31.05.2023: If one compares the torpor of different animal species, two different patterns emerge. A recently published Australian–Austrian study led by the University of Veterinary Medicine Vienna investigated how this difference affects the long-term survival of the animals. The researchers conclude that not all torpor is the same and that the ability to enter a state of decreased physiological activity probably evolved for different reasons in different species.
Torpor is a highly effective method in mammals and birds to reduce energy expenditure. The extent of energy savings achieved, however, and thus the long-term survival of the animals, appears to differ among species. Animals that remain in a state of torpor for several days at a time (hibernation) seem to have an advantage over daily heterotherms, which are species that remain torpid for only part of the day (daily torpor).
Tests at different temperatures
A joint study conducted by Vetmeduni and the University of New England (Armidale, New South Wales, Australia) recently investigated this concept. The researchers tested how long-term survival on stored body fat – crucial for overcoming adverse periods – is related to the pattern of torpor.
The researchers investigated the pattern of torpor expressed by the eastern pygmy possum (Cercartetus nanus), a small mouse-sized marsupial, under different ambient temperatures, with 7 °C being typical of hibernation and 15 °C and 22 °C typical of daily torpor.
Pronounced differences in torpor patterns and survival times …
The pygmy possums expressed torpor at all temperatures and survived without food for an average of 310 days at 7 °C, 195 days at 15 °C and 127 days at 22 °C. At 7 °C and 15 °C, the duration of reduced body temperature (torpor bout duration, TBD) increased from < 1–3 to 5–16 days over two months; at 22 °C, TBD remained at less than 1 to about 2 days. At all temperatures, daily energy use was substantially lower and TBD and survival times much longer (3 to 12 months ) than in daily heterotherms (around 10 days).
… indicate different ecological purposes
The study’s final author, Thomas Ruf from the Research Institute of Wildlife Ecology at Vetmeduni, concludes: “Such pronounced differences in torpor patterns and survival times even under similar thermal conditions provide strong support for the concept that torpor in hibernators and daily heterotherms are physiologically distinct and have evolved for different ecological purposes.”
The article “Long‑term survival, temperature, and torpor patterns” by Fritz Geiser and Thomas Ruf was published in Scientific Reports.