Monkeys May Tune In to Basic Melodies

Some tunes stick in one’s memory, sometimes with remarkable persistence. Think of “Happy Birthday,” “Old MacDonald,” and “Row, Row, Row Your Boat.” In laboratory experiments, even infants exhibit a keen memory for such songs.

A dozen of these childhood classics prove as memorable to rhesus monkeys as they do to people, a new study finds. This represents the first well-controlled demonstration that any nonhuman animal perceives simple melodies, say psychologist Anthony A. Wright of the University of Texas Medical School at Houston and his colleagues. Their report appears in the September Journal of Experimental Psychology: General.

“The perception of melodies depends on the structure of our nervous systems, not just on childhood and cultural experiences,” Wright contends. “It’s anybody’s guess why this ability evolved in monkeys as well as humans.”

Wright’s group studied octave generalization, a critical facet of melody perception. In Western musical scales, moving from one eight-note octave to the next highest octave represents a doubling of acoustic frequencies. If a melody’s other acoustic properties, such as its pitch patterns and its key, stay the same, people easily recognize the tune when it shifts by one or more octaves.

Two earlier studies, one in 1943 with rats and one in 1988 with dolphins, also yielded evidence of octave generalization. However, those efforts lacked definitive experimental controls, Wright holds. Several other tests, including some with monkeys and songbirds, found no evidence for octave generalization.

The new study focused on a pair of adult rhesus monkeys that learned to report whether one sound matched another heard after a 1-second delay. Sounds included a boat whistle, owl hoots, and sonar pings. If the sounds were the same, a touch to a loudspeaker on the right yielded a food pellet; if the sounds differed, a touch to a speaker on the left earned the prize.

Using this approach, both monkeys accurately identified repeats of any of 12 childhood songs—including those cited above—even when the melody shifted by one or two octaves. The monkeys succeeded whether the tunes sounded as if they’d been played on a piano, guitar, or other instruments.

The animals also displayed octave generalization for new melodies, created according to a mathematical formula for tonal, well-structured tunes. Tonality refers to the relationship of a melody’s tones to a central tone, or key, that gives the passage a musical or songlike nature.

As previously reported for people’s musical skills, the animals’ ability to identify childhood songs plummeted when they heard tunes that shifted by either one-half octave or 1 1/2 octaves. In these trials, the monkeys touched the speakers randomly. Similar problems arose when they heard octave shifts of single notes, random sets of notes, or atonal sequences, in which there’s no key.

“Wright’s group makes a good case that these monkeys perceive whole tunes, not just isolated notes,” remarks psychologist Sandra Trehub of the University of Toronto. Trehub, who studies infants’ musical perception, theorizes that many mammals are sensitive to basic musical patterns.

Scientists have yet to reach a consensus on how to quantify the tonality of a sequence of musical notes, a concept that’s crucial in the new study, comments psychologist Diana Deutsch of the University of California, San Diego. Although octave generalization probably occurs in nonhuman animals, Deutsch says the new experiments don’t establish that monkeys perceive music.