MY RITUAL evening circumnavigation of Heron Island ends with a walk out onto the breakwater of the marina, heart beating a little faster at the prospect of a moonlight assignation with beautiful Ben. I've missed the turtle season but Ben - a giant green turtle, about the size of a dodgem car - is not one of those fly-by-nighters who come and go with the breeding cycle.
He's a long-term resident who in his dotage enjoys the easy pickings of the shallows, unbothered by the company of the tourist snorkellers who join him whenever the tide is right. Most nights I find him and follow him with a thin beam of torchlight, watching as he paddles about, apparently immune to the comings and goings of the busy harbour.
Every year, anywhere from 40 to 2000 of Ben's kin return to Queensland's Heron Island, guided onto the beach by primitive instinct. They are returning to lay their eggs - 100 or so at a time - as close as they can manage to the site where they first emerged as hatchlings themselves maybe 25, 35 years earlier. They are miraculous survivors in a hostile world, their homecoming the culmination of an epic life journey.
Evading predators and overcoming astounding odds even to get beyond the hatchling phase, they then spend decades floating and maturing out in the distant sea, grazing on algae and plankton and growing large in the surface waters. As they get older they swim back to closer waters to feed on the richer pickings on the sea bottom, to find mates and breed.
They are the winners in the turtle life lottery - it's estimated that only one in 1000 hatchlings makes it to maturity.
Not so long ago, having survived all that nature could throw at them, these creatures met a cruel fate when they came back to Heron and nearby islands to breed. They became livestock, rounded up for turtle soup.
In the summer of 1926-27, as a generation of giant female turtles paddled across the reef in the dark and lumbered onto the sand, one by one they were set upon by men who flipped them on their backs. The great creatures were left stranded out of reach of the tide, sometimes for days.
''It is a pitiful thing to see these poor beasts … their blood-injected eyes filmed with mucus, their strained and stiffened flippers flapping hopelessly in their attempts to right themselves, their heavy sighs audibly expressing the torture they are enduring,'' observed a journalist when he visited the canneries in the 1920s. The end would come with decapitation with an axe. After the 1926-27 season, the factory on Heron faltered because there were not enough turtles coming up on the beach to viably put into cans.
The new generation of females making up the protected, revived turtle population are still greeted when they arrive on the beach at Heron by men - and many women too - intent on using them, but rather more benignly. The people loitering on the sand now are scientists and surveyors who want to determine what the turtles can tell them about the changing oceans, and what humans might do to help protect them from a fraught future.
Warming is conjuring some formidable new obstacles to survival. For the early decades of their life, the turtles are pelagic fossickers, working the sea surface, so as with so many species of seabirds there are questions about what changing water temperatures might mean for them. Shifts in circulation and oceanographic behaviour might also influence them, especially hatchling distribution. On land, more frequent and stronger cyclones and rising sea levels would damage and erode rookery sites, although against these odds the patient and resilient turtles might have a fighting chance-if they wash up on new beaches, and deliver a new generation back there 30 or 40 years hence. But what conditions will await them?
The biggest factor blighting the future of the marine turtle is how warming sands will affect the embryos of the next generation. Variations in the temperature of the sand where the eggs are laid determine whether the hatchling that emerges is male or female, as well as influencing its physical strength.
Destiny - in terms of gender and physical rigour - is determined by a tight thermal range. If the eggs incubate in conditions that are too warm, the ratio swings strongly to production of females. Too hot, and the hatchlings don't emerge at all. Present sand temperatures have already reached the point where they strongly skew sex ratios towards females. It might be 30 years or more before the consequences become apparent, but the fear is that heating sands might doom the turtles within a generation or two.
Led by Mariana Fuentes, a James Cook University team working up in the northern end of the Great Barrier Reef has been evaluating the various climatic threats facing the green turtles.. Under the worst-case scenarios for climate change - which is pretty much the trajectory we are on - sea-level rise, and the consequent impact on nesting sites, shapes up as the biggest threat for the turtles from now until 2030. But by 2070, the models anticipate sands will have reached a temperature which would bring about a near-complete feminisation of hatchlings. A few male enclaves are likely to survive where conditions provide some respite from the heat. But the overall picture is grim.
There are also indications of trouble now. Two species of turtle come to Heron Island to breed. The islands of the Capricorn-Bunker group collectively host Ben's kin, Chelonia mydas, as well as one of only two breeding populations in eastern Australia for the loggerhead turtle, Caretta caretta, which can be found all over the world. In Queensland, its population has been monitored yearly since the late 1960s, and in that time has shrunk by some 85 per cent.
DAVID Booth is a turtle specialist from the University of Queensland. Over three years - from 2006 to 2009 - he and his team dropped data loggers into nests on Heron Island to measure temperatures over the incubation season. The conditions the eggs were exposed to varied a bit depending on whether they were laid late or early in the season. At each end, as the heat built up or eased, there were still conditions capable of producing males. But through the peak season, it was hard to find a nest likely to nurture a single male heir to the line.
In recent years Booth has broadened his exploration to look not just at gender but at the effect of higher temperatures on the physiology of the hatchlings, and what impact that might have on their ability to survive. The little hatchlings who push their way out of their shells after six to eight weeks of incubation, and who scramble across the sand towards the powerful beacon of the moon over the reef, have the race of their lives in front of them, Booth explains.
Only those in peak form have even a sporting chance. Most of them are destined for a very short existence - minutes or hours.
First they have the frantic dash down to the water. In the shallow fringing reef, all manner of hungry predators wait for their yearly feast of raw turtle soup - reef sharks and stingrays and parrotfish. Those first few hundred metres are murder. The green turtles ''have only one strategy when swimming the gauntlet of these fish predators'', Booth says: ''Put your head down and swim as fast as you can. They take no evasive action.'' (Some studies have reported that loggerhead hatchlings do try to dodge some perils.)
On Heron, studies have found that one in three hatchlings is eaten before it can get to the relative safety of deeper water.
Booth became curious about the mechanics of that first hatchling swimming frenzy, because it was so crucial to survival. He retrieved a few hatchlings as they emerged from their shells in the sand, and took them back to the nearby laboratory. There he slipped them into little Lycra swimsuits which acted as a harness, and dropped them into swimming tanks.
Once inside, they were tethered but launched into the frenzy of their first swim, all the while the scientists recording and monitoring their stroke, their flipper movements, their oxygen consumption, their energy levels.
''What we've found is that the optimum nest temperature - the 28 to 30 degrees Celsius range - is not only where you produce roughly 50-50 sex ratio but is also optimal for hatchling swimming performance,'' he explains. The warmer the nest, the weaker the hatchlings' performance.
''If they are not good at swimming, especially on coral cays like Heron, there is a really horrific predation rate on the reef flat. It averages around 30 to 40 per cent but can be up to 80 per cent. The better swimmer you are, the faster you get across that reef flat, the higher the chances that you will make it.''
All in all, things don't look good for turtles. Scientists are writing papers discussing what they might do to help them - by putting shade over their rookeries, or building structures to protect them from rising tides. But maybe, in the end, the turtles will find a way to survive despite humanity's efforts, good and bad.
In late 2010 the Royal Society tried to sound the alarm on the scale of extinctions being observed across the planet, publishing a special volume of findings - Biological Diversity in a Changing World - and painting a shatteringly bleak picture. What is occurring, the authors wrote, is a mass-extinction event of a speed far in excess of anything found in the fossil record. Extinctions are an inevitable part of life, they state pragmatically.
It's the sheer scale of what is now under way ''caused by over-exploitation of natural resources, that needs to worry us … never before has a single species driven such profound changes to the habitats, composition and climate of the planet''.