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SCIENCE Vulnerable loggerhead sea turtle gender ratios finally documented

Published 12 Feb 2018 11:23AM

Words by Joellie Hale | Staff Writer

The world of conservation is rarely rife with good news.

And so it shouldn’t be. The painstaking, meticulous labours of the saviours battling to preserve biodiversity are usually heart-breaking.

Indeed, last month, the devastating effects of climate change upon ecosystems were demonstrated once again. A study on the green sea turtles of Australia made waves by showing on the warmer, Northern beaches, over 99% of juveniles and sub-adults were female. A whopping 86% of the adult population in this area was also female.

With convenient timing, a 3-year study on the gender ratios of breeding loggerhead sea turtles has just been published. And, the news is tentatively good.

In Florida, the gender ratio of loggerhead hatchlings echoes that of the green sea turtles: over 90% of hatchlings in some coasts off the Americas are female. Until now, it was unknown whether this skewed ratio resonated in the adult population.

Due to the behaviour of the species, the gender ratio of adult sea turtles is challenging to document. Even estimating the general adult population is not easy, due to migratory movement and confusion with other sea turtle species in observational surveys. Females come to land to lay their eggs, while males generally spend their lives in the open ocean and rarely come to land. This results in a limited understanding of male sea turtle reproduction. Adult gender ratios cannot be directly estimated.

The study, published by Lasala and colleagues, investigated the paternity of 989 hatchlings from 51 nests. Due to the female-biased nature of sea turtle populations, the researchers expected to find the same individuals as fathers across several nests. If so, this may add to the extinction risk of the species, as gender-skewed populations are inclined to fall prey to genetic disorders associated with inbreeding. The results were, thankfully, not as expected.

No males had offspring in more than one clutch, over the entire 3 year period. This means that there are more adult males than expected within the breeding population.

But, why? If the hatchlings demonstrate extreme gender bias, why is this not reflected in the adult, breeding population?

Firstly, it takes loggerhead sea turtles around 30 years to reach sexual maturity. This means that any recent gender skews observed in hatchlings will not yet be affecting the breeding population.

It is also possible that differential survival rates between the genders reduce the skew as the turtles mature. One study found that during the perilous journey from nest to ocean, males were 4 times more likely to survive. This is supported by another study, which upon examining the genders of in-water juveniles, discovered the female: male ratio was 2:1. This is a far cry from the 9:1 ratio demonstrated in some hatchling populations.

Of course, it is probable that beach temperatures have only just reached a tipping point. The extreme hatchling gender ratio is only seen in some areas. While the gender of human embryos is determined genetically, the gender of sea turtle embryos is determined by temperature.

Nest temperatures above 29°C produce mostly, or all, females. This makes sea turtles particularly vulnerable to extinction from climate change. So, while today we celebrate the pleasingly unexpected balance of breeding loggerhead sea turtles, it is essential to their persistence that conservation efforts continue.

For tomorrow, we may well find that as temperatures continue to rise, the turtles are unable to sustain this balance. The key to surviving extinction events is adaptability. The hatchling skew of sea turtles demonstrates their vulnerability to temperature fluctuations. Sea level and temperature rise are projected to accelerate as we progress through the century.

We do not know how long the species will manage to absorb the hatchling gender bias. One contributor to the adaptability - and so survivability - of a species is time taken to reach sexual maturity. If favourable survival traits quickly become commonplace among populations due to rapid maturity, the species is much more likely to survive.

Sea turtles do not mature quickly. Due to our overhunting, destruction and fragmentation of habitats, toxic pollution, the introduction of alien species and of course, acceleration of climate change, we are in the sixth mass extinction event. In this period of multifaceted environmental stress, 30 years is a long time. The traits which favour the survival of certain hatchlings may not be preferable in 30 years when the babies of today are finally ready to reproduce.

Observations of this vulnerable species are set to continue. And now, thanks to the work of Lasala, future populations can be compared. Hopefully, if the skewed gender ratio of the hatchlings begins to spread to the adult population, we will afford them with the protection they deserve in time to save them.


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