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.