How many males do you need for a species to survive? That is the question at the heart of the latest study on sea turtle populations.
A study published in Current Biology last week found that in certain parts of the Great Barrier Reef, 99 per cent of the sea turtles were female. While it seems unlikely that any of the male turtles are complaining, it is currently unknown how many of them are needed to sustain a sea turtle population.
Climate change appears to be the culprit, according to the researchers led by Dr Michael Jensen and Dr Camryn Allen of the National Oceanic and Atmospheric Administration. The conclusion of the abstract paper is that warming temperatures may result in the “the complete feminisation of this population”.
Previous studies have observed a biased sex ratio in other sea turtle populations (though nowhere near as severe). But this study uses a new method for estimating the sex of a hatchling, which is difficult – because hatchlings do not have any external genitalia.
In fact, when previous researchers determined the sex of a hatchling, they did so by first killing the turtles and then examining their gonads. This study, thankfully does not do that. Instead, Jensen and Allen use keyhole surgery and hormone analysis to determine the hatchling’s sex.
The turtles were caught in shallow water, “rodeo-style”, which, yes, does involve a researcher flinging themselves into the water and grabbing a turtle. They were then categorised into three different sizes: juvenile, sub-adult and adult. By doing this, the researchers could estimate the sex-ratios of rookeries (another name for a colony of breeding turtles) for at least the last 50 years. Using a computer model to then determine the temperatures each turtle would have been born in, they could determine whether the sex ratio of the sea turtles had changed as the climate had changed.
The scientists also compared turtles from two sides of the Great Barrier Reef, the cooler southern side and the warmer northern side. What they found was that juveniles and sub-adults in the warmer climate were 99 percent female. By contrast, only around 65 percent of the turtles of that age and size in the cooler Southern Great Barrier Reef were female.
This is due to a quirk in the turtle embryonic development. Unlike humans, this is dependent on the temperature that the egg is incubated in (which is usually buried in the warm sand).
Though the exact mechanisms for how this occurs is still not fully known, Dr Jacques-Olivier Laloë, a research fellow at Australia’s Deakin University, tells me that the temperature “determines which set of enzymes are turned on or off throughout development” and that different enzymes produces will result in different expression levels of hormones, some of which, Laloë goes on say “like oestrogen, cause the gonads to differentiate towards being female”.
Many reptilian species have their sex determined by temperature, though higher temperatures do not necessarily mean more of the female population
The Tuatara for example, a reptile from New Zealand that resembles a lizard but has its own distinct and ancient lineage, actually has a male bias in hotter climates. This might make sense in terms of the passing of the seasons – it is believed to be an adaptive trait that allows the earlier-born and larger male tuatara hatchling to find refuge and reproduce with more females, which would help the species to survive. But this only works if there are any females to mate with.
The female bias seen in sea turtles is likely due to the notion that more females means more eggs, which allow the species to continue. In theory, sea turtles could adapt to new temperatures by moving to new nesting locations. However as the climate is changing at such a rapid pace, it is feared that that these turtles which are extremely slow to reach sexual maturity (between 20 to 50 years), will fail to keep up with the times. Soon entire populations will be female.
Yet this is not the only problem facing sea turtles. As well as altering the global climate, humans are meddling in local habitats as well. Dr Rui Patrico, a marine ecologist at Exeter University points out that “degradation of nesting habitat, such as the removal of vegetation and coastal development coupled with the existence of other anthropogenic pressures (fisheries bycatch, illegal harvesting, ingestion of plastic debris)” is making it harder for the turtles to face the pressures of climate change. Furthermore, rising sea levels are flooding the nesting grounds of these turtles. Hatchlings are more likely to die before birth at higher temperatures.
In order to mitigate these risks, biologists at WWF Australia are using shade cloth to protect hatchlings during heatwaves. Scientists are even planning to trial artificial rain on the Northern Great Barrier Reef to make it easier for turtles to nest but also to lower the incubation temperatures of the eggs, and hopefully result in more males being produced.
It is still unclear whether total feminisation of a population is a major problem, and even if it is, it may be one of the relatively easier and inexpensive problems to solve.
The total feminisation of a species may be one of the most novel and eye-catching ways in which climate change is affecting animals on our planet. But the sadly humdrum effects of rising temperatures continue to pose a far more immediate threat.