What is rhythm and how can we study it?

Rhythm. What is it? We all have an intuitive understanding of rhythm: The term is used widely to describe temporal structures of any kind. These range from the rhythm of the tides, or seasons, the random - yet rhythmic - drumming of rain on our windowsill, to the movements of animals, heartbeats or breathing, or the captivating rhythm of a skilled drummer. As of yet, the term "rhythm" lacks established technical usage in behavioral biology and other research fields and has therefore been applied to a wide range of behavioral phenomena in the temporal domain. This wide scope and lack of terminological specificity easily confound the study and discussion of the distribution of rhythmic behaviors across species including humans.
In this chapter, we tackle this issue across animal species including humans mostly in the context of cognitive and behavioral biology and neurobiology. First, we identified three general perspectives to study rhythm, starting with the physical domain, describing a purely physical phenomenon purposefully detached from any biological example. A second perspective, moving into biology, includes the perception of these physical signals. A third perspective is based on higher-order concepts and structures such as music and group interactions. Starting from the most general sense in which the term rhythm is used for phenomena in time, we add successive constraints to increase specificity or specialization. Thus, we proceed by stages from a sense of rhythm in which it is present in all life forms, to one which, to our current knowledge, might only apply to humans. Lastly, we provide an extensive overview of issues and perspectives necessary to grasp the full concept of "rhythm" in modern rhythm research across all domains.