December 2019 BBC Wildlife 65
the case for hazel dormice and fat dormice.
Bouts of torpidity are regularly interrupted
by periods of so-called ‘euthermia’, when
the animal heats up, wakes up and may
move around for several hours, or even
longer, breaking its hibernation. This is a
good opportunity to expel waste products
and, in certain conditions, have a snack.Bats, especially pipistrelles, will
sometimes forage outside for
insectson warm winter nights, soon
returning to hibernation with a slightly
fullerstomach. Hedgehogs are roused
naturally about once every 5–27 days.
Twotothree times each winter, they will
usethese breaks to relocate to another nest.
Bats,too, sometimes change roost sites
duringthe winter, especially if their roost
itebecomes too hot or cold.
Theidea that hibernation includes
regularwaking up and activity breaks
comesas a surprise to many people. But the
rules of hibernation are more flexible still.
Take the example of the hazel dormouse,
an animal that is famous for its long sleeps.
In Britain, it usually hibernates between
October and April or May. However, that
same dormouse species also occurs in parts
of the Mediterranean, where it doesn’t enter
hibernation at all. Similarly, some bats insouthern or even central Europe eschew
hibernation, or only remain dormant for
a month or so. Other species, including
the hedgehog, practise what is known as
facultative hibernation – they hibernate
when it is apposite to do so. If they are
warm and well-fed, they don’t. Incidentally,
the fat dormouse is what is known as an
obligate hibernator and invariably enters into
dormancy, typically for at least six months.Summer siestas
The plot thickens, too, when you look at
the activity of some of these animals in
warmer seasons. Dormice sometimes
become torpid – in which the metabolic
rate drops as in hibernation – in times of
food shortage in the summer, and bats
routinely become torpid for the same
reason. It is known as aestivation. It isn’t
true hibernation, because the animal rarely
remains torpid for more than 24 hours.Left: fat dormice
spend most of
their time in
the tree canopy
but come down
to the ground
to hibernate.
Above: taking
the opportunity
to have a stretch
in a nest of fat
dormice. Right:
the common
frog mayretreat under
rocks or embed
themselves in
mud underwater
as winter takes
hold. Below:
brimstone
butterflies shun
cold days –
spotting one in
flight is taken
as a sign that
the seasons
are changing.clockwise from top left: Klaus Echle/naturepl.com; ARCO/naturepl.com; frog: Alamy; David Plummer/Alamy