Size at Maturity and Tail Elongation of Loggerhead Turtles (Caretta caretta) in the
North PacificAuthor(s) :Takashi Ishihara and Naoki Kamezaki
Source: Chelonian Conservation and Biology, 10(2):281-287. 2011.
Published By: Chelonian Research Foundation
DOI:
URL: http://www.bioone.org/doi/full/10.2744/CCB-0893.1

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NOTES AND FIELD REPORTS

281

Chelonian Conservation and Biology, 2011, 10(2): 281–287
g 2011 Chelonian Research Foundation

Size at Maturity and Tail Elongation of
Loggerhead Turtles (Caretta caretta) in the
North Pacific
TAKASHI ISHIHARA1,2 AND NAOKI KAMEZAKI1,3
1

Graduate School of Agricultural and Life Sciences, University ofTokyo, Yayoi 1-1-1, Bunkyo, Tokyo 113-8657, Japan[ishihara@umigame.org; Fax: 81-72-864-0535;
kamezaki@umigame.org];
2
Sea Turtle Association of Japan, Nagaomotomachi 5-17-18-302,
Hirakata, Osaka 573-0163, Japan;
3
Suma Aqualife Park Kobe, Wakamiyacho 1-3-5, Suma, Kobe,
Hyogo 654-0049, Japan

ABSTRACT. – Understanding reproductive biology isimportant to elucidate the ecology and life history ofanimals and is needed to predict population dynamics
and demography for conservation. The loggerhead
turtle (Caretta caretta) is an endangered sea turtle
species, and the vast majority of what is known about
their reproductive biology has been learned fromnesting females, with little information available onmature males and the time before reaching maturation.
In order to increase the understanding about biology ofmaturation, size at maturity and puberty and therelationship between tail elongation and sexual maturation in loggerhead turtles in the North Pacific Ocean
were investigated. The maturity stages of 54 males and
106 females, mostly captured at Kochi and Mie
prefectures, Japan, were determined on the basis of
gonadal development. There is no significant size
difference between males and females in all maturity
stages. The straight carapace length notch to tip (SCLnt) at maturity was estimated as 82.1 cm. Tail elongationis the main secondary sexual characteristic of mature
males. The elongation appears to begin when a malereaches to 65.8 cm in SCLn-t. The SCLn-t coincides
with the size at puberty, which was estimated as 66.0 cm
in this study.
KEY WORDS. –Reptilia; Testudines; Cheloniidae; demography; maturity; puberty; tail elongation
Reproduction is one of the most important phases in
the life history of an animal, and information on
reproduction can also inform conservation strategies forendangered species. The loggerhead turtle (Caretta
caretta) is an endangered sea turtle species and is widelydistributed in the world’s temperate and tropical oceans(Pritchard 1997). Loggerhead turtles exhibit substantial
phylogeographic structure among their major nesting
populations (Bowen 2003), one of which is in the North
Pacific Ocean (Bowen et al. 1995). Understanding thevariation in life history and the reproductive biology of all

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CHELONIAN CONSERVATION AND BIOLOGY, Volume 10, Number 2 – 2011

Figure 1. Sites within Japan (dark gray) of the large pound nets
in which loggerhead turtles used in this study were caught
incidentally. At both sites, pound nets were set at 50–75 m depth
and 1–2 km apart from the shore.

