On the Pelvic Cartilages of the Balaenoptera-Foetuses, with
Remarks on the Specifical and Sexual Difference
BY
H. Hosukawa· (Anatomical Department, Medical School,
1:-niveisity of Tokyo, Profeg;or Dr. T. Ogawa)(Anatomical studies on the whalebone-whale foetuses, No. 1)

Introduction
It is well known that the whales have no hind-limb protruding on the surfaceof the body. But probably all of them have the rudimentary pelvis as a pair of
slender bones. 1l Unlike in most mammals, whose pelvic girdle consists of threeelements (ischium, pubis and ilium) coalescing into a single bone, the cetacean
pelvis is made of only one element, a fact stated first by Eschricht and Reinhardt(1866), who found a single center of ossification in it. This bone is believed _torepresent the ischium, for it is connected with the crura penis or clitoridis, andalso with the musculus ischiocavernosus.2J3J
While in the Odontoceti the pelvis is the sole remnant of the hind-limb, mostof the Mysticeti have one or two other pairs of very small bones or cartilages
placed near the middle of pelvis. One pair of them is attached to the pelvis byligaments and accompanies the third pair at its caudal end. They were foundat first by Reinhardt ( 1848) in a newborn Greenland Right-whale and asserted
then by Eschricht and Reinhardt (1866)
in a half-grown and a full-grown whaleof the same species (Fig. 1). Later,
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Struthers (1881) observed eleven pairs\
pof the rudimentary bones, studying
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minutely the articulations and neighbouring muscles. The supernumerary
pelvic bones are nowadays supposed toFig. I Side view of bones of posterior extremity of Greenland Right-whale
represent femur and tibia, as sug(from Eschricht and Reinhardt)
gested by the discoverers of them.
P ...... Pelvis F ...... Femur T ...... Tibia
I) In Cogia the presence of the pelvis is not yet ascertained. (van Oort: Zoo!. Medel.
Rijksmuseum Nat. Hist. Leiden. 9, 1926.)
2) Schneider (1795) took the cetacean pelvis for pubis.
3) The Eschricht and Reinhardt's interpretation of the cetacean pelvis, that its cranial andcaudal portions represent respectively pubis and ischium, while the lateral process corraiponds
to ilium, may be also true, if we take in mind the relations to neighbouring structureB.

6

H. HOSOI{AWAOn the other hand, in the adult Humpback Eschricht found only the second
pair, and afterwards Struthers (1888) confirmed also the absence of the third pairin this species.
The same result was mentioned by Burmeister (1867). and 'stru.
thers (1872, 93) also for the Fin-whale, in which the only existing femur was
extremely vestigial. Further, the lesser Fin-whale (Balaenoptera acuto-rostrata,
Lac.) and the Sei-whale show, according to Struthers etc., no trace of femur andtibia, just like in the Odontoceti (Fig. 2).

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dCetacean skeletom of hind-limbs (from Struthers)
LE$er Fin-whale (14.5 feet, male)b. Sei-whale (36 feet, male)Fin-whale (64 feet, male)d. Greenland Right-whale (48 feet, male)
Humpback-whale (40 feet, male)

During my Antarctic whaling voyage on board the Nissin-Mam No. 1 (1947-48),
I examined the cartilages in question in the Balaenoptera-foetuses and found somenoteworthy facts.
The materials examined by me are as follows:
Foetuses of the Blue whale (Balaenoptera musculus, L.) 9.
(male, 6; female, 3; max. length, 22 feet 2 inches; min. length, 3 'feet 11
inches)Foetuses of the Fin-whale (B. physalus, L.) 9.
(male, 5; fel)lale, 4; max. length, 15 feet 1 inch; min. length, 2 feet 1 inch)
Additionally for compa~ison, each one pair of pelvic bones of adult Blue andFin-whales.

Pelvic cartilages of the Blue and ~Fin foetuses

On the Pelvic Cartilages of the Balaenoptera-FoetusesrBoth species have besides a pair of pelvis another pair of very small cartilages,
the third pair being completely absent. The pelvis (Fig. 3, 4) is elongated, somewhat curved, hammerlik.e in form, and has a very short lateral process, corresponding to "promontry" of Struthers. The upper process, processus cranialis,is pointed at the end and deserves the name " apex cranialis ", while the lower
one, proc. caudalis, is shorter, with its somewhat rounded extremity. The lateralprocess, the shortest of all processes, ends also 1not sharpened, but has here amore or less eminent tubercle, tuberculum laterale.
Near the tubercle, a little more ventrad and craniad, there is a small, oval
piece of cartilage, the upper end of which is pointed in some cases, especially
P. er.'

