A TAXONOMIC STUDY OF THE MINKE WHALE IN THE
ANTARCTIC BY MEANS OF HYOID BONE
YUKO SATAKE AND HIDEO OMURAWhales Research Institute, TokyoABSTRACT

Hyoid bones of the minke whale in the Antarctic were studied fromthe standpoint of taxonomy, comparing those in the North Pacific. A distinction was noted in the length and thickness of the stylohyals. Greatest
height of the ankylosed bone of basihyal and thyrohyals as well as the heightat middle of the right and left wings are greater in the specimens from theAntarctic than in those from the North Pacific. But there is a possibility
that this is a difference according to age of the whale. Hence samples fromthe Antarctic are biased towards older ages and those from the North Pacifictowards younger ages, no conclusion was reached in this respect.

INTRODUCTION

The minke whale in the Antarctic, though most closely related to Balaenoptera
acutorostrata, differs from the latter in having flippers of a uniform pale gray colour
instead of showing the characteristic white band of the true minke whale. Furthermore the baleen of B. acutorostrata is of a uniform yellowish-white colour whereas
the baleen of the minke whale in the Antarctic is white at the front of the series
and gray and white at the back (Williamson, 1959, 1961; Utrecht and Spoel,1962; Kasuya and Ichihara, 1965; Ohsumi et al., 1970).
Williamson (1961) describes that it is uncertain whether they (B. bonaerensis)
represent a true species, or a subspecies of B. acutorostrata, but Utrecht and Spoel(1962) and Ohsumi et al. (1970) report that B. bonaerensis is a synonym of B. acutorostrata, mainly based on studies on the external characteristics.
In this report the hyoid bones of the minke whales in the Antarctic are compared with those of the North Pacific (B. acutorostrata) from the taxonomic standpoint, following after the method adopted by Omura (1964).
MATERIAL

In the 1971-72 whaling season Dr. Seiji Ohsumi went to the Antarctic on board
Jinyo Maru, a minke whaling expedition. He collected hyoid bones from 25
minke whales and made them at our disposal. These whales were taken in theAntarctic area IV and the sex and body length of each whale are shown in Appendix Table.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974, 15-24

SATAKE AND OMURA16MEASUREMENTS

After extraction of oil and cleaning measurements of various parts were made.
These are compared with those from the North Pacific, cited from Omura (1964).
There is rather a wide difference in body length of samples between both hemispheres.
Samples from the Antarctic range from 710 cm to 980 cm in length, with an averageof 850 cm, whereas those from the North Pacific are from 500 cm to 768 cm andin average 624 cm. Thus samples from the Antarctic are biased towards olderages and those from the North Pacific towards younger ages, which should be bornin mind in direct comparison.

I !piFig. 1.

Showing positions of the measurements.

The following measurements were made on each hyoid bone collected (seeFig. I).
A. Ankylosed bone of basihyal and thyrohyals.
a. Overall length. Curved length measured along the outer surface andpassing centers of basihyal and thyrohyals.
b. Straight length. Straight length between tips of right and left wings.
c. Greatest height. Greatest height between tips of the anterior and posterior
projections of the basihyal.
d. Height at center. Height measured at the center of the bone.
e. Forward notch, depth. Depth of the notch between right and left forward
projections of the basihyal.
f. Height at middle of wing, right. Height measured at middle between thecenter of the basihyal and tip of right wing.
g. Height at middle of wing, left. Height measured at middle between thecenter of the basihyal and tip of left wing.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

MINKE WHALE

17Thickness at middle of wing, right. Thickness of right thyrohyal on the lineof measurement f.
i. Thickness at middle of wing, left. Thickness of left thyrohyal on the lineof measurement g.
j. Height at distal end, right. Height of right thyrohyal at its distal end.
k. Height at distal end, left. Height of left thyrohyal at its distal end.
Stylohyal.
1. Total length, right. Straight length between tips of right stylohyal.
m. Height at middle, right. Height at middle of right stylohyal.
n. Thickness at middle, right. Thickness of right stylohyal on the line ofmeasurement m.

h.

B.

Measurements

no.

h,.

ge',E\:Jdc
0102030

40% against overall length
Fig. 2. Measurements of various parts of the ankylosed bone of basihyal andthyrohyals of the minke whale, expressed as percentages against its overall length.
For measurements no. see text. Horizontal line indicates ranges of measurements, vertical midline the arithmetic mean, square the 95% confident limitsof the mean. Square with hatched lines denotes samples from the minke whalefrom the North Pacific and white square those from the Antarctic.

o.

