Development of suckling controls in the rat: independence from thyroxine

Until 2 weeks of age, rat pups attach to the nipples of their dam with the same avidity regardless of whether they are fed or fasted. During the 3rd postnatal week, some mode of control of suckling appears such that fasted pups attach much more rapidly than fed pups. In this study we have investigated the effects of hyper- and hypothyroidism on the maturation of suckling controls. Surprisingly, the results indicate that this aspect of behavioral development is independent of the thyroxine status of the pup. We conclude that the maturation of suckling controls is not cued by the developmental rise of serum thyroxine that occurs during the 2nd postnatal week.