loggerhead populations would be a valuable conservation
asset. However, the vast majority of what we know about
reproductive biology of sea turtles has been learned fromnesting females, while little information is available onmales and the time before reaching maturation.
In the Pacific and the Atlantic oceans, loggerhead
nesting sites are restricted to the western side of thebasins. Posthatchling turtles migrate to the eastern watersof the basins (Bolten et al. 1998; Boyle et al. 2009) andreturn to western waters after they reach to approximately
50 cm curved carapace length (CCL) in the North Atlantic(Bjorndal et al. 2000), approximately 70 cm CCL in theSouth Pacific (Limpus and Limpus 2003), and between 56
and 75 cm straight carapace length (SCL) in the North
Pacific (Ishihara et al. 2011). The size at maturity ofloggerhead turtles is often represented by the carapace
length of nesting females, which varies among nesting
populations but is larger than the size at which juvenile
turtles recruit back to the western sides of each ocean
basin (Dodd 1988; Kamezaki 2003). Thus, althoughloggerhead turtles grow into maturity after coming backto neritic habitats near reproductive areas, little is known
about the maturation process.
In order to understand the reproduction of loggerhead
turtles, information about maturation in both the male andthe female is essential; however, data on males are
extremely scarce. Tail elongation is induced by sexual
maturation, and the level of maturation in many freshwater
turtles is indicated by elongation of the tail (Kuchling1998). For sea turtles, the carapace length at the beginningof tail elongation has been studied in the loggerhead turtles
in the North Pacific and the Mediterranean (Hirate 2000;

Casale et al. 2005), green turtles (Chelonia mydas) in the
North Pacific (Hirate 2000), and hawksbill turtles (Eretmochelys imbricata) in the Caribbean (van Dam and Diez
1998). However, those studies were unable to determine
the level of sexual maturity in each individual.
The relationship between tail length and maturation
has only been described for loggerhead turtles from theSouth Pacific by Limpus (1985) and Limpus and Limpus(2003), who classified maturation levels on the basis of
the morphological characteristics of the testis andepididymis using laparoscopy. They also showed thatthe tail length measured from the carapace (TLC; i.e., thelength of the tail from the posterior midline junction of
the supracaudal scutes to the tip of the straightened tail) in
pubescent males is 9–19 cm, whereas the TLC of mature
males was larger than 19 cm in mature turtles. In thispopulation, there was no difference in the size of mature
males and females (Limpus 1985). Likewise, the size ofpresumed mature males based solely on tail elongation
was similar to the size of nesting females in the North
Pacific (Kamezaki 2003) and the Mediterranean (Casale
et al. 2005). Because the size of nesting females differs
among loggerhead populations, data on size-based
reproductive status on each population are needed.
However, in the absence of corroboration from gonadal
histology, the detailed relationship between tail lengthand the actual level of maturity is unclear.
In order to expand the knowledge of the reproductive
biology of loggerhead turtles, the aims of this study are 1) toexamine and compare the sizes of males and females, 2) to
identify the size of males and females when puberty begins,
and 3) to clarify the relationship between the maturity andtail length of males in the North Pacific population.
METHODS. —We used 160 loggerhead turtles caught as bycatch inlarge pound nets in Japan. The pound nets were set along the
Pacific coast of Muroto in Kochi prefecture and Shimakatsu
in Mie prefecture. Both locations are near the edge of thecontinental slope and close to the Kuroshio Current (Fig. 1).
We measured the straight carapace length notch to tip(SCLn-t) and tail length (TL) of all turtles. SCLn-t wasmeasured from the anterior point at the midline (nuchal
scute) to the posterior tip of the supracaudals (Bolten 1999),
and TL is the linear distance from the end of the plastron to
the center of the cloaca.
One hundred and fifty-two of the specimens were
found dead, and their reproductive organs were examined.
A total of 46 turtles were determined to be male, and therest (n 5 106) were female. They were then classified into
3 maturity stages on the basis of the morphologicalcharacteristics of the gonads and genital ducts: ‘‘mature’’
when the gonads and genital ducts were fully developed,‘‘pubescent’’ when reproductive system undergoes a seriesof morphological changes, or ‘‘prepubescent’’ when theturtle has not reached to puberty (Fig. 2; Limpus andNOTES AND FIELD REPORTS