Degree of curvature, right. Greastest distance between the straight line
passing the most prominent parts of right stylohyal and its forward surface.
p. Total length, left. Straight length between tips of left stylohyal.
q. Height at middle, left. Height at middle of left stylohyal.
r. Thickness at middle, left. Thickness of left stylohyal on the line of measurement q.
s. Degree of curvature, left. Greatest distance between the straight line passing
the most prominent parts of left stylohyal and its forward surface.
Results of these measurements are shown in Appendix Table, together withthe serial number of the whale, sex, and body length.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

18SATAKE AND OMURA

These measurements were then calculated of their percentages against overall
length in the case of the ankylosed bones of the basihyal and thyrohyals. For thestylohyals also percentages against overall length were calculated for total length,
but in other measurements percentages against the length of stylohyals were calculated. Further for each measurement the arithmetic mean, its standard deviation
and 95 % confident limits of the mean were calculated.
DISCUSSION

The results of the calculations are shown in Figs. 2-4, comparing with those fromthe North Pacific. As seen in these figures differences are noted between the minke
whales from the different oceans in the following measurements:
Measurements no.
p455055606570% against overall length

Fig. 3. Measurements of stylohyal of the minke whale, expressed as percentages
against the overall length of the ankylosed bone of basihyal and thyrohyals. For
explanation see Fig. 2.
Measurementsno ~------------------,
s0_r·~

n·Ebq·•·'m51015Eb20

25% against length of stylohyal

Fig. 4. Measurements of various parts of the stylohyal of the minke whale, expressed as percentages against its total length. For explanation see Fig. 2.

A.

Ankylosed bone of basihyal and thyrohyals (Fig. 2).
c. Greatest height.
f. Height at middle of wing, right.
g. Height at middle of wing, left.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

MINKE WHALE

19

B.

Stylohyal (Fig. 3, Fig. 4).
I. Total length, right.
p. Total length, left.
n. Thickness at middle, right.
r. Thickness at middle, left.
In the other measurements 95 % confident limits are overlapping each otherand they cannot be said as distinct. In conclusion above the minke whales in theAntarctic have more higher combined bone and larger and thicker stylohyals,
hence more massive hyoid bones than those in the North Pacific.

~ 20E
u"'c0-"'-0"'<ll0>.

-"'c"'£"'

0...,..c:·~10..c:

0500

600
700 800
Body length (cm)

900 1000

Fig. 5. Relative growth of the greatest height of the ankylosed bone of basihyal
and thyrohyals and body length of the minke whale. Circles indicate minke whalesin the North Pacific and black dots those from the Antarctic.

As already stated average body lengths of the sample whales are widely different between the two oceans. More larger or older whales were sampled fromthe Antarctic than from the North Pacific. The reason for such discrepancy inbody length should be sought in the segregation of the minke whales according tothe size or age. Samples from the North Pacific were collected at Ayukawa on
the east coast of Japan where sexually mature animals are far less than in thewaters of higher latitudes (Omura and Sakiura, 1956). Also in the Antarctic
similar segregation by age groups was noted in males (Ohsumi et al., 1970).
Consequently there is a possibility that the above noted differences in hyoid bones
are in fact due to differences according to age.
In Fig. 5 the greatest heights of the ankylosed bone of basihyal and thyrohyals
have been plotted logarithmically against body length of whales. In the figure
different symbols were used for the minke whales from both hemispheres. This
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

SATAKE AND OMURA

20

figure shows that there are significantly different gradients for regressions of log
height of the bone on log length between whales from the different oceans, and thelines of regression intersect at the body length of 709 cm.
In Figs. 6 and 7 relative growth of the height at middle of wing of the combined bones and body length are shown. In these cases two regression lines in•

1110/

9'E(.)·~8bJJc'j;-0 7"''O~ 64/'500600700 800 900 1000
Body length (cm)Fig. 6. Relative growth of height at middle of right wing of the ankylosed boneand body length of the minke whale. For symboles see Fig. 5.

