Light pollution is killing desert rodents - study

The rodents that have been dying like flies are the diurnal golden spiny mouse, and the nocturnal common spiny mouse.

A mouse, illustrative image. (photo credit: INGIMAGE)
A mouse, illustrative image.
(photo credit: INGIMAGE)

Pollution from even low-intensity artificial lights is killing off two species of desert rodents, according to researchers in the School of Zoology at Tel Aviv University (TAU), who made this first-time discovery in their lab. This poses a possible danger from light pollution which can be extremely harmful not only to these species but also to ecosystems, biodiversity and even human health.

The researchers warned that “the results clearly demonstrate the severe adverse effects of light pollution. This is an important step toward understanding the impact of human activity on the environment, as a means for promoting science-based decisions and policies.”

The rodents that have been dying are two species of spiny mouse: diurnal golden and nocturnal common. Study leader Prof. Noga Kronfeld-Schor, chief scientist of the Environmental Protection Ministry, and doctoral student Hagar Vardi-Naim, both from the School of Zoology and the Steinhardt Museum of Natural History, said that the findings were highly disturbing.

The study was published in the prestigious journal Scientific Reports under the title “Fitness consequences of chronic exposure to different light pollution wavelengths in nocturnal and diurnal rodents.”

On two different occasions, entire colonies exposed to ALAN (Artificial Light at Night) died within days, and their reproduction also decreased significantly compared to control groups.

 Prof. Noga Kronfeld-Schor. (credit: Courtesy of Tel Aviv University) Prof. Noga Kronfeld-Schor. (credit: Courtesy of Tel Aviv University)

“We have been studying these closely related rodent species for years. They both live in Israel’s rocky deserts: the golden spiny mouse (Acomys russatus) is diurnal [awake during the day], and the common spiny mouse (A. cahirinus) is nocturnal [awake at night],” Kronfeld-Schor explained.

“The two species share the same natural habitat but use it at different times to avoid competition. By comparing closely related species that differ in activity times, we gain new insights into the biological clock and its importance to the health of both animals and humans,” she said.

“Our understanding of the adverse effects of ALAN is based mostly on observational or lab studies, and its effects are probably underestimated. Demonstration of direct experimental fitness consequences of ALAN on mammals is missing. We studied the effects of chronic light pollution at different wavelengths on fitness and glucocorticoid hormone levels under semi-natural conditions,” they wrote in the journal.

“In most species studied to date, including humans, the biological clock is synchronized by light. This mechanism evolved over millions of years in response to the daily and annual cycles of sunlight – day and night and their varying lengths that correspond to the change of seasons,” Vardi-Naim said. “Different species developed activity patterns that correspond to these changes in light intensity and day length and developed anatomical, physiological and behavioral adaptations suitable for day or night activity and seasonality.”

Changing the rules 

HOWEVER, over the last decades, humans have changed the rules by inventing and extensively using artificial light, which generates light pollution, she said. “According to latest studies, about 80% of the world’s human population is exposed to ALAN, and the area affected by light pollution grows annually by two to six percent. In a small and overcrowded state like Israel, very few places remain free of light pollution.”

The team placed 96 spiny mice of both species – males and females in equal numbers – in eight spacious outdoor enclosures at TAU’s Zoological Research Garden. The enclosures simulated living conditions in the wild.

All animals were exposed to natural environmental conditions, including the natural light/dark cycle, ambient temperatures, humidity, and precipitation. Each enclosure contained shelters, nesting materials and access to sufficient amounts of food.

The experimental enclosures were exposed to low-intensity ALAN (similar to a street lamp in urban areas) of different wavelengths (colors) for 10 months. Two enclosures were exposed to cold white light, two to warm white (yellowish) light and two to blue light, while two enclosures remained dark at night and served as controls.

All animals were marked to enable accurate monitoring of changes in behavior and physical condition. The experiment was conducted twice in two successive years.

“The average life expectancy of spiny mice is four or five years, and our original plan was to monitor the effects of ALAN on the same colonies, measuring the effects on reproductive output, well-being and longevity,” noted Kronfeld-Schor. “But the dramatic results thwarted our plans. On two unrelated occasions, in two different enclosures exposed to white light, all animals died within several days. We had seen no preliminary signs, and autopsies at TAU’s Faculty of Medicine and the Kimron Veterinary Institute in Beit Dagan revealed no abnormal findings in the dead spiny mice.”

Impaired immune response

The team suggested that “exposure to ALAN had impaired the rodents’ immune response, leaving them with no protection against some unidentified pathogen. No abnormal mortality was recorded in any of the other enclosures, and as far as we are aware, no similar event has ever been documented by researchers before.”

OTHER FINDINGS also indicated that exposure to ALAN disrupts the reproductive success of spiny mice. “In the wild, both species of spiny mice breed mainly during summer, when temperatures are high, and the newborn pups are most likely to survive,” said Vardi-Naim.

“Artificial light, however, seemed to confuse the animals. The common spiny mice began to breed year-round but produced a lower number of pups per year. Pups born during winter are not expected to survive in nature, thus further reducing the species’ reproductive success in the wild.

“The reproduction of golden spiny mice was affected in a different way – colonies exposed to ALAN continued to breed in the summer, but the number of young was reduced by half compared to the control group, which continued to thrive and breed normally,” she said. “These findings are in accordance with the fact that in seasonal long day breeders, the cue for reproduction is the length of the day.”

“These findings are in accordance with the fact that in seasonal long day breeders, the cue for reproduction is the length of the day.”

Hagar Vardi-Naim

Additional tests revealed that exposure to ALAN caused physiological and hormonal changes, most significantly in the level of cortisol – an important stress hormone involved in the regulation and operation of many physiological pathways including the regulation of the immune system. Lab tests indicated that exposure to blue light increased cortisol levels of golden spiny mice, while white light reduced cortisol levels of golden spiny mice males in winter.

Our findings show that light pollution, especially cold white and blue light, increases mortality and disrupts reproduction, and thus may be detrimental to the fitness and survival of species in the wild,” concluded Kronfeld-Schor.

“This adverse effect can have far-reaching consequences at the current wide distribution of light pollution. Our clear results are an important step toward understanding the impact of light pollution on biodiversity and will help us promote science-based policies, specifically with regard to the use of artificial light in both built up and open areas.

“In future studies,” she added, “we plan to investigate what caused the extensive deaths in the enclosures exposed to ALAN, focusing on the effect of light pollution exposure on the immune system.”