Temperature explains variation in seasonal temporal activity of Miniopterus natalensis more than moonlight or humidity

Abstract

Animals partition their daily activities based on a variety of internal and external factors. For insectivorous bats, the availability of prey, weather conditions, moonlight and reproductive energy demands are proposed as the major influencers of temporal activity. This study investigated the temporal variation in activity of the subtropical cave-dwelling, migratory Natal long-fingered bat, Miniopterus natalensis, by temperature, humidity, lunar illumination and the time of moonrise per night. Using echolocation survey data collected 2014–2018 near a maternity roost in north-eastern South Africa, differences in activity between the summer maternity (1 September–28 February) and winter non-breeding (1 March–31 August) seasons were described. In the non-breeding season, M. natalensis exhibited peak activity during the early evening, with decreasing activity throughout the night. In the maternity season, a bimodal activity pattern was observed, probably related to peaks in insect activity and the energy requirements of lactating females. Predictably, temperature had the strongest positive influence on the magnitude of M. natalensis activity across both seasons. Humidity also positively affected activity in both seasons to a lesser extent. The time of moonrise did not affect the activity of M. natalensis during either of the seasons. Lunar illumination did not affect activity during the maternity season, but activity was diminished on nights with bright moonlight in the non-breeding season. Emergence and return times were not affected by lunar illumination in either season. The need of M. natalensis to maximise resource acquisition during the maternity season is likely a strong modulator in the temporal activity of this species and outweighs predation risk in high moonlight conditions. Subsequently, this species exhibits nightly activity behaviours that balance risk factors with intrinsic needs during different seasons.