When thinking about a city at night, chances are that you include some twinkling streetlights in your mental image. Billboards, streetlamps, and neon signs are no novel presence to city dwellers. These light sources are in technical terms called artificial lights at night, or ALAN for short. While thinking about a city at night can conjure a quite picturesque image, ALAN’s impacts on animals and people alike are not all that dazzling. For humans, as for other animals that are active during the daytime (diurnal), ALAN lengthens the daily active period into the nighttime which disturbs natural sleeping and waking cycles.1 For animals that are active during nighttime, ALAN can have an opposite effect, which can make nocturnal animals less active at night.2 In addition to diurnal and nocturnal animals, animals exist which show heightened activity specifically during dawn and dusk. These animals are called crepuscular animals.
In an article published last year, researchers Tasha Oosthuizen, Neville Pillay and Maria K Oosthuizen investigated the effect of ALAN on a suspected crepuscular animal, the single-striped grass mouse (Lemniscomys rosalia).3 In nature, this African small rodent lives a herbivorous life in grasslands that provide it with cover. Oosthuizen and her colleagues report that little is known about the effect of ALAN on African small mammals, or about the behaviour of the single-striped grass mouse in general. According to Oosthuizen and colleagues, this lack of knowledge may leave these animals particularly vulnerable to the impacts of urbanization, with possible unknown consequences for the broader ecosystem of which these small mammals are a basal part.3 To help bridge the knowledge gap, and to show the impact of ALAN, the researchers conducted a series of experiments using wild caught single-striped grass mice.
Mice were caught in peri-urban areas, the areas where the city meets the surrounding countryside. The researchers used three experimental setups. In the first setup, the researchers used laboratory conditions with alternating cycles of 12 hours of darkness and 12 hours of light. The temperature in the lab remained constant during the experimentation. The second setup was to test the effect of ALAN, or artificial light at night. In this setup, the mice were also housed inside the lab, but this time a dim light was always on during the 12 hours of nighttime to simulate ALAN. In the third setup, researchers used natural conditions by placing cages outside, so that temperature and light matched the natural day-night cycle. To monitor activity, researchers used infrared cameras mounted above the cages in which the mice were housed.
In the experiments, the researchers saw that the mice were showing mostly crepuscular activity. When comparing the three setups, mice under the ALAN circumstances were least active compared to the other setups. The authors discuss that this could indicate adverse effects of ALAN, and that the decreased activity may have resulted from heightened stress due to the constant light. In the wild, it may lead to more mice being eaten by predators, or to less offspring being born. Since these small rodents are both the consumers of local vegetation, as well as food for other animals, a change in their population numbers could have ripple effects in the larger ecosystem.
But, as the authors stress in their article, a laboratory study is not the same as the natural environment of an animal. For the single-striped grass mouse, the authors tell us that high vegetation cover and tall grasses likely protect mice from predation risks by providing cover from predators, but also by protecting against ALAN exposure4. Climate change and encroaching urban expansion could however lead to a loss of vegetation cover. This could make the single-striped grass mouse vulnerable, which potentially effects the ecosystem that it is a part of. Not all is lost however, as the preservation of dense vegetation coverage can protect against ALAN.
By making the impacts of ALAN on these understudied African small rodents visible, Oosthuizen and her team add to what we know about the diverse impacts of urbanization on wildlife. This knowledge can help policy makers, researchers and others alike to make informed decisions about the natural environment surrounding cities.
Want to know more? Read the paper here:
References
- Russart, K. L. G. & Nelson, R. J. Artificial light at night alters behavior in laboratory and wild animals. J. Exp. Zool. Part Ecol. Integr. Physiol. 329, 401–408 (2018).
- Viljoen, A. & Oosthuizen, M. K. Dim light at night affects the locomotor activity of nocturnal African pygmy mice (Mus minutoides) in an intensity-dependent manner. Proc. R. Soc. B Biol. Sci. 290, 20230526 (2023).
- Oosthuizen, T., Neville, P. & Oosthuizen, M. K. Wild Mice in an Urbanized World: Effects of Light at Night Under Natural and Laboratory Conditions in the Single-Striped Grass Mouse (Lemniscomys rosalia). Chronobiol. Int. 41, 347–355 (2024).
- Teckentrup, L., Grimm, V., Kramer-Schadt, S. & Jeltsch, F. Community consequences of foraging under fear. Ecol. Model. 383, 80–90 (2018).
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