Exposure to extremely hot and dry conditions damages the DNA of nestling birds during their first few days of life, so they age faster, die younger and produce fewer offspring
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I recently shared the disturbing story about how some desert birds are going to be roasted into local extinction within five years, thanks to accelerating climate change (more here). It almost goes without saying that birds and mammals (including humans) who experience heat waves can die en masse. But how is global warming (global cooking?) affecting those birds that actually manage to avoid being roasted to death?
In general, birds are quite vulnerable to climate warming because they are small, they have a high metabolism, and most of them are active during the daytime. Although adult birds have a variety of behavioral and physiological strategies to reduce their chances of heat stroke and death, their hatchlings and nestlings have far fewer options because they are stuck in a nest where they are totally dependent upon others for life-sustaining care, they are experiencing rapid growth (rapid cell divisions) and their bodies are still developing. Thus, the effects of extreme heat are likely to be exaggerated in young birds and the resulting damages may persist into adulthood.
A team of scientists at Monash University investigated and found that animals exposed to hot and dry climate conditions can indeed suffer long-term health consequences (ref). The team, led by postdoctoral research fellow Justin Eastwood, came to this conclusion after examining the long-term effects of heat waves on an Endangered Australian songbird, the purple-crowned fairy-wren, Malurus coronatus (EPBC Act 2015). This recent study reports that high heat damages the DNA of very young nestlings, so they age faster, die younger and produce fewer offspring overall. But what does this DNA damage look like?
One way to understand what’s happening with the DNA of birds is to look at their telomeres. Telomeres are portions of DNA located on the ends of chromosomes. They are non-coding DNA “caps” that protect the ends of chromosomes from becoming frayed or tangled. Each time a cell divides, its telomeres become slightly shorter. When telomere lengths have been shortened beyond a critical limit, the cell can no longer successfully divide and it either shuts down or dies.
Not only do telomeres shorten as a cell ages, but shortened telomeres also act as biomarkers indicating exposure to adverse environmental conditions — an increased metabolic rate, cellular stress, dehydration, or reduction in nutrition (more here). Similarly, exposure to intensely high temperatures during early development has also been shown to shorten telomeres (ref). This DNA damage is irreversible: even after the initial stressor has ended, shortened telomeres are still short, and this telomere shortening is associated with reduced life spans as well as other fitness consequences.
There are two subspecies of purple-crowned fairy-wrens, each living in separate ranges in the northern reaches of Australia (Figure 1). The study population of purple-crowned fairy-wrens consists of roughly 300 individuals that reside at the Australian Wildlife Conservancy’s Mornington Wildlife Sanctuary, where they have been the focus of ongoing long-term ecological research since 2005, making them an ideal study subject.
“Our long-term study has followed the birds from hatching and observed everything we could about their reproductive behavior until death”, Dr Eastwood pointed out.
Formerly known as Mornington Station, this 3,582 square kilometer (1,383 square mile) nature reserve includes the upper catchment of the Fitzroy River and portions of the King Leopold Ranges located in the Kimberley region of northwest Western Australia.
Purple-crowned fairy-wrens are tiny (∼11 g) insectivorous songbirds. They are highly social, living in groups composed of a dominant breeding pair and up to nine of their offspring from previous nests. These birds breed cooperatively where subordinates help raise their younger siblings and defend the nest against predators. Nests contain from one to four nestlings, and the young birds reach sexual maturity at one year of age. Purple-crowned fairy-wrens live in dense vegetation alongside riverbanks, which they defend from intruders.
Purple-crowned fairy-wrens live in a tropical monsoon climate where most of the annual rainfall occurs during a short rainy season (November to April). At the same time, air temperatures often exceed 45°C (113°F). Although they do nest throughout the year, their breeding efforts reach a peak during this annual monsoonal wet season.
To conduct this study, Dr Eastwood and his collaborators collected a tiny blood sample from the wing veins of week-old nestling purple-crowned fairy-wrens. The team measured the nestlings’ telomere lengths and compared them to ambient air temperatures. Dr Eastwood and his collaborators found that nestlings exposed to hot, dry conditions where maximum ambient air temperatures exceeded 31℃ (88°F) during their first few days of life had shorter telomeres than those that were exposed to cooler maximum temperatures. (Curiously, this study also found that nestlings appeared to tolerate extreme heat better when it coincided with rain.)
Using these data, the researchers then constructed a mathematical model to simulate the effects of nestling telomere shortening upon their future reproductive success.
“We found that even under relatively mild climate warming scenarios that the non-lethal effects on nestling telomere length alone could result in population decline”, Dr. Eastwood said (Figure 3).
They found that nestling telomere length acted as a biomarker of lifespan and the number of offspring they have over their lifetime.
“But in contrast, the math also showed that breeding during wetter conditions or evolving longer telomeres could possibly mitigate the effect”, Dr Eastwood added.
Realistically, these ‘escape scenarios’ are highly unlikely given the speed at which the climate is warming. The number of rainy days in the region is predicted to rapidly decline, and the birds already time their major breeding efforts to coincide with rain. Further, extremely hot and dry conditions are predicted to become even more common throughout Australia.
Moreover, no one really understands how telomeres evolve, so preventing DNA damage caused by soaring temperatures by evolving longer telomeres — especially in a few short years — is doubtful.
These ominous findings suggest that purple-crowned fairy-wrens — which are already Endangered and declining — will be living under a constant and growing threat from global temperature increases and more frequent and more deadly heat waves. This study serves to highlight the urgent need to include hidden long-term impacts, such as irreversible DNA damage, of climate warming in conservation actions alongside habitat loss and lethal heat waves.
This deeply worrying study documents how the costs of climate warming can be subtle and difficult to detect, whilst at the same time, its effects upon biodiversity are powerful and potentially unalterable.
The team are already busy with more studies. The team is investigating whether females can discern which microhabitats are coolest so they select the best sites for their nests so their nestlings are shielded from the worst intense heat. The researchers also are looking at the effects of habitat management practices on the number and quality of microhabitats that might be best suited for nests. Ultimately, Dr Eastwood and his collaborators are hopeful that their research will better inform the design of conservation strategies of this iconic Australian bird and other animals before they go extinct.
Justin R. Eastwood, Tim Connallon, Kaspar Delhey, Michelle L. Hall, Niki Teunissen, Sjouke A. Kingma, Ariana M. La Porte, Simon Verhulst, and Anne Peters (2022). Hot and dry conditions predict shorter nestling telomeres in an endangered songbird: Implications for population persistence, Proceedings of the National Academy of Sciences of the United States of America 119(25)e2122944119 | doi:10.1073/pnas.2122944119
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