Sperm don't like heat, so climate change could damage male fertility, study says

The latest problem to be linked to global warming: male sperm counts.

It's not good news. According to a new study published Tuesday, male fertility appears to decline as temperatures soar. The study showed "clear evidence" that stress from heat waves reduces "sperm number and viability" in bugs.

Yes, the scientists used beetles to test their theory. But researchers say the insects can be used as a proxy for people. 

Beetles are one of the most common species on Earth, "so these results are very important for understanding how species react to climate change," said study co-author Matt Gage, an ecologist at the University of East Anglia in the U.K.

Heat waves are predicted to be more frequent and more extreme this century as human-caused climate change continues.

"Research has also shown that heat shock can damage male reproduction in warm-blooded animals too, and past work has shown that this leads to infertility in mammals,” added lead author Kirs Sales, also of the University of East Anglia.

An illustration of human sperm. Male fertility could decline as the planet warms due to climate change.  

ERAXION, GETTY IMAGES/ISTOCKPHOTO

"Our research shows that heat waves halve male reproductive fitness, and it was surprising how consistent the effect was,” he added.

In human males, the testicles make sperm and, to do this, the temperature of the testicles needs to be cooler than the inside of the body, according to the University of Rochester Medical Center.

In the study published Tuesday, the researchers found "that heat wave conditions (9 to 13 degrees above the typical high temperature for 5 days) damaged male, but not female, reproduction. Heat waves reduce male fertility and sperm competitiveness, and successive heat waves almost sterilize males," the study said.

In addition, the offspring of the dads who'd endured the heat lived shorter lives.

"When it comes to heat waves and reproduction, males can't stand it," Sales concluded.

The study was published in Nature Communications, a peer-reviewed British journal.