The loggerhead turtle

The loggerhead (Caretta caretta) is the sea turtle that ties this coast's sea country to its land country in the most literal way there is. A big, blunt-headed, hard-shelled turtle, it spends its life at sea — much of it far offshore, feeding on the crabs, shellfish and other hard-bodied prey its heavy jaws are built for — and it is the predominant sea turtle nesting on the mainland beaches of south-east Queensland. The nesting here is small-scale, watched over by local volunteer programs, and modest against the great regional rookery at Mon Repos near Bundaberg to the north; but it is real, and it means a stretch of quiet summer beach on this coast is not just scenery but a nursery. On warm nights a female hauls out above the tideline, digs a chamber in the dry sand, lays a clutch of roughly a hundred or so eggs, covers them and returns to the sea, leaving the sand to do the incubating.

And the sand does more than warm the eggs — it decides what they become. Sea turtles have temperature-dependent sex determination: there is no sex chromosome, and the sex of each hatchling is set by the temperature of the nest during a critical window of its development. It is a well-established rule right across the sea turtles — cooler nests run to males, warmer ones to females, with only a narrow band of temperatures producing a healthy mix of both. That makes a nesting beach a thermometer with consequences: a warming climate, or a beach stripped of the shade and vegetation that once cooled it, skews clutch after clutch toward females, and a rookery can slide toward too few males to sustain it long before any adult turtle looks to be in trouble. It is one of the quieter ways a warming world reaches an animal — not by killing it, but by unbalancing the sex of the next generation before it has even hatched.

Then there are the odds facing the hatchling itself, which are long past believing. Of the hundred eggs in a nest, only a fraction hatch, and of those that do, only a sliver survive the first hours and years — a hatchling's chance of ever returning as a breeding adult is often put at something like one in a thousand. The gauntlet begins the moment it digs free. Ghost crabs and foxes take them on the sand, seabirds and fish in the shallows and beyond. And to the natural gauntlet we have added our own: newly-hatched turtles find the sea by crawling toward the brightest, lowest horizon, which on a dark natural beach is the pale sky over the water — but a lit-up esplanade behind the dunes overrides that ancient cue and draws them inland to exhaustion, roads and death, a straightforward and fixable kind of harm called light pollution. Vehicles compact the sand over nests and cut ruts a hatchling cannot cross; dogs dig; feral predators raid. So the loggerhead is exposed, like every migrant, along the whole span of a life lived in two worlds at once (see species-dependence): it needs healthy sea to feed in and a dark, quiet, undeveloped beach to nest on, and it can be undone in either. The clearest thing this coast can do for it is also the cheapest — keep some beach dark.