Wednesday, July 30, 2025

The unusual head of a fish and the puzzle of its genes

University of Konstanz

Cichlid fish Perissodus microlepis, hunting behaviour — image:
The favoured direction of the cichlid's attack corresponds to its laterally asymmetric head shape: It prefers to attack the prey fish either from the right or from the left side, depending on the direction in which its head is bent.
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Credit: Weiwei Li

Almost all animals have symmetrical bodies: If we look at the left and right halves of our body, the limbs, eyes and ears are arranged evenly along the axis that runs through the centre of our body. This bilateral symmetry is almost universal in all animals and is only very rarely broken – with exceptions like the five-armed starfish or crab species that have one large and one small claw. One example of broken bilateral symmetry is the cichlid fish Perissodus microlepis, which is native to Lake Tanganyika in Africa. Its head and especially its mouth are shifted sideways in one direction, depending on the individual, either to the left or to the right. The fish have a permanently "crooked head", so to speak, which they use to their advantage when hunting. Axel Meyer, an evolutionary biologist from Konstanz, and his team conducted extensive analyses of the genome to find out why this is the case and which genes are responsible. The research results have now been published in the journal Science Advances. The research was funded by several funding sources, particularly by the Hector Fellow Academy.

The cichlid fish Perissodus microlepis feeds almost exclusively on the scales of other fish: it bites them off the living animals from their sides by attacking torpedo-like from behind and below and scrapes off the scales with backward-curved teeth. The favoured direction of the cichlid's attack corresponds to its laterally asymmetric head shape: It prefers to attack the prey fish either from the right or from the left side, depending on the direction in which its head is bent. In the cichlid population, the numerical ratio between "left-headed" and "right-headed" fluctuates around every four to five years, but in the long term it remains very consistent at 50:50. First author Xiaomeng Tian has a plausible explanation for this: "If, for example, the proportion of left-headers grows, their prey that survives these attacks pays more attention to the side that is more frequently attacked, so that right-headers, in this case, are at an advantage – and vice versa". This is why none of the variants is permanently more successful than the other and the ratio stabilizes at 50:50. "This is a prime example of rare frequency-dependent selection in which the more common form is at a disadvantage", explains the biologist.

Genetic tracing
But what is the genetic basis of this very unusual asymmetrical head shape? In a comprehensive study, Axel Meyer's research team has now investigated the genetic puzzle of the broken symmetry. They examined 102 cichlids, carried out extensive genetic analyses and performed around 1,000 micro-computed tomography analyses per fish to analyze its morphology in 3D. They were able to identify 72 regions in the genome that are associated with the development of the asymmetrical head shape. "Previous studies assumed that the polymorphism in the cichlid Perissodus microlepis was based on a simple Mendelian trait with a bimodal distribution. However, our investigations show that the situation is actually more complex: Not one single gene is responsible for the asymmetry, but many genes that are spread throughout the entire genome of this species", explains Axel Meyer.

When deciding what came first – the shifted head shape or the one-sided preference in hunting behaviour – it's like the question of the hen and the egg. First author Xiaomeng Tian provides an answer: "The two probably evolved together and reinforced each other", says the evolutionary biologist from Konstanz. The preferred direction of attack is therefore a result of both genetic causes and behaviour-related experiences. Further studies of the fishes’ brains in the Konstanz laboratory show that the preferred direction in hunting is partly the result of an asymmetrical activation of genes (gene expression) on the left or right side of the brain. "Our results suggest that both morphological and behavioural asymmetries have a measurable genetic component and have a common – or linked – genetic basis", confirms Axel Meyer.

"Based on our study, we have come to the following conclusion", summarizes Xiaomeng Tian: "Firstly, the asymmetry of the head shape is not caused by a single gene, but we identified 72 regions in the genome that each contribute a measurable effect. Secondly, the unilateral preference in the cichlids' hunting behaviour affects the asymmetrical morphology of its head through developmental plasticity – but, the genes in turn influence the fishes' behaviour as well. There is thus an interaction between the asymmetry of head shape and behaviour, both of which have strong genetic components".