F.

bdFig. 3 Pelvic cartila5es of the Fin and Blue foetusesa. Fin, male (13 feet 6 inches long) b. Fin, female (15 feet I inch long)c. Blue, male (12 feet 3 inches long) d. Blue, female (14 feet 11 inches Jong)I) In the adult whales the cranial process is rather flattmed mediolaterad, in opposite to
the club-like caudal process (Fig. 4).

8

H. _HOSOKAWA

in the Fin-whale.

This small

cartilage, being nothing but theremnant of femur, is covered
with a thick capsule of connective tissue.

The remnant of-femur does not increase in size
proportionately with the growthof the foetus: in other words,
much more remarkable indivibaFig. 4 Pelvic bone of adult whalebone-whalesa, b, Fin-whale, lateral and vantrel viewc, d, Blue whale, lateral and ventral view
P. er., Proc. cranialis; P. ea., Proc. caudalis;
1'. I., Tuberculum laterale

dual variation is seen in thesize of this cartilage than in- that of the pelvis. This difference might be understood by
the more vestigial nature of theformer. It is moreover likely,that the femur is more rudimentary in the Blue whale than in the Fin-whale. For,according to my observation, the femur of considerably larger foetuses of Blue
whale (12 feet 4 inches, male; 13 feet 4 inches, male; 15 feet 5 inches, female;
22 feet 2 inches, male) is sometin1es maue solely of connective tissue, containing
no cartilage.

The ligamentous band between femur and pelvis is very loose, andhere I found no articular formation, though Eschricht, Reinhardt and Struthers
mentioned an articulation respectively in an adult Greenland Right-whale and inan adult Fin-whale.

Ossification of the pelvic cartilages
Neither pelvis nor femur of all the foetuses, measuring from 2 feet 11 inches
up to 22 feet 2 inches in length, showed any trace of ossification. It is therefore
supposed that these skeletons are ossified only after the parturition. For, consulting the statistics published by Mackintosh, Wheeler (1927) and Matthews (1937),
it seems certain that the foetus of 22 feet is only a little before the birth. Rein·
hardt found in a newborn Greenland Right-whale an ossified portion of the femur
in its middle part, but no ossification in pelvis and tibia.

Position and locality of the pelvic cartilages
The cartilaginous pelvis exists on each side, rather laterad and caudad, to theexternal genital orifice, with its axis nearly parallel to that of the whale-body.
We can palpate them after removal of the skin as somewhat hard objects.

Allof my specimens were not so small as to settle the problem upon the presence ofan external protrusion representing the hind-limb reported by Guldberg (1894, 99)
and Kiikenthal (1895) in Phocaena communis (Fig. 5). I have ,known, however,On the Pelvic Cartilages of the Balaenoptera-Foetuses
acnstic pore

9that the pelvis is located insmall foetuses under the skinmore superficially, while thosein larger ones lie more deeply

exrernalfore-limb---

nasal aperture__ umbilical cord

embedded in the muscular layerand so more hardly palpable.

Difjerence of the foetal pelvis in form between Fin- andBlue whale
In the Fin foetus the caudal

process of the cartilaginous pelvis is relatively shorter thangenital member

external rudimentary hind-extremity

in the Blue, viz. · the pelvis

Fig. 5 The 17 mm. long Phocaena-enibryo seenfrom the right side. ( Guldberg)of the former has the lateralprocess placed more caudad
than that of the latter (Fig. 3).

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My measurements of the distances 1) from the cranial apex
to the caudal end (a), 2) fromthe cranial apex to the lateral\i

tubercle ({3), 3) from the caudal end to the lateral tubercle

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(r), and the relative length of~\(3 andFig. 6. Measurements of the pelvic cartilage

shown in Table L
While the average of this in---;_--~r (f3/r) . (Fig. 6) aredex f3!r is in the foetal Fin-whale 2.01 (max., 2.67; min., 1.63), it is in the
foetal Blue whale only 1.06 (max., 1.25; min., 0.71); there is accordingly a distinct difference between the both species.

