J-STAGE Advance published date: December 2, 2021

DORSAL FIN SHAPE OF PACIFIC WHITE-SIDED DOLPHINSSEXUAL DIMORPHISM IN
THE DORSAL FIN OF PACIFIC WHITE-SIDEDDOLPHINS (LAGENORHYNCHUS OBLIQUIDENS)FROM COASTAL WATERS OFF JAPAN
Etsuko KATSUMATA*, Saeko NARUSE, Toru HOSONO andHiroshi KATSUMATA
Kamogawa Sea World, 1464–18 Higashicho, Kamogawa-shi, Chiba 296–0041, Japan*Corresponding author: etsuko_katsumata@granvista.co.jp

Abstract
Records of external measurements of 35 male and 40 female Paciﬁc white-sided dolphins
collected from 1970 to 2020 were examined for the shape change of the dorsal ﬁn and the difference between males and females. For mature individuals based on a body length of 200 cm,
which is an approximate guideline for maturity, a signiﬁcant diﬀerence was detected in theratios of the front edge length and the tip height which were 3.8 on average with a maximum
value of 5.6 in males, and 2.8 (average) and 3.1 (maximum value) in females, respectively. It
was concluded that sexually mature males could be distinguished by measuring the shape ofthe dorsal ﬁn.
Key words:

allometry, dorsal ﬁn, growth, morphology, Paciﬁc white-sided dolphin.

The Paciﬁc white-sided dolphin (Lagenorhynchus obliquidens) is an endemic species of the North
Paciﬁc Ocean, and even a mature individual is relatively small, with a body length of 2.5 m or less.
Length at birth is reported to be about 80–95 cm. The distribution area on the western side of the
Paciﬁc Ocean is along the Sea of Japan and the Paciﬁc coast of Japan on the western Paciﬁc Ocean(Leatherwood and Reeves, 1982, Brownell et al., 1999). Hayano et al. (2004) identiﬁed a distribution
gap around the longitude of 150°E or between Japanese coastal waters and the oﬀshore western andcentral North Paciﬁc, and concluded that samples from these two geographical areas came from diﬀerent populations. The authors suggested that the low genetic diﬀerence could be due to the short timesince reproductive isolation between the two geographical populations oﬀ Japan. The origin of theJapanese name “Kama-iruka” is that the dorsal ﬁn resembles a “sickle” (Kasuya, 2017).
Regarding individual identiﬁcation of dolphins and whales, the dorsal ﬁn is one of the most obvious traits to consider whether in the wild or in captivity. The dorsal ﬁn shape, size, color, scratches,and cuts are used to identify an individual. Killer whales (Orcinus orca) are known as an example
where individual identiﬁcation and identiﬁcation by sex and maturity status is possible by observing
the dorsal ﬁn. The male dorsal ﬁn (height to width ratio) could be readily distinguished from that ofa female, and a height to width ratio of 1.4 or greater was indicative of a mature male (Olesiuk et al.,1990, 2005).
In the case of the Paciﬁc white-sided dolphin dorsal ﬁn morphology, it has been found that sexuallymature males have a pronouncedly curved and round dorsal ﬁn (Walker et al., 1986). Furthermore,Kasuya and Yoshida (1990) and Kasuya (2017) clariﬁed that the tip of the dorsal ﬁn hangs down inmature males from the point of measurement of the height of the tip of the dorsal ﬁn and the overall
height. Attempts have been made to identify sexually mature males from photographs of the dorsal
ﬁns of wild Paciﬁc white-sided dolphin herds using this method (Morton, 2006). However, there are
Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

273

KATSUMATA E., NARUSE, HOSONO AND KATSUMATA H.