Key facts:

Embargoed until 30. July 2024, 20:00 CEST (14:00 US Eastern Time)
Original publication: Xiaomeng Tian, Ming Li, Axel Meyer, Insights into the genetic basis of bilateral head asymmetry in a scale-eating cichlid fish, published in Science Advances on 30. July 2025
DOI: 10.1126/sciadv.adw4406
Press contact: Professor Axel Meyer, Professor of Zoology and Evolutionary Biology at the University of Konstanz, phone +49 7531 88-4163, email: axel.meyer@uni-konstanz.de
The research was funded by the German Research Foundation (DFG) and the Hector Fellow Academy.

Cichlid fish Perissodus microlepis, which is native to Lake Tanganyika in Africa. Its head and especially its mouth are shifted sideways in one direction, depending on the individual, either to the left or to the right.

Credit

Ad Konings

Note to editors
You can download photos here:

1) https://www.uni-konstanz.de/fileadmin/pi/fileserver/2025/der_schiefe_fischkopf/2.jpeg
https://www.uni-konstanz.de/fileadmin/pi/fileserver/2025/der_schiefe_fischkopf/3.jpeg
https://www.uni-konstanz.de/fileadmin/pi/fileserver/2025/der_schiefe_fischkopf/4.jpeg

Caption: Cichlid fish Perissodus microlepis, which is native to Lake Tanganyika in Africa.
Copyright: Ad Konings

2) https://www.uni-konstanz.de/fileadmin/pi/fileserver/2025/der_schiefe_fischkopf/1.png

Caption: The head and especially the mouth of cichlid fish Perissodus microlepis are shifted sideways in one direction, depending on the individual, either to the left or to the right.
Copyright: Axel Meyer, Universität Konstanz

3) https://www.uni-konstanz.de/fileadmin/pi/fileserver/2025/der_schiefe_fischkopf/5.png

Caption: The favoured direction of the cichlid's attack corresponds to its laterally asymmetric head shape: It prefers to attack the prey fish either from the right or from the left side, depending on the direction in which its head is bent.
Copyright: Weiwei Li

Journal

Science Advances

DOI

10.1126/sciadv.adw4406

Article Title

Insights into the genetic basis of bilateral head asymmetry in a scale-eating cichlid fish

Article Publication Date

30-Jul-2025

How does metformin lower blood sugar?

Baylor College of Medicine

Although metformin has been the go-to medication to manage type 2 diabetes for more than 60 years, researchers still do not have a complete picture of how it works. Scientists at Baylor College of Medicine and international collaborators have discovered a previously unrecognized new player mediating clinically relevant effects of metformin: the brain. By uncovering a brain pathway involved in metformin’s anti-diabetic action, researchers have discovered new possibilities for treating diabetes more effectively and precisely. The study appeared in Science Advances.

“It’s been widely accepted that metformin lowers blood glucose primarily by reducing glucose output in the liver. Other studies have found that it acts through the gut,” said corresponding author Dr. Makoto Fukuda, associate professor of pediatrics – nutrition at Baylor. “We looked into the brain as it is widely recognized as a key regulator of whole-body glucose metabolism. We investigated whether and how the brain contributes to the anti-diabetic effects of metformin.”

The team focused on a small protein called Rap1, found in a specific part of the brain known as the ventromedial hypothalamus (VMH). The researchers discovered that metformin’s ability to lower blood sugar at clinically relevant doses depends on turning off Rap1 in this brain region.

To test this, the Fukuda lab and his colleagues used genetically modified mice that lacked Rap1 in their VMH. These mice were fed a high-fat diet to mimic type 2 diabetes. When given low doses of metformin, the drug failed to lower their blood sugar. However, other diabetes medications like insulin and GLP-1 agonists still worked.

To further show that the brain is a key player, the researchers injected tiny amounts of metformin directly into the brains of diabetic mice. The result was a significant drop in blood sugar, even with doses thousands of times smaller than what’s typically given by mouth.

