Inactivity/sleep in two wild free-roaming African elephant matriarchs – Does large body size make elephants the shortest mammalian sleepers?

Abstract

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.

When elephants fall asleep: A literature review on elephant rest with case studies on elephant falling bouts, and practical solutions for zoo elephants

“…Table gives an overview on the literature of quantitative studies on elephant resting and sleeping behavior under free‐ranging and different captive conditions. For the African species, daily lying rest durations have been identified in two adult females, while such quantitative results from the wild are lacking for the Asian species (Gravett et al, ). In the wild, African elephants have been observed to go for up to 9 days without lying down to sleep (Paul Manger, personal communication).…”

Nightly selection of resting sites and group behavior reveal antipredator strategies in giraffe

“…The correlation to natural light conditions may be species‐specific, and as such further studies are required. Gravett et al () showed that light and sunset played no significant role in sleep onset in African elephants, but that environmental conditions (ambient air temperature, relative humidity) had a larger impact. Many ecological factors influence the frequency and duration of sleep.…”

Most sleep does not serve a vital function. Evidence from Drosophila melanogaster

“…Absence of movement is therefore routinely used as a proxy to measure sleep across a wide range of animals, spanning from jellyfish to elephants (16,24,25). In Drosophila too, sleep can be estimated by measuring the absence of walking bouts, generally using a commercially available device to detect whenever an isolated fly crosses the midline of a tube (26).…”

“…We know that some species -such as elephants (16) or giraffes (17) -have evolved to cope with limited amount of sleep and several genetic mutations conferring short sleeping phenotypes in flies, rodents, and humans have been characterised in the past two decades (reviewed in (18)); some animals are also able to forego sleep for days or weeks in particular ecological conditions (16,(19)(20)(21), but the identification of a constantly sleepless animal can be considered a holy grail of the field.…”