D
o pigs fly? Perhaps not, but it may come as a
surprise that some frogs can. To be precise, they rather
glide or ‘parachute’ from the high canopy down to the
lower levels of the rainforest. Gliding is characterised by
descent at an angle of less than 45 degrees from the
horizontal (aided by lift generated by an aerofoil), while
parachuting involves an angle of greater than 45
degrees.
Some years ago, on an assignment in the Danum Valley
in Sabah, I was participating in a scientific project
involving tropical moths. One evening while checking
moth traps I got caught in a torrential downpour. It
looked like there was no end in sight, so I turned back on
an overgrown walking track towards the Danum Valley
Field Centre and shelter. About halfway there I came
across a large pig wallow. The muddy depression had
filled with yellow water and was set against a high bank
covered with overhanging vegetation. I glanced around,
and the beam of my headlamp struck a pair of bright
eyes some three metres above the wallow. Although its
body wasn’t clearly visible, I could see enough to know
that I was looking at a very large, green frog. It took me
some time to negotiate the steep bank, pushing through
the thick undergrowth towards my target, but when I
got to the spot, I could not find the frog. I tried moving
backwards and forwards, shining the light from different
angles, and had almost given up when suddenly the frog
appeared right in front of my face. It was a stunning
animal! At first, I wasn’t sure what species I was looking
at because it had large white spots and blotches all over
its body – I had never seen anything like it. Upon closer
examination, the distinctive feet revealed the frog’s true
identity – it was the elusive Wallace’s Flying Frog,
Rhacophorus nigropalmatus. Despite many visits to
Borneo, I had never previously encountered this
beautiful frog and the pictures in books and on the
internet showed only plain green specimens, some with
fine white dusting but none with such large white spots.
Wallace’s Flying Frog has a fascinating history. It was
first illustrated by Alfred Russell Wallace in 1869 in
Sarawak, Borneo, but amazingly the species was only
formally described by Boulenger more than 25 years
later, in 1895. Wallace wrote in his book The Malay
Archipelago: ‘One of the most curious and interesting
reptiles which I met with in Borneo was a large tree-frog,
which was brought me by one of the Chinese workmen.
He assured me that he had seen it come down in a
slanting direction from a high tree, as if it
flew. On examining it, I found the toes very
long and fully webbed to their very extremity,
so that when expanded they offered a
surface much larger than the body.... This is, I
believe, the first instance known of a "flying
frog," and it is very interesting to Darwinians
as showing that the variability of the toes
which have been already modified for pur-
poses of swimming and adhesive climbing,
have been taken advantage of to enable an
allied species to pass through the air like the
flying lizard.’
But R. nigropalmatus is by no means the only
‘flying frog’ in existence. Other species from
the families Hylidae (in the New World) and
Rhacophoridae (Old World) have also evolved
this peculiar adaptation for gliding; all having
enlarged, fully-webbed feet that provide
physical resistance when travelling through
the air. Wallace’s Flying Frogs can apparently
glide a horizontal distance of more than 15
metres and are quite adept at mid-air
manoeuvres. Nevertheless, there are many
species of canopy-dwelling frogs all around
Left: original watercolour painted by Wallace
in Sarawak. This was used as the basis for the
woodcut illustration that appeared in The
Malay Archipelago.
Right: all flying frogs have enlarged, fully-
webbed feet.