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Determinants of rhythm and rate in suckling.

Suckling was studied in infant miniature pigs to determine (a) the necessary stimulus for eliciting rhythmic behavior and (b) whether the rhythm of the feeding movements could be entrained with a rhythmic pulsed delivery of milk. The animals fed on an automated milk delivery system, which supplied pulses of milk either at fixed, predetermined rates or on demand. The rhythm of the suckling response was quantified from the teat pressure changes produced by the animal, which were highly correlated with jaw movement. Suckling frequency was measured as the dominant frequency in the teat pressure wave, determined by fast Fourier transform. When each animal was allowed to determine its own rate of milk delivery, the preferred frequency of suckling was approximately 3.8 Hz. When animals attempted to suckle on the teat but milk was not delivered, suckling was erratic and arrhythmic. The first aliquot of milk delivered to the animal elicited rhythmic suckling at approximately 4.6 Hz, which was maintained when milk was delivered at a range of fixed rates (0.2-0.56 Hz) an order of magnitude below the preferred suckling frequency. When milk was delivered at a fixed rate (2.0-5.6 Hz) close to the animals' preferred rhythm, suckling proceeded at a lower frequency (3.9 Hz) than when the milk was delivered at the much lower rate. However, variation in the delivery rate (2.0-5.6 Hz) did not cause a significant difference in the suckling frequency. These findings provided evidence against entrainment. The higher suckling frequency elicited by the slower delivery rate was suggestive of a negative feedback loop; in the infant/sow relationship, such a mechanism could favor a particular volume delivery per unit time.