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Heatwaves significantly impact male fertility, says huge study

As the world gets hotter, men may have fewer and fewer viable sperm

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  • New research on beetles shows that successive exposure to heatwaves reduces male fertility, sometimes to the point of sterility.
  • The research has implications both for how the insect population will sustain itself as well as how human fertility may work on an increasingly hotter Earth.
  • With this and other evidence, it is becoming clear that more common and more extreme heatwaves may be the most dangerous aspect of climate change.

When we hear that climate change causes "extreme weather events," our minds jump to hurricanes ripping palm trees out of the ground or snowstorms burying cars. But one of the most dangerous kind of extreme weather event is much less visible than, say, a tornado tearing the roof of a house. Heatwaves kill more people than all other weather events combined, and they're on the rise.

During the summer of 2003, an estimated 70,000 Europeans died when a deadly heatwave swept through the region. As the globe continues to warm, 150 Americans are projected to die every summer day by the mid-2040s. Aside from directly causing death from heatstroke, dehydration, other heat-related conditions, high heat also weakens cognition. Violent crimes, too, tend to spike on hotter-than-average days.

Now, a new study has uncovered another threat posed by heatwaves. It turns out the prolonged exposure to high heat reduces male fertility, and repeated exposure to successive heatwaves can even cause sterility.

Studying reproductive performance under heatwave conditions

This image shows the temperature anomalies across Europe during the devastating 2006 heatwave.

Wikimedia Commons

The study, recently published in Nature, examined the sperm viability of red flour beetles under heatwave conditions. In a corresponding blog post, study co-author Matthew Gage wrote, "a wealth of research since the start of last century, mainly on warm-blooded mammals (including humans), has shown that environmental or experimental warming by even a few degrees can cause declines in sperm quality and male ability to fertilise."

He points out that most of these studies have been conducted on endothermic, or warm-blooded, animals, but very few studies have been conducted on exothermic, or cold-blooded, animals like insects. Given that the vast majority of animals are ectothermic and that insects serve as food or pollinators for many animals and plants, understanding how climate change affects these creatures is critical to preserving our ecosystem. What's more, studying sperm viability in insects serves as a model for how heatwaves will affect other animals—like humans.

What did they find?

The researchers exposed red flour beetles (like this one) to high-heat conditions to observe their reproductive performance. (Wikimedia Commons)

The researchers exposed their beetles to heatwave-like conditions over the course of five days. Male red flour beetles like temperatures at about 35 degrees Celsius, or 95 degrees Fahrenheit. The experiment exposed them to an environment that was 42 degrees Celsius, or 107.5 degrees Fahrenheit. Depending on where you live, that may sound like an unlikely temperature, but the authors noted that over 90 countries experience heatwaves in that temperature range or higher.

After their extended sauna session, the beetles were given the chance to mate with a female of the species. Compared to male beetles in the control group, the heat-exposed beetles didn't do so well. They produced 75% less sperm, and only one third of their sperm was viable. Overall, the heat-exposed beetles' reproductive performance was cut in half.

But nature is adaptable, right? One might expect the beetles to eventually get over the initial shock of their new, hot environment and perform better. This research showed that, if anything, the opposite is true, at least in the short term. A second exposure to heatwave conditions made the beetles almost completely sterile.

It gets worse; not only did the heat-exposed male beetles do worse than their cooler peers, their male offspring were also less fertile than the offspring of the control beetles. Even when this new generation of beetles was not exposed to heatwave conditions, they had a 25% reduction in mating success. What's more, they lived significantly shorter lives than the control beetles, too.

We've known for a while that certain environmental conditions can affect the health and fertility of offspring, but usually this is in response to irradiation, exposure to toxic chemicals, or stress. When an animal is exposed to an unhealthy environment, the genetic material contained in their sperm becomes damaged, hamstringing the next generation's ability to reproduce. This research suggests that exposure to high heat damages offspring's reproductive ability in the same way.

The environmental impact

While this study looked specifically at beetles, evidence suggests heatwaves will affect mammals in much the same way. Exposing mice to hotter temperatures reduced fertility by 75%, for instance. Overall, "this work reveals that sperm are very sensitive to heat," writes Gage. "Environmentally-relevant experimental heatwaves damage sperm function, leading to reduced fertility and a decline in offspring performance."

If this effect is widespread enough, it could throw a serious monkey wrench into the inner workings of our ecosystem. The importance of insects in the diets of large animals cannot be overstated. As the globe continues to warm, insect populations are crashing. One study in Germany found that the local insect population had dropped by 78% in just 24 years, which appears to be part of a global trend. While it can't be said that this is entirely due to increasingly common and extreme heatwaves, this research suggests that they may play a major role.



Neom, Saudi Arabia's $500 billion megacity, reaches its next phase

Construction of the $500 billion dollar tech city-state of the future is moving ahead.

Credit: Neom
Technology & Innovation
  • The futuristic megacity Neom is being built in Saudi Arabia.
  • The city will be fully automated, leading in health, education and quality of life.
  • It will feature an artificial moon, cloud seeding, robotic gladiators and flying taxis.
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Human brains remember certain words more easily than others

A study of the manner in which memory works turns up a surprising thing.

Image Point Fr / Shutterstock
Mind & Brain
  • Researchers have found that some basic words appear to be more memorable than others.
  • Some faces are also easier to commit to memory.
  • Scientists suggest that these words serve as semantic bridges when the brain is searching for a memory.

Cognitive psychologist Weizhen Xie (Zane) of the NIH's National Institute of Neurological Disorders and Stroke (NINDS) works with people who have intractable epilepsy, a form of the disorder that can't be controlled with medications. During research into the brain activity of patients, he and his colleagues discovered something odd about human memory: It appears that certain basic words are consistently more memorable than other basic words.

The research is published in Nature Human Behaviour.

An odd find

Image source: Tsekhmister/Shutterstock

Xie's team was re-analyzing memory tests of 30 epilepsy patients undertaken by Kareem Zaghloul of NINDS.

"Our goal is to find and eliminate the source of these harmful and debilitating seizures," Zaghloul said. "The monitoring period also provides a rare opportunity to record the neural activity that controls other parts of our lives. With the help of these patient volunteers we have been able to uncover some of the blueprints behind our memories."

Specifically, the participants were shown word pairs, such as "hand" and "apple." To better understand how the brain might remember such pairings, after a brief interval, participants were supplied one of the two words and asked to recall the other. Of the 300 words used in the tests, five of them proved to be five times more likely to be recalled: pig, tank, doll, pond, and door.

The scientists were perplexed that these words were so much more memorable than words like "cat," "street," "stair," "couch," and "cloud."

Intrigued, the researchers looked at a second data source from a word test taken by 2,623 healthy individuals via Amazon's Mechanical Turk and found essentially the same thing.

"We saw that some things — in this case, words — may be inherently easier for our brains to recall than others," Zaghloul said. That the Mechanical Turk results were so similar may "provide the strongest evidence to date that what we discovered about how the brain controls memory in this set of patients may also be true for people outside of the study."

Why understanding memory matters

person holding missing piece from human head puzzle

Image source: Orawan Pattarawimonchai/Shutterstock

"Our memories play a fundamental role in who we are and how our brains work," Xie said. "However, one of the biggest challenges of studying memory is that people often remember the same things in different ways, making it difficult for researchers to compare people's performances on memory tests." He added that the search for some kind of unified theory of memory has been going on for over a century.

If a comprehensive understanding of the way memory works can be developed, the researchers say that "we can predict what people should remember in advance and understand how our brains do this, then we might be able to develop better ways to evaluate someone's overall brain health."

Party chat

Image source: joob_in/Shutterstock

Xie's interest in this was piqued during a conversation with Wilma Bainbridge of University of Chicago at a Christmas party a couple of years ago. Bainbridge was, at the time, wrapping up a study of 1,000 volunteers that suggested certain faces are universally more memorable than others.

Bainbridge recalls, "Our exciting finding is that there are some images of people or places that are inherently memorable for all people, even though we have each seen different things in our lives. And if image memorability is so powerful, this means we can know in advance what people are likely to remember or forget."

spinning 3D model of a brain

Temporal lobes

Image source: Anatomography/Wikimedia

At first, the scientists suspected that the memorable words and faces were simply recalled more frequently and were thus easier to recall. They envisioned them as being akin to "highly trafficked spots connected to smaller spots representing the less memorable words." They developed a modeling program based on word frequencies found in books, new articles, and Wikipedia pages. Unfortunately, the model was unable to predict or duplicate the results they saw in their clinical experiments.

Eventually, the researchers came to suspect that the memorability of certain words was linked to the frequency with which the brain used them as semantic links between other memories, making them often-visited hubs in individuals's memory networks, and therefore places the brain jumped to early and often when retrieving memories. This idea was supported by observed activity in participants' anterior temporal lobe, a language center.

In epilepsy patients, these words were so frequently recalled that subjects often shouted them out even when they were incorrect responses to word-pair inquiries.

Seek, find

Modern search engines no longer simply look for raw words when resolving an inquiry: They also look for semantic — contextual and meaning — connections so that the results they present may better anticipate what it is you're looking for. Xie suggests something similar may be happening in the brain: "You know when you type words into a search engine, and it shows you a list of highly relevant guesses? It feels like the search engine is reading your mind. Well, our results suggest that the brains of the subjects in this study did something similar when they tried to recall a paired word, and we think that this may happen when we remember many of our past experiences."

He also notes that it may one day be possible to leverage individuals' apparently wired-in knowledge of their language as a fixed point against which to assess the health of their memory and brain.

Does conscious AI deserve rights?

If machines develop consciousness, or if we manage to give it to them, the human-robot dynamic will forever be different.

Videos
  • Does AI—and, more specifically, conscious AI—deserve moral rights? In this thought exploration, evolutionary biologist Richard Dawkins, ethics and tech professor Joanna Bryson, philosopher and cognitive scientist Susan Schneider, physicist Max Tegmark, philosopher Peter Singer, and bioethicist Glenn Cohen all weigh in on the question of AI rights.
  • Given the grave tragedy of slavery throughout human history, philosophers and technologists must answer this question ahead of technological development to avoid humanity creating a slave class of conscious beings.
  • One potential safeguard against that? Regulation. Once we define the context in which AI requires rights, the simplest solution may be to not build that thing.

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