Hummingbirds have Excellent Control over their Body Temperatures

Hummingbirds have a wide range of unique traits, including one of the largest brain-to-body size ratios known and a vast species-specific diversity of vocalizations, some of which reflect local dialects. With the discovery of a cerebral vocal system in hummingbirds that is not found in non-vocal learning species, the solution appears to be in the patterning of neuron connectivity rather than total brain size.

Scientists from several colleges have discovered that there are two levels of hummingbird torpor: shallow and deep, as well as a transition stage between levels of torpor and the usual sleep state.

To meet the aerobic demand of hovering, the hummingbird possesses the biggest heart of any bird in relation to its size. In comparison to the mouse, which has a heart that accounts for 0.5 percent of its body mass, its heart accounts for 2.4 percent. It can attain a heart rate of over 1000 beats per minute while hovering.

Hummingbirds substantially reduce their body temperature and metabolism at night by entering an energy-saving condition of inactivity known as torpor. Scientists from several universities have discovered that there are two levels of torpor: shallow and deep, as well as a transition stage between levels of torpor and the typical sleep state. Their research was published in the Journal of Experimental Biology.

Hummingbirds substantially reduce their body temperature and metabolism at night by entering an energy-saving condition of inactivity known as torpor. In our investigation, we discovered that the tiniest bird used deep torpor every night. The larger birds utilize deep torpor at times, shallow torpor at others, and no torpor at all!

Anusha Shankar

“There were a few signs that this ability to fine-tune thermoregulation was feasible,” said lead author Anusha Shankar, a Rose Postdoctoral Fellow at the Cornell Lab of Ornithology. “However, the tests were conducted in a laboratory, not in the field, where a bird might experience different situations. It was thrilling to discover that hummingbird torpor may be varied and adaptable.”

Shankar and colleagues from Stony Brook University, the Swiss Federal Research Institute, and George Fox University tracked the body temperatures of three hummingbird species in Arizona: the Blue-throated Mountain-gem, Rivoli’s Hummingbird, and Black-chinned Hummingbird. They measured the warmth emitted by the skin around the birds’ eyes. The disparities in heat generation at different stages are striking.

Hummingbirds exert fine control over body heat

Even in colder climates, a hummingbird’s usual daytime body temperature is more than 100 degrees Fahrenheit. Their body temperature dips by around 20 degrees Fahrenheit during shallow torpor. The bird in deep torpor maintains a body temperature 50 degrees Fahrenheit lower than its normal daylight temperature. If our body temperature dropped just 3 degrees Fahrenheit from the normal 98.6 degrees Fahrenheit, we’d be in hypothermia and, unlike hummingbirds, would require outside assistance to warm up again. Size is also important.

“In our investigation, we discovered that the tiniest bird used deep torpor every night,” Shankar explained. “The larger birds utilize deep torpor at times, shallow torpor at others, and no torpor at all!”

Shallow torpor may have evolved to balance the benefits of energy savings with the drawbacks of deep torpor. A hummingbird in deep torpor is undoubtedly more susceptible to sickness because its immune system shuts down, and it is also prone to predation and sleep deprivation. This ability to use torpor at varied depths suggests that at least some birds are capable of fine-tuning how chilly they get in torpor and how they balance energy savings with the possible consequences of dropping deep into torpor.

Torpor is not synonymous with sleep. Sleep consumes significantly more energy and performs numerous restorative activities. Daily torpor differs from hibernation as well. Hibernating animals go into a low-energy state for weeks or months at a time, whereas hummingbirds can sleep every night.

Many concerns remain regarding what happens in the brain during torpor, how body fat levels may cause torpor, and how hummingbirds create heat to warm up again – a process that can take up to 30 minutes.

“At least 42 bird species employ torpor,” Shankar explained, “but only hummingbirds, nightjars, and one mousebird species descend into deep torpor.” The study of torpor’s range could help us understand the evolution of thermoregulation in birds.”

The mass-specific metabolic rate of hummingbirds is the highest of any homeothermic animal. When food is short and they are not foraging, they can enter torpor, a state comparable to hibernation, and lower their metabolic rate to one-fifth of its regular pace.