What happens if you don't turn off the lights at night? It may kill small animals...

What happens if you don’t turn off the lights at night?

As human lighting equipment becomes more and more advanced, artificial light turns the dark night into a place that never sleeps . This brings us a sense of security, a rich nightlife, and endless overtime work; but for many animals, this light that does not conform to the natural circadian rhythm is completely different from their millions of years of evolutionary experience. They suffer many survival problems as a result...

The dazzling city night scene is a nuisance to animals｜Pixabay

Kangaroos: Belated babies

Eugene's wallaby (Macropus eugenii) is one of the victims of light pollution - artificial light at night can delay their birth , causing their newborns to miss the time when food is most abundant.

Eugene kangaroos live on islands in western and southern Australia and feed on grasses and leaves. During the day, Eugene kangaroos stay in the shade of bushes; when night falls, they jump out to more open grasslands.

Eugene kangaroo mother and baby | Mehgan Murphy/Smithsonian's National Zoo.

The length of daylight is the reproductive activation signal of Eugene kangaroos, which determines when they give birth . Eugene kangaroos have the phenomenon of "embryo diapause", and their pregnancy process can be suspended midway. Under normal circumstances, after mating, the fertilized egg of Eugene kangaroos will stop dividing first. It is not until after the summer solstice in the southern hemisphere (December 23), the nights begin to get longer, and the melatonin in the mother kangaroo's body will increase due to the extension of darkness, stimulating the embryo to continue to develop, and finally giving birth to a kangaroo baby in the early second year.

Garden Island in Western Australia is one of the habitats of Eugene kangaroos. It is occupied by the Australian Department of Defense as a naval base. Only the jungle in the northernmost part of the island is free from human activities and artificial light. Scientists observed kangaroos living near the naval base and in the northern jungle respectively, and found that artificial light affects the reproductive activation of kangaroos. Kangaroo mothers living near the naval base have delayed their births .

The two groups of kangaroo mothers observed came from a jungle without artificial light interference and a naval base with severe light pollution. Reference [1]

Delayed childbirth is more than just a matter of giving birth later. Originally, the secretion of melatonin can help animals synchronize their reproduction with the seasons, allowing kangaroo mothers to give birth to their cubs in the summer when food is most abundant. However, once the birth of the cubs is delayed, they may face the dilemma of food shortage as soon as they are born .

Eugene kangaroos in the jungle give birth in January, while most kangaroos at the naval base give birth later in February and April. ｜Reference [1]

The researchers also put collars with light sensors on the Eugene kangaroos. The results of the sensors showed that the light intensity at the naval base was 10 times that of the jungle kangaroos . Kangaroo mothers must be very troubled. The sky is always so bright, and they watch the food become less and less. The physiological mechanism that has evolved for tens of millions of years still does not intend to allow the birth of cubs...

Too bright｜Michelle McFarlane

Firefly: Hard to find your other half

When we encounter glaring light, we feel dazzled and cannot see clearly. Many animals will also feel dazzled, such as fireflies that like to move at night. When the night is too bright, fireflies have to "wear sunglasses", which will also make them miss the objects right in front of them .

Fireflies are insects that use light signals for sexual communication. Female European common fireflies (Lampyris noctiluca) have no wings. They emit green light from their abdomens and wait for males to find them based on the light. In the dark environment of nature, females usually wait for a night for males to come and propose marriage . After successful mating, the females stop emitting light.

Female fireflies attract males by emitting green light from their abdomens | Tuchong Creative

However, the scientists found that when exposed to artificial white light, the females glowed for much longer, sometimes for up to 15 nights - suggesting that they were having trouble successfully mating .

When male fireflies encounter artificial white light, they have to cover their compound eyes with a hood. The hood is like their sunglasses, which can reduce the strong light they see; but once the sunglasses are worn, it is more difficult to see the fluorescence of female insects . When male insects are in the dark, they only wear sunglasses 0.5% of the time; when exposed to white light, the male insects wear sunglasses for a significantly increased time to 25%.

Left: The fake female used in the experiment in a white light environment. Right: The real female glows in the dark | Jeremy Niven

The scientists also recorded the success rate of male insects in finding female insects under different lighting conditions. Under the intensity of street lighting, 70% of male insects successfully flew to the female insects; but when the white light was increased to the intensity of spotlights in parks and squares, only 20% of male insects could successfully find female insects .

If more and more artificial light invades the habitat of fireflies, their reproductive capacity will continue to decline, and the grass that has been flashing fluorescently for millions of years will gradually dim...

The forest cannot be without fluorescent lights! | Tuchong Creative

Migrating birds: lost in the bright night

For birds that migrate or hunt at night, moonlight and starlight are their natural navigation. However, when artificial lights illuminate the night, they may confuse artificial light with natural light, deviate from their course, and get lost in dangerous high-rise buildings in the city. Every year, millions of birds die from colliding with illuminated buildings.

