An ultrasonic fingerprint sensor in a mobile device is operated to capture an initial snapshot of reflection from a finger's surface, of acoustic energy transmitted at a first frequency. Additionally, the ultrasonic fingerprint sensor is operated repeatedly to capture over time, a sequence of sets, each set including one or more additional snapshots of reflection from one or more depths within the finger, of acoustic energy transmitted at a second frequency significantly lower than the first frequency. Measurements in the additional snapshots are processed to determine whether any signal oscillating at a rate in a normal range for heart rate or respiration rate is present. When a signal is found, its rate may be used in several ways, e.g. to enable functionality when a fingerprint in the initial snapshot matches a reference fingerprint, or to identify and track a heart rate (or a respiration rate) based on the signal's rate.