1. How Six Andean Hummingbird Species Cooled Differently on the Same Cold Mountain Night
At 3,800 m in the Andes, scientists measured the body temperature of 26 hummingbirds from six species. The night air temperature ranged from 2.4°C to 5.9°C (36°F to 43°F). All six species entered a deep state of rest called torpor. However, the way they controlled their body temperature differed by species. [1]
One species, *Colibri coruscans*, appeared to maintain its body temperature at about 8°C, even as the air got colder. It acted like a room with a thermostat set to a specific temperature. Another species, *Metallura phoebe*, let its body temperature drop along with the air temperature. The difference between its body temperature and the air temperature was only 0.87°C on average. Its lowest average body temperature was 5.13°C. This bird acted like a room with the heater turned off, cooling down to match the outside. [1]
It is important to note that birds do not have a simple switch for this. These are estimates based on observations. The question remains: why do different species on the same mountain cool down in different ways? [1]
2. Why Lowland Brazilian Hummingbirds Kept Their Body Temperature Steady While Resting
In southeastern Brazil, at an elevation of about 700 m (2,300 ft), eight species of hummingbirds were studied. During their nightly rest, these birds held their body temperature at 12–20°C (54°F to 68°F). If the air temperature dropped below this range, their bodies produced heat to keep their temperature up. [2]
The way they enter this state is also interesting. The drop in their metabolism seems to happen passively as their body cools down. It is thought that they do not need to actively suppress their metabolism to start torpor. Instead, they stop producing heat, and the body cools. When the body temperature gets close to the target range, they start producing heat again to stop the cooling. [2]
These results are from birds at 700 m. It does not mean all lowland tropical hummingbirds stop at 12–20°C. In Panama, another study found that six species entered torpor even when the air temperature was above 28°C (82°F). [3] So, the temperature they hold differs between the Andes (about 8°C) and Brazil (12–20°C). Is this difference due to altitude, or is it due to the species? [2]
3. Does Body Temperature at Night Depend on the Mountain Air or on the Species?
If cold air alone determined body temperature, there would be no differences between species on the same mountain. But there were differences. Researchers found that evolutionary history, or how closely related the species are, plays a significant role. The influence of family ties on minimum body temperature was measured at 0.620. [1]
Three species in the "coquette" group (*Oreotrochilus melanogaster*, *Polyonymus caroli*, and *Metallura phoebe*) tended to have lower body temperatures and longer periods of torpor. The conclusion is that while deep torpor is common in high-Andean hummingbirds, inherited differences between species affect how deep it gets. [1]
Other records support this. *Oreotrochilus estella* has been reported at about 6.5°C, with one individual at 5.0°C. *O. melanogaster* and *M. phoebe* hold the records for the lowest temperatures in hummingbirds. Generally, a bird's body temperature during torpor gets close to the cold night air. [1]
However, this was not an experiment comparing the same species at high and low altitudes. It is difficult to separate the effect of altitude from the effect of evolution. We cannot say it is decided only by altitude or only by evolution. [1]
4. How Body Size Affects How Deep and How Long Panama Hummingbirds Cool Down
In the lowlands of Panama, 37 birds from six species were studied. All species used torpor, even at temperatures above 28°C. Body weight was the best predictor of how deep and how long the torpor lasted. Smaller birds went into deeper and longer torpor. [3]
But size alone is not the only factor. In a study of 249 birds from 29 species in the western Andes of Colombia, across a height difference of 1,920 m, smaller birds were more likely to use torpor than larger birds. However, this was only when the air temperature was low, and in those cases, the torpor lasted longer. The use of torpor changes with the bird's physical condition and its family lineage. [4]
Smaller birds likely have a higher cost to maintain their body temperature in the cold, so they gain more benefit from torpor. This is a guess, not a direct fact from the data. With body size, air temperature, and lineage all playing a part, it is hard to explain the depth of torpor with just one cause. [3]
5. Why How Long a Hummingbird Stays Cool Matters More Than How Deep It Cools
Returning to the first question: if cooling deeper saves more energy, why do high-altitude and lowland birds save similar amounts? Researchers reported that the overnight weight loss, which indicates energy savings, was similar for both groups. The authors suggest that in colder places, the cost of warming the body back up in the morning is higher. [1]
Let's look at the numbers. The fastest rate of rewarming was about 1 to 1.5°C per minute. To raise body temperature by 30°C at this speed would take 20 to 30 minutes. This is a calculation for this article; actual speeds vary. [1]
It seems that the length of torpor is related to energy saving. Overnight weight loss was negatively correlated with the length of torpor when body temperature was below 30°C. The duration varied from 5.7 to 10.6 hours on average for species, and from 2.3 to 12.9 hours for individuals. Birds that stayed cold longer lost less weight. [1]
In the Panama lowlands, overnight weight loss was about 8 to 10%. This was larger than the about 4% reported in high-altitude studies. They suggest that when nights are warm, the benefit of torpor may be limited. Since these studies are separate, these numbers are rough estimates. [3]
6. What Scientists Still Do Not Know About How Hummingbirds Control Deep Night Cooling
We have looked at set points, lineage, body size, and temperature as factors that influence how deep torpor gets. However, because there is no data comparing the same species at high and low altitudes, we cannot determine exactly how much each factor contributes. [1]
The paper by Wolf et al. is available in a university repository. You can look at the graphs showing the relationship between body temperature and air temperature for each species. Compare the difference between *Colibri coruscans* and *Metallura phoebe* with your own eyes. Comparing this with the 12–20°C range from the Brazil study makes the idea of "holding a temperature" more concrete. [1]