Pollution from even low-intensity artificial light kills two species of desert rodents. Those who respond by saying “it’s just mice” should be aware of the potential danger that light pollution can be extremely damaging not only to these species, but also to ecosystems, biodiversity, and even human health.
The researchers at Tel Aviv University’s (TAU) Faculty of Zoology, who made this first-ever laboratory discovery, warn: “The results clearly show the serious negative effects of light pollution. This is an important step in understanding the impact of human activities on the environment as a means of promoting science-based decisions and policies.”
The rodents that have died like the flies are the diurnal golden spiny mouse and the nocturnal common spiny mouse. The findings are extremely worrying, said Prof. Noga Kronfeld-Schor, scientific director of the Ministry of the Environment, who led study and doctoral student Hagar Vardi-Naim, both from the Zoological Faculty and the Steinhardt Natural History Museum.
The researchers have just published their results in the renowned journal Scientific Reports under the title “Fitness results of Chronic Exposure to Different Light Pollution Lengths in Nocturnal and Diurnal Rodents”.
On two separate occasions whole colonies exposed to ALAN (Artificial Light at Night) died within days and reproduction also decreased significantly compared to control groups.
Prof. Noga Kronfeld-Schor. (Image credit: Courtesy of Tel Aviv University)
“We have been studying these closely related rodent species for years. Both live in the rocky deserts of Israel: the golden spiny mouse (Acomys russatus) is diurnal [awake during the day]and the common spiny mouse (A. cahirinus) at night [awake at night]”, explained Kronfeld-Schor. “The two species share the same natural habitat but use it at different times to avoid competition. By comparing closely related species with different activity times, we gain new insights into the biological clock and its importance for animal and human health.”
“Our understanding of ALAN’s side effects is based primarily on observational or laboratory studies, and its effects are likely to be underestimated. Evidence of direct experimental fitness consequences of ALAN in mammals is lacking. We examined the effects of chronic light pollution at different wavelengths on fitness and glucocorticoid hormone levels under near-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 changing seasons,” added Vardi-Naim. “Different species evolved activity patterns that correspond to these changes in light intensity and day length, and evolved anatomical, physiological, and behavioral adaptations appropriate for diurnal or nocturnal activity and seasonality.”
changing the rules
However, in the past few decades, people have changed the rules by inventing and making extensive use of artificial light, which creates light pollution, she added. “According to recent studies, around 80% of the world’s population is exposed to ALAN, and the area affected by light pollution is growing at 2-6% annually. In a small and crowded country like Israel, very few places are free from light pollution. In our study, we closely monitored the long-term effects of ALAN on individuals and populations under near-natural conditions.”
The team placed 96 spiny mice of both species – equal numbers of males and females – in eight spacious outdoor enclosures in the TAU 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 adequate amounts of food.
The experimental pens were exposed to low-intensity ALAN (similar to a streetlight in urban areas) with different wavelengths (colors) for 10 months. Two pens were exposed to cool white light, two to warm white (yellowish) light, and two to blue light, while two pens remained dark at night and served as controls. All animals were tagged to allow close monitoring of behavioral changes and physical condition. The experiment was conducted twice in two consecutive years.
“The average lifespan of spiny mice is four or five years, and our original plan was to monitor the effects of ALAN on the same colonies and measure the effects on reproductive performance, welfare and longevity,” noted Kronfeld-Schor. “But the dramatic results thwarted our plans. On two independent occasions, in two different enclosures exposed to white light, all animals died within a few days. We had seen no preliminary signs, and autopsies at TAU School 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 compromised the rodents’ immune response, leaving them without protection against an 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.”
Other results also suggested that exposure to ALAN disrupted reproductive success in spiny mice. “In the wild, both species of spiny mice breed primarily during the summer, when temperatures are high and the newborn pups are most likely to survive,” Vardi-Naim said. “However, artificial light seemed to confuse the animals. The common spiny mice began breeding year-round, but produced a smaller number of pups per year. Winter-born pups are not expected to survive in the wild, further reducing the species’ reproductive success in the wild. Reproduction in golden spines was affected in another way – colonies exposed to ALAN continued to breed through the summer, but the number of young was reduced by half compared to the control group, which continued to thrive and breed normally. These findings are consistent with the fact that the cue for reproduction in seasonal long-day breeders is length of day.”
Additional testing revealed that exposure to ALAN caused physiological and hormonal changes, most notably in levels of cortisol — an important stress hormone involved in the regulation and operation of many physiological pathways, including regulation of the immune system. Laboratory tests showed that exposure to blue light increased cortisol levels in golden spines, while white light lowered cortisol levels in winter golden spines.
“Our results show that light pollution, particularly cold white and blue light, increases mortality and disrupts reproduction, and thus can affect the fitness and survival of species in the wild,” Kronfeld-Schor concluded. “This disruption can have far-reaching consequences given the current prevalence of light pollution. Our clear results are an important step towards understanding the impact of light pollution on biodiversity and will help us to promote science-based strategies, especially regarding the use of artificial light in both built and open spaces. In future studies, we plan to investigate what caused the numerous deaths in the pens exposed to ALAN, focusing on the effects of light pollution on the immune system.”