“We also investigated which cells in the VMH were involved in mediating metformin’s effects,” Fukuda said. “We found that SF1 neurons are activated when metformin is introduced into the brain, suggesting they’re directly involved in the drug’s action.”

Using brain slices, the scientists recorded the electrical activity of these neurons. Metformin made most of them more active, but only if Rap1 was present. In mice lacking Rap1 in these neurons, metformin had no effect, showing that Rap1 is essential for metformin to “switch on” these brain cells and lower blood sugar.

“This discovery changes how we think about metformin,” Fukuda said. “It’s not just working in the liver or the gut, it’s also acting in the brain. We found that while the liver and intestines need high concentrations of the drug to respond, the brain reacts to much lower levels.”

Although few anti-diabetic drugs act on the brain, this study shows that widely used metformin has been doing so all along. “These findings open the door to developing new diabetes treatments that directly target this pathway in the brain,” Fukuda said. “In addition, metformin is known for other health benefits, such as slowing brain aging. We plan to investigate whether this same brain Rap1 signaling is responsible for other well-documented effects of the drug on the brain.”

Other contributors to this work include Hsiao-Yun Lin, Weisheng Lu, Yanlin He, Yukiko Fu, Kentaro Kaneko, Peimeng Huang, Ana B De la Puente-Gomez, Chunmei Wang, Yongjie Yang, Feng Li and Yong Xu. The authors are affiliated with one or more of the following institutions: Baylor College of Medicine, Louisiana State University, Nagoya University – Japan and Meiji University – Japan.

This work was supported by grants from: National Institutes of Health (R01DK136627, R01DK121970, R01DK093587, R01DK101379, P30-DK079638, R01DK104901, R01DK126655), USDA/ARS (6250-51000-055), American Heart Association (14BGIA20460080, 15POST22500012) and American Diabetes Association (1-17-PDF-138). Further support was provided by the Uehara Memorial Foundation, Takeda Science Foundation, Japan Foundation for Applied Enzymology and the NMR and Drug Metabolism Core at Baylor College of Medicine.

Journal

Science Advances

DOI

10.1126/sciadv.adu3700

Method of Research

Experimental study

Subject of Research

Animals

Article Title

Low-dose metformin requires brain Rap1 for its antidiabetic action

Article Publication Date

30-Jul-2025

Black Death offers window into how childhood malnutrition affects adult health

University of Colorado at Boulder

The Black Death arrived on the shores of England in May 1348 and, in less than two years, spread throughout the country, killing an estimated 2 million people. The death toll from the disease, which was caused by the bacterium Yersinia pestis, got so high that officials in London and other cities opened new cemeteries where hundreds of bodies were interred every day.

According to a new study, those who died around the time of the Black Death may help scientists answer a decidedly modern question: How can malnutrition early in life shape the health of humans far into adulthood?

The answer may be more complicated than scientists once suspected, said Sharon DeWitte, lead author of the study and a professor in the Institute of Behavioral Science and Department of Anthropology at the University of Colorado Boulder.

In the new research, DeWitte and her colleagues examined chemical clues hidden in the teeth of nearly 275 people buried in English cemeteries before, during and after the Black Death. The team discovered something surprising: People who experienced malnutrition early in their lives may have survived threats to their health, like plague, at greater rates than their peers up until young adulthood, or roughly before the age of 30.

Those survival advantages, however, could have dropped significantly when the same individuals entered their middle and late adult years.

“What this might indicate is that if people experienced a period of starvation early in their childhoods or adolescence but survived, that could have shaped their development in ways that were beneficial in the short term but led to poor outcomes once they got older,” DeWitte said.

She and her colleagues will publish their findings July 30 in the journal Science Advances.

The research is part of DeWitte’s ongoing effort to understand the past to help humans living today.

