Offspring pay the price when their parents overheat

In our latest Insight, Andreas Nord talks about hardworking parents, cooling birds and the dangers of overheating. You can also read his recent Functional Ecology paper here.

Andreas Nord. Photo by Henrik Högström
Andreas Nord. Photo by Henrik Högström

What determines how hard animals can work?

Researchers have always been interested in what sets the limit on how hard animals are able to work when caring for their young. This traditionally focused on the limits set by the amount of food animals could gather from their environment, and how successfully they could digest and extract nutrients for subsequent delivery to different tissues and organs in the body. There is much support for such limits in the literature, but over the last decade, a range of studies on captive rodents presented evidence that was not compatible with any environmentally enforced or animal-specific constraints on the capacity for work.


A new theory emerges

Rodents were traditionally believed to work at their maximum capacity when producing milk for their pups because, in response to experiments enlarging litter size, females could not increase food intake, milk production or energy content of the milk, which ultimately leads to decreased pup condition in large litters. But when the animals were allowed to breed at a lower than normal temperature, they could suddenly increase both intake of food and production of milk beyond levels that, at normal temperature conditions, appeared limiting. Letting them breed at above-normal temperatures produced the opposite results.

These observations eventually led to the formation of the heat dissipation limit (HDL) theory, which states that an animal is limited in its work output by the rate at which it can dissipate metabolic heat produced as a by-product of metabolism. The HDL theory subsequently gained experimental support from a range of studies that found that experimental facilitation of heat dissipation rate, such as shaving the back of the animal, allowed females to increase their work load. Conceptually, the idea that animals should balance how hard they work against the risk of overheating makes sense, because above-normal body temperature can be very harmful to integrity of the body.


Does the risk of overheating matter for wild animals?

There are few data exploring how the risk of overheating affects animals’ capacity for work in the wild, where temperatures in many parts of the world is often lower than those in indoor laboratory conditions. In a recent study of our own (Nilsson, J.-Å. & Nord, A. Proceedings of the Royal Society of London B, 285, 20180652, doi: 10.1098/rspb.2018.0652), we found that marsh tits that were experimentally manipulated to work harder sired smaller offspring and maintained higher body temperatures when feeding their young. The temperatures recorded were sometimes over 45 °C, which is several degrees above normal. This suggests that hard-working birds may be prone to overheating, and that the risk of incurring too high a body temperature may affect the effort put into parental care.

blue tit female before feather clipping
Blue tit female before feather clipping. Photo by Andreas Nord

We wanted to extend this study by directly manipulating the parents’ ability to rid themselves of excess heat. We experimentally removed some of the feathers covering the abdomen and pectoral muscles in nestling-feeding blue tits. This species was excellently suited for the purpose because they produce large broods (around 15 nestlings per pair is not uncommon), which requires parents to deliver food to the nest several hundred times a day.

Blue tit female before feather clipping.
Blue tit female before feather clipping. Photo by Andreas Nord

Once we had trimmed some of their plumage away, we attached a small transponder to the leg of the birds to measure how often they visited their nests when delivering food to nestlings. We then left the nests undisturbed until the chicks were close to fully grown, at which point we recaptured parents to measure their body mass and body temperature, and also measured and weighed the nestlings to see if any detected increase in parental effort would impact their offspring.


So what happened?

Blue tit nestling at two weeks of age.
Blue tit nestling at two weeks of age. Photo by Andreas Nord

Parents that were experimentally manipulated for improved capacity to dissipate heat had a lower body temperature. They also lost less body mass during the study. This suggests that blue tits are prone to overheating when they work hard and that, when this constraint is lifted, birds can maintain their body condition to a larger extent. Higher body mass at the end of the breeding season means birds go into the energetically demanding period of replacing body feathers in better condition. Not only that, but nestlings sired by parents with part of the plumage removed grew larger, and sometimes heavier. These results support the predictions one would make if the working capacity of a bird was limited by how fast it could rid itself of heat.

Interestingly, birds experimentally manipulated to stay cooler did not deliver food at a faster rate than unmanipulated birds. This surprised us greatly, because flight is a costly, heat-generating means of transport, and we speculated that relieved constraints on overheating risk would translate into increased feeding rates. Now, instead, we believe that these birds took advantage of the relieved thermal constraints to forage more effectively – and either brought more food to the nest on each visit or only the very best food items.


Why is this study important?

We believe this study is important because it shows that birds ecology and life-history can be affected by their ability to withstand risks of overheating, even in an environment that is relatively cold. In a world likely to experience increased frequency of extreme weather events, such as the 2018 heat wave that swept across Europe, the reproductive success of hard-working animals may be impaired. This could result in behavioral, morphological or physiological adaptations to mitigate any such negative effects, and might even lead to fewer offspring.

In summary, there is no shortage of reasons to keep investigating how animals deal with high temperature.


Andreas Nord is based at Lund University, Sweden. Read Nord and Nilsson’s new paper, Heat dissipation rate constrains reproductive investment in a wild bird in Functional Ecology.


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