In a match-to-sample task study, a single California sea lion was able to demonstrate an understanding of symmetry, transitivity and equivalence; a second seal was unable to complete the tasks.[140] They demonstrate the ability to understand simple syntax and commands when taught an artificial sign language, though they only rarely used the signs semantically or logically.[141] In 2011, a captive California sea lion named Ronan was recorded bobbing its head in synchrony to musical rhythms. This "rhythmic entrainment" was previously seen only in humans, parrots and other birds possessing vocal mimicry.[142] In 1971, a captive harbor seal named Hoover was trained to imitate human words, phrases and laughter.[143] For sea lions used in entertainment, trainers toss a ball at the animal so it may accidentally balance it or hold the ball on its nose, thereby gaining an understanding of the behavior desired. It may require a year to train a sea lion to perform a trick for the public. Its long-term memory allows it to perform a trick after at least three months of non-performance.