“Mortality varied during a catastrophe 700 years ago in ways that might have been preventable,” she said. “My hope is that we can absorb that lesson and think about how human health can vary across different social categories today, and figure out the points of intervention where we can do something to reduce that burden.”

Childhood health

How experiences early in life shape our health long into the future is far from clear cut.

Some studies of modern humans, for example, have linked low birth weights in infants to health problems later in life. Babies born small, a possible sign of nutritional stress, seem to be more prone to illnesses like cardiovascular disease and diabetes in adulthood than the population at large.

The Black Death, sometimes known as the second pandemic of plague, might be an ideal laboratory for studying these questions, DeWitte noted. In part, that’s because the death toll around Europe varied drastically—in some parts of England, for example, about 30% of the population died, while mortality rates reached 75% in Florence, Italy.

“It raises questions about why mortality was higher in some populations than others,” she said.

To pursue those questions, DeWitte and her colleagues turned to teeth.

Environment matters

She explained that what humans eat as infants and children leaves a mark in the development of our adult teeth—subtly shifting the types, or “isotopes,” of carbon and nitrogen atoms present in the dentine. In particular, when people experience extreme nutritional stress, their bodies will begin to break down their own fat stores and muscle, which have a different signature of isotopes than food that is eaten.

In the current study, DeWitte’s team examined the isotopes present in the teeth of hundreds of people buried in English cemeteries between 1100 to 1540 AD. They included the East Smithfield Black Death Cemetery, which opened in London in 1348 and where the bodies of hundreds of plague victims were stacked in a mass burial trenches.

DeWitte emphasizes that the team’s results are far from definitive—in many cases, the group doesn't have any records about the humans included in the research, so it’s hard to know for sure how they died or how healthy they were in life.

But the findings carry hints that malnutrition early in life may shape the health of adults in ways that aren’t necessarily good or bad—it all depends on context.

When infants or children don’t have enough to eat, DeWitte said, their bodies may develop in ways that prime them for hardship later in life. They may have altered metabolism, for example, so that they use calories, which may be scarce, more efficiently.

Those changes can be beneficial—that is, until the environment changes and food becomes more plentiful. Some evidence, for example, suggests that in the wake of the Black Death, conditions for survivors in England improved as laborers demanded higher wages.

“People who experienced nutritional stress as children may have had a mismatch with their environments later in life,” DeWitte said. “If there’s now a resource abundance, but their bodies were shaped for an environment of scarcity, they may have poor health outcomes, like packing too many fat stores, which can lead to cardiovascular disease.”

For DeWitte, the study is another example of what humans living today can learn from people who died hundreds of years ago:

“For a very long time, I've been interested in this question of why some people experience good health and others living in the exact same society don’t.”

Co-authors of the new research include Julia Beaumont and Jacqueline Towers at the University of Bradford in the United Kingdom; Brittany Walter of the Defense POW/MIA Accounting Agency; and Emily Brennan at the University of South Carolina.

Journal

Science Advances

DOI

10.1126/sciadv.adw7076

Article Title

Childhood nutritional stress and later-life health outcomes in medieval England: evidence from incremental dentine analysis.

Article Publication Date

30-Jul-2025

Researchers map where solar energy delivers the biggest climate payoff

Using advanced computational modeling, a Rutgers professor, in collaboration with researchers from the Harvard T.H. Chan School of Public Health and Stony Brook University, reveal both the immediate and delayed climate benefits of solar power.

Rutgers University

Increasing solar power generation in the United States by 15% could lead to an annual reduction of 8.54 million metric tons of carbon dioxide emissions, according to researchers at Rutgers, the Harvard T.H. Chan School of Public Health and Stony Brook University.

The study, published in Science Advances, found that the climate benefits of solar power differ markedly throughout U.S. regions, pinpointing where clean energy investments return the greatest climate dividends.

In 2023, 60% of U.S. electricity generation relied on fossil fuels, while 3.9% came from solar, according to the U.S. Energy Information Administration. Because fossil fuel-generated electricity is a leading source of carbon dioxide, or CO2, and harmful air pollutants such as fine particulate matter, expanding solar could not only mitigate CO2 but help reduce illness, hospitalizations and premature deaths linked to air pollution exposure.