283

Figure 2. Gonads and genital ducts of loggerhead turtles. Maturity stages were defined on the basis of the morphologicalcharacteristics of gonads and genital ducts (Limpus and Limpus 2003; Miller and Limpus 2003). Mature males (A) had cylindricaltestes with each epididymis distinctly enlarged and pendulous. Pubescent males (B) had elliptical testes, and epididymis formed anonpendulous ridge on the body wall. Prepubescent males (C) had testes that were flat or ellipsoidal; each epididymis that was notbulging from the body wall. Mature females (D) had yellow, vascularized, vitellogenic follicles; corpora lutea, corpora albicantia, oratretic follicles may be present in the ovary; eggs may be present in the oviduct. Pubescent females (E) had ovaries with an expanding
or approximately fully expanded stroma; each oviduct was partly convoluted, oval, and 0.3–1.5 cm in diameter adjacent to the ovary.
Prepubescent females (F) had ovaries with a nonexpanded stroma; each oviduct was white, straight, or slightly convoluted; cylindricalto oval in cross section; and , 0.3 cm wide opposite the ovary. Photo by Takashi Ishihara.

Limpus 2003; Miller and Limpus 2003). In addition to 4
male turtles for which sex was identified by direct
observation of reproductive organs, 8 turtles that wereaccidentally caught alive in the pound nets in Muroto werealso considered mature males on the basis of their fully
elongated tail and the presence of sperm in the cloaca. As apreliminary examination, we searched for sperm in the
cloaca of 2 mature and 8 pubescent dead males using lightmicroscopy, but sperm was confirmed only in mature
males. However, the presence of sperm in the cloaca can be
an unreliable indicator of maturation for males because
sperm are not always found in the cloaca of mature males.
The size at maturity was defined by the body size at
which the number of mature turtles exceeded the number ofpubescent turtles. Likewise, the size at puberty was defined by
the size at which pubescent turtles exceeded the number ofprepubescent turtles. The turtles were grouped by SCLn-t
with an interval of 2.0 cm to calculate the following ratios:

ratio of mature turtles 5 mature turtles/(mature turtles +pubescent turtles) and ratio of pubescent turtles 5 pubescentturtles/(pubescent turtles + prepubescent turtles). The ratios
were then fitted to a logistic equation: ratio of mature or
pubescent turtles 5 K/(1 + b3e2c3SCLn-t). The parameters
K, b, and c were evaluated by solver of Microsoft Excel.
The relative tail length (rTL) was then calculated as
TL/SCLn-t. Regression lines for the relationship betweenrTL and SCLn-t were established for both males andfemales, and the SCLn-t value at the intersection of themale and female regression lines was considered as thecarapace size at which both tail elongation and development of secondary sexual characteristics begin.
RESULTS. —Examination of the morphological characteristics of the
gonads and genital ducts of 46 dead male and 106 dead284

CHELONIAN CONSERVATION AND BIOLOGY, Volume 10, Number 2 – 2011

Table 1. The size of mature, pubescent, and prepubescent loggerhead turtles. Maturity was determined by morphological examinationof the gonad and genital duct except right live mature males, which classified on the basis of the presence of sperm in the cloaca.
SCLn-t: straight carapace length; rTL: relative tail length calculated as tail length/SCLn-t.

MaturePubescent
Prepubescent

SCLn-trTL
SCLn-trTL
SCLn-trTL

Male

Female83.8 ± 4.3 cm (77.8–90.2 cm, n 5 12)0.423 ± 0.061 (0.302–0.501, n 5 11)74.5 ± 3.8 cm (68.6–82.6 cm, n 5 36)
0.249 ± 0.070 (0.157–0.446, n 5 29)
68.0 ± 6.5 cm (57.7–75.0 cm, n 5 6)
0.149 ± 0.030 (0.121–0.187, n 5 5)82.1 ± 4.3 cm (73.8–91.9 cm, n 5 27)
0.183 ± 0.036 (0.136–0.277, n 5 20)74.4 ± 4.4 cm (63.6–85.3 cm, n 5 67)
0.165 ± 0.027 (0.113–0.228, n 5 55)
68.8 ± 3.8 cm (60.1–72.6 cm, n 5 12)0.146 ± 0.026 (0.108–0.174, n 5 8)