Fig. 7 indicates my data diagram-

matically. From this figure we know that the index f3/r is in the Fin foe·tuses, without exception, more than 1.5, while in the Blue foetuses it lies always
under 1.5. It is interesting that these intimately related two species of
Balaenopterides show such a remarkable difference in the form of pelvis.

I can'tdecide, however, whether the same difference is present also in the adult whales,
but the postnatal existence of approximately the same relation is unplausible.1 l1) Struthers (1893) stated that the characteristics in the form of pelvic bones of some
adult whales C\iegapte~a, Right-whale) are to be se~n 'also in the immature; younger:whales.

H. HOSOKAWA

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According to Eschricht, Reinhardt and Struthers, the posterior part of the pelvic
bone exceeds considerably in length the anterior part in the Greenland
Right-whale. Hence the ratio in question seems to be in this' whale by far lessthan 1.0. After Struthers' measurement of eleven specimens, this ratio seems to

On the Pelvic Cartilages of the Balaenoptera-Foetuses

11fall between 0.3 and 0.7 (average, 0.41) for the Greenland Right-whale. And I
calculated, using Struthers' data, that this ratio is for a male Megaptera of 40
feet long 0.77, i.e. an intermediate -value between t!l.e Greenland Right-whale
and the Blue foetus. In this way we can probably determine the kind of whalebone-whales from the form and size of pelvis, especially by consulting the relative length of the upper and lower processes.

Sexual difference of the cartilaginous pelvis
In both the Fin and Blue foetuses the {1/r index of the pelvis is larger in thefemale than in the male (Fig. 7).

As to the Fin-whale, this index is in thefemale almost always more than 2.0 (average, 2.34; max., 2.67; min., 1.95.', whilein the male it is, without exception, less than 2.0 (average, 1.75; max., 1.97;
min., 1.63). In the Blue whale, the critical point of the ratio for the sexual
difference is 1.0 (average for the male, 0.91; max., 0.98; min., 0.71: average forfemale, 1.14; max., 1.25; min., 1.00). Further investigation of the pelvic bones
in the adult Fin- and Blue whales is from this viewpoint very desirable.1>

Angle between the cranial and caudal process of the pelvis
These processes make each otherat the level of the lateral process an

Fig. 8.

Angle of the pelvic cartilage

obtuse angle opened mediad (Fig. 8).
This angle, measured with goniometer, shows no remarkable specificalor sexual difference (Table 1), theaverage being in the Fin and Bluefoetuses respectively 136°(male, 136°;
female, 137°) and 133° (male, 124°;
female, 137°); the individual variation is in the Fin-whale 127°~14£ ,0

and in the Blue whale 120°-149°. Neither exists any remarkable difference between the right and left pelvis (average for the Fin-male: right, 136°, left 136°;
female: right, 138°, left, 136°; average for the Blue- male: right, 123°, left, 125°;
female: right, 140°, left, 138°), though some instances showed the angle tolerably
asymmetrical.
I) From Struthers' data no distinct sexual difference seems to exist in the relative len5th
between anterior and posterior parts in the Greenland Right-whale, because this ratio becomesprobably 0.30 ( -'f) and 0.45 ( o ). In his opinion, however, it is easie1" to tell the sexual8
difference of the pelvic bone in M:ysticetus than in human beinss, for the hinder end o! thibone is much thickened in the male, while it is flattened, usually with expansion, in thefemale.

12

H. HOSOKAWA

N.B. .According to Struthers, this angle appears to be in the female Greenland Right-whale somewhat larger than in the male, the average being respectively 139° and 124 °.
Development >f the pelvis in length

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13

The length of the cartilaginous pelvis in various intrauterine stages is shownin Fig. 9. By the length of the pelvis I mean the distance from the cranial apex
to the caudal end (a in Table 1), and by the body length the distance from therostral extremity to the median notch of the caudal flipper. We see from thisfigure that the relative length of pelvis to the whole body remains, regardless of
size, sex and species of the foetuses, almost constant. 1 >2> The cartilaginous pelvis
occupies about 1.6-1.8% of the total length of the body (average, Fin-whale-male,
1.8, female, 1.7; Blue whale-male, 1.6, female, 1.7, max., 1.9. min., l.4)(Table 1).
Nearly the same ratio seems to hold good in the adult stage, for from..

