By Holly Moeller on May 7, 2014

For years, I dreamed of seeing a baby sea turtle. I’ve swum with dozens of juveniles — and one or two behemoths — over the years in several different oceanic basins. But I was still missing that nature-documentary experience of watching a nest full of hatchlings scurry across the beach into the sea.

I never thought that my first baby sea turtle sighting would be one of the saddest memories of my life.

Dad and I were wandering down a beach on Moloka’i, one of the least developed Hawaiian islands, when we discovered the hatchling, which couldn’t have been more than a few days old. Hatchlings are supposed to head to water as soon as they can, but the infant turtle was being tumbled around by the waves splashing at the edge of the beach, seemingly unable to orient itself and swim out to sea. There was no sign of a nest or siblings.

It was abundantly clear that this turtle was not going to make it. It was on its last legs, too exhausted to do more than blink once or twice when a particularly strong wave finally forced it up the beach beyond the lapping ocean. I felt my heart break.

Dad reminded me that this hatchling was still a sign of hope: Somewhere, a whole nestful of brothers and sisters had successfully hatched. While this turtle would be one of the many hatchlings that don’t survive that initial, high-mortality period, some of its siblings had likely made it out to the relative safety of the sea.

As usual, Dad was right.

Even so, many hatchlings are not expected to survive to maturity, and in fact most sea turtles are endangered, so ensuring successful nesting is a major conservation goal. Many turtle-nesting beaches are afforded extra protections and monitoring because every population source is vital.

Yet in a few years, even ensuring the safety of every single turtle nest might not be enough.

That’s because climate change is warming the beaches on which the turtles nest and, as a result, potentially changing the sex ratio of the turtles that hatch out of those nests.

Turtles are reptiles, so their body temperatures and physiologies are affected by the ambient temperature of their environment. Unlike humans, who have X and Y sex chromosomes that determine biological sex, turtle gender is controlled by the temperature of the nest in which its egg is incubated. The girls like it hot, so warmer nests mean relatively more female turtles than males.

Climate change is expected to skew the turtle population’s sex ratio: Future turtle generations, hatched on warmer beaches, are going to be increasingly dominated by females.

But if the sex ratio is going to be skewed, perhaps that’s the best direction it could go in. Sea turtles don’t pair for life like, say, albatrosses. So a single male turtle could potentially mate with multiple females, compensating for a male-female ratio that is no longer 1:1.

Scientists are already finding evidence that behavioral compensation may play a role in sea turtle biology. According to one study, males return to the waters offshore of key breeding beaches more frequently than females do, allowing each male to do the work of several.

Over time, natural selection will likely also act to counteract the sex imbalance. Those females whose eggs produce a more balanced sex ratio, even at warmer temperatures, will produce sons who are more reproductively successful, thus passing on those warm-adapted genes. And sea turtles may also begin to expand their breeding ranges poleward, nesting on beaches that were previously too cold but now are just right for producing a crop of boys.

But will all this happen in time? Thanks to human activity, the climate is changing at an unprecedented rate — even within the scope of sea turtles’ ancient evolutionary lineage. And since it takes decades for turtles to reach reproductive maturity, shifting sex ratios in the breeding population will lag behind nest temperatures by many years. That’s why it’s important to do everything we can to reduce other pressures on sea turtle populations — pressures such as lighting hazards that distract hatchlings from heading toward the water, or fishing nets that snarl older turtles at sea — so that populations are as healthy as they can be in the face of this new threat.

For my part, although I am a biologist and know something of the ways of death, I couldn’t stand the thought of leaving one baby turtle stranded and helpless on the beach. While Dad waited on the shore, I swam, turtle resting on my belly, backstroking out past the breakers toward the mouth of the cove. I left the turtle there, bobbing upright and lifting its head every couple of moments to take a breath, hoping for a miracle that would be impossible on the shore.

Holly welcomes reader remarks and turtle caretaking tips at hollyvm “at” stanford.edu.