female loggerhead turtles revealed that 4 males and 27
females were mature, 36 males and 67 females werepubescent, and 6 males and 12 females were prepubescent.
Since only 4 turtles were identified as mature males by theexamination of reproductive organs, we increased the sample
size by including 8 live mature males for which the maturity
and sex were determined by the presence of elongated tail andsperms in cloaca. The SCLn-t (mean ± SD) of mature turtles
was 83.8 ± 4.3 cm for males and 82.1 ± 4.3 cm for females,that of pubescent turtles was 74.5 ± 3.8 cm for males and74.4 ± 4.4 cm for females, and that of prepubescent turtles
was 68.0 ± 6.5 cm for males and 68.8 ± 3.8 cm for females.
We found no significant difference in the SCLn-t between
males and females in any maturity stage (p . 0.05, MannWhitney U-test; Table 1).

The relationship between SCLn-t and the proportionof mature turtles or pubescent turtles were estimated bylogistic regression analysis (Fig. 3). The SCLn-t at which
50% of turtles are expected to be mature or pubescent wasconsidered as the size at maturity or puberty, respectively.
Data on males and females were combined for analysis
because there was no gender-based difference in the size ofloggerhead turtles at all maturity stages. The ratio of thenumber of upper maturity stage was expressed as following equations: ratio of mature turtles 5 1.028/(1 + 1.2023
102133e20.36623SCLn-t) and ratio of pubescent turtles 5
0.9585/(1 + 1.2983102303e21.0523SCLn-t). The size atmaturity and the size at puberty were then estimated as82.1 and 66.0 cm SCLn-t, respectively.
We also evaluated rTL of 45 males and 83 females toinvestigate the relationship between maturity stage and taillength. The maximum rTL was 0.501 for a mature malewith SCLn-t of 87.7 cm, and the minimum rTL was 0.108
for a prepubescent female with SCLn-t of 69.6 cm. Sexual
difference for rTL was not detected in prepubescent turtles,
whereas the rTL values of mature and pubescent males
were significantly greater than that of females. For males,rTL increased significantly with maturation (df 5 42, error
variance 5 0.004228, p , 0.05; Tukey method). For
females, the difference between rTL values of matureand prepubescent turtles was significant (df 5 80, error
variance 5 0.0008736, p , 0.05, Tukey method).
There was a linear relationship between the SCLn-t
and rTL values for both males and females (Fig. 4). The
point where the male and female regression lines
intersected was at 65.8 cm SCLn-t and 0.155 rTL.
DISCUSSION. —Figure 3. The size at maturity and the size at puberty ofloggerhead turtles. Data on males and females were combined
for analysis because there was no gender-based size difference(see ‘‘Results’’). The sizes at maturity and puberty were definedas the SCLn-t where 50% of turtles transition out of pubescent tomature and out of prepubescent to pubescent, respectively(dotted lines). Ratio of mature and pubescent turtles were unifiedin each 2.0-cm SCLn-t and then regressed by logistic curves.
The SCLn-t marked 82.1 cm at maturity and 66.0 cm at puberty.

There was no significant difference in size between
males and females among all maturity stages in thisstudy, which is consistent with Limpus (1985), Kamezaki(2003), and Casale (2005), who reported no gender-relateddifference in carapace length in loggerhead turtles inmature individuals. In contrast, Pritchard and Trebbau(1984), quoting from Deraniyagala (1939), mention thatthe carapace length of male loggerheads is greater than thatof females. For another sea turtle species, males of green
turtles were observed to be smaller than the females in 12
breeding populations (Godley et al. 2002). Females are

