When you're small, some things are inherently risky. Some animals solve this problem by performing these tasks, such as venturing out for food, under the cover of darkness, when it's less likely that they'll be seen. As humans continue to artificially illuminate the night sky, the risk of being caught and potentially eaten becomes higher. This leaves animals in a bit of a conundrum: they need to find food and mates, but they also need to live long enough to find them. So how might tiny animals such as the plum woodlouse (Porcellionides pruinosus), that must venture out from under the cover of fallen leaves, cope with moving about during the brighter nights? This question led Andrea Dissegna and Cinzia Chiandetti of the University of Trieste, Italy, to find out how bold these tiny crustaceans are when they need to move about in light that's brighter than the moon.
To see how bold the woodlice are when it's brighter, they first needed to know how bold they are in the dark. They put each isopod under a black box with an end open as an exit and waited to see how long it took them to come out. In the dark, it took the small woodlice ∼193 s before they were brave enough to leave the shelter of the box. Afterwards, the scientists allowed the isopods to move around outside of the box and recorded how much time they spent stationary and how quickly they walked. Then, Dissegna and Chiandetti made the nights brighter for woodlice, with some experiencing the brightness of a full moon (1 lx) while others experienced the brightness of being near a streetlamp (10 lx). The scientists then tested the tiny crustaceans again. This time, it took the woodlice nearly 3 min longer to emerge from their shelter. Once out, they walked at a slightly slower speed, suggesting that they were less confident moving about without the shelter of the box.
Next, the researchers tested how the woodlice would react to a presence looming overhead like a predator and how long it took them to realise that there was nothing to worry about. So, the team placed the isopods in a Petri dish and moved a black paddle over them. Normally, the woodlice would freeze as the paddle loomed above them, but with each successive pass of the paddle, they would spend less time immobile. After experiencing the light pollution, the isopods that had the full moon brightness stayed frozen in place for 624.98 ms longer, while those whose nights were as bright as a streetlamp remained still for more than 1 s longer than they normally would. This suggests that the woodlice took longer to learn that the paddle was not a predator, potentially making it take longer for them to find food. But the light pollution might not only change the behaviours themselves, it might also change the relationship between the behaviours.
For instance, normally, the woodlice that emerged the earliest were less likely to spend time standing still. However, for the isopods that experienced the brightest nights, this relationship was reversed, with the animals that spent the most time standing still being the ones that came out from under their shelter first. For the ones that experienced the full moon, there was no relationship at all between these behaviours. ‘This is puzzling for me and suggests that there is still a lot of work to do to understand how light pollution influences an invertebrate’s behaviour’, says Dissegna. Although light pollution makes woodlice take fewer risks, these crustaceans will need to be a bit bolder if they are to survive in our illuminated world.
