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.
Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in evolution in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of afrotherians, believed to be the first clade to radiate from the common eutherian ancestor. We find that afrotherians share non-neuronal scaling rules with rodents, primates and eulipotyphlans, as well as the coordinated scaling of numbers of neurons in the cerebral cortex and cerebellum. Afrotherians share with rodents and eulipotyphlans, but not with primates, the scaling of number of neurons in the cortex and in the cerebellum as a function of the number of neurons in the rest of the brain. Afrotheria also share with rodents and eulipotyphlans the neuronal scaling rules that apply to the cerebral cortex. Afrotherians share with rodents, but not with eulipotyphlans nor primates, the neuronal scaling rules that apply to the cerebellum. Importantly, the scaling of the folding index of the cerebral cortex with the number of neurons in the cerebral cortex is not shared by either afrotherians, rodents, or primates. The sharing of some neuronal scaling rules between afrotherians and rodents, and of some additional features with eulipotyphlans and primates, raise the interesting possibility that these shared characteristics applied to the common eutherian ancestor. In turn, the clade-specific characteristics that relate to the distribution of neurons along the surface of the cerebral cortex and to its degree of gyrification suggest that these characteristics compose an evolutionarily plastic suite of features that may have defined and distinguished mammalian groups in evolution.
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