Girls like it hot, but boys don't

How do Florida's loggerhead turtles decide where to lay their eggs? Researchers think it could be critical in keeping a balance between males and females. Malcolm Smith reports

Having a sex change in the shade of an apartment block on a hot Florida beach may sound a touch implausible. Not, it seems, if you are a loggerhead turtle and these are your breeding beaches.

For these threatened animals look as if they are attempting to balance the ratio of male to female offspring they produce by seeking shaded locations on beaches in which to lay their eggs. As with many other reptiles, the sex of the hatchlings depends on the temperature at which their eggs are incubated. In sea turtles like the loggerhead, higher temperatures produce females; cooler ones males.

Nick Mrosovsky, Craig Lavin and Matthew Godfrey of the University of Toronto and Florida Atlantic University, had noticed that most loggerheads laying eggs on developed beaches at Boca Raton, Florida, dug their nests in the shaded sand beneath apartment blocks.

In such spots the researchers found the sand - at the one- to two-foot depth that eggs are laid - to be up to two degrees cooler. At about 29C, the so-called pivotal temperature, half of the hatchlings are male, half female. In midsummer, whether shaded by apartments or not, sand temperatures were mostly 30C, guaranteeing a female-dominated society.

But in the early part of the season, sand temperatures are often around the sex-balancing 29C or even lower. At such times, the males become dominant again.

The problem with loggerhead turtles is knowing what sex ratio among hatchlings is natural. On undeveloped Florida beaches, around 90 per cent of young loggerheads are female because there is no respite from the hot sun.

But is this full sun natural anyway? The only clues come from scant information about how these Florida beaches looked a century ago when there was no development to speak of.

Vegetation, dominated by Saw Palmettos perhaps a few feet high, grew at the head of most beaches, and taller vegetation - perhaps up to 15ft - farther back still. None of this would have cast much shade on the beach, certainly far less than multistorey apartment blocks do. So are turtles that seek out deep shade acting unnaturally? Perhaps not.

Old photographs show Florida beaches with piles of sargassum seaweed washed up on them. Today it is cleared away because people don't want to lie on rotting vegetation. Could it be that loggerheads would have dug their nests among the sargassum, which would have kept the eggs cooler than if they were exposed to the sun? No one can be sure.

What the researchers are sure about is that loggerheads breeding on undeveloped stretches of beach nearly always dig their nests much closer to the sea. There the underlying water table is closer to the eggs, perhaps helping to keep them cool.

Life is generally hard for loggerhead turtles. They are hunted for food and for their shells. Like other sea turtles, they often get tangled in fishing nets. And disturbance by noise and night lights on their breeding beaches is a serious problem.

Most conservationists believe that seaside apartment blocks are a menace for turtles, too. But if they help loggerheads to attain a more natural sex ratio, they could be good news. Better still, though, will be to leave some beaches just as nature intended, seaweed and all. Then the loggerheads can get back to whatever their normal sex ratio happens to be.