The effect of group size on sleep in a neotropical bat, Artibeus jamaicensis

Abstract

Sleep is associated with many costs, but is also important to survival, with a lack of sleep impairing cognitive function and increasing mortality. Sleeping in groups could alleviate sleep-associated costs, or could introduce new costs if social sleeping disrupts sleep. Working with the Jamaican fruit bat (Artibeus jamaicensis), we aimed to: (1) describe sleep architecture, (2) assess how sleeping in groups affects sleep, and (3) quantify total sleep time and identify rapid eye movement (REM) sleep using behavioral indicators that complement physiological evidence of sleep. Twenty-five adult bats were captured in Panama and recorded sleeping in an artificial roost enclosure. Three bats were fitted with an electromyograph and accelerometer and video recorded sleeping alone in controlled laboratory settings. The remaining 22 bats were assigned to differing social configurations (alone, dyad, triad, and tetrad) and video recorded sleeping in an outdoor flight cage. We found that sleep was highly variable among individuals (ranging from 2 h 53 min to 9 h 39 min over a 12-h period). Although we did not detect statistically significant effects and our sample size was limited, preliminary trends suggest that male bats may sleep longer than females, and individuals sleeping in groups may sleep longer than individuals sleeping alone. We also found a high correspondence between total sleep time quantified visually and quantified using actigraphy (with a 2-min immobility threshold) and identified physiological correlates of behaviorally-defined REM. These results serve as a starting point for future work on the ecology and evolution of sleep in bats and other wild mammals.

Keywords: chiroptera; electromyography; electrophysiology; sleep; social behavior; sociality.

Figure 1

Setup of social and EMG bat sleep filming at STRI laboratories. EMG filming was conducted inside the laboratory to protect equipment, and social sleep filming was conducted in outdoor flight cages, with (a) window into the flight cage, and (b) observation roost where (c) the bats were filmed. (d) Sony Handycams ~4.5 m from bats and (e) security cameras ~ 0.6 m (fitted with infrared light) were used to film the bats, and food and water were provided beneath the cameras (f). Security camera footage was recorded onto a Windows computer for analysis and allowed researchers to monitor the bats in real time (g). Cameras were setup approximately 1.5 m from the bats. Illustration by Hannah B. Tilley. See Figure S1 for photographs of this setup. EMG, electromyography; STRI, Smithsonian Tropical Research Institute.

Figure 2

Mean times spent (a) sleeping and (b) in REM‐like sleep for male and female bats. The midline of the boxplots denotes the median, and the data points represent individual bats. These plots were created using the subset data set, so the times displayed are not across a continuous 12‐h period. REM, rapid eye movement.

Figure 3

Mean total time spent sleeping and in REM‐like sleep for bats sleeping alone, in dyads, triads, and tetrads. The midline of the boxplots denotes the median, and the data points represent the overall means of each group (e.g., a single bat, or a single dyad, triad, or tetrad). These plots were created using the subset data set, so the times displayed are not across a continuous 12‐h period. REM, rapid eye movement.

Figure 4

Time each individual bat spent (a) asleep and (b) in REM‐like sleep, colored by group size. A single data point represents the duration of a sleep bout or REM‐like sleep bout; the sum of all sleep bouts for a single bat thus reflects the total sleep duration or REM‐like sleep duration. The midline of the boxplots denotes the median. These plots were created using the subset data set, so the times displayed are not across a continuous 12‐h period. REM, rapid eye movement.

Figure 5

Timing and duration of sleep bouts across the full 12‐h subjective day period (data were only collected during these 12 h), for each of the fully scored bats. Showing bats sleeping (a) alone, and in (b) dyads, (c) triads, and (d) tetrads. Data points represent the total duration of individual sleep bouts.

Figure 6

Physiological changes associated with the transition to REM‐like sleep in a solitary housed bat. (a) Bar charts in clockwise order from top‐left show muscle tone (EMG power 30−100 Hz relative to the mean of the pre/post‐REM onset periods), movement intensity (standard deviation in accelerometer magnitude), heart rate (mean inter‐beat interval) and heart rate variability (standard deviation in interbeat interval) in the 2‐min periods before and following the onset of REM‐like sleep. Standard error of the mean (SEM) bars are shown for metrics in which multiple animals were measured. Muscle tone is represented relative to the mean muscle tone between time periods for each animal. The black lines represent the individual values for each animal. (b) Muscle tone is decreased but interspersed with high amplitude muscle twitches that initiate whole body movements detectable in accelerometer recordings. Heart rate is decreased whilst heart rate variability is increased during the REM event. The REM event is followed by a brief awakening characterized by sustained movement and recovery of muscle tone and cardiac behavior. EMG, electromyography; REM, rapid eye movement.