A new study has found that hummingbirds use the hibernation-like state of torpor in varying ways.
The research, published in Proceedings of the Royal Society B, showed that hummingbirds use torpor depending on their physical condition and what is happening in their environment. Torpor allows them to ramp down energy consumption to well below what they normally use during the day, helping them make it through cold nights.
For the study, researchers caught 249 hummingbirds from 29 different species living in the mid- to high-altitude mountains. They collected measurements related to torpor in the wild, as opposed to studying the birds under laboratory conditions.
Andean Emerald is one of many hummingbird species found in the Colombian Andes (Stuart Reeds).
"Torpor is somewhere between a power nap and hibernation," said Justin Baldwin, a PhD candidate in biology in Arts and Sciences at Washington University, first author of the study. "Earlier research had suggested that torpor was a way of completely shutting metabolism down to minimal levels.
"Our findings join a growing body of evidence to suggest that when animals enter torpor, they have diverse options to calibrate aspects of torpor to their environment."
The researchers found that hummingbirds can enter into deep or shallow torpor. This means entering the state for merely a few hours, or an entire night, with the birds coming back from torpor hours or minutes before sunrise. When they exit torpor, some birds warm up quickly, with others more gradually.
Smaller individuals were more likely to use torpor than larger birds, but only at low ambient temperatures, where torpor lasted three hours or more.
"Torpor can be the difference between surviving through cold night-time temperatures encountered at high elevations, or dying trying to regulate their body temperature using fuel from their internal reserves," said Gustavo Londoño, co-author of the study and a professor at the Universidad Icesi in Cali, Colombia.
"One of the new things we learned with this study is that hummingbirds start exiting torpor more or less one hour before sunrise. Being out of torpor and ready to fly with the first sunlight is key to ensuring that their first meal will be from flowers that are full of nectar. The flowers then get depleted through the morning."
Hummingbirds need to be in good physical condition to use torpor, otherwise they might not have enough resources to exit from torpor or survive cold nights. When the scientists caught hummingbirds that were in poor condition, they noted that these unhealthy birds exited torpor right at (or even after) sunrise, apparently waiting for heat from the sun to provide the extra energy.
Because the scientists studied so many species at once, they were able to address how torpor may have contributed to hummingbird evolution and the colonisation of the harsh environments of the high Andes.
Baldwin added: "Species that more readily deployed torpor at our study sites also occurred at higher elevations across the entire Andean region. This is exciting, because it suggests that a characteristic of bird physiology can tell us something about their distribution across the entire continent.
"Even though we don't know which came first – an increase in readiness to use torpor or the ability to persist at high elevations – we think that hummingbirds' readiness to use torpor is likely tied to their evolutionary conquest of mountain habitats."
Reference
Baldwin, J W, Hernández, D C R, & Londoño, G A. 2023. Ecological drivers and consequences of torpor in Andean hummingbirds. Proceedings of the Royal Society B: Biological Sciences. DOI: https://doi.org/10.1098/rspb.2022.2099
