A mouse eating in the illumination chamber. Photo credit: Sophia Anne Marie B. Villanueva.
A mouse eating in the illumination chamber. Photo credit: Sophia Anne Marie B. Villanueva.
As civilisation has progressed, it has become more and more important for us to be active well into the dark hours of the night. Doing so has also caused us to illuminate the nighttime world. However, we have neglected the problems that our bright nights have on animals that are active when the sun goes down. Many wild rodents rely on the darkness to hide them from predators while they go about their nightly activities and are even reluctant to be out and about under the light of a full moon. But rodents don't see like humans do. While we have three types of color-sensing cells in our eyes (called cones), mice only have two – which sense green light and blue/ultraviolet light. As the eyes of many rodent species are very similar, Travis Longcore, Sophia Villanueva, Kyle Nguyen-Ngo, Cristina Ghiani and Christopher Colwell of the University of California Los Angeles, USA, and Benjamin Harrison of Korrus, Inc., USA, believe that the house mouse (Mus musculus) might be able to tell us whether the brightness or the color of the lights we use at night are causing problems for wild rodents.
Two hours after turning off the lights for the night, Longcore and colleagues interrupted the darkness in the lab by giving their mice an hour-long illumination using custom LED lights that can change the brightness and color of the light. Each mouse experienced light that was 0.01 lux (similar to a quarter moon), 0.5 lux (a bit brighter than a full moon), 5 lux (similar to twilight) and 50 lux (similar to a lit room in a house) in white, yellow and orange in hue. The researchers made sure each mouse experienced each combination of brightness and coloration, waiting 6 days in between. The team then recorded each mouse's activity by noting the number of revolutions they ran on a wheel during the time they experienced the artificial illumination. Unsurprisingly, the mice ran more when the lighting was dimmer, running the most when the lights were as bright as a quarter moon. They also ran more when the light's hue was closer to the red color that the mice don't see as well. However, when the lights were dimmest, the color of the light didn't seem to matter much at all. This suggests that the brightness of the light is more important to the mice than the color. These somewhat brief illuminations are enough to stop the mice from running as much, but are they enough to change their daily cycles as well?
To find out, the scientists gave some mice hour-long illuminations of the brightest light in both yellow and white hues, and then looked to see whether this changed the activity of the nerve cells in the suprachiasmatic nucleus – the part of the brain that controls their internal clock. Surprisingly, Longcore and colleagues didn't see any changes in the activity of the nerve cells. It could be that ‘the colors are too similar to elicit a response’, explains Longcore. He goes on to say that ‘perhaps the behavioral response is just more sensitive’.
Understanding how the brightness and color of the lights we use to illuminate our night skies affect animals can help governments decide how best to conserve certain species. In this case, it seems that dimming the lights is more important, but if the light needs to be brighter, then changing the color can help. Although we still need to illuminate the night, maybe dimming our lights will ensure a brighter future for mice and other animals that go about their business in the darkness.
