After years of debate, gray wolves were reintroduced (重新引进) to Yellowstone National Park. Fourteen wolves were caught in Canada and moved to the park. By 2016, the Yellowstone wolf population had grown to more than 170.

Gray wolves once were seen here and there in the Yellowstone area and other pans of United States, but they were gradually displaced by human development. By the 1920s, wolves had disappeared from the Yellowstone area. They went farther north into the deep forests of Canada, where there were fewer humans around. The disappearance of the wolves had many unexpected results. Deer and elk populations—major food sources(来源) for the wolf—grew rapidly. These animals ate large amounts of plants, which reduced plant diversity(多样性) in the park. With the disappearance of gray wolves, coyote (草原狼) populations also grew quickly. The coyotes killed a great number of red foxes in the park, and completely drove away the park’s beavers (河狸).

As early as 1966, biologists asked the government to consider reintroducing wolves to Yellowstone Park. They hoped that wolves would be able to control the elk and coyote problems. Many framers were against the plan because they feared that wolves would kill their farm animals or pets.

The government spent nearly 30 years thinking lip a plan to reintroduce the wolves. The U.S. Fish and Wildlife Service carefully monitors and manages the wolf packs in Yellowstone. Today, the debate continues over how well the gray wolf is fitting in at Yellowstone. Elk, deer, and coyote populations are down, while beavers and red foxes have made a comeback. The Yellowstone wolf project has been a valuable experiment to help biologists decide whether to reintroduce wolves to other parts of the country as well.

In a new study, shark researchers working off the eastern coast of New Zealand have found that three species of deep-sea shark are bioluminescent （生物性发光的），producing a soft blue-green light with specialized cells in their skin.

One of the species, the kitefin shark, grows to a length of nearly six feet, making it the largest known bioluminescent animal. The other two species are somewhat smaller than the kitefin, and all are occasionally caught as unintended by-catch by fishers. None are considered in danger of extinction, but little is known about their lifestyles and biology.

Bioluminescence had previously been documented in only around a dozen shark species, so this discovery significantly adds to our knowledge,^,, says Jerome Mallefet, lead author of the new study.

In the deep sea, where scientists estimate three-quarters of all creatures are bioluminescent, having the ability to create light can be extremely advantageous. In the depths of the ocean, which receives minimal amounts of sunlight, bioluminescent animals can hide themselves from enemies by producing enough light to match their surroundings. All three species examined in this study have large concentrations of photocytes （发光细 胞）on their undersides, which suggests that these sharks may hide from enemies in just this way.

"The discovery that these three species produce light is not surprising", says David Ebert, director of the Pacific Shark Research Center. That's because researchers think many more species of sharks are likely capable of producing light一Mallefet estimates that perhaps 10 percent of the 540 known species of sharks are bioluminescent. But Ebert thinks even this is far from the truth. "As the deep-sea shark research advances, that number will go even higher," he says.

Both Ebert and Mallefet hope that more attention will be paid to deep-sea sharks in the future, as the creatures and their habitat are understudied and under threat. "A lot of people know that sharks can bite," says Mallefet, "but few people know that they can produce light in the dark. ”

How can an air-breathing animal sleep safely in the open ocean, with killer whales and great whites hanging around near the surface? For elephant seals, we now know the solution. Jessica Kendall-Bar at the Scripps Institution of Oceanography in California and her colleagues have developed a system for recording the electrical brain activity of elephant seals, as well as their heart rates and movements. After tests on a few captive (圈养的) elephant seals, they applied this system to eight wild female seals with a removable tool. The team didn’t do this with males as their much larger size makes it harder to attach any kind of device.

The team firstly used the devices to record how the seals move while in slow-wave, or deep sleep, and also in rapid-eye-movement (REM), or dream sleep. And then, with this, they looked back at previous time-and-depth recordings from more than 300 wild females and finally found out when they were sleeping and which sleep stage they were in.

The study shows that females out in the open sea sleep for less than 10 minutes at a time during dives and get only around 2 hours of sleep per day in total, compared with around 10 hours when they are on land. At around 100 metres or so below the surface they go into slow-wave sleep and start sinking. But when the animals go into REM sleep, they do lose control. Their bodies turn upside down and sink in a characteristic spiral, with one animal going as deep as 377 metres. Then the seals wake and swim back to the surface. In waters less than 250 metres deep, the seals instead sleep still on the seafloor, like other seals do.

Other marine mammals have solved the how-to-sleep-at-sea problem in a different way. Only half of the brains of whales and dolphins sleep at a time, allowing the other half to watch out for danger and maintain breathing at the surface. Fur seals use the same trick. Unusually, though, fur seals can do both half- and whole-brain sleeping -- when they are on land the proportion (比例) of whole-brain sleep increases.