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(a) Vocalization’s sonogram of piglets under normal welfare (baseline); (b) vocalization’s sonogram of piglets being squeezed (assumed to produce pain); (c) vocalization's sonogram of piglets feeling cold; (d) vocalization's sonogram piglets feeling hunger. The formants (dotted red line), the sound intensity (yellow line), and the frequency of pitch (in blue line) are shown in each sonogram.