1110'E-'.:. 9bJJ

c'j; 8.....0"' 7'O-0

.E~

"' 6~

.s::

bJJ·a;

:r:

5

4500

600
700 800 900 1000
Body length (cm)Fig. 7. Relative growth of height at middle ofleft wing of the ankylosed bone andbody length of the minke whale. For symbols see Fig. 5.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974MINKE WHALE

21each figure intersect at a body length of 761 cm (right side, Fig. 6) and 727 cm(left side, Fig. 7) respectively. Ohsumi et al. describe that the sexual maturity ofthe minke whale in the Antarctic is attained at a body length of 710 cm in maleand 790 cm in female. According to Omura and Sakiura (1956) in the minkes
on the coast of Japan these lengths at sexual maturity of male and female are 660690 cm and 720 cm respectively.
One possibility, therefore, is that the antero-posterior growth of the combinedbone becomes greater proportionally after the attainment of the sexual maturity.
But this cannot be concluded at present due to lack of sufficient materials. Morematerials, i.e. in the case of the minke whale in the Antarctic samples from immature animals and in that in the North Pacific more samples from mature animals,
are needed.

35E~ 30//

15500

600700 800 900 1 OOO
Body length (cm)Fig. 8. Relative growth in length of right stylohyal and body length of the minke
whale. For symbols see Fig. 5.

In Figs. 8 and 9 relative growth of the stylohyal and body length are shown.
As clearly seen in these figures two lines of regression are nearly parallel and wemay safely conclude that the minke whale in the Antarctic has more longer stylohyals than the minke in the North Pacific, though there is rather a wide individualvariation.
We are not in a position, however, to conclude at present whether the minke
whale in the Antarctic, Balaenoptera bonaerensis, represent a true species, or a subspecies of B. acutorostrata, as stated by Williamson (1961). Further studies especiallyon the skull and other postcranial bones are needed in order to reach a more de.finite conclusion.
Omura (1964) made a key to the genera and species of mystacoceti by meansof hyoid bone. In the key he separates B. acutorostrata from B. borealis and B. edeni
by percentage of the greatest height of the combined bone against its overall length.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

SATAKE AND OMURA

22

The critical proportion is 29 percent and those less than this value was assigned toB. acutorostrata and others to B. borealis and B. edeni. B. bonaerensis has more greatervalue than this and the key should be or should not be revised in this respect,depending upon the final conclusion.

.

•//

015500600700 800 900 1000
Body length (cm)Fig. 9. Relative growth in length of left stylohyal and body length of the minke
whale. For symbols see Fig. 5.

ACKNOWLEDGEMENTS
Our sincere thanks are due to Dr. Seiji Ohsumi who collected hyoid bones of theminke whale in the Antarctic for the present study. The crew of Jinyo Maru andthe staff of Taiyo Gyogyo K.K. are deeply acknowledged for their cooperation insampling and also in transportation of the material.
REFERENCES
KASUYA, T. and T. IcHIHARA, 1965. Some informations on minke whales from the Antarctic. Sci. Rep.
Whales Res. Inst., 19: 37-43.
0HSUMI, S., Y. MASAKI and A. KAWAMURA, 1970. Stock of the Antarctic minke whale. Sci. Rep. Whales Res.
Inst., 22: 75-125.
OMURA, H., 1964. A systematic study of the hyoid bones in the baleen whales. Sci. Rep. Whales Res. Inst.,

18: 149-170.
OMURA, H. and H. SAKIURA, 1956. Studies on the little piked whale from the coast of Japan. Sci. Rep.
Whales Res. Inst., 11: 1-33.
UTRECHT, W. L. VAN, and S. VAN DER SPOEL, 1962. Observations on a minke whale (Mammalia, Cetacea)
from the Antarctic. Zeit. Siiugetier., 27(4): 217-221.
WILLIAMSON, G. R., 1959. Three unusual rorqual whales from the Antarctic. Proc. Zoo!. Soc. (London).

133: 134-144.
WILLIAMSON, G. R., 1961.

Two kinds of minke whale in the Antarctic. Norsk Hvalfangst-Tid., 50(4): 133-141.
Sci. Rep. Whale Res. Inst.,
No. 26, 1974

MINKE WHALE

APPENDIX TABLE.

