Brain waves during REM sleep solidify memories in mice, scientists report in the May 13 Science.

Scientists suspected that the eye-twitchy, dream-packed slumber known as rapid eye movement sleep was important for memory. But REM sleep’s influence on memory has been hard to study, in part because scientists often resorted to waking people or animals up — a stressful experience that might influence memory in different ways.

Richard Boyce of McGill University in Montreal and colleagues interrupted REM sleep in mice in a more delicate way. Using a technique called optogenetics, the researchers blocked a brain oscillation called theta waves in the hippocampus, a brain structure involved in memory, during REM sleep. This light touch meant that the mice stayed asleep but had fewer REM-related theta waves in their hippocampi.
Usually, post-learning sleep helps strengthen memories. But mice with disturbed REM sleep had memory trouble, the researchers found. Curious mice will spend more time checking out an object that’s been moved to a new spot than an unmoved object. But after the sleep treatment, the mice seemed to not remember objects’ earlier positions, spending equal time exploring an unmoved object as one in a new place. These mice also showed fewer signs of fear in a place where they had previously suffered shocks.

Interfering with theta waves during other stages of sleep didn’t seem to cause memory trouble, suggesting that something special happens during REM sleep.

Sea turtles do not have an easy start to life. After hatching, they have to break out of their shell, dig their way out from beneath the sand, then make a mad dash across the beach to the water where they may or may not find food and safety — hopefully without getting snapped up by a predator. All of this requires a bit of luck and a lot of energy. And the energy a hatchling expends on breaking out of the nest is energy that can’t be used on surviving the rest of the journey.

Now, a new study has quantified the amount of energy a baby sea turtle uses to dig itself to the surface. Having lots of siblings — and, thus, lots of help — can really be a time and energy saver, researchers report May 18 in the Journal of Experimental Biology. That also implies that the conservation technique of dividing clutches may instead make hatchlings worse off.

Figuring out the energy expenditure of baby sea turtles took some trial and error. Mohd Uzair Rusli of the University of Malaysia Terengganu and colleagues started by burying newly hatched green turtles beneath 40 centimeters of beach sand, but the hatchlings never started digging and the researchers abandoned the experiment after 48 hours. They suspected that the turtles might need a pocket of air, something that would naturally be found in between eggs.

The team then tried eggs that were just starting to hatch, orienting them so that the top of the egg — where a turtle had started to emerge — would be toward the sand surface. But instead of digging upwards, many of the turtles dug toward the side of the big sand-filled chamber. The researchers thought that the babies may have been drawn to light entering through the transparent chamber walls. “It appears that they can be attracted to light even when buried underground,” they note. This is perhaps not all that surprising given that researchers knew that baby turtles use cues from the sun to emerge most often at night or on cloudy days.

For the final experiment, the scientists buried clutches of eggs just about to hatch beneath 40 centimeters of beach sand in a chamber with opaque walls. Just above the eggs sat a strip of aluminum foil that, when broken, signaled the start of the digging-out process. A 24-hour webcam monitored the top of the sand so researchers could see when digging ended. The whole setup was then enclosed so that the scientists could measure oxygen consumption — a stand-in for energy expenditure. And the team was careful to stay quiet near the experiment, because they learned that talking near the buried turtles prompted the tiny hatchlings to dig.

Escaping from the sand took between 3.7 and 7.8 days, with larger clutches taking less time to emerge and also using less oxygen per hatchling. Digging behavior was not consistent during the whole time; the oxygen consumption rate rose and fell in peaks as the turtles dug and dug and dug together, rested and then started again. “In nature, it is likely that hatchlings receive a significant benefit by belonging to a large clutch,” the team concludes. They use less energy in their escape, leaving more for the mad dash to the sea and finding a first meal.

The researchers note that in some regions of the world, it is a common conservation strategy to split up clutches when relocating them into hatcheries. But this practice, they warn, could leave baby turtles with reduced energy reserves when they reach the ocean.

Americans have climate change to thank for a decades-long spate of milder winters. Around 80 percent of U.S. residents live in counties where the weather has become more pleasant over the last four decades (see map). That trend won’t last, however: Researchers predict in the April 21 Nature that 88 percent of Americans will experience noticeably worse weather by 2100 than they do today.

The researchers created a weather pleasantness index to rank weather conditions. Hot, humid summers cost points, while mild winters added points. In the contiguous United States, winter warming has outpaced increases in summertime temperature and humidity. But if greenhouse gas emissions continue unabated, summer weather will become less pleasant over the coming decades, potentially sparking increased public interest in combating climate change, the researchers predict.