no studies that have examined the body length and dorsal ﬁn measurements of this species in comparison with the sex.
In this study, we investigated changes in dorsal ﬁn shape and curvature using proportion measurements of Paciﬁc white-sided dolphins collected in the waters of both the Paciﬁc Ocean side and theSea of Japan side of Honshu, Japan, over the past 50 years from 1970 to 2020. The purpose of thisstudy was to clarify the diﬀerence in dorsal ﬁn shape between males and females with changes inbody length, and to contribute to the biological database of Paciﬁc white-sided dolphins.
The measurements used in this study are shown in Fig. 1. The proportion measurement records of35 males and 40 females were used. These measurement records include post-mortem measurements(bycatch or after death in captivity) and live individuals’ latest measurements data at Kamogawa Sea
World (KSW). It is a characteristic of Paciﬁc white-sided dolphins distributed in the coastal watersof Japan that individual diﬀerences in body length are large in all age ranges. The range shared bysexually mature and immature individuals was 180–234 cm for males and 195–209 cm for females(Kasuya, 2017). From this, the guideline for sexually mature body length was roughly set to 200 cm inthis paper.
The following allometric equation was used to extract growth patterns at each point:y = βxα
where x deﬁnes the body length; y, the length (cm) of the measurement points; α, the allometric coeﬃcient; and β is a constant. The t-test was conducted using allometric coeﬃcients to reveal growth
patterns at each measurement point, and the data were grouped into three diﬀerent growth patterns:
positive allometry (hyperallometry) when the allometric coeﬃcient was signiﬁcantly greater than 1,
isometric allometry and negative allometry (hypoallometry) when the allometric coeﬃcient was lessthan 1. These statistical analyses were conducted by using the packaged tools in the statistical software R (R Core Team, 2021). Fig. 2 shows the obtained body length plotted against each measurement point of dorsal ﬁns for males and females. The body length (measurement point No. 1, Fig. 1)
ranged from 71 cm (fetus) to 231 cm for males and 70.5 cm (fetus) to 225 cm for females. As the bodylength increased, each measured value also increased, and the slope of the graph increased. For differences between males and females in each graph, the length of dorsal ﬁn base (measurement pointNo. 2, Fig. 1) diﬀered between males and females. The maximum value was 49.0 cm for males and41.0 cm for females. However, the height of the dorsal ﬁn tip (measurement No. 3) was conspicuously
narrower in individuals of body length 200 cm or more for both males and females (Fig. 2). In otherwords, the tip of the dorsal ﬁn tended to hang down as individuals sexually matured in both males andFig. 1. External measurement points of Paciﬁc white-sided dolphin used in the present study: No. 1, from thetip of the upper jaw to notch in the tail ﬂuke (body length); No. 2, length of dorsal ﬁn base; No. 3, height ofdorsal ﬁn tip; No. 4, length from the front edge to ﬁn tip and No. 5, length from ﬁn tip to trailing edge.

274

Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

DORSAL FIN SHAPE OF PACIFIC WHITE-SIDED DOLPHINS

Fig. 2. Relative growth of measured points in dorsal ﬁn and body length of the Paciﬁc white-sided dolphin.
Closed circles (left) represent males and open circles (right) represent females, respectively. The dotted line
indicates the approximate body length of a sexually mature (200 cm) individual.
Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

275

KATSUMATA E., NARUSE, HOSONO AND KATSUMATA H.

females. The front edge length (measurement point No. 4, Fig. 1) was also diﬀerent between males
and females. For individuals with a body length over 200 cm, dorsal ﬁns grew longer in males than infemales. The maximum value was 59.4 cm for males and 47.5 cm for females. The dorsal ﬁn trailingedge length (measurement point No. 5) values varied widely for both males and females. This may be
due to the variety of trailing edge shapes. The maximum value was 39.0 cm for males and 29.6 cm forfemales.
Table 1. Allometry of the length of dorsal ﬁn measurement points and body length of Paciﬁc white-sided dolphin males and females.
MaleMeasurement points

αβR2Growth patternP-value

No. 2
No. 3
No. 4
No. 5

Length of dorsal ﬁn base
Tip height
Front edge length
Trailing edge length

1.06
0.631.09
0.87

0.14
0.56
0.15
0.25

0.96
0.50
0.930.72

isometric allometry
negative allometry
isometric allometry
isometric allometry

0.135
0.003**
0.097
0.204

FemaleMeasurement points

αβR2

Growth patternP-value

No. 2
No. 3
No. 4
No. 5

Length of dorsal ﬁn base
Tip height
Front edge length
Trailing edge length

1.01
0.45
0.92
0.66

0.16
1.55
0.32
0.72

0.93
0.32
0.91
0.47

isometric allometry
negative allometry
isometric allometry
negative allometry

0.782
0.000**
0.137
0.010**** indicates a statistically signiﬁcant diﬀerence (p<0.01) between sexes in the length of dorsal ﬁn measurement points
and body length (see also Fig. 2).