Struthers' data, this ratio becomes 1.7 in an adult male Fin-whale of 50 feet, 1.6 inan adult male Sei-whale of 36 feet, 1.9 both in an adult male lesser Fin-whale of
14.5 feet and in an adult male Megaptera of 40 feet. But in the Greenland Rightwhale the pelvis seems to be relatively larger, occupying 2.3% (max., 3.0, min.,2.0) of the total length of the body.
Summary

1) From the viewpoint of the rudimentary skeletons pertaining to the hindlimb, the whalebone-whales can be classified into the following three groups: Group
1, to which the Greenland Right-whale belongs, has besides the pelvis two pairsof subsidiary bones or cartilages (femur and tibia). Group 2, in which other thanthe pelvis only the femur is present, comprises the Humpback, the Fin-whale andperhaps also the Blue whale. Group 3, which has neither femur nor tibia but has
the pelvis only, seems to comprise the Sei-whale and the lesser Fin-whale.
2) Many foetuses of Fin- and Blue whales studied in this paper have twopairs of cartilages corresponding to pelvis and femur, which show no trace of thebony nucleus within them. Very likely, therefore, the ossification of them takes
place only after the parturition.
3) The pelvis of whales, though generally it is hammerlike in form, shows
remarkably different shapes according to the species. Especially the ratio oflength between the anterior and posterior portions of the pelvis is characteristic
to each species.
4) In the Fin and Blue foetuses the sexual difference is marked in the formof the pelvis. The ratio mentioned in the preceding paragraph is always· greaterin the female than in the male; namely, the female pelvis has relatively longer
cranial process than the male.
1) Flower (1876) stated that the pelvic bone of whales is usually more largely developedin the male than in the female.
·2) I never found such an asymmetrical developmettt of pelvis as observed by Struthers(1893) in an adult Sei-whale,

14

H. HOSOKAWA 'Table 1.

-( Foetu~ 'Measurements of hind-limb cartilages of Blue and Fin-whale foetuses

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On the Pelvic Cartilages of the Balaenoptera-Foetusffi

155) The obtuse angle between the cranial and caudal portions of pelvis is in
the average 136° and 133° respectively for the Fin and the Blue foetuses. It
shows no remarkable variation as to species, sex, age and body-side of whales.
6) The length of pelvis is in the Fin and the Blue ~foetuses about 1.6-1.8%
Df the bo:ly length. This ratio seems to be nearly constant for almost all whalebone whales, regardless of species, sex: and age, probably with the exception ofGreenland Right-whale, which shows a ~atiO of far greater than 2.0%.

Literaturel) Schneider, J. G.; Beitrage zur Naturgeschichte der Walfischarten. Leipzig. 1795.
2) Eschricht, D. F. and Reinhardt, J.; On the Greenland Right-Whale (Balaena
mysticetus, L.). Ray Society, 1866. Recent Memoirs on the Cetacea. London 1866.
3) Burmeister, H.; Preliminary descriptions of a new species of Finner whale(Balaenoptera bonaerensis). Proc. ZooL Soc. London. 1867. pp. 707-713.
4) Flower, W. H.; An introduction to the osteology of the mammals. London.
1870. Second edition. 1876.
5) Struthers, J.; On some points in the anatomy of a great Fin-whale (Balaenoptera musculus). Journ. Anat. Physiol. 6. pp. 107-125. 1872 (1871).
-6) Struthers, J.; On the bones, articulations, and muscles of the rudimentaryhind-limb of the Greenland Right-whale(Balaena mysticetus). Ibid. 15. pp. 141176, 301-321. 1881.
7) Struthers, J.; On some points in the anatomy of a Megaptera longimana. Part
2. The limbs. Ibid. 22. pp. 240-282. 1888.
8) Struthers, J.; On the rudimentary hind-limb of a great Fi_n-whale (Balaenoptera rnusculus). Ibid. 24. pp. 291-335. 1893.
9) Kiikenthal, W.; Ueber Rudimente von Hinterflossen bei Embryonen von Walen.
Anat. Anz. 10. pp. 534-537. 1895.
10) Guldberg, G.; Ueber temporare aussere Hinterflossen bei Delphinembryonen.
Verh. anat. Gesellsch. 8. pp. 92-95. 1894.
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