It was 2005 and scientists in Cape Town made a shocking discovery. Their tracking data showed a great white shark moving from South Africa to Australia and back again in a near straight line. It was the fastest transoceanic return migration ever recorded and it was carried out with near pinpoint accuracy. Today, it’s well known that sharks make yearly returns such as this to specific locations, but how exactly they do it has escaped consensus.

A group of scientists from Florida State University has taken on the question and concluded that sharks have an internal, GPS-like navigation system that allows them to read the Earth’s geomagnetic field. To conduct the research, the team first got 20 juvenile bonnethead sharks in St George Sound off the Florida Panhandle, and placed them in a small pool surrounded by copper wire. The wire allowed the researchers to create a custom magnetic field in the centre of the pool. Exposed to the magnetic field from the capture location, the sharks swam in random directions at leisure; but when exposed to the geomagnetic field that would be found 600 kilometres south of that spot, they swam north in a “homeward orientation”.

Researchers have suspected that sharks and sawfish detect magnetic fields since the 1970s, but the exact mechanism by which they do so, and the prevalence of this skill in nature has proven elusive, partly because it’s so difficult to study. “We’ve known for some time that sharks have the ability to detect the magnetic field, but this is the first time it has been tested successfully,” says Bryan Keller, a scientist at the US National Oceanic and Atmospheric Administration. “We expect these abilities are also observed in other species, like the great white, which migrate 20,000 kilometres out and back to the same spot.” The results mean that some sharks can be added to the growing list of animals that navigate by magnetic sensation, which includes sea turtles, lobsters and birds.

With the shark navigation system now demonstrated, scientists want to understand the mechanism behind it. Two theories have emerged: some researchers believe that it depends on an iron mineral called magnetite; others believe it’s based on a magnetic-field-sensing molecule in the retina (视网膜) of the eye called cryptochrome. Both theories, or a combination of the two, are plausible. Magnetite has been isolated from many animal tissues, while evidence from studies in birds suggests that they sense the inclination of the magnetic field using cryptochrome molecules in their retinas; the direction of the field is transmitted by the optic nerve to the brain, which allows them to “visualize” north and south. But scientists don’t yet know the precise location of the cryptochrome receptors, or the brain centres that process the information on the magnetic field. There’s more work to do to truly understand these masterful navigators.

Are Bamboo-Eating Pandas Really Herbivores?

On the outside, giant pandas look like herbivores(食草动物). They spend nearly all of their waking hours eating bamboo. But on the inside, they're built like carnivores(食肉动物). About half of the calories they eat come from protein, according to a new study.

The ancestor of giant pandas were omnivorous(杂食的). They are both animals and plants, and had the digestive system and gut bacteria to metabolize(使发生新陈代谢)them. They had ''umami taste receptors'', to appreciate the flavors of meat.

However, about 2.4 million years ago, things began to change. 【小题1】 Their jaw and teeth evolved to help them crush bamboo, and their wrist bone became capable of grasping the stalks(杆)of their favorite plant. Scientists think pandas switched to eating bamboo partly because they didn't have to fight with other animals to get it. Bamboo is high in fiber but has a low concentration of nutrients, so pandas have to eat 20 to 40 pounds of the plant every day just to get by.

David Raubenheimer, a nutritional ecologist at the University of Sydney, and his colleagues put GPS trackers on two giant pandas and followed their movement throughout the year. They discovered that the pandas followed the protein. 【小题2】 At the start of the cycle, they ate Bashania fargesii leaves until they got the chance to feast on young shoots, which contained more protein.

The more the shoots grew, the more their protein was diluted(冲淡)by fiber. That caused the pandas to move to higher ground, where Fargesia qinlingensis grew. First, they ate the shoots, but these, too, went from being protein - rich to fiber-rich as they grew. The panda responded by switching to the leaves. 【小题3】 The researchers found that about half of the calories the pandas ate were in the form of protein.

【小题4】 ''They can know exactly where to go, and when to go, so they can get the most of the nutrients that their ecosystem can provide, '' said Silvia Pineda - Munoz, who was not involved in the study.

The work also shows that classifying an animals as herbivore or carnivore is more complex than one might sassume. ''It's not whether you’re eating plants but what of the plants you’re eating, '' said Pineda - Munoz.

In order to save the rare northern spotted owl, biologists are doing something that goes against their heart — shooting another owl that’s rapidly taking over spotted owls’ territory (领地) across the northwest.

During the 1990s, a few barred owls showed up in an area of forests along Redwood Creek that was spotted owls’ territory. Barred owls, which reproduce much faster than spotted owls, now claim nearly all this territory. No spotted owls have nested here in recent years.

“It’s very annoying and there’s nothing that’s going to stop this expansion of barred owls,” says Diller, who has studied spotted owls for 25 years. The only possible solution forces him to go against his nature.

Diller’s a hunter, but he was taught never to kill a bird or anything you didn’t plan to eat. Diller remembers the first time he took a shot. “I was so nervous about what I was doing that I had to steady myself against a tree.” Over the past five years, Diller has killed more than 70 barred owls with a shotgun. “I hate it every time I go out and do it,” he says. People recognized that there’s a crisis for spotted owls and those barred owls are part of the cause of that crisis. So, they unwillingly attempted to kill the barred owls.

A group, Friends of Animals, doesn’t believe the government can make a moral argument for shooting an animal, even if it would benefit another animal.

“I don’t see that as being a solution. At some point you have to allow these species to either figure out a way to coexist or for nature to run its course,” says Michael Harris, director of Friends of Animals.

But Diller argues that is an absurd thing to say after all the way humans have changed nature. People cut down most of the forests that used to host barred owls. They made lots of changes to the Great Plains, which he believes helped the barred owl move across the continent.

For Diller, seeing rare spotted owls increase in the forests is worth the pain of shooting barred owls.

“Probably what makes spotted owls so special is the fact that they fly right up to you,” Diller says. “You get to interact with them. It’s almost impossible for a biologist not to fall in love with these birds — they’re just the neatest animal.”