23

MEASUREMENT OF HYOID BONE OF THE MINKE WHALE
IN THE ANTARCTIC.
Measurement in mm*. I Body
Senalength Sexno.
(m)abcdefghjkmnopqr71]2721 7. I F 364
71J2692 7. 5 F 446
71]0188 7.8 F 459
71J2849 7.9 M 489
71J2685 7. 9 F 414

334404425
452
399

127125127
12613298868887105

21 62
38 66
37 65
40 67
24 6261646565
62303230322929 3430 38
30 43
31 35
30 36

3438453533

220 46 23
266 55 21288 54 22274 62 20271 47 23

1825351730219 45271 56
292 55276 63
275 4822202221231926351828

71J2727 8.171]2323 8.371]2324 8.471]0157 8.4
71J0312 8.4

F 478
F 450
M 503
F 466
F 433

451
420
435438
405

161
166
131
171
171

106 52 76 77
138 23 87 84
107 19 76 77
96 70 79 77
129 35 79 77

4941323731306 62
285 63289 59275 61
318 64

32 25
34 33
23 33
38 50
30 52

311 64286 65289 62
280 62324 603133
2437332530
3152
5571]0459 8.4
71J0174 8.5
71J2415 8.5
71J2793 8.5
71J2322 8.6F 477 425
M 522 475
F 502 462
M 514 491
F 427 397

143164147155153

90 43 69 71 34
Ill 49 65 66 30
94 52 65 65 38
113 40 66 66 31123 31 73 73 38

34 32 32 285 55 29 27 287 57 30 24
29 30 29 294 61 24 37 298 66 26 36
39 39 41 267 62 24 17 272 63 24 18
32 35 39 294 61 30 41 302 60 30 45
37 37 36 257 59 34 22 256 60 36 2171]0165 8.671]2373 8.671]2667 8.671]2197 8. 771J2228 8.7

F 506 460
M 471 438
M 507 453
F 505 460
F 480 435

14812415015115997 46
96 2799 46
105 43
121 387371767271

312447343132 29
26 3646 41
34 44
31 29

30 294 67 23 2537 286 65 25 23
41 315 69 35 23
41 300 67 29 33
30 290 61 20 18299 70
308 65316 70307 68292 642325363021262625
34
1871]0145 9.071]0154 9.2
71J0276 9.4
71J0458 9. 7
71J2883 9.8

F 503 454
F 511 479
F 485 433
F 556 489
F 543 493

145
162176184207106 35 70 71
104 49 75 76
147 22 91 94
130 44 89 88
157 48 111 107

3832394640413240
46403841445340296
312
314
346
336

27222728
282320244739* For measurement number see text.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

727076
71
73

38 36
42 41
32 3142 41
35 36

48
41
32
34
33

37
39
47
51
38

297
306
318
335
339

62
63
84
69
63

27
24
29
27
27

24
19
23
43
33

62
62
82
67
64

24

SATAKE AND OMURAEXPLANATION OF PLATESAll the photographs were taken from the ventral side.
PLATE IThe ankylosed bone of basihyal and thyrohyals of the minke whale from the Antarctic.
Fig. I. specimen 71]2721
Fig. 8. specimen 7 IJ2324
Fig. 2. specimen 71]2692
Fig. 9. specimen 71]0157
Fig. 3. specimen 71]0188
Fig. 10. specimen 71]0312
Fig. 11. specimen 71]0459
Fig. 4. specimen 7 IJ2849
Fig. 5. specimen 71]2685
Fig. 12. specimen 71]0174
Fig. 6. specimen 71J2727
Fig. 13. specimen 71]2415
Fig. 14. specimen 7 IJ2793
Fig. 7. specimen 7 IJ2323
PLATE II
The ankylosed bone of basihyal and thyrohyals of the minke whale from the Antarctic.
Fig. 21. specimen 71J0145
Fig. 15. specimen 71]2322
Fig. 22. specimen 7 IJ0154
Fig. 16. specimen 71]0165
Fig. 17. specimen 71]2373
Fig. 23. specimen 71]0276
Fig. 18. specimen 71]2667
Fig. 24. specimen 71J0458
Fig. 19. specimen 71]2197
Fig. 25. specimen 71]2883
Fig. 20. specimen 71]2228
PLATE III
Stylohyals of the minke whale from the Antarctic.
For explanation see PLATE I.
PLATE IV
Stylohyals of the minke whale from the Antarctic.
For explanation see PLATE II.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

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