BBC Wildlife - UK (2019-12)

HIBERNATION

66 BBC Wildlife December 2019

So far, we have only considered

endotherms, animals that generate their

own body heat. In its strictest sense, true

hibernation only refers to endotherms

and, in Britain, in the absence of any

hibernating birds, it is confined to dormice,

bats and hedgehogs. However, as we all

know, exotherms, or cold-blooded animals,

also need to pass the winter. Many of

these, such as reptiles, amphibians and

insects, also spend large amounts of the

year in a dormant state. Strictly speaking,

this isn’t hibernation, because while

mammals, as we’ve seen, may ‘choose’

to lower their metabolic rate and body

temperature, exotherms have no choice

• the outside temperature lowers it for
  them. Nonetheless, many aspects of their
  behaviour are equivalent to hibernation.
  For example, frogs, toads and newts all
  change their behaviour as soon as the frosts
  start,inOctober.Allretreattosecluded
  spotsonland,awayfromdirectexposure
  totheelements– underlogsorpilesof
  stones,insidea holeinthegroundorina
  compostheap,forexample.Thelatterare
  particularlyfavouredbyslow-worms,often
  ingroups,whileotherlizardshibernate
  aloneinsmallhollows.Natterjacktoads
  buryintothesand,whileallBritish
  snakesselectsitessuchasdisusedrabbit
  burrowsforcommunalquartersknown
  ashibernacula.Onoccasion,toads,newts,
  lizardsandevensnakeswillallgravitateto
  thesamehollow,formerenemiesentering
  intoa sleepytruce.Alloftheseexothermic
  vertebratescanberousedbywarmwinter

days – frogs may hunt for food and snakes

bask in the weak sunshine.

The big exception among herptiles is the

common frog, the adult males of which

often winter in the mud at the bottom of

ponds. They can breathe simply by the

exchange of gases through their skin, rather

than the lungs, and since they are inactive,

they burn very little internal fuel. In most

winters, the arrangement is perfect, but

fatal if the pond freezes solid.

Delayed development

Insects utilise many different strategies for

overwintering. We are often accustomed

to asking the question of an insect: “How

long does it live?” The typical answer for

a butterfly, moth or dragonfly tends to

be about two weeks, but this, of course,

is quite wrong, because that’s only the

adult stage. An insect is alive whether

it’sanegg,a larva,a pupaoranadult,

orwhateverinstarit mightbein.One

ofthestrategiesforinsectspassingthe

winteris topausedevelopmentduring

oneoftheimmaturestages.Thisdelayin

developmentis calleddiapauseandthough

it isn’thibernation,it hasthehelpfuleffect

ofavoidingseasonsofcoldandlackof

food,andpromotingsurvival.

Thepreparationfordiapausecanbeas

profoundasanychemicalchangeintrue

hibernation.Manyinsectsproducean

anti-freezingcompound,suchasglycerol,

intheirtissuestoallowthemtosuper-cool,

theirbodytemperaturesdroppingbelow

freezingpointbutnotactuallyfreezing.

Though a likely candidate

for hibernation, shrews

are active year-round.

Right: badgers take

refuge underground to

avoid the sting of winter.

Shrew: Colin Varndell/naturepl.com; badger: Naturfoto-Online/Alamy