City light pollution causes spiders’ brains to shrink, new study finds

As night falls, the nocturnal half of the animal kingdom begins its day. Nocturnal species are perfectly adapted to navigate and survive in the darkness of the night that has existed for millions of years.

What happens to these creatures when the darkness they live in is transformed by streetlights and other artificial nighttime lighting?

In new research published in Biology lettersWe studied the effect of light pollution on the development of Australian garden web-spiders. We found that it reduces their brain size, particularly in regions dedicated to vision, with unknown effects on their behavior.

What light pollution means for animals

Artificial light is one of the increasingly widespread forms of pollution by humans. It has many effects on animals, plants and ecosystems. Recent studies suggest that stress from light pollution can harm the growth and development of the brains of some birds and mammals.

This could be catastrophic. To survive in new environments with more light pollution, such as cities, animals may need larger and more complex brains.

But what about insects, spiders and other smaller creatures that live at night? Could light pollution affect their brain growth and development in the same way?

Our study of the Australian garden spider suggests that this is the case.

More insects, but fewer offspring

The Australian cobweb spider is a perfect species to study this question. It lives happily in cities and rural areas where it builds its webs every night in large open spaces (even under streetlights).

In previous studies, we found that urban spiders that build their webs under streetlights catch more insects. We also showed that nighttime light comes at a cost, as it accelerates juvenile development, resulting in smaller adults that produce fewer offspring.

In this current study, we examined whether developing under light at night equally affects brain size in men and women.

To investigate this question, we collected late juvenile spiders from relatively dark parks in Melbourne, Australia, and raised them in the laboratory until they became adults.

During rearing, we kept half of the spiders in the dark at night and exposed the other half to nighttime illumination equivalent to the brightness of a street lamp.

Smaller brains, but why?

A few weeks after the spiders reached adult size, we assessed whether nighttime light had affected their brain development. Since a spider’s brain is about the size of a ballpoint pen tip (less than a cubic millimeter), we used micro-CT imaging technology to visualize what was inside.

We found that short-term exposure to light at night led to an overall decrease in brain volume in spiders. The most pronounced effects were seen in the area of ​​the brain linked to vision in the spider’s primary eyes.

These results are a first for invertebrates (animals without backbones, such as insects and spiders), but they mirror what has been described in vertebrates. One can only speculate about how these differences arose.

It is possible that the presence of light at night created a stressful environment that disrupted hormonal processes related to growth and development. However, if this were the case, one would expect to see all parts of the brain affected, which is not the case.

Another possible explanation is that spiders forced to develop under light at night have altered their “investment” in different parts of the brain. Proper brain function is essential for an animal to navigate its environment. So, under stressful conditions, limited resources may be directed to the most important parts of the brain. For spiders that don’t rely on vision, such as the spherical-armored spiders, they may compensate by reducing investment in the visual parts of the brain, as we found here.

Other invertebrates, such as desert ants (Cataglyphis fortis), exhibit similar “neuroplastic changes” in the visual center of their brains when they switch from maintaining an underground nest to foraging above ground, based on vision.

Why Spiders and Their Brains Are Important

This is all pretty interesting, but you might be wondering why we should care about light pollution affecting a spider’s brain size.

Spiders play a very important role in an ecosystem. They feed on other invertebrates, including many pest species such as flies and mosquitoes. Spiders are also important prey for other predators, such as birds and lizards.

If spiders’ brains shrink, it may affect their cognitive function and ability to perform these essential roles. We know from other species of birds and mammals that larger brains can help individuals survive in new urban environments, and it’s likely that this is true for spiders as well.

This study also shows that the effects of light pollution on brain development extend to invertebrates as well as birds and mammals. The effects of humanity’s infatuation with artificial light are probably much greater than we think.

This article is republished from The Conversation under a Creative Commons license. Read the original article.