Sound frequency patterns and sound pressure levels of distress calls from pups of several litters of albino mice were analysed. Practically all the characteristics measured (length of the calls, initial, terminal, minimum and maximum sound frequency, total bandwidth and sound pressure) changed with successive days of age. However, only the length of the calls, their bandwidth and their sound pressure developed in a regular way and all decreased as the pups grew older. The other measures fluctuated considerably but did not show any definite trend. It is suggested that reduction in the acoustic energy of the calls (intensity × duration) could decrease their effectiveness in eliciting maternal responses and could be the result of reduced cold stress as the pups develop homoiothermy. © 1969.
