Estivation in Protopterus

Estivation in Protopterus is an episodic event characterized by elaboration of a cocoon as ambient water is withdrawn, a state of torpor, and distinctive cardiorespiratory and metabolic changes. Among the more striking of these features is a decrease in oxygen consumption, a complete reliance on air breathing to satisfy metabolic need, a slowing of the heart rate, and a drop in blood pressure. The initiating mechanism for these dramatic changes is not known. As yet, specific “estivating factors” have not been identified. However, the pattern of decrease in oxygen uptake during estivation and starvation are quite similar, suggesting that a common factor may be involved in both. Attempts to implicate suppression of thyroid function in the onset of estivation have been unconvincing. Although initiating mechanisms for estivation in Protopterus remain uncertain, once estivation sets in a variety of adaptive changes occur that enable the estivating lungfish to survive for months to years without ingesting food or water. Among these are oliguria and a shift in metabolic pathways. Although estivation in Protopterus has been characterized with respect to cardiorespiratory and metabolic parameters, no attempt is made to extrapolate from the biologic processes in Protopterus to other lepidosirenid lungfish or to other genera.