In just five hours, an average farm pig can learn how to interpret an image in the mirror and use it to find hidden food.
Scientists consider the ability to use a mirror a sign of complex cognitive processing and an indication of a certain level of awareness. In addition to humans and some primates, dolphins, elephants, magpies and a famous African grey parrot named Alex have all been known to retrieve objects or remove marks on their body using a mirror. Now it looks like pigs should be added to the list of clever critters that can master a mirror: After spending five hours with a mirror in their pen, seven out of eight pigs could use the reflection to find a hidden bowl of grub.
"This is the first demonstration of the ability of pigs to use mirrors," animal behavior expert Donald Broom of the University of Cambridge wrote in an e-mail. "Finding sophisticated learning and awareness in animals can alter the way that people think about the species and may result in better welfare in the long run." Broom co-authored the paper published this month in Animal Behaviour.
Like most animals, the pigs were immediately curious when researchers placed the shiny, reflective object in their pen. They approached the mirror until they bumped into it with their snout, and then checked to see what was behind the mirror. The pigs spent an average of 20 minutes gazing at their reflection, often turning in different directions to inspect themselves from several angles.
"These kind of movements suggest that the pigs were correlating the movements of their body with the visual stimulus they were receiving from the mirror, and so learning the contingency between the two," biologist Louise Barrett of the University of Liverpool wrote in a commentary about the paper, also published this month in Animal Behaviour.
After five hours with a mirror, the pigs were placed in a new test area that contained a food bowl hidden behind a barrier. Although the pigs could see the reflection of the bowl in the mirror, they couldn't see the food directly. A fan above the bowl circulated the scent of food around the room, prohibiting the pigs from smelling their way to the treat.
Seven out of eight of the pigs with previous mirror experience spotted the reflection of the food bowl and correctly interpreted its location: Instead of searching for the food in its apparent position behind the mirror, the pigs headed around the barrier and straight for the true location of the bowl. When the researchers tested pigs with no prior mirror exposure, however, nine out of 11 of them became confused, searching behind the mirror for the food.
"These results suggest not only that pigs learn the contingency between their own movements and their image in the mirror," Barrett wrote, "but that their knowledge incorporates the layout of the environment as well, so that they can locate objects in space."
The researchers say their experiment is more than a nifty trick: The fact that pigs can learn to use a mirror means they are capable of a type of awareness called assessment awareness, which means they can understand the significance of a situation in relation to themselves, over a short period of time. In this case, the pigs remembered how their own movements appeared in the mirror, and were able to apply that knowledge to a separate situation involving a hidden food bowl.
"Having a sense of self and using it is a form of assessment awareness," Broom wrote. Although the mirror experiment doesn't directly prove that pigs have a sense of self, the researchers suggest that given how quickly the pigs learn to recognize their own movements in a mirror, they may have some degree of self-awareness. "We have no conclusive evidence of a sense of self," Broom wrote, "but you might well conclude that it is likely from our results."
Other mirror tests have been used to more directly examine an animal's sense of self - if researchers apply a yellow mark to the black feathers of a magpie, for instance, the bird will use a mirror to clean itself off. Unfortunately, Broom says the mark experiments just don't work on pigs: Pigs are so accustomed to being streaked with mud, they don't much care if researchers apply extra marks on their bodies. "We have put marks on pigs," Broom wrote. "They take little notice of them."
Combined with a host of other research studies demonstrating the keen intelligence of pigs, the researchers hope their study will lead to better treatment of the farm animals. "If an animal is clever," Broom wrote, "it is less likely to be treated as if it is an object or a machine to produce food, and more likely to be considered as an individual of value in itself."
"这种移动表明猪把自己身体的动作和镜中的影像联系在一起，然后学会这两者间的关联性。"来自利物浦大学的生物学家Louise Barrett 在一篇也是这个月发表在《动物行为》中的论文的评论中写道。
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