Researchers examined five years of hourly electricity generation, demand and emissions data from the Energy Information Administration starting July 1, 2018. They focused on the 13 geographic regions in the United States.

With this dataset, the researchers constructed a statistical model to explore how increases in hourly solar energy generation would affect CO2 emissions within a given region and in its neighboring regions.

The study quantified both immediate and delayed emissions reductions resulting from added solar generation. For example, the researchers found that in California, a 15% increase in solar power at noon was associated with a reduction of 147.18 metric tons of CO2 in the region in the first hour and 16.08 metric tons eight hours later.

“It was rewarding to see how advanced computational modeling can uncover not just the immediate, but also the delayed and far-reaching spillover effects of solar energy adoption,” said the lead author Arpita Biswas, an assistant professor with the Department of Computer Science at the Rutgers School of Arts and Sciences. “From a computer science perspective, this study demonstrates the power of harnessing large-scale, high-resolution energy data to generate actionable insights. For policymakers and investors, it offers a roadmap for targeting solar investments where emissions reductions are most impactful and where solar energy infrastructure can yield the highest returns.”

The researchers said their methods provide a more nuanced understanding of system-level impacts from solar expansion than previous studies, pinpointing where the benefits of increased solar energy adoption could best be realized. In some areas, such as California, Florida, the mid-Atlantic, the Midwest, Texas and the Southwest, small increases in solar were estimated to deliver large CO2 reductions, while in others, such as New England, the central U.S., and Tennessee, impacts were found to be minimal – even at much larger increases in solar generation.

In addition, the researchers said their study demonstrates the significant spillover effects solar adoption has on neighboring regions, highlighting the value of coordinated clean energy efforts. For example, a 15% increase in solar capacity in California was associated with a reduction of 913 and 1,942 metric tons of CO2 emissions per day in the northwest and southwest regions, respectively.

“I am very excited about this study because it harnesses the power of data science to offer insights for policymakers and stakeholders in achieving CO2 reduction targets through increased solar generation,” said Francesca Dominici, director of the Harvard Data Science Initiative and Clarence James Gamble Professor of Biostatistics, Population and Data Science and a corresponding author of the study.

Journal

Science Advances

Method of Research

Computational simulation/modeling

Subject of Research

Not applicable

Article Title

Quantifying Effects of Solar Power Adoption on CO2 Emissions Reduction

Article Publication Date

30-Jul-2025

Increasing solar power could lead to significant cuts in CO2 emissions

Harvard T.H. Chan School of Public Health

Key points:

Researchers estimated that a 15% increase in U.S. solar power generation could reduce CO2 emissions by 8.54 million metric tons annually, offering major climate benefits.
The benefits of added solar power varied widely by region. Areas like California, Florida, Texas, Mid-Atlantic, Midwest, and Southwest exhibited major reductions in emissions from solar increases, while other areas, such as Central, New England, and Tennessee, saw minimal impact.
Solar expansion in one region can reduce emissions in neighboring regions, highlighting the importance of regional collaboration in clean energy planning.

Boston, MA—Increasing solar power generation in the U.S. by 15% could lead to an annual reduction of 8.54 million metric tons of carbon dioxide (CO2) emissions, according to a new study led by Harvard T.H. Chan School of Public Health. The researchers found that the climate benefits of solar power differ markedly across U.S. regions, pinpointing where clean energy investments return the greatest climate dividends.

“This is an exciting study in that it harnesses the power of data science to offer insights to policymakers and stakeholders on how we can achieve CO2 reduction targets,” said corresponding author Francesca Dominici, director of the Harvard Data Science Initiative and Clarence James Gamble Professor of Biostatistics, Population, and Data Science.

The study will be published July 30 in Science Advances.

