2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Male red-billed weaver (Quelea quelea)
Red-billed weavers, small passerines native to Africa, were able to survive a temporary increase in body temperature of up to 48 degrees without harm to themselves. This is a record high value, which is about two degrees higher than the upper limit of the temperature tolerance of the vast majority of birds and mammals. According to ornithologists, the "superpower" of weavers is associated with life in huge flocks, which only occasionally visit a watering hole: in order to save water, birds need to reduce its costs for cooling the body. The research results are published in Scientific Reports.
Body temperatures above 45-47 degrees Celsius are lethal for most vertebrates. It disrupts the supply of oxygen to the body and leads to the degradation of large biological molecules. Few lizards are able to withstand body temperatures up to 51 degrees Celsius, while for birds and mammals the upper limit rarely exceeds 46 degrees.
A team of researchers led by Andrew McKechnie of the University of Pretoria set out to evaluate the thermal stability of red-billed weavers (Quelea quelea). These small passerines, distributed throughout tropical Africa, are considered the most numerous wild birds in the world: the total number of representatives of the species is about one and a half billion individuals.
Red-billed weavers often move in flocks of millions. This provides protection from predators, but leads to other difficulties. For example, a flock goes to a watering hole only after a sufficiently large proportion of birds encounter a lack of liquid. This means that many individuals wait for a long time to quench their thirst.
Bird watchers suggest that in such a situation, weavers temporarily cut back on fluid consumption, including for cooling the body. This reduces the risk of dehydration, but leads to an increase in body temperature. If this idea is correct, then red-billed weavers can face very high body temperatures.
To test this hypothesis, the researchers captured twenty red-billed weavers and conducted a series of experiments with them. The birds were placed in individual plastic chambers with a volume of about three liters, where atmospheric air purified from water vapor was supplied. They were set at 28 degrees Celsius and then gradually raised in four-degree increments. After reaching the forty-degree mark, each step was equal to two degrees. The maximum air temperature in the experiment was 52 degrees Celsius.
The weavers spent at least ten minutes on each step. During this time, scientists managed to measure their body temperature using an implanted transponder, as well as the loss of water with evaporation and the volume of exhaled carbon dioxide. If ornithologists saw a bird showing signs of overheating, it was immediately removed from the chamber, rubbed the feathers on its belly with alcohol (to cool it), and placed in a cage with access to water and food. After the end of the study, all the weavers were released in the same place where they were caught.
At the beginning of the experiment, the average body temperature of red-billed weavers was 40.9 degrees Celsius, which corresponds to the values typical for most small passerines. After the air warmed up to 38 degrees, the body temperature of the experimental birds began to rise, on average by half a degree, with an increase in the ambient temperature by one degree.
At an air temperature of 51 degrees Celsius, the birds warmed up to an average of 48 degrees. In some individuals, the body temperature was even higher - 49 degrees. This is 2-3 degrees above the tolerance limits of the vast majority of birds and mammals. Thus, the hypothesis that red-billed weavers can tolerate extreme temperatures without harm to their health was confirmed. It is possible that these birds rely on large-scale production of heat shock proteins, as well as the developed vascular system of the eyes, which serves to cool the brain. At the same time, the ability of weavers to cool due to evaporation turned out to be rather modest.
Interestingly, other small passerine species that live in hot, arid environments are not adapted to such high body temperatures. According to the authors, this indicates that adaptations to extreme heating, such as large-scale production of heat shock proteins, are too energy-intensive. They are probably only beneficial for birds that form large flocks. To test this assumption, experts intend to study other schooling species from arid zones - for example, budgerigars (Melopsittacus undulatus). And red-billed weavers will become an excellent model organism that will help to study the mechanisms and limits of temperature tolerance of vertebrates.
Unfortunately, not all living organisms are so tolerant to high temperatures. For example, fish embryos, as well as individuals during the spawning stage, are very vulnerable to excessive heat. If global warming continues at the same pace as it is now, by 2100, about 60 percent of marine and freshwater fish species could face temperature conditions in which they would not be able to reproduce, scientists predict.