Artificial light can also cause birds to migrate too early or too late and miss ideal climatic conditions for nesting and foraging. | Wim Wiskerke

Moreover, birds that chirped during nighttime migration were more likely to collide with buildings than birds that did not chirp. When they were disoriented, they chirped more frequently, which would attract more flock members and cause more collisions.

Even if they don’t collide with a building, a disoriented bird may be forced to land, where it may be unable to take off again and is vulnerable to predation, being hit by a vehicle, or dying from starvation and dehydration.

The distribution of artificial nighttime lighting around the world. The lines in the figure connect the breeding and wintering grounds of some nocturnal migratory birds; birds have to pass through many areas with dense artificial lighting when migrating｜Cabrera-Cruz et al. (2018)

Statistics from coastal areas show that among birds stranded due to artificial light every year, the number of young birds far exceeds that of adult birds . For example, when their young birds fly out to sea for the first time at night, their visual systems are not yet developed, and they are prone to getting lost. In addition, they have lived in dim caves since childhood, and the only light comes from the cave entrance, where the light is the fresh prey brought home by their parents. Therefore, young birds are easily attracted by light and mistakenly believe that any light is food.

Up to 4,000 Calonectris borealis chicks may become stranded during a breeding season｜Beneharo Rodríguez/X

In fact, even birds that are used to city life don't like the dark nights when lights are on. Pigeons and magpies in cities will shorten their sleep time because of the brighter nights. Next time, don't blame the magpies and pigeons for waking you up with their chirping. They may be just as sleep-deprived as you are.

I can’t sleep again today｜Doug Gimesy

Restoring darkness in the city

The slow evolution of nature often cannot keep up with the rapid development of human society. In addition to the kangaroos, fireflies and birds mentioned in the article, there are too many animals that have not yet gotten used to the increasingly bright nights and are suffering from the impact of light.

For example, baby turtles can easily mistake street lights for moonlight and drift further away from the sea; the survival rate and growth rate of many fish and amphibians will be reduced under the interference of artificial light; moths gather under street lights and are easily eaten by natural enemies, and plants that rely on insects for pollination will find it difficult to reproduce...

A newly hatched loggerhead turtle follows an artificial light and turns inland instead of heading toward the sea. | Blair Wellington

In fact, to alleviate the impact of artificial light on animals, we do not need to completely give up human night life . Take the problem of birds hitting buildings, for example. Scientists point out that even if it is not possible to turn off all artificial lights, just turning off bright lights intermittently can effectively encourage birds to fly away from buildings and reduce collisions.

Controlling light pollution does not mean completely shutting down nighttime lighting. Light pollution refers to overused lighting. For example, we can turn off lighting designed purely for landscape purposes, turn off lighting in unoccupied office buildings, stop using dazzling, scattered light, and choose voice-activated sensor lights. At the same time, warm-colored light is less harmful to many nocturnal animals and can be used as an eco-friendly alternative to white light.

These behaviors can create a friendlier environment for nocturnal animals, and at the same time, it is more conducive to our nighttime rest. | Michael Nagle

In recent years, many cities and regions have launched "lights-off" activities . For example, Shenzhen City has issued the "Shenzhen City Urban Lighting Special Plan (2021-2035)", proposing to build a dark night protection demonstration area in Shenzhen. Specifically, the design and lighting hours of landscape lighting near dark night communities will be strictly controlled; during the bird migration season from October to May of the following year, ecological avoidance will be actively carried out. As a result, Shenzhen has become the first city in China to have a "dark night community".

Maybe one day, we and animals will be able to see the starry sky in the city again. Do you have similar concepts and practices of "dark night city" where you live? Feel free to share with us in the comments section!

Who doesn't want to see the sky full of stars at home? | Tuchong Creative

References

[1] Robert, KA, Lesku, JA, Partecke, J., & Chambers, B. (2015). Artificial light at night desynchronizes strictly seasonal reproduction in a wild mammal. Proceedings of the Royal Society B: Biological Sciences, 282(1816), 20151745.

[2]https://royalsociety.org/blog/2015/09/wallaby-babies-arrive-late-due-to-artificial-light-at-night/

[3]https://darksky.org/resources/what-is-light-pollution/effects/wildlife-ecosystems/

[4]https://www.latrobe.edu.au/news/articles/2020/release/artificial-light-keeps-birds-up-at-night

[5] Atchoi, E., Mitkus, M., Vitta, P., Machado, B., Rocha, M., Juliano, M., Bried, J., & Rodríguez, A. (2023). Ontogenetic exposure to light influences seabird vulnerability to light pollution. Journal of Experimental Biology, 226(7)https://doi.org/10.1242/jeb.245126

[6] Moubarak, EM, David Fernandes, AS, Stewart, AJ, & Niven, JE (2023). Artificial light impairs local attraction to females in male glow-worms. Journal of Experimental Biology, 226(11), jeb245760.

[7]https://theconversation.com/light-pollution-is-taking-the-sparkle-out-of-glow-worm-mating-207514

[8] https://royalsocietypublishing.org/doi/10.1098/rspb.2021.0454

Author: Bowl

Editor: Mai Mai

This article comes from GuokrNature (ID: GuokrNature)