Male Insect Fertility Plummets After Heat Waves

Karen Weintraub, The New York Times, Nov. 13, 2018

Researchers say that temperature’s effects on sperm production could be one factor behind the decline in the world’s insect populations

Image from article, with caption: A simulated heat wave in a lab caused sperm production in a type of beetle to drop by half, and a second heat wave nearly sterilized them.

For years, insect populations have been dropping worldwide without a clear explanation. A new paper suggests male infertility is at least one factor behind that decline, as warmer-than-usual temperatures take a disproportionate toll on males of some insect species.

After a lab-simulated heat wave, researchers from England’s University of East Anglia found that male flour beetles produced vastly less sperm. But they also found that the damage wasn’t confined to the males. Sperm inside a female’s reproductive tract became less viable and the sons of the males that endured the hotter temperature became less fertile, too.

Matt Gage, an evolutionary ecologist who led the work published Tuesday in Nature Communications, said he was surprised by the findings, and by how quickly male fertility plummeted.

It’s long been known that heat can affect sperm quality in mammals. “There’s a good reason the testes are outside the body,” noted Dr. Gage, adding that this keeps the sperm 6 to 10 degrees cooler than body temperature.

But no one had previously looked to see whether coldblooded males were also affected, even though most of life on Earth is coldblooded, he said. And the flour beetles that Dr. Gage studied are used to warm environments, living in places that regularly reach 95 degrees Fahrenheit.

His team simulated a heat wave in their lab, raising temperatures by about 9 to 12 degrees Fahrenheit for five days — roughly equivalent to a temperature spike in England last summer, Dr. Gage said.

After the heat exposure, sperm production in the flour beetles dropped by half, the study showed. A second heat wave nearly sterilized them.

“We thought they might have hardened to temperature extremes,” Dr. Gage said. “We found the opposite.”

Females appeared unaffected by the heat themselves. But if they had already been inseminated, their fertility fell by 30 percent after the heat exposure. This suggests that heat affects not just the manufacture of sperm, but also its later viability, Dr. Gage said.

The sons of the males who endured the heat wave produced 20 percent fewer offspring than males that hadn’t undergone that stress. Dr. Gage said he’s not sure whether the sperm suffered DNA damage from the heat wave that could not be repaired, or if changes on top of the DNA — so-called epigenetic changes — affected their sons’ fertility.

“The transgenerational effects are exciting and scary,” said Scott Pitnick [JB note: Will he be the renamed professor in a remake of My Fair Lady"?:)], a professor of biology at Syracuse University in New York, who was not involved in the research. He added that sperm with DNA damage might still be able to fertilize females and yield offspring.

Dr. Gage said he thinks that such heat response may account for at least some of the population decline seen in insects.

A 2017 study found that the population of flying insects fell by 75 percent over 27 years in German nature preserves.

Curt Stager, a professor at Paul Smith’s College in Paul Smiths, New York, said he’s not ready to accept the idea that global climate change is the primary cause of this drop.

“Global-scale insecticide usage is, to me, a more convincing cause for a widespread, across-the-board insect decline,” Dr. Stager said. Heat waves, he said, have not occurred universally enough to cause widespread decline.

Dr. Pitnick said that given how sensitive sperm is to temperature change, and that coldblooded insects are less able to protect their sperm, it was smart to “explore detrimental effects on sperm as a candidate cause of widespread decline of invertebrate populations in response to climate change.”