Number of breeders and population density
The operational sex ratio (OSR) was slightly biased toward females
(0.87:1 male: female), indicating that more females than males breed in
this population. OSR in sea turtles has typically been reported as
skewed towards males or even between both sexes (Hays et al., 2022). For
example, Prakash et al. (2022) found an OSR of 1:1 in a population of
hawksbill turtles in Fiji, and Wright et al. (2012b) found an OSR of
1.4:1 in a green turtle population in Cyprus. The primary sex-ratio of
this population in Tetiaroa does not seem to suffer heavy female bias as
observed elsewhere (Laloë et al., 2020), and so the discrepancy between
the number of male and female breeders is likely small. However, a low
level of multipaternity was observed (7% of the clutches, 16% of the
females) and green turtles demonstrated multipaternity in as much as
92% of their clutches (Alfaro-Núñez et al., 2015). Our finding
constitutes the lowest estimation of multipaternity level in this
population, considering that 70 clutches were analysed over a total of
549 observed. Plus, the number of hatchlings genetically sampled was
uneven in the clutches (most often < 10), thus the
contribution of some fathers may have been missed. With no apparent
fitness benefits, a low level of multipaternity may indicate low chances
of encountering multiple males, directly linked to the density and
number of male breeders (Lee et al., 2018). However, a high density of
breeders is observed near the island (Gaspar, pers. comm.), and 11% of
the males mated with multiple females within a season, indicating that
males may encounter several available females. Coupled with the low
level of multipaternity, it suggests that not all males are equally
capable of mating with multiple females. In contrast, Phillips et al.
(2013) found no male hawksbills that mated multiple times within a
season and concluded that a low density and high turnover of males
prevented this from occurring.
Another deduced behaviour of interest arose from the two reconstructed
males *53 and *62. They both sired female CMY1384 in 2018/19 and a
different female CMY2419 in the next season. This finding is surprising
given the low number of breeders retained and the low level of
multipaternity observed, though the probability of such an occurrence
increases when the number of male breeders is low and the density in the
mating area is high. This may indicate that some males are dominant on
the breeding ground. Dominance hierarchies in males have been described
in other species of freshwater turtles like the common snapping turtle
(Chelydra serpentina ) and the wood turtle (Clemmys
insculpta ), and in the gopher tortoises (Gopherus agassizii )
(Pearse and Avise, 2001). Such dominance could occur regularly on the
breeding ground, or be year-specific in the case where CMY2419 was sired
in 2018/19 and able to store sperm until the next season. Alternatively,
non-random mating choices driven by a restrictive criterium may limit
the number of suitable partners for a given female, increasing the
probability of mating with the same pair of males. Nonetheless, these
contrasting results are the first to look into the number of male
breeders in French Polynesia and show that differences between the two
sexes are likely to be small and that this number seems sufficient to
maintain fitness in the population despite high levels of relatedness.