Study of beetles could explain global decline – and also be a warning to humankind

Heatwaves severely damage the fertility of male beetles and consecutive hot spells leave them virtually sterilised, according to research.

Global warming is making heatwaves more common and wildlife is being annihilated, and the study may reveal a way in which these two trends are linked. The scientists behind the findings said there could also be some relevance for humans: the sperm counts of western men have halved in the last 40 years.

Researchers studied beetles because their 400,000 species represent about a quarter of all known species. Insect populations are plunging worldwide as temperatures rise, falling by about 80% in 30 years in Puerto Rico’s rainforest and by 75% in German nature reserves.

Insects are such an integral part of life, as pollinators and prey, that scientists say their decline could lead to “ecological Armageddon”. Little is known about the precise causes of the decline, though climate change, habitat destruction and global use of pesticides are considered probable factors.

The research, published in the Nature Communications journal, found that exposing beetles to a five-day heatwave in the laboratory reduced sperm production by three-quarters; females were unaffected.

“Beetles are thought to constitute a quarter of biodiversity, so these results are very important for understanding how species react to climate change,” said Kris Sales, at the University of East Anglia, who led the work.

Other research has shown that heat can damage male reproduction in humans as well as cows, sheep and other mammals.

“There could be relevance for human fertility,” said Prof Matt Gage, co-leader of the UEA research group. “The paradox is that one of the reasons the climate is warming up and we are having more heatwaves is there are too many humans. So maybe this is a leveller.”

Stuart Wigby, of the University of Oxford, who was not involved in the study, said: “Given what we already know about the generality of the sensitivity of sperm to heat, there is every reason to expect that similar effects would be seen in other insects and also in mammals including humans.”

The red flour beetle used in the experiments is a tropical species that thrives at 35C. When exposed to temperatures of 5C and 7C higher, male fertility plummeted. While these were high temperatures, 90 countries had experienced them in recent years, said Gage.

After being exposed to a heatwave, the number of offspring produced by males fell by half. Even more worryingly, harmful effects were inherited by the males they produced – their lives were 20% shorter and they in turn produced fewer offspring.

The most extreme impact was in male beetles exposed to two heatwaves 10 days apart – their offspring production fell by 99%. “It kind of wipes them out,” Gage said.

Sales added: “Insects in nature are likely to experience multiple heatwave events, which could become a problem for population productivity if male reproduction cannot adapt or recover.”

Further experiments are under way to find out whether the beetles can adapt to heat shocks and to examine how other species, such as butterflies, cope with raised temperatures.

The research is among the first of its type on cold-blooded creatures. “There is a big gap, as most of life on Earth is cold-blooded and it is much more directly affected by temperature variations,” Gage said. “We’ve shown that sperm function is an especially sensitive trait when the environment heats up, and in [species] representing a huge amount of global biodiversity.”

How the heat damages the sperm is not fully understood, though the researchers noticed damage to sperm membranes. It is likely that the DNA in the sperm is also damaged, meaning harm can be passed to offspring. Most warm-blooded male animals protect sperm from excessive heat by having testes outside the body, leaving them several degrees cooler.

Wigby said the understanding of factors that reduced male fertility was poor. “So any studies that add to our understanding of the basic mechanisms underlying variation in male fertility is valuable. Showing that the harm can occur in a heatwave-like scenario is new and important, as is the demonstration of transgenerational effects. However, the applicability to the real world is always an issue for lab studies.”