Hummingbirds: Literally cool state

During the day, the shiny black-bellied tail whizzes nimbly from flower to flower, but in the cold night the Andean hummingbird falls into a motionless rigidity. (Image: neil bowman / iStock)

Researchers report that hummingbirds in the cool heights of the Andes go into an astonishing rigor at night. In order to save energy, a type of state called torpor lowers its body temperature to up to 3.3 degrees Celsius, the measurements showed. This is the lowest body temperature that has ever been measured in living birds, say the scientists.

From flower to flower they whiz through the warm tropical forests of the world – but some hummingbird species have also conquered extreme locations in the South American Andes, where they are exposed to enormous temperature fluctuations: in their habitats up to around 5000 meters above sea level, temperatures can rise at night Approaching freezing point. In the case of the tiny birds, this challenge seems particularly great, because they are among the smallest warm-blooded creatures on earth and also have the highest relative metabolic rate of all vertebrates. In order to cover the energy costs for heat generation and the strenuous hovering flight, they have to collect enormous amounts of nectar. The following applies: if they consume more energy than they can absorb, they are quickly threatened with extinction.

Hummingbirds in cold energy saving mode

But how are the tiny creatures able to maintain their normal body temperature of around 36 degrees Celsius with their low energy reserves on cold nights? It has been known for some time that some hummingbird species avoid this by falling into a “hibernation” at night. Other vertebrates and some bird species also use this state, known as torpor, to temporarily bridge unfavorable environmental conditions. The hummingbird species, adapted to the mountain regions, can adjust their body temperature to the ambient values ​​in their cold gate in order to save energy. The tiny birds lapse into a rigidity in the evening, from which they wake up again in the morning to go looking for food again. The details of this interesting concept are, however, not yet clear. A team led by Blair Wolf from the University of New Mexico in Albuquerque has now devoted itself to further research into the cold gate in hummingbirds.

Her focus was on six species whose distribution areas include high altitudes of the Andes. The investigations were carried out at around 3800 meters above sea level in Peru. The test animals consisted of 26 specimens of each species that were temporarily held. The researchers gave them food and then placed them in special cages at the natural night temperature. As they report, the ambient values ​​fell to 5.9 to 2.4 degrees Celsius during the test period. The scientists used tiny sensors to record the changes in body temperature with which the test animals reacted.

It was found that all six species became frozen in the course of the nights, but there were marked differences between them in the depth and duration of sleep. The temperature measurements showed that the maximum cooling rates when entering the torpor reached around 0.6 degrees Celsius per minute – the peak heating rates were again around one degree Celsius per minute. The duration of the condition varied greatly between the species: from 2.3 to just under 13 hours, the researchers report.

Cold record

In most species, the body temperature dropped to around five to ten degrees Celsius every night. The record was delivered by the shiny black-bellied tail (Metallure phoebe). These birds, about twelve centimeters tall, let their body temperature drop to 3.3 degrees Celsius. “As far as we know, these minimum body temperature values ​​measured as part of the study are the lowest that have been recorded in birds to date,” the scientists write. According to them, minimum temperatures of less than five degrees Celsius are only found in some mammal species in a state of hibernation. “The hummingbird torpor is a spectacular example of how birds adapt to extreme environmental conditions,” the researchers said.

So far, however, it remains unclear whether 3.3 degrees Celsius is already the lower limit for shiny black-bellied tails. “In the present study showed Metallure phoebe no signs of metabolic processes to maintain a lower limit value for the gate temperature, which gives the possibility that there could be lower values ​​in even colder environmental conditions ”, write Wolf and her colleagues.

Source: Biol. Lett., Doi: 10.1098 / rsbl.2020.0428

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