The current study provides details of sleep (or inactivity) in two wild,
free-roaming African elephant matriarchs studied in their natural habitat with
remote monitoring using an actiwatch subcutaneously implanted in the trunk, a
standard elephant collar equipped with a GPS system and gyroscope, and a
portable weather station. We found that these two elephants were polyphasic
sleepers, had an average daily total sleep time of 2 h, mostly between 02:00 and
06:00, and displayed the shortest daily sleep time of any mammal recorded to
date. Moreover, these two elephants exhibited both standing and recumbent sleep,
but only exhibited recumbent sleep every third or fourth day, potentially
limiting their ability to enter REM sleep on a daily basis. In addition, we
observed on five occasions that the elephants went without sleep for up to 46 h
and traversed around 30 km in 10 h, possibly due to disturbances such as
potential predation or poaching events, or a bull elephant in musth. They
exhibited no form of sleep rebound following a night without sleep.
Environmental conditions, especially ambient air temperature and relative
humidity, analysed as wet-bulb globe temperature, reliably predict sleep onset
and offset times. The elephants selected novel sleep sites each night and the
amount of activity between sleep periods did not affect the amount of sleep. A
number of similarities and differences to studies of elephant sleep in captivity
are noted, and specific factors shaping sleep architecture in elephants, on
various temporal scales, are discussed.
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