Australia’s hot conditions, already famous for its extremes, is only set to intensify with climate change. Recognising the potential implications for reproductive health, researchers at the University of Newcastle and HMRI’s Infertility and Reproduction Research Program, Dr. Natalie Trigg, Dr. John Schjenken, and Professor Brett Nixon are investigating how heat stress impacts sperm.  
 
Their findings, recently published in PNAS, focused on what happens to sperm after they have left the testes during heat wave conditions of 35 degrees during the day and 25 degrees overnight. 

Dr Trigg says, “We know that heat affects sperm development in the testes but we were interested in finding out what happens to sperm during their subsequent maturation in the male reproductive tract (i.e. epididymis).  

“We found that while the emulated heat wave conditions didn’t affect the ability of sperm to swim or to fertilise an egg in laboratory models, there were changes to the small RNAs that the heat-exposed sperm carry” says Dr Trigg.  

What does this mean if the sperm goes on to fertilise an egg?

Sperm carry small RNAs that are delivered to the egg, which help control how genes are expressed in the embryo and influence the health of the offspring.

In the heat model, higher temperatures cause changes to embryonic gene expression, faster development before implantation, altered hatching of the blastocyst (a key stage in early development), and a higher fetal-to-placental weight ratio, which indicates better placental efficiency.

These results show that even small changes in temperature can impact male reproductive function, highlighting how sensitive sperm small RNAs are to environmental stress.

Practical takeaways for men and couples 

Sperm production is vulnerable: Sperm production, essential for reproduction, is highly sensitive to environmental stress like heat. 

Impact of heatwave conditions: Simulating heatwave conditions (35°C during the day and 25°C at night for 7 days) showed no visible damage to reproductive organs, but sperm were still affected in important ways. 

Changes in sperm’s RNA: Heat exposure altered the small RNA in sperm, which plays a key role in regulating genes during early embryo development. 

Effects on embryos: Sperm from heat-exposed males caused embryos to: 

• Develop faster than normal early on. 

• Have issues with blastocyst hatching, a crucial step for implantation. 

• Show a higher fetal-to-placental weight ratio, which could affect how nutrients are delivered to the fetus. 

 
Proof from RNA injection: Injecting altered RNA from heat-exposed sperm into normal embryos caused the same developmental changes, proving the direct impact of heat stress on sperm RNA. 

 
Broader risks: Rising temperatures and more frequent heatwaves due to climate change could threaten the reproductive health of humans and animals, even with relatively small temperature increases. 

What’s Next in Research? 

“We know that paternal preconception health has an impact on the health of the offspring so the next steps will be exploring strategies to protect sperm during heat wave conditions, as well as understanding how quickly the small RNAs revert to normal once the heat episode ends,” says Dr Trigg.  

Why It Matters 

This research serves as a reminder of the interconnectedness of environmental health and human health. As global temperatures rise, understanding and mitigating the effects of heat stress on fertility will be vital—not just for individuals and families, but for broader public health. 

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