In 2023, 60% of U.S. electricity generation relied on fossil fuels, while just 3.9% came from solar, according to the U.S. Energy Administration. Since fossil fuel-generated electricity is a leading source of both CO2 and harmful air pollutants such as fine particulate matter, cutting emissions by expanding solar could not only mitigate CO2 but also help reduce illness, hospitalizations, and premature deaths linked to air pollution exposure.

For this study, the researchers examined five years of hourly electricity generation, demand, and emissions data from the Energy Information Administration, starting July 1, 2018. They focused on 13 regions: California, Carolinas, Central, Florida, Mid-Atlantic, Midwest, New England, New York, Northwest, Southeast, Southwest, Tennessee, and Texas. Using this dataset, they constructed an advanced statistical model to explore how increases in hourly solar energy generation would affect CO2 emissions within a given region and in its neighboring regions.

The study quantified both immediate and, for the first time, delayed emissions reductions resulting from added solar generation. For example, the researchers found that in California, a 15% increase in solar power at noon was associated with a reduction of 147.18 metric tons of CO2 in the first hour and 16.08 metric tons eight hours later.

The researchers’ methods provide a more nuanced understanding of system-level impacts from solar expansion than previous studies, pinpointing where the benefits of increased solar energy adoption could best be realized. In some areas, such as California, Florida, Mid-Atlantic, Midwest, Texas, and the Southwest, small increases in solar were estimated to deliver large CO2 reductions, while in others, such as New England, Central, and Tennessee, impacts were found to be minimal—even with much larger increases in solar generation.

In addition, the study demonstrates the significant spillover effects solar adoption has on neighboring regions, highlighting the value of coordinated clean energy efforts. For example, a 15% increase in solar capacity in California was associated with a reduction of 913 and 1,942 metric tons of CO2 emissions per day in the Northwest and Southwest, respectively.

“Our study offers policymakers and investors a roadmap for targeting solar investments where emissions reductions are most impactful and where solar energy infrastructure can yield the highest returns,” said lead author Arpita Biswas, assistant professor in the Department of Computer Science at Rutgers University. “From a research perspective, our findings also demonstrate the power of harnessing large-scale, high-resolution energy data to generate actionable insights.”

Other Harvard Chan authors included Danielle Braun and Daniel Mork.

The study was supported by the National Institutes of Health (grants R01ES030616, RF1AG074372, R01MD016054, RF1AG080948, R01ES034021, U24ES035309, R01ES034373, R01ES037156, R01AG066793, R01ES035735).

“Quantifying Effects of Solar Power Adoption on CO2 Emissions Reduction,” Arpita Biswas, Minghao Qiu, Danielle Braun, Francesca Dominici, Daniel Mork, Science Advances, July 30, 2025, doi: 10.1126/sciadv.adq5660

Visit the Harvard Chan School website for the latest news and events from our Studio.

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Harvard T.H. Chan School of Public Health is a community of innovative scientists, practitioners, educators, and students dedicated to improving health and advancing equity so all people can thrive. We research the many factors influencing health and collaborate widely to translate those insights into policies, programs, and practices that prevent disease and promote well-being for people around the world. We also educate thousands of public health leaders a year through our degree programs, postdoctoral training, fellowships, and continuing education courses. Founded in 1913 as America’s first professional training program in public health, the School continues to have an extraordinary impact in fields ranging from infectious disease to environmental justice to health systems and beyond.

Journal

Science Advances

DOI

10.1126/sciadv.adq5660

Method of Research

Observational study

Subject of Research

Not applicable

Article Title

Quantifying Effects of Solar Power Adoption on CO2 Emissions Reduction

Article Publication Date

30-Jul-2025

How cumulative heat exposure affects students

A systematic review synthesizes research on the long-term effects of heat on cognitive performance

PLOS

A holistic approach reveals the global spectrum of knowledge on the impact of cumulative heat exposure on young students, according to an article published July 30 in the open-access journal PLOS Climate by Konstantina Vasilakopoulou from the Royal Melbourne Institute of Technology, Australia, and Matthaios Santamouris from the University of New South Wales, Australia. The article aims to shed light on the social and economic inequalities caused within and across countries, the potential adaptive measures to counterbalance the impact of overheating, and forecasts about the cognitive risks associated with future overheating.

