“Our findings are consistent with the hypothesis that the strong drive to eat prior to metamorphosis is due to the absence, or the relative immaturity of hypothalamic feeding control circuits,” the authors write in their paper. This lack of inhibition helps allow “the animal to maximize growth during this critical life-history stage.”
Previous work by the researchers had implicated a role for leptin, a hormone that acts as a hunger inhibitor in vertebrates, in regulating the changing feeding habits of toads during early development. To investigate how this hormone might prepare juvenile amphibians for metamorphosis, the team analyzed levels of mRNA transcripts for leptin receptor proteins and for the hormone itself in tadpoles of the African clawed frog (Xenopus laevis).
The researchers found that the tadpoles were essentially unresponsive to leptin, unlike their adult counterparts, and showed minimal expression of the leptin receptor in the hypothalamus—a key brain region in the regulation of feeding behavior. Instead, these responses develop as the relevant neural circuits mature during metamorphosis, the authors write.