Fig. 3. Paciﬁc white-sided dolphin dorsal ﬁn growth-related ratio change No. 4/No. 3 shows front edge length
to tip height ratio and No. 5/No. 3 shows trailing edge length to tip height ratio. Closed circles (left) represent
males and open circles (right) represent females, respectively. The dotted line indicates the approximate bodylength of a sexually mature (200 cm) individual.

276

Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

DORSAL FIN SHAPE OF PACIFIC WHITE-SIDED DOLPHINS

Table 1 shows the values applied to the relative growth curve (relationship between the length of
dorsal ﬁn measurement points and body length) for males and females. Tip height in males, and tipheight and front edge length in females showed negative allometry.
Fig. 3 shows front edge length to tip height (No. 4/No. 3) and trailing edge length to tip height (No.
5/No. 3) ratios by body length for males and females. The ratio rises sharply after a body length of
200 cm in males, while on the other hand such a change is not observed in females.
Fig. 4 shows the front edge length to tip height (No. 4/No. 3) and the trailing edge length to tipheight (No. 5/No. 3) ratios of individuals over 200 cm in body length. The ratios of front edge length
to tip height were signiﬁcantly higher in males (3.8±0.2, mean±SE, n=12) than in females (2.8±0.1,n=10) (t-test, p<0.01). No signiﬁcant diﬀerence was found in the trailing edge length to tip heightratios.
This study found that the dorsal ﬁns of Paciﬁc white-sided dolphins inhabiting the coastal waters ofJapan diﬀer in shape and size between males and females. Especially in length of dorsal ﬁn base (measurement point No. 2) and front edge length (measurement point No. 4), males clearly grew more thanfemales. When the ratio of dorsal ﬁn was examined using these measurements (Fig. 1), a signiﬁcantdiﬀerence was found in the front edge length to tip height ratio (No. 4/No. 3). This diﬀerence in theshape of the dorsal ﬁns of males and females was eﬀective for individual identiﬁcation of wild Paciﬁcwhite-sided dolphins and identiﬁcation of sexually mature males. Suzuki et al. (2020) suggested that
there are at least two forms on the coastal waters of Japan, and from the result of genetic structural
analysis of 64 Paciﬁc white-sided dolphins kept at aquariums in Japan, they concluded that in the Japanese coastal waters, genetic exchange between populations with diﬀerent genetic backgrounds seems

Fig. 4. Comparison of the dorsal ﬁn ratios between mature males and females. No. 4/No. 3 shows front edge
length to tip height and No. 5/No. 3 shows trailing edge length to tip height. The vertical black bars indicate
the average error and * indicates a statistically signiﬁcant diﬀerence (p<0.01) between sexes.
Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

277

KATSUMATA E., NARUSE, HOSONO AND KATSUMATA H.

to occur in the wild. In the future, it is hoped that diﬀerences in body length and external morphology
between individuals of the forms identiﬁed by genetic structural analysis will be investigated.
As a conclusion, we compared the measured values of the dorsal ﬁn for the Paciﬁc white-sided dolphin using records from the coastal waters of Japan. It was shown that the ratio of the dorsal ﬁn canbe used to identify sexually mature males. Their dorsal ﬁn (front edge length to tip height ratio) could
be readily distinguished from that of females, and a ratio of 3.8 or greater was indicative of a sexuallymature male.
Acknowledgements

We would like to express our deep gratitude to all the staﬀ who have been involved in the continuous care of dolphins,
and the measurement of proportions and the storage of records for over 50 years at Kamogawa Sea World. We would like tothank Masayuki Muramatsu and Naoko Suhara for their cooperation in preparing the ﬁgures and the signiﬁcance test. Finally,
we deeply appreciate the anonymous reviewers for their helpful suggestions.

