Bouts of rest and physical activity in C57BL/6J mice

The objective was to exploit the raw data output from a scalable home cage
(type IIL IVC) monitoring (HCM) system (DVC®), to characterize the pattern
of undisrupted rest and physical activity (PA) of C57BL/6J mice. The
system’s tracking algorithm shows that mice in isolation spend 67% of the
time in bouts of long rest (≥40s) and 59% of the time was interpreted as
sleep. Twenty percent is physical activity (PA), split equally between
local movements and locomotion. Decomposition revealed that a day contains
~6500 discrete bouts of short and long rest, and local and locomotor
movements. Mice travel ~330m per day, mainly during the dark hours, while
travelling speed is similar through the light-dark cycle. Locomotor bouts
are usually <0.2m and <1% are >1m. Tracking also
revealed fits of abnormal behaviour. The starting positions of the bouts
showed no preference for the rear over the front of the cage floor, while
there was a strong bias for the peripheral (75%) over the central floor
area. The composition of bouts has a characteristic circadian pattern,
however, intrusive husbandry routines increased bout fragmentation by
~40%. Extracting electrode activations density (EAD) from the raw data
yielded results close to those obtained with the tracking algorithm, with
59% of the time in long rest (<1 EAD s-1) and 20% in PA. We confirm
that EAD correlates closely with movement distance (rs>0.95) and
the data agreed in ~96% of the file time. Thus, albeit EAD is less
informative, it may serve as a proxy for PA and rest, enabling monitoring
group-housed mice. The data show that a change in housing density from one
to two, and up to three mice had the same effect size on EAD (~2) with no
difference between sexes. The EAD deviated significantly from this
stepwise increase with 4 mice per cage, suggesting a crowdedness stress
inducing sex-specific adaptations. We conclude that informative metrics on
rest and PA can be automatically extracted from the raw data flow in
near-real time (< 1 hrs). These metrics relay useful longitudinal
information to those who use or care for the animals.