Vowels and voiced consonants of human speech and most mammalian vocalizations consist of harmonically structured sounds. The frequency contours of formants in the sounds determine their spectral shape and timbre and carry, in human speech, important phonetic and prosodic information to be communicated. Steadystate partitions of vowels are discriminated and identified mainly on the basis of harmonics or formants having been resolved by the critical-band filters of the auditory system and then grouped together. Speech-analog processing and perception of vowel-like communication sounds in mammalian vocal repertoires has not been demonstrated so far. Here, we synthesize 11 call models and a tape loop with natural wriggling calls of mouse pups and show that house mice perceive this communication call in the same way as we perceive speech vowels: they need the presence of a minimum number of formants (three formants-in this case, at 3.8 ؉ 7.6 ؉ 11.4 kHz), they resolve formants by the critical-band mechanism, group formants together for call identification, perceive the formant structure rather continuously, may detect the missing fundamental of a harmonic complex, and all of these occur in a natural communication situation without any training or behavioral constraints. Thus, wriggling-call perception in mice is comparable with unconditioned vowel discrimination and perception in prelinguistic human infants and points to evolutionary old rules of handling speech sounds in the human auditory system up to the perceptual level.auditory perception ͉ evolution of speech perception ͉ formant filtering and grouping ͉ mouse ͉ sound communication M ouse pups produce so-called wriggling calls when struggling in the nest, mainly when pushing for the teats during suckling by the mother (1). These calls release three types of maternal behavior: namely, licking of pups, changes of suckling position, and nest building, and thus show that they are important communication sounds (2). Wriggling calls usually consist of a fundamental frequency near 4 kHz and several (a minimum of two) overtones reaching to a maximum frequency of about 20 kHz, if overtones within a 30-dB range are considered (Fig. 1). This basic harmonic structure may be modified by frequency modulations of the harmonics and rapid amplitude modulations leading to side bands of the harmonics or to rather noisy partitions of the calls (Fig. 1).Mouse calls are structurally similar to vocalizations of many mammals (3), including cries and other nonverbal sounds of humans, especially infants (4). Because little is known about the perception of these types of vocalizations, we will compare the perceptual properties of wriggling calls with the perception of vowels of human speech that also have a frequency structure similar to that of wriggling calls of mice (5, 6). We hypothesize that mice perceive the wriggling calls in the frequency domain by following the same rules as humans in analyzing, identifying, and grouping formants together to a vowel percept (7-9). The term ''form...