Berardius bairdi of Japan, with notes on :(iphiuscavirostris. Sci. Rep. Whales Res. Inst., No. 10: 89-132.
OMURA, H. and KAWAKAMI, T. (1956). Japanese whale marking in the North Pacific. Norsk HvalfangstTid., 1956(10): 555-563.
RrcKER, W. E. (1958). Handbook of computations for biological statistics of fish populations. Fish. Res.
Board Canada Bull., No. 119: 300 pp.
ScHEFFER, V. B. (1950). Growth layers on the teeth of pinnipedia as an indication of age. Science, 112:309-311.
SERGEANT, D. E. (1959). Age determination in Odontocete whales from dental growth layers. Norsk
Hvalfangst-Tid., 1959(6): 273-288.
- - (1962). The biology of the pilot or pothead whale globicephala melaena (Trial) in Newfoundlandwaters. Bull. Fish. Res. Board Canada, No. 132 : 84 pp.
TAVOLGA, M. 0. and EssAPIAN, F. S. (1957). The behavior of the bottle-nosed dolphin (Tursiops truncatus):
mating, pregnancy, parturition and mother-infant behavior. :(oologica, 42 (1): ll-31.
YABLOKOV, A. V. (1958). The Cetaceans' dental structure and type of teeth. Bull. Moscow Soc. Naturalists,
Biol. Section, 63(2): 37-48.

EXPLANATION OF PLATESPLATE I
Longitudinal ground sections of the maxillary and mandibular teeth of sperm whales.
B: Dark band, C: Cement, D: Dentine, E: Enamel, NL: Neonatal line, 0:Osteo-dentine.
Left: Female, 10.8 m in body length, ovaries lost, 11 growth layers, taken in the coastal
waters to Japan on July 26, 1961. ( X 4.4)
Right upper: Female, 5.0 m in body length, immature, no growth layer, stranded in Enoshima Beach on July 27, 1959. ( X 4.4)

DENTINAL GROWTH LAYER OF SPERM WHALE
Right lower: Female, 3.7 m in body length, fetus, taken in the coastal waters to Japanon Sept. 27, 1960, mandibular tooth. (x4.4)
PLATE II
Longitudinal ground sections of the maxillary teeth of male sperm whales.
Left: Male, 13.5 m in body length, 2.0 and 2.2 kg in weight of each testes; 22 growthlayers, taken in Aleutian waters on June 16, 1961. ( x 2.4)
Right: Male, 13.2 m in body length, 3.0 and 3.3 kg in weight of each testes, 25 growthlayers, taken in Aleutian waters on June 21, 1961. (x2.2)
PLATE IIILongitudinal ground sections of maxillary teeth of sperm whales.
Left: Female, 10.7 m in body length, one corpus luteum and three corupus albicans
in the ovaries, pregnant, 17 growth layers, taken in the coastal waters to Japan in July 26,
1961. (x3.8)
Right: Female, 8.5 m in body length, one corpus luteum and three corpus albicantia ina ovary, pregnant, 11 growth layers, taken in the coastal woters to Japan on July 30,
1961. (x3.5)
PLATE IV
Longitudinal ground sections of maxillary tooth of a sperm whale. Marks are same as Plate I.
Female, 10. 8 m in body length, ovaries lost, 11 growth layers, taken in the coastal waters
to Japan on July 26, 1961.
Upper: Tip portion of the tooth( X 17 .4)
Lower : Pulp portion of the tooth( x 17 .4)
PLATE V
Longitudinal ground sections of maxillary tooth of a sperm whale.
Female, 8.5 m in body length, one corpus luteum and one corpus albicans in a ovary,pregnant, 11 growth layers, taken in the coastal waters to Japan on July 22, 1961.
Upper: Tip portion of the tooth( X 18)Lower: Pulp portion of the tooth ( X 18)
PLATE VI
Longitudinal ground sections of the tip portions of maxillary teeth of sperm whales.
I: interglobular space, other marks are same as Plate I.
Upper: Male, 13.5 m in body length.
( X 224)
Lower: Female, 10.8 m in body length. ( x 134)
PLATE VII
Longitudinal ground sections of a male sperm whale. Dentinal tubules are running throughthe dentine.
P: predentine, other marks are same as Plate I.
Upper: Male, 12.9 m in body length, 2.4 and 2.4 kg in the weight of each testes, takenin the Aleutian waters on July 4, 1961. ( X 112)
Lower: The same tooth with the upper figure. ( x 268)

35

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