Koalas
are adorable and that
alone makes them a wor thy topic to write
about. These animals spend approximately 20
hours each day sleeping or gripping trees and
have evolved specialised features for climbing
and t ree - hugg i ng. However, i t
is unlikely that koalas hug trees
just to be adorable. So, why do
they spend so much time hugging
trees? More specifically, what are
the evolutionary advantages of tree-
hugging?
Australian summers are brutally
hot. Some terrestrial animals maintain a stable
body temperature by migrating to cooler
subterranean areas, or by losing heat to the
ground through conduction. Other animals
such as dogs and kangaroos lick themselves
and rely on evaporative heat loss. When
water molecules evaporate off of the animal’s
skin, latent heat of vaporisation is absorbed
by the water, i.e., the heat is absorbed from
the sur roundings when liquid is conver ted
to gas, leavi ng the sur face temperature
cooler than it was before. Humans also use
this mechanism of thermoregulation through
sweat evaporation.
Koalas, on the other hand, are arboreal
animals, meaning that they are adapted
to living on trees. They are physically tied
to their habitats and cannot easily migrate
to cooler areas during Australia’s extreme
summers. These summers are also incredibly
dr y and water sou rces are scarce. Wi ld
koalas rarely dr ink, instead they
obta i n mos t of t he i r wate r
intake through the eucalyptus
leaves they eat. The scarcity of
water therefore limits the koala’s
ability to thermoregulate through
evaporation.
Tree bases and tree trunks are significantly
cooler compared to the ai r, presumabl y
because the xylem (transpor t tissue) stores
large amounts of ground water. The surface of
the trunk provides a conductive heat sink for
koalas in the summer. A recent report showed
that under hot conditions, koalas were more
likely to adopt a tree-hugging posture, which
exposed more of their body surface area to the
cool surface. During the experimental period,
koalas most frequently hugged a non-food
tree species,
Acacia mearnsii
, which had the
coolest temperatures.
I s this tree hugging method ef fective?
On a 35°C day with a wind speed of 0.1 m/
s, a completely shaded 11.3 kg male koala is
required to lose 10.33 watts of heat in order
to maintain a constant body temperature (a
koala’s body temperature ranges from 35°C
to 37°C). The koala can lose up to 7.07 watts
of heat by hugging the lower trunk of an
A.
mearnsii
alone. In other words, it only needs to
lose an additional 3.26 watts of heat through
evaporation – 68% less than no hugging!
E v a p o r a t i v e h e a t l o s s i s a c o s t l y
thermoregulatory mechanism for it exhausts
Why do
Koalas
Hug Trees?
By Wai Lam Raphaella So
蘇韋霖