Exposure to elevated temperatures, both indoors and outdoors, is strongly associated with adverse health outcomes. The impact of high temperatures and heat stress on human productivity and cognitive performance is well-documented. Most studies indicate that exposure to excessive heat detrimentally affects working memory, information processing, and knowledge retention, thereby impairing overall cognitive performance. The impact of high temperatures on students’ academic performance is profound, influencing their educational, intellectual, and professional achievements.

Although there is a plethora of investigations on the momentary and short-term exposure of students to heat and its effects, there is a lack of knowledge and information on the impact of long-term exposure on student cognitive performance. Given the rapid increase in temperature caused by global and regional climate change, understanding the consequences of cumulative exposure to high temperatures on the cognitive ability of students is an urgent priority.

To address this knowledge gap, the authors reviewed seven studies, described in six peer-reviewed articles, that investigated the effects of prolonged heat exposure on students’ cumulative cognitive performance. Collectively, these studies covered an extensive dataset comprising nearly 14.5 million students from 61 countries, linking individual learning outcomes to heat exposure. Overall, the findings suggest that long-term heat exposure negatively impacts students’ cumulative learning, with complex tasks (e.g., mathematics) more affected than simpler ones (e.g., reading).

Six of the seven studies identified a significant negative relationship between extended heat exposure and cognitive performance, while one study found the impact to be minimal. Adaptation via acclimatization and increased air conditioning use showed protective effects. However, lower socioeconomic groups faced disproportionately greater impacts, underlining critical inequalities.

According to the authors, impairments related to cognitive and human capital loss of the young generation may affect the future progress of nations because of the associated dramatic economic, social and cultural implications caused by persistent disruptions to the learning process. The social cost of global overheating will unfortunately affect equity and the quality of life of the vulnerable low-income population. It will accelerate societal discrepancies and will impede economic progress in less developed countries suffering from excessive heat exposure. There is an urgent need to adopt a new perspective on the cognitive implications of climate change by advancing technologies and implementing robust, targeted policies to safeguard both current and future human capital.

The authors add: “This study critically reviewed the existing literature on the effects of long-term heat exposure—primarily driven by climate change—on students’ cognitive performance. Prolonged exposure to elevated temperatures was consistently linked to reduced learning outcomes, particularly in tasks that require complex cognitive processing, such as mathematics. The findings revealed that students in socioeconomically disadvantaged areas are disproportionately affected. Factors contributing to this disparity include higher localized temperatures in low-income communities, limited access to air conditioning at school and at home, and fewer opportunities for academic support services like private tutoring to offset learning losses. Projected climate scenarios indicate that these disparities will only deepen over time, further disadvantaging already vulnerable student populations. To mitigate these effects, the literature highlights several adaptive strategies to enhance learning conditions—such as implementing indoor and outdoor cooling systems and improving classroom ventilation.”

They summarize: “This review, conducted over the course of a year, examined a comprehensive range of sources on the subject. The evidence underscores how global warming can produce far-reaching and often overlooked social consequences. Most critically, it reiterates that those bearing the brunt of climate change's impacts are often those least responsible for it—and least equipped to combat its effects.”

###

In your coverage please use this URL to provide access to the freely available article in PLOS Climate: https://plos.io/4l1kVPL

Citation: Vasilakopoulou K, Santamouris M (2025) Cumulative exposure to urban heat can affect the learning capacity of students and penalize the vulnerable and low-income young population: A systematic review. PLOS Clim 4(7): e0000618. https://doi.org/10.1371/journal.pclm.0000618

Author Countries: Australia

Funding: The author(s) received no specific funding for this work.

