Ryan Photographic - Essay - Fiji frogs
Fijian Frogs
The Fiji tree frog, Cornufer vitiensis, formerly known as Platymantis vitiensis, is probably rarely
seen rather than rare. Because it spends most of its life above ground it
does not compete with the giant toad for food and it is not frequently exposed
to predation, except perhaps from the Pacific boa and a very lucky bat. The eggs are laid in the leaf axils of the pandanus, and presumably
in the common lily Collospermum montanum from which I have collected
a large number of adults. There doesn’t seem to be any evidence to
suggest that the adults stay with the eggs although I have seen adult females nearby. Little is known about their mating
habits although it is possible that fertilisation is internal. I used to
think that they were mute, as in over 50 hours in the rain forest studying
them I had never heard them produce a sound. This view changed rapidly when
I showed Fang, my pet boa, some captive specimens. Before I had a chance
to do anything, Fang had a struggling frog in her jaws. This animal produced
a loud and penetrating scream, which is presumably designed to warn other
frogs of danger. The unfortunate tree frog puffed up with air and resisted
all of Fang’s attempts to swallow it. I released the hapless frog and
it settled back down, full of puncture holes from Fang but apparently otherwise
just fine.
Fergus Clunie, the ex-Director of the Fiji Museum kept a pet tree
frog. It started to call … a soft double tap drip noise, repeated at
regular intervals. This was exciting enough but better was to follow. Fergus
thought the animal was a male, a view that Fergus had to change when it laid
a clutch of eggs. Calling in female frogs is not totally unheard of but there
are few species in which this behaviour is known. Sometime later John Gibbons
heard a male softly replying to a female call. In 1997 French researchers
Boistel and Sueur confirmed these observations and published additional information
about the calls. The female tree frog lacks a vocal sac and her call is not
as complex as that of the male.
We know a little more about their breeding habits now too. I was fortunate
enough to observe a captive pair in amplexus (the term for amphibian mating).
They looked rather strange as the female weighed six times as much as the male.
The large 10g female was clamped around her groin by the little 1.5 g male. Frogs
are a little more restrained than human beings as regards mating position;
no Kama Sutras for them. The majority use an axillary grip in which the male
clasps the female around the armpits, while the rest use an inguinal position
in which he grasps her just above her hind legs. The tree frog uses an inguinal
position but we know nothing as yet about the ground frog.
I have collected and released over 500 tree frogs from various localities.
They display a bewildering variety of colours and patterns. With 70 frogs
from Monasavu in front of me I tentatively sorted them into 18 different
colour patterns, but even then the decisions were totally subjective. Colours
range from light creamy grey through browns, tans, orange, greeny-greys (but
no true greens) to brilliant yellow. Hourglass shaped dark markings between
the shoulders and the eyes may or may not be present. Some have a thin white
streak down the back, others a broad yellow stripe. Reproductively mature
individuals have a brilliant yellow flash inside the thigh and groin.
A few Fiji tree frog color morphs
From field experiments I have found that frogs spend much of their
time in the same tree. If they do wander, it is usually to low shrubs adjoining “their” tree.
The wetter it is the further afield they appear to move. These little frogs
are accomplished jumpers and nