6 Dec 2022


Drone surveys of green turtles off Queensland’s Heron Island have found there is not yet a “man drought” as there were just as many males as females turning up to mate.

That’s despite an estimate that more than 80% of turtle hatchlings from Heron Island are female.

The new research, published in the international journal Marine Biology, is good news for the southern Great Barrier Reef population of green turtles.

“Females require huge amounts of energy to reproduce, meaning they can only breed every 5 to 7 years whereas males can breed every 2 to 3 years,” said lead author, University of Queensland PhD candidate, Melissa Staines.

“This likely explains why the number of males and females arriving to mate – the operational sex ratio – was about one-to-one as males can breed 2-3 times more frequently."

“In addition, males mate with multiple females and females mate with multiple males."

“The equal number of breeding males and females was despite an estimate that female hatchlings outnumber the amount of males produced from the region."

“This one-to-one operational sex ratio is good news, as it means that there is no immediate need to intervene on Heron Island to produce more males,” she said.

Using drones, the research team filmed in October 2021, during the peak of the breeding season, when the overall density of green turtles observed increased five-fold compared to when the surveys were done in July, before the breeding season.

It’s likely some had travelled to the southern Great Barrier Reef from as far away as New Caledonia and other areas of the Indo-Pacific.

To calculate the operational sex ratio, the researchers focused on an inshore section where adult green turtles’ preferred foods (seagrass or algal turf) are scarce. That excluded individuals who might be foraging rather than mating.

They filmed 38 males and 36 females engaging in courtship and breeding behaviours. Ms Staines said there is nothing gentle when turtles get together to reproduce.

“They can be quite aggressive towards each other as they compete for a mate. If a male has mounted a female, a rival will bite at his flippers and neck and try to pull him away. Females will even take a nip at a male if they get too annoyed."

“Using drones was an innovative way to observe mating activity from a distance without influencing turtles’ natural behaviours,” Ms Staines said.

Turtle populations globally face the threat of “feminisation” due to human-induced climate warming that is increasing sand temperatures where the turtles nest.

The sex of turtle embryos is determined by nest temperature – the hotter the nest the more females.

In 2018, scientists revealed that for the past few decades green turtles hatching on Raine Island – about 1500 km north of Heron Island – have been more than 99% female.

The majority of northern Great Barrier Reef green turtles nest on Raine Island and researchers now fear for the long term survival of this distinct population.

A modelling study published earlier this year estimated that under an extreme climate warming scenario there will be so few adult northern GBR males that each one will have to mate with 15–30 different females per season just to prevent population decline.

Concern over the lack of male hatchlings led to the Turtle Cooling Project, a partnership between the World Wide Fund for Nature-Australia, The University of Queensland, and the Conflict Islands Conservation Initiative with funding support from furniture company Koala.

The project has already shown that pouring sea water on a nest can cool eggs enough to generate male hatchlings and the next phase of the research was the drone surveys at Heron Island.

Co-author Christine Madden Hof, WWF’s Global Marine Turtle Conservation Lead, said discovering the operational sex ratio at Raine Island will be crucial to conserve what is the largest green turtle rookery in the world.

“We’ve shown the drone methodology can work. It will now be important to apply this approach in other regions where feminising is occurring, in particular Raine Island,” said Ms Madden Hof.

“The operational sex ratio results for Raine Island, compared to Heron Island, may be drastically different and alarming,” she said.