Estimating sex ratios in Caribbean hawksbill turtles: testosterone levels and climate effects

Abstract

Evolutionary theory predicts that male and female offspring should be produced at a 1:1 ratio, but this may rarely be the case for species in which sex is determined during incubation by temperature, such as marine turtles. Estimates of primary sex ratio suggest that marine turtle sex ratios are highly skewed, with up to 9 females per male. We captured juvenile hawksbill turtles Eretmochelys imbricata in waters around Anegada, British Virgin Islands, a regionally important foraging aggregation, and analysed concentrations of plasma testosterone and oestradiol-17β from 62 turtles to estimate sex ratio. There were 2.4 to 7.7 times more females than males. Testosterone concentrations correlated with sampling date and sea surface temperature (SST), with higher con centrations in the late summer when SST was highest, suggesting that assigning sex through threshold values of sex hormones must be carried out cautiously. The sex ratio in the juvenile foraging aggregation around Anegada is more male biased than at other locations, suggesting that turtles at Anegada have resilience against feminising effects of climate change. Future work should (1) integrate the relative contributions of different genetic stocks to foraging aggregations and (2) investigate the annual and seasonal cycles of sex hormones, and differences among individuals and life history stages. KEY WORDS: Marine turtles · Sex ratio · Climate change · HormonesResale or republication not permitted without written consent of the publisher Aquat Biol 18: 9-19, 2013 Skewed sex ratios may be advantageous under specific biological circumstances (e.g. if pa rental investment in the sexes is unequal, if one sex helps their parents or if male siblings compete for mates), but also in some environmental circumstances (Warner & Shine 2008). The Charnov-Bull model (Charnov & Bull 1977) was proposed to ex plain this and has been demonstrated for a lizard species, such that offspring raised in conditions that promote high fitness for females had higher lifetime reproductive success if they were female than male (males were produced by hormonal manipulation at temperatures that usually lead to the development of females; Warner & Shine 2008). Temperature-dependent sex determinationIn marine turtles, sex is determined by the temperature experienced during the middle third of the embryo incubation period (temperature-dependent sex determination [TSD], a type of ESD), where males are produced at lower temperatures and females at higher temperatures (Yntema & Mrosovsky 1980, 1982, Mrosovsky 1988. The majority of research into sex ratios of marine turtles has been carried out on the nesting beach at the hatchling stage, and approaches using the modelled relationship between incubation temperature and sex ratio, as well as incubation duration and sex ratio, have enabled estimation of 'primary' sex ratio for a large number of rookeries (reviewed in Hawkes et al. 2009). Primary sex ratios vary among beaches and within clutches, as well as during the course of a nest...

High rates of growth recorded for hawksbill sea turtles in Anegada, British Virgin Islands

“…Hawksbill turtles were captured as part of an in-water sampling program using the rodeo technique (Limpus 1981) in waters around Anegada, British Virgin Islands (Fig. 1;(McGowan et al 2008;Hawkes et al 2013)). Surveys took place over 109 irregularly spaced days between the 16 November 2003 and the 8 August 2006, comprising a total effort of 543 h. Surveying for turtles took place in waters shallow enough for capture to take place (generally <20 m depth) and within the vicinity of reefs (see also (McGowan et al 2008;Witt et al 2010)).…”

“…On occasions where curved carapace length (CCL) measurements were not collected (n = 26 turtles), they were estimated using straight carapace length (CCL = 1.1 9 SCL + 0.1; R 2 = 0.99, t = 171.0, P < 0.01 in 267 turtles for which we had both measurements), accepting that inaccuracies in the straight carapace length measurements would be carried forward to CCL estimates. Blood samples were collected from turtles for analysis of testosterone and oestradiol-17b (results reported in (Hawkes et al 2013)), and hormone values compared with thresholds reported in Geis et al (2003), Diez and van Dam (2003) and Blanvillain et al (2008) to estimate sex of captured individuals.…”

Incubation and water temperatures influence the performances of loggerhead sea turtle hatchlings during the dispersal phase

“…Because sea turtle eggs laid in the early and late nesting seasons are generally incubated under cool IT conditions (producing males) and warm IT conditions (producing females), respectively 41 – 53 , taken together, we suggest that hatchling survival rates during early life stages will differ by seasons as well as sexes. Such differences might account for why the sex ratio of juveniles is less female-biased than that of hatchlings in some populations 54 – 56 . Notably, sea turtles have survived many changes in climatic conditions over the past hundred million years 57 , suggesting that such sex differences of survival during early life stages might be one of the adaptive mechanisms to adjust the sex ratio at later life stages in response to changes in environmental condition.…”

Assessing Nesting Status of Green Turtles, Chelonia Mydas in Perak, Malaysia

“…In addition, equal hatchling sexes produced from the hatchery’s plot is very important for adult mating process in future when the turtle reaches maturity [approximately 20 years, Davenport (1997) ]. Evolutionary theory predicts that male and female offspring should be produced at a 1:1 ratio ( Hawkes et al . 2013 ).…”

“…Higher incubation temperature (> 30°C) tends to produce more females while low incubation temperature (< 28°C) tends to produce more males ( Mrosovsky & Yntema 1980 ). This may be based on a mechanism known as temperature-dependent sex determination, TDS ( Hawkes et al . 2013 ) where equal sexes between males and females will be produced.…”