References
Brownell, R.L. Jr., Walker, W. and Forney, K.A. 1999. Paciﬁc white-sided dolphin, Lagenorhynchus obliquidens. pp. 57–84.
In Ridgway S. H. and Harrison, R. (eds.). Handbook of Marine Mammals. Vol. 6: The Second Book of Dolphins and thePorpoises. Academic press, San Diego. 486 pp.
Hayano, A., Yoshioka, M., Tanaka, M., and Amano, M. 2004. Population diﬀerentiation in the Paciﬁc white-sided dolphin
Lagenorhynchus obliquidens inferred from mitochondrial DNA and microsatellite analyses. Zool. Sci. 21(9): 989–999.
doi: 10.2108/zsj.21.989.
Kasuya, T. 2017. Paciﬁc white-sided dolphin. pp. 437–443. In: Kasuya, T. and Perrin, W.F. (eds.). Small Cetaceans of Japan:
Exploitation and Biology. CRC Press, New York. 475 pp.
Kasuya, T. and Yoshida, S. 1990. Secondary sexual characteristics of the Paciﬁc white-sided dolphin. Abstracts of the Spring
Meeting of the Japanese Society of Scientiﬁc Fisheries, Tokyo, Japan, 2–5 April 1990.
Leatherwood, S. and Reeves, R.R. 1982. Paciﬁc White-sided Dolphin pp. 204–207. In: The Sierra Club Handbook of Whalesand Dolphins. The Sierra Club Books, San Francisco. 302 pp.
Morton, A. 2006. Occurrence, photo-identiﬁcation and prey of Paciﬁc white-sided dolphins (Lagenorhynchus obliquidens) inthe Broughton Archipelago, Canada 1984–1998. Mar. Mam. Sci. 16: 80–93. doi: 10.1111/j.1748-7692.2000.tb00905.x.
Olesiuk, P.F., Bigg, M. A. and Ellis, G.M. 1990. Life history and population dynamics of resident killer whales (Orcinusorca) in the coastal waters of British Columbia and Washington State. Rep. int. Whal. Comm. (Special issue) 12:
209–242.
Olesiuk, P.F., Ellis, G.M. and Ford, J.K.B. 2005. Life History and Population Dynamics of Northern Resident Killer
Whales (Orcinus orca) in British Columbia. Canadian Science Advisory Secretariat, Fisheries & Oceans, Canada.
Research document 2005/045, 75 pp.
R Core Team 2021. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing,Vienna.
Suzuki, M., Morinaga, M., Sawayama, E., Matsumoto, T. and Kato, H. 2020. Preliminary analysis of the genetic population
structure of Paciﬁc white-sided dolphins kept in Japan. Abst. Spring Meeting of the Japanese Society of Fisheries Science. p. 56 (in Japanese).
Walker, W.A., Leatherwood, S., Goodrich, K.R., Perrin, W.F. and Stroud, R.K. 1986. Geographical variation and biology of
the Paciﬁc white-sided dolphin, Lagenorhynchus obliquidens. pp. 441–465. In: Bryden, M.M. and Harrison, R. (eds.).
Research on Dolphins. Oxford Science Publications, Oxford. 478 pp.

Received: January 21, 2021
Accepted: August 19, 2021
Published online: December 2, 2021

278

Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

DORSAL FIN SHAPE OF PACIFIC WHITE-SIDED DOLPHINSAppendix
Dorsal ﬁn shape photos; ID, sex, sexual status, body length.
The photos are shown based on body length left to right, from top to bottom by male (this page) andfemale (next page). The individuals in the photos have been kept at KSW and conﬁrmed to be matureand immature by reproductive history, sex hormones, and semen collection in the case of males.

Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280

279

KATSUMATA E., NARUSE, HOSONO AND KATSUMATA H.

Appendix (Continued.)280

Cetacean Popul. Stud. (CPOPS), Short paper
Vol. 3, 2021, 273–280