NOTES AND FIELD REPORTS

Figure 4. Differences in the rTL (relative tail length) against
SCLn-t (straight carapace length, measured from notch to tip;
see text for details) between the male and female loggerhead
turtles. Black squares: mature males (n 5 11), gray squares:
pubescent male (n 5 29), white squares: prepubescent males(n 5 6), black circles: mature females (n 5 20), gray circles:
pubescent females (n 5 55), and white circles: prepubescentfemales (n 5 8). The SCLn-t at the intersection of the male andfemale regression lines was 65.8 cm beyond this point; tailelongation is assumed to be induced in males. The rTL of
females was 0.228 or less. Therefore, specimens with rTL values
exceeding 0.250 were considered male.

usually larger than males for aquatic chelonians, whereas
males of terrestrial turtle species tend to be the same size asor larger than females (Berry and Shine 1980; Kuchling
1998; Miller and Dinkelacker 2008). Larger body size infemales is related to increases egg size or annual clutch
frequency, while larger size in males might confer anadvantage in male–male encounters or female choice forlarger males (Gibbons and Lovich 1990; Kuchling 1998).
In contrast, smaller size at maturity could also result in an
increase in the number of reproductive opportunities in a
lifetime because turtles could reach maturity at a younger
age. Further research into the life history trade-offs
between age and size at maturity for male and female sea
turtles is needed to elucidate observed relationships in wild
populations of different species.
The size of mature males and females in our study was
smaller than that of mature turtles of the same population
measured at Yakushima Island: 86.2 ± 6.9 cm (n 5 10) for
males and 85.6 ± 4.68 cm (73.5–101.5 cm; n 5 690) for
females (Kamezaki 2003). Nesting females of some populations, including the North Pacific population, show size
differences linked to foraging strategy, with small females
feeding on planktonic prey in oceanic habitats and large
females feeding on benthic prey in neritic habitats (Hatase
et al. 2002, 2007; Hawkes et al. 2006; Hatase and Tsukamoto
2008; Reich et al. 2010). Despite no significant gender-based
size difference within each study site, there appeared to be ahabitat-related size difference between Muroto (this study)
and Yakushima Island (Kamezaki 2003). This difference
could be explained by distance from main foraging habitats tostudy sites, as Yakushima Island is close to the East China
Sea, which is known as neritic habitat with nutrition-richbenthic prey, whereas Muroto is close to oceanic habitats, andthe turtles caught at Muroto fed on planktonic prey such as

285

Pyrosomida (Iwamoto, unpubl. data). This intriguing possibility warrants further exploration.
The size of pubescent sea turtles has been reported
only for loggerhead turtles in the South Pacific (Limpus
and Limpus 2003). The sizes of pubescent turtles in theSouth Pacific that range from 80 to 85 cm and from 100 to
105 cm CCL were larger than the size of pubescent turtles
estimated in this study (mean SCLn-t 5 66.0 cm, or
CCL 5 71.1 cm based on the conversion equation: SCL 5
21.442 + 0.9483CCL; Teas 1993). The size difference
between these populations is likely to be related to the size
at maturity; the size at maturity in Muroto was also smaller
than the size at first breeding in the Great Barrier Reef
(Limpus 1990; Limpus and Limpus 2003).
As for the relationship between maturation and taillength, tail elongation in males appeared to begin after
puberty and continue during sexual maturation because therTL values of prepubescent males and females were similar,
but there were significant differences between the rTL
values of each maturity stage for males, and tails of females
also exhibit positive allometric growth against body size.
Casale et al. (2005) also reported a positive relationshipbetween TL with the growth of CCL. The positive allometric
growth of tail has several functional implications. For males,
tail elongation has a clear purpose: the elongated tail of themale enables the penis to reach the female’s cloaca during
copulation. For females, there are 3 possibilities that explain
tail elongation: 1) it could be beneficial for nesting, 2) toenable rejection of copulation attempts, and 3) physiologicaleffects of hormones without significant functionality. We
explore each possibility in turn.
First, laying eggs is one of the most important
reproductive behaviors for female sea turtles, and a longer
tail might reduce the risk of eggs being damaged while
dropped into a nest, which can be 35–85 cm below thesurface of the sand (Dodd 1988). However, shells of
newly laid eggs are pliable and regularly withstand thefall from cloaca to nest chamber, rendering this possible
explanation of tail elongation in females unlikely.
Second, female sea turtles in captivity are often
observed to interfold the tail and protect it by crossing
the hind limbs to avoid mating. An elongated tail could
make it easier to defend against courtship from males ascompared with shorter tail of prepubescent turtles. Since
the sexually receptive period for female sea turtles is only 1
to several days (Owens 1980; Wood and Wood 1980),
females should reject courtship from males the rest of thetime. Behavioral observations of how females use theirtails during courtship, mating, and nesting are needed todetermine any benefit of longer tails related with female
reproduction.
Third, there might be no functional meaning. Tailelongation was induced in immature, 42-month-old malegreen turtles that were given testosterone-propionate(Owens 1974). Testosterone also functions for female
reproductive physiology in relation to reproductive migration and ovulation (Wibbels et al. 1990; Blanvillain et al.

286

CHELONIAN CONSERVATION AND BIOLOGY, Volume 10, Number 2 – 20112011). Therefore, tail elongation of females could be
influenced by testosterone, even at relatively low levels
compared to that of males.
An rTL of 0.228, the highest value obtained forpubescent and prepubescent females, was considered thethreshold value for discerning males from females.
Pubescent and prepubescent individuals with rTL values. 0.250 were considered male; however, the rTL values
could not be used to determine the sex of individuals withshorter tails. In contrast, Hirate (2000) examined loggerhead turtles in Okinawa and found that tails in males begin
to elongate at about 68.0 cm SCLn-t. This difference mightvary from site selection (i.e., Okinawa is close to the neritic
habitat of the East China Sea) or sexing method because
Hirate (2000) defined long-tailed individuals as male andthus probably skewed sex determination. Furthermore, thesize of the beginning of tail elongation in our study wasconsistent with the size at puberty. Thus, this result
suggests that male North Pacific loggerhead turtles begin todevelop their secondary sexual characteristics concurrently
with the beginning of sexual maturation.
The present study reveals the size at maturity andpuberty, the absence of sexual size dimorphism, and therelationship between maturation and tail elongation ofloggerhead turtles in the North Pacific, all of which
expand our knowledge of loggerhead reproductive
biology. Similar studies should be undertaken for otherpopulations and species to continue filling important
information gaps about sea turtle reproductive biology,
life history strategies, and population demography.
ACKNOWLEDGMENTS
This study is a part of the research conducted by the Sea
Turtle Association of Japan. We are grateful to K. Ebisui, Y.
Yasuoka, K. Hashimoto, S. Yamashita, K. Kusakabe, H.
Tanaka, K. Saito, A. Abe, T. Nakazatomi, H. Mizuno, andH. Fukuie for their help in examining these turtles. We
appreciate the cooperation of the following organizations
and institutions: Takaoka Pound Net Association, Shiina
Pound Net Association, Mitsu Pound Net Association,
Shimane Aquarium ‘‘Aquas,’’ Tottori Prefectural Museum,
Tottori Prefectural Fisheries Center, Tottori Prefectural
Fisheries Experimental Station, Minamichita Beach Land
Aquarium, Hiwasa Chelonian Museum ‘‘Caretta,’’ Himeji
City Aquarium, Himeji City Zoo, the Kameppuri researchclub of Mie University, and the Laboratory of Aquatic
Conservation at the University of Tokyo. We are grateful to
Kiki Tziouveli and Takahiro Shimada for proofreading thismanuscript. We would also like to acknowledge the funding
from Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists.
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Received: 18 October 2010
Revised and Accepted: 4 August 2011