Energy saving in day-roosting female Myotis emarginatus during reproduction (Chiroptera: Vespertilionidae)

Abstract

From May to June 2012, we studied the behaviour of day-roosting reproductive female Geoffroy’s bats in a maternity roost in eastern Burgenland (Austria) which is under surveillance from 2011 till to date. By using a remote-controlled infrared-illuminated video camera, we conducted six weekly sessions of direct observation and instantaneous scan sampling, each lasting 16 hours. Based on a total of 384 sampling sessions, we quantified the amount of time adult females spent in the activities resting, alert, grooming and relocating during pregnancy and lactation. Ambient and roost temperatures were recorded hourly, the numbers of individuals returning to the roost in the mornings were registered constantly by using an infrared light barrier. Over the entire study period, all bats arriving in the maternity roost in the morning formed immediately a single huddling cluster. As a rule, this cluster was large, multilayered, three-dimensional and tight. It did not change in size and form until the onset of pre-emergence activities. It consisted of an interior part in which about 50% of all bats roosted and the periphery consisting of those bats which had not succeeded in entering the interior. Over the entire diurnal stay in the roost, resting – which causes the least energy output – was the predominant behaviour of all roost mates. Significant differences were found, however, in the amount of time allocated to some activities by bats occupying different positions in the cluster. While bats in the interior of the cluster spent the estimated 90–95% of the entire day-roosting period resting, bats on the periphery spent only 57–73% resting. The average percentage of time allocated by peripheral bats to grooming decreased from 27% in the first week to 19–13% in the following weeks of pregnancy and stayed at 16% during the two weeks of lactation. The mean percentages of being alert and of relocating ranged between 7–10% and 4–7%, respectively. During the last two weeks of pregnancy and the two weeks of lactation, roost temperatures, daily colony size and reproductive states did not influence the huddling behaviour significantly. However, activities performer in the first and second week were probably influenced by unrest due to colony formation after the return from hibernation (week 1) and by cold ambient temperatures during the week 2. Our study supports the hypothesis that the short duration and notable timing of reproduction typical for Myotis emarginatus (Spitzenberger & Weiss 2020) is achieved by maximal energy saving through continuous huddling in a large, three-dimensional, multilayered and tight cluster over both the entire day-roosting and entire reproductive period, differences in the behaviours of bats located in the interior and on the periphery of the cluster and lack of social interactions between roost mates.