Reproduction frequency and long-term sperm storage
Parentage assignment revealed that at least 5 females (16%) returned to
lay once or twice during the 11-year study period. These returning
events were not observed in field surveys, as recapture identification
found that only 2 females were resampled during a returning event. One
of these females was among the 5 females identified from their genotype,
matching the genetic approach (CMY1113, Table 1). Interestingly, these
females were found nesting in consecutive seasons. According to data
available so far, annual reproduction frequency has never been observed
in female marine turtles. For example, Hays et al. (2014) revealed with
satellite tracking that while no female loggerhead turtles migrated
annually to breeding grounds in the Mediterranean Sea, 76% of the males
did. Studies focusing on female reproduction frequency are based on
field observations, and thus it is possible that in some cases, annual
reproduction has been overlooked due to the absence of direct
observation in the field.
Satellite tracking on adult females in French Polynesia revealed that
several individuals stay within the territory, while others migrate to
Fijian neritic feeding grounds between the nesting seasons (Craig et
al., 2004; Piovano et al., 2019; DIREN personal comm.). Whether those
breeding annually are those that remain in French Polynesia throughout
the year needs to be determined, but it is difficult to imagine long
annual migrations to Fiji (> 2000km), given the time and
energy that it requires, adding to the energy needed for the
reproduction itself. Interestingly, the same fathers consistently sired
the clutches of annually breeding females over the given period (from
359 to 673 days). This may indicate that females are capable of storing
sperm across seasons or exhibiting narrow male choice for reproduction,
implying fidelity to what are currently unknown mating criteria.
Alternatively, cross-seasonal sperm storage has neither been
demonstrated nor quantified in marine turtles, although it is suspected
to occur. For example, Wright (2013) found that multipaternity was more
common in returning females than in primary nesters, and proposed
cross-seasonal sperm storage as one of the possible causes.
Cross-seasonal sperm storage over several years has been observed in
other taxa, including birds (Feldschuh et al., 2005), insects (Baer et
al., 2003), and reptiles (Ewing, 1943; Levine et al., 2021). Although
the possibility of multiple mating among the same individuals cannot be
ruled out (Sakaoka et al., 2013), cross-seasonal sperm storage coupled
with relatively sedentary life traits may help females to minimize
energy costs linked with migration and mating. To date, however, adult
green turtles are rarely observed outside the nesting season in French
Polynesia, and no feeding ground has been identified throughout French
Polynesia or nearby. Hatase et al. (2006) showed, using satellite
tracking and stable isotope analysis, that 31% (n=89) of the females
nesting on Ogasawara Islands, Japan, were oceanic planktivores rather
than neritic herbivores. Further investigation using these complementary
techniques should reveal whether females in French Polynesia also feed
in oceanic habitats close to nesting grounds.
For males, 6 (22%) sired different females in consecutive seasons,
indicating a possible annual reproduction frequency. However, caution
should be taken as this might also reflect long-term sperm storage in
the case when the females could not be identified in one of the
consecutive seasons. Male reproduction interval is thought to be shorter
than for females due to a smaller energy cost of reproduction that
allows them to reproduce more often (Hays et al., 2022). For example,
the male green turtle remigration interval was 2.1 years in the southern
Great Barrier Reef (Limpus, 1993), and 1 year in Hawaii (Balazs, 1983),
based on tagged recapture. Wright et al. (2012a) determined the
reproduction frequency of male green turtles in northern Cyprus with
parentage analysis and found that 3% bred more than once within the 3
years. Overall, annual male reproduction frequency linked to a
residential strategy must be considered as a strong possibility in
French Polynesia, and in this population, a discrepancy between male and
female reproduction frequency is not observed.