Journal

PLOS Climate

DOI

10.1371/journal.pclm.0000618

Method of Research

Observational study

Subject of Research

People

Article Publication Date

30-Jul-2025

Weathering change: Fewer cold fatalities, more heat emergencies in California

Temperature extremes affect rates of illness and death differently, leading to potential economic shifts from climate change

University of California - San Diego

As temperatures rise, California is experiencing fewer deaths from cold temperatures, which outweigh increased deaths from extreme heat. However, hotter temperatures sharply increase emergency department visits – a previously overlooked consequence of climate change that could place a greater burden on the healthcare system.

Using data covering all deaths, emergency department (ED) visits, hospitalizations and daily temperatures in California from 2006 to 2017, researchers from the University of California San Diego and Stanford University reported that hot and cold days influence illness and deaths differently in California. The findings were published online in the journal Science Advances on July 30, 2025.

“Heat can harm health even when it doesn’t kill,” said Carlos F. Gould, Ph.D., assistant professor at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego and first author of the study. “Warmer temperatures were consistently associated with more trips to the emergency department, so studies and planning that only consider mortality miss a big slice of the burden.”

Varied Health Impact by Age

The study found that emergency room visits, which reflect a wider range of health impacts across age groups, rise sharply with hotter days. Conditions like injuries, mental health issues, and poisonings show clear increases with heat but are not major causes of death, so they are often missed in studies that focus only on mortality.

“Age plays a critical role in shaping health risks from temperatures,” said Gould. “Older adults are particularly vulnerable to cold temperatures, whereas younger adults and children are more affected by heat.”

While California may see fewer cold‑related deaths as the state experiences fewer extreme cold days, that benefit will be partly offset by more trips to the emergency room as a result of more extreme heat. Researchers suggest that health policy needs to account for differences to address temperature-related impacts in the full population – hospitals, insurers and public health agencies should prepare for heavier heat demand and tailor warnings and resources to different age groups.

“Understanding who is affected, how, and at what temperatures is critical for planning appropriate responses to protect health,” said study co-author Marshall Burke, Ph.D., associate professor of environmental social sciences at the Stanford Doerr School of Sustainability. “This is true with or without climate change, but a warming climate makes it more important and alters who is exposed to what.”

Economic and Social Burden of Climate Change

Healthcare spending in the United States on chronic disease alone is estimated to exceed $3 trillion annually, which accounts for 17.6% of US gross domestic product, according to the National Health Expenditure Accounts.

Using projections based on moderate climate change scenarios through 2050, researchers estimate California will see around 53,500 fewer deaths overall due to less cold weather —saving approximately $30 billion annually. However, this is partially offset by an estimated additional 1.5 million heat-driven emergency department visits, costing an extra $52 million annually in healthcare spending.

“We often think about only the most extreme health impacts of heat waves: deaths. This work is showing that many things that we may not think about being sensitive to extreme heat are, like poisonings, endocrine disorders, injuries and digestive issues,” said Alexandra K. Heaney, Ph.D., assistant professor at the Herbert Wertheim School of Public Health and co-author of the paper. “We need to focus on the full spectrum of health impacts when we think about heat waves, now and in the future.”

Co-authors include: Carlos F. Gould and Alexandra K. Heaney at UC San Diego; Sam Heft-Neal, Eran Bendavid, Christopher W. Callahan, Mathew V. Kiang, and Marshall Burke at Stanford University; and Josh Graff Zivin, UC San Diego and the National Bureau of Economic Research.

Funding for this research came, in part, from the Robert Wood Johnson Foundation, Stanford’s Center for Population Health Sciences, the National Institutes of Health (R01HD104835), and the National Institute of Environmental Health Sciences (K01ES036991).

Disclosures: The authors declare that they have no conflicts of interest to disclose.

DOI: 10.1126/sciadv.adr3070

Journal

Science Advances

DOI

10.1126/sciadv.adr3070

Method of Research

Data/statistical analysis

Subject of Research

People

Article Title

Temperature extremes impact mortality and morbidity differently

Article Publication Date

30-Jul-2025