In this study, we demonstrate for the first time in a nocturnal primate that light pollution changes the preference of use of corridors, modifies the locomotor pattern, limits the ability of animals to move from one area to another and to efficiently exploit their environment according to light intensity, and thus fragments the habitat.
Indeed, during the experiment, the total number of crossings through both corridors, illuminated or not, decreased according a light intensity-dependent relationship. Especially, the number of crossings decreased and the speed of crossing significantly increased when using the illuminated corridor. Consequently, light pollution limited the ability to move and changed the locomotor pattern with animals avoiding illumination. In addition, according to light intensity, mouse lemurs reduced the time spent in cage 2, i.e. the time allocated to foraging. Similar results have been previously reported in male mouse lemurs exposed to light pollution. Indeed, animals under illumination reduced their locomotor activity, spent less time outside their nest box, tended to spend less time feeding outside and brought more fruits into the nest box 35. However, in both the previous and present study, there was no effect neither on the daily caloric food intake nor on body mass, most likely because food was provided ad libitum. Nevertheless, in another study, under constant illumination (free-running conditions), animals did not feed at all because of the strong inhibitory effect of light on general activity 27. Similarly, in the Darwin’s leaf-eared mouse (Phyllotis darwini), animals exposed to simulated moonlight carried 40% of their food to the refuge site, consumed 15% less food during the experiment and lost 4.4 g in body mass (around 10% of the mean body weight in this species) in only one trial night 36. This response probably reflected an anti-predator behaviour because light affects visual abilities in both nocturnal predators and preys and the perceived risk of predation increases with illumination of the environment 37,38. Consequently, to minimize predation risk, preys limit their general activity even at the cost of loss of body mass. In the wild, including Madagascar, where food availability is highly unpredictable 39, the impact on body mass could be increased. All these results confirm that light pollution fragments the habitat and limits the ability of animals to move and to effectively exploit their environment and its resources and could, therefore, threaten survival. In Madagascar, light pollution could exacerbate the detrimental effects of habitat loss and fragmentation observed in the past century on all lemur species. As they become increasingly confined to smaller and isolated forest patches, their population numbers, genetic diversity, and distribution have been altered, sometimes leading to local extinctions 40–42. Given that light pollution in Malagasy protected areas has been relatively low but gradually increasing in the past decades 43, it is crucial to promptly address and mitigate its potential future effects on habitat fragmentation.
In our experiment, the apparatus provided two corridors. Interestingly, while the number of crossings through the illuminated corridor decreased according to light intensity, it increased proportionally through the black corridor. In addition, the speed of crossing through the black corridor did not change throughout the study. Concretely, when conditions of locomotion through one corridor were not optimal (i.e. illumination), mouse lemurs reversed their preference on the other (i.e. without illumination). However, this compensation was only partial because despite the presence of an optimal corridor, the total number of crossings through both corridors decreased according to light intensity. This point is crucial for the implementation of conservation plans. For example, in France, following the multiparty debate on the environment that took place in 2007, the conservation plan ‘green and blue Frame’ was initiated. This plan has for objective to reconstitute a network (reserves and corridors) on the national territory allowing animal species to communicate, to circulate, to feed and to reproduce within protected natural areas 44. However, this plan has been initiated without recommendations relative to light pollution. Today, several associations for environmental protection ask for the implementation of a complementary ‘nocturnal frame’ to protect living organisms from the impacts of light pollution (ANPCEN, 2013), which has been included in the french Biodiversity Act since 2016 (Assemblée nationale, 2016). Considering that 28% of vertebrates and 64.4% of invertebrates live exclusively or partially at night, i.e. many species susceptible to be disturbed by light pollution 47 and considering the extent of light pollution, this inclusion is relevant. Indeed, in our study light pollution extended on 50 cm only and habitat fragmentation occurred on a fine scale. However, for a city of 10,000 inhabitants, the sky glow extends up to 20 km and can extend up to 120 km for a city of 1 million inhabitants 48. Consequently, urban light pollution could fragment the habitat on a broad scale. Recently, Mu et al published an evaluation of light pollution in global protected areas from 1992 to 2018 and reported that there was a significant or a trend of increase in nighttime light in 53% of polluted protected areas in Europe, 78% in South America, and 81% in Africa 49. In these impacted areas, light pollution, homogenizing the physical environment and being detrimental to photosensitive species, could reinforce biotic homogenization, threaten biodiversity and defeat conservation plans 50. On the other hand, Japan and the United States of America (USA) exhibit opposite trends (In Japan, 85% of polluted protected areas demonstrated a significant or trend of decrease in nighttime light, while in the USA, the figure stood at 65%), highlighting the significant and positive impact that well-planned ecological conservation policies can have on mitigating light pollution 49.
Finally, our study shows that light intensities commonly found in urban area or recommended by the European standard EN-13201 on public lighting are sufficient to fragment the habitat. This point demonstrates the necessity to find a trade-off between human needs and environmental protection, by implementing public lighting policies, urbanization plans and conservation plans that take the light pollution factor into account.