You might have more in common with the chicken on your plate than you realize. Sure, you've also got two thighs, two legs, two breasts, and two wings (sort of). But new research suggests that chickens might like to rock out to the same tunes you've got on your iPod. The kinds of sounds that humans tend to find pleasant is called consonant, which are different from from unpleasant sounds, which are called dissonant. Think of the difference between a Mozart sonata and fingernails on a chalkboard, and you're on the right track. Consonant notes sound - to the untrained ear - as if they were a single tone, while a you can identify multiple tones within a dissonant note. This might be related to the human preference for harmonics, since in humans, the preference for consonant sounds are associated with preferences for harmonic spectra (harmonic relationships between frequencies), while dissonant sounds are not.

It might be easiest to understand by listening to these melodies. The melodies are the same, but the first one is consonant (composed of minor and major thirds) and the second one is dissonant (composed of minor seconds). Turn your speakers up:

Two-month-old human babies prefer to listen to consonant music rather than dissonant music. As early as one to three days after birth, human infant brains can distinguish between consonant and dissonant music - though it is unclear if there is a preference at that early age. Songbirds like Java sparrows (Padda oryzivora) and European starlings (Sternus vulgaris) can distinguish consonant from dissonant music as well, though, like day-old human infants, it is unclear if there is a preference. Japanese macaques (Macaca fuscata) can distinguish the two types of tones, though no preference has been observed in cotton-top tamarins (Saguinus oedipus). There was one human-raised chimpanzee (Pan troglodytes) that preferred consonant music. Taken together, the evidence for the musical preferences of humans and non-human animals is a bit...dissonant. No harmony to be found here.

It is therefore still an open question whether the preference for consonant music is fundamental to the properties of the auditory system or if the preference for consonant music is a product of culture or experience. Even though two month old babies haven't had that much experience with music, it is still possible that their preference for consonant music could have developed very fast after birth or even while still in utero. It is likewise possible that songbirds prefer consonant music simply because they have experience listening to the songs of other birds, which tend to be consonant.

The best way to test whether or not the preference for consonant music is innate or learned due to experience would be a controlled rearing study, where animals are raised from birth in a highly controlled environment. It is impossible to raise a human in this way, but it is possible to raise certain animals in a controlled way. And that is exactly what Cinzia Chiandetti and Giorgio Vallortigara, from the Italian University of Trento, did.

The animal they used is one we are all intimately familiar with: the chicken (Gallus gallus).

Chickens are an ideal species to use for controlled rearing studies: they need no parental care. As soon as they hatch, they are able to survive on their own. They can eat and drink on their own, and they can walk around beginning on the first day of life. Their vision is also very good right out of the shell. Compare this to human children who take months before they can even crawl around on their own. Chickens, like many birds, also imprint onto the first moving object they see - usually, that's mom. This turns out to be really useful to researchers. If you imprint a chick to an object (say, a red ball), then that red ball will become a source of comfort. If you briefly separate the chick from the ball, the chick will become motivated to reunite with it. And researchers can use that motivation to their advantage.

Here's how they did it: the researchers took some fertilized eggs from a local hatchery and incubated them in a soundproof room. On the last day of incubation, each egg was placed in its own little cage so that the chicks would not hear each others' chirps upon hatching. In this way, the researchers were able to control every sound that the chick heard both while incubating and after hatching.

In each cage was a small red object suspended from a string, to which each chick would imprint.

Then, the chick was moved into the center of a testing cage. At the ends of the testing cage were two speakers. One speaker played a consonant version of a melody, and the other played a dissonant version of that same melody (like the ones you listened to at the top of this post). Next to each speaker was an object identical to the one to which the chicks had been imprinted. After listening to the sounds for 34 seconds, the chick was given six minutes to approach one of the two sides of the cage, while the music continued to play.

The researchers reasoned that since the imprinted objects could be found on either side of the testing cage, the chicks should have been equally motivated to approach either the right side or the left side. If day-old chickens prefer one type of music over the other, however, that ought to skew the results in favor of the side of the cage where that type of music was playing. And that's exactly what they found. For the first four minutes of the six minute testing period, the chicks tended to freeze where they had been placed in the center of the testing cage - this is consistent with a natural fear response due to the new situation. However, after calming down a bit, the chicks chose to approach and spent more time on the side of the cage playing consonant music more often than the side playing dissonant music, during minutes five and six.

What does it mean that chickens seem to prefer consonant music? For one thing chickens and related species (the galliformes) do not sing. So the preference for consonant music is probably not limited to species that have organized systems of speech or song. The researchers noted that it is possible that - while deprived of external sounds - the chicks in this experiment could have heard the sounds that they, themselves, produced either while developing in the egg or shortly after hatching. Therefore, the results of this experiment lead to two possibilities: either there is an innate preference for consonant sounds that is independent of experience, or the preference is a by-product of early exposure to sounds produced by the chicks themselves.

Chiandetti and Vallortigara place these findings in the context of earlier work on the preference for consonant music and make the following proposition: given that preferences for harmonic sounds are correlated with preferences for consonant sounds, it is possible that the observed preference for consonant music also reflects a preference for harmonic music. Further, they note that harmonic spectra are prominent features of sounds that occur in nature. They suggest that harmonic spectra could serve as auditory cues that guide newly hatched chicks to distinguish between animate and inanimate objects, similar to the way that biological motion serves as a visual cue to distinguish agents from objects.

As usual, more research must be done to further investigate these claims, which could lead to an important conclusion: that the preference for consonant music is not due to enculturation. Indeed, even if some early exposure to consonant sounds was required for the preference to develop, this could be a case of experience-expectant learning, much like language or three-dimensional stereo vision, rather than experience-dependent learning. In other words, the auditory system might require exposure to consonant sound in early life (or in utero, or in the eggshell) to perform optimally, but given certain constants in the world (such as, for example, the types of sounds that infant chickens naturally make), that exposure is virtually guaranteed. Which would mean that the preference for consonant music was innate.

Chiandetti C, & Vallortigara G (2011). Chicks like consonant music. Psychological science, 22 (10), 1270-3 PMID: 21934134

Chick photo via Flickr/KRO-Media. Piano keys photo via Flickr/zoomed_in.