Sleep in the lesser mouse-deer (Tragulus kanchil)

The mouse-deer or chevrotains are the smallest of the ungulates and ruminants. They are characterized by a number of
traits which are considered plesiomorphic for the Artiodactyla order. The objective of this study was to examine sleep in
the lesser mouse-deer (Tragulus kanchil), which is the smallest in this group (body mass < 2.2 kg). Electroencephalogram,
nuchal electromyogram, electrooculogram, and body acceleration were recorded in four adult mouse-deer females using a
telemetry system in Bu Gia Map National Park in Vietnam. The mouse-deer spent on average 49.7 ± 3.0% of 24 h in nonrapid
eye movement (NREM) sleep. REM sleep occupied 1.7 ± 0.3% of 24 h or 3.2 ± 0.5% of total sleep time. The average
duration of REM sleep episodes was 2.0 ± 0.2 min, the average maximum was 5.1 ± 1.1 min, and the longest episodes lasted
8 min. NREM sleep occurred in sternal recumbency with the head held above the ground while 64.7 ± 6.4% of REM sleep
occurred with the head resting on the ground. The eyes were open throughout most of the NREM sleep period. The mousedeer
displayed polyphasic sleep and crepuscular peaks in activity (04:00–06:00 and 18:00–19:00). The largest amounts of
NREM occurred in the morning (06:00–09:00) and the smallest before dusk (04:00–06:00). REM sleep occurred throughout
most of the daylight hours (08:00–16:00) and in the first half of the night (19:00–02:00). We suggest that the pattern and
timing of sleep in the lesser mouse-deer is adapted to the survival of a small herbivorous animal, subject to predation,
living in high environmental temperatures in the tropical forest undergrowth.