The Tawny Frogmouth conserves its energy entering daily torpor
Living organisms are either ectothermic or endothermic. Ectotherms have low metabolic rates, lack insulation and therefore their body temperature is a function of ambient temperature. Because they do not ‘waste’ energy on internal heat production for thermoregulation, their energy and nutrient requirements are low, but they and their bodily functions are directly affected by the temperature of their environment.
Endotherms, on the other hand, have metabolic rates that are many-fold higher than those of ectotherms and therefore their energy requirements are high. Endotherms usually insulate their bodies to minimize heat loss.
Endotherms include most mammals and birds. Some species, however, are heterothermic, it is they can switch between ectothermic (or poikilothermic) and endothermic (or homeothermic) strategies to deal with energetic and other challenges, and, during certain times of the day or year, enter a state of torpor.
Mammalian and avian torpor (including hibernation and daily torpor) is characterized by temporal, substantial but controlled reductions in body temperature, metabolic rates, water loss, heart rate and other physiological functions and is the most effective means for energy conservation available to endotherms.
The Tawny Frogmouth, Podargus strigoides (Caprimulgidae - Podargidae) is an endemic, nocturnal bird species widespread throughout Australia. They are heterotherm and the largest bird know to use torpor. Like Nightjars, Tawny Frogmouths enter daily torpor at night and/or in the early morning. This birds remain torpid for only part of the day, but usually continue to forage during their active phase.
Unlike hibernators, daily heterotherms, such as the Nightjars and the Tawny Frogmouth, always express daily torpor independent of ambient temperature, season and trophic state. Daily torpor lasts only for hours rather than days or weeks, is usually not as deep as hibernation, and is often interrupted by activity and feeding.
Photo credit: ©Peter Nijenhuis | Locality: The Rainforest Habitat Wildlife Sanctuary, Port Douglas, Craiglie, Queensland, Australia