Green fluorescent protein, or G.F.P., highlights sperm in the female reproductive tract following mating with control (left) and male subject to heat wave (right). (Matthew Gage and Martin Taylor)

One of the ways that heat kills is by increasing pressure in the skull, constricting blood flow to the brain. Damaged tissue can also enter the bloodstream and cause kidney failure. At a certain point, an elevated internal temperature simply incinerates cells in the body.

In contrast to extreme weather events so visible and violent that they hardly escape public notice, such as hurricanes and tornadoes, heat waves are more of a “silent killer,” as the National Weather Service has called the prolonged periods of hot weather.

But kill they certainly do. Heat fatalities in the United States exceed all other weather-related deaths in the 30 years since such data has been available. In the U.K., Parliament’s Environmental Audit Committee recently warned of 7,000 annual deaths by heat by 2050 unless quick action is taken, the need for which was underscored by last month’s Intergovernmental Panel on Climate Change report.

Heat doesn’t just kill. It is also diminishes the vitality of sperm, curtailing the capacity to reproduce, as scientists have documented.

“Heatwaves reduce male fertility and sperm competitiveness, and successive heatwaves almost sterilise males,” wrote the authors of a study published Tuesday in the peer-reviewed Nature Communications.

But the research points newly to an even longer-lasting effect. Ecologists and evolutionary biologists at the University of East Anglia in Norwich, England, found that heat stress appears to be associated with transgenerational fertility problems.

That means that organisms may bear the effects of elevated temperatures long after the initial exposure — in the form of reduced lifespans, reproductive challenges and other types of defects passed to offspring.

The scientists found that heat waves undermine sperm production and viability, and also interfere with movement through the female. They further discovered that extreme heat “reduced reproductive potential and lifespan of offspring when fathered by males, or sperm, that had experienced heatwaves.”

The researchers used red flour beetles to test sensitivity to temperature in cold-blooded ectotherms, species that don’t regulate their own body temperature, in contrast to endotherms, such as humans. Warm-blooded mammals have been the primary focus of existing research on warming and sperm quality, their paper noted.

They set out to fill this gap, while nevertheless finding applications to the human case. Most terrestrial species are insects, and most life on Earth is cold-blooded, making the findings especially relevant to the question of climate change’s effect on biodiversity. As the lead author, Matthew Gage, observed in an accompanying blog post on the study, “we know that insect numbers are crashing, but we understand remarkably little about the particular mechanisms driving population declines.”

To examine one possible mechanism, reproduction, the scientists exposed mature adult beetles to experimental heat waves lasting five days at about 40 to 42 degrees Celsius, or about 104 to 107.5 degrees Fahrenheit — above their optimum by about 10 degrees. “It’s worth noting that these temperatures have been exceeded in the natural environment in half the world’s countries over recent years,” Gage wrote. Each male was paired with a female for 15 minutes, before being transferred to the next mate.

Male reproductive reproductive performance halved after a first heat wave. After a second, males became almost completely sterile — contradicting theories of acclimation or hardening as a response to environmental stress.

Meanwhile, female potential was unchanged. However, inseminated sperm already within the female tract were vulnerable to elevated heat, and caused a reduction in female fertility by one-third.

Most surprising, Gage told The Post, was the effect they observed across generations.

“It suggests there could be problems for sons,” he said. “We know that in humans, heat can damage sperm DNA, and we know that men with damaged DNA in their sperm have problems with fertility, but you can’t really do an experiment to heat males up and look at whether that damages human offspring performance, so this is one way to get at that.”

The results, he said, primarily indicate problems with fertility — with the clearest implications for insect biodiversity — but there is also evidence of an “underlying, longer-term damage as a result of damaged sperm DNA.”

“In much the same way that radiation causes damage, and that can lead to offspring problems, there could be that kind of damage operating as a consequence of heat conditions,” Gage said. “You’re looking at possible population viability problems, which need to be studied more."

When it comes to humans, not all populations will be equally affected, studies have shown. The elderly, low-income people, those who are immobile or have other preexisting health issues, are especially vulnerable.

While the results of the experiment don’t offer conclusions about population viability, they offer insight into how a particularly sensitive trait, sperm function, reacts to heat waves, which scientists say will continue to be among the severe effects of global climate change.

“If sperm function goes down, reproduction goes down,” Gage said. “If reproduction goes down, you’re looking at population viability problems.”

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