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Studies of whales in captivity (囚禁；被拘留的状态) have taught us much about their physiology (生理学) as well as their complex social behavior. Their learning and problem-solving skills, together with their curiosity and often eager interactions with humans, have led some scientists to believe that whales possess a high level of intelligence.
The large brains of whales also suggest a high intelligence. The sperm whale (抹香鲸)’s brain is considerably larger than an elephant’s, brain and six times the size of a human brain. However, relative to the rest of the body, a sperm whale’s brain is only about 0.02 percent of its body weight, while a human brain is about 2 percent. These animals tend to rely on hearing more than vision, which explains why the visual centers of their brains are smaller proportionally than those in humans. Another feature is the degree of folding of the upper surface of the brain. This folding increases the surface area and has been observed in more intelligent animals. Whale brains generally show as much or more folding area as is seen in humans.
Complex behavior may reveal more about whale intelligence than brain size. Perhaps the most interesting indications of whale intelligence came with the discovery in the 1970s of whale singing. Singing is most common on the winter mating grounds, for example in the Caribbean and Hawaii.
There is no solid evidence that the songs encode language in an intellectual sense. The whale songs may be simply longer versions of the mating songs also noted in birds and amphibians (两栖动物).

"There is no solid evidence that the songs encode language in an intellectual sense." in the sentence "solid" means（）.

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