Dim light at night affects the locomotor activity of nocturnal African pygmy mice (Mus minutoides) in an intensity-dependent manner

Abstract

Rodents are integral components of ecosystems as they provide several important ecosystem services. Despite their importance as prey, pollinators and seed distributors, African rodents are largely understudied. The effect of anthropogenic changes such as artificial light at night extends past urban areas to peri-urban and rural habitats, and can have profound effects on entire ecosystems. We investigated the effect of dim light at night (dLAN) on the locomotor activity rhythms of the African pygmy mouse (Mus minutoides). Pygmy mice showed a dramatic, intensity-dependent reduction in their locomotor activity when subjected to dLAN, which was accompanied by a delay in the activity onset. We also considered masking responses with a dark pulse (DP) during the day and a light pulse at night. All animals became inactive in response to a light pulse during the night, whereas approximately half of the animals showed activity during a DP in the day. Our results suggest that the African pygmy mouse is highly sensitive to light and that their activity is strongly masked by light. In their natural environment, vegetation could shield pygmy mice against high light levels; however, other anthropogenic disturbances can alter the behaviour of these animals and could affect their survival.

Keywords: dLAN; light pollution; locomotor activity; masking; nocturnal; pygmy mouse.

Figure 1.

(a) Mean activity counts (± s.e.) of pygmy mice during a 1 h DP during the day (black bar) and 2 h prior and 2 h after the pulse (white bars). (b) Mean activity counts (± s.e.) of pygmy mice during a 1 h light pulse during the night (white bar) and 2 h prior and 2 h after the pulse (black bars). (c) An actogram of a pygmy mouse (MM13♂) that displays positive masking during the day and negative masking during the night. (d) Animal MM15♀ shows negative masking during the night and positive masking on some days during the day. (e) Animal MM2♂ did not show positive masking during the day. Animals were subjected to a 14L : 10D light cycle, including a dawn and dusk period; animals commence activity when it is completely dark and cease activity before the light comes on again. Black bars on top of the graph indicate light phases, and consecutive days are on the y-axis.

Figure 2.

(a) Mean activity hourly counts (± s.e.) during the dark, twilight and light phases of each of the light cycles pygmy mice were subjected to. (b) Mean hourly activity counts for each of the different light cycles over the 24 h period of the day.

Figure 3.

Double-plotted actograms of a representative mouse (MM1♂) for each of the different LAN cycles: (a) control, (b) 0.5 lux light at night, (c) 1 lux light at night and (d) 2 lux light at night. Consecutive days are depicted on the y-axis; greyed-out areas are the twilight and dark phases.

Figure 4.

Mean time of activity onset for 8 days after the light cycles changed, for each light cycle. The onset was later when light at night was present.