The absence of adverse health outcomes later in development and the similarity of defining features of "colic" across cultures suggest that an evolutionary perspective may give us some insight into the nature of this puzzling condition. Evidence suggests that the larynx evolved first as a protective valve and later as a means to stabilize the upper thorax momentarily for precision upper arm movements. Recently, we found another physiological role for the larynx in regulating respiratory function to promote the recovery of young rats from severe hypothermia. In the process, bursts of calling were emitted by unconscious pups, which were nevertheless effective in eliciting maternal search and retrieval. These unexpected findings reveal how infant calling may have evolved as a communicative signal derived from more primitive physiological functions of the larynx. Repetitive calling in the normal young rat when isolated from its littermates and mother is regulated by multiple sensory cues present in the infant's social interactions, and in the paper it is described how this sensory input projects to central neuromodulatory systems known to be active in the control of anxiety behaviors in adult rats and humans.
Conclusion: This broad range of functions for infant calling in other mammals suggests several new ways to approach the further study and treatment of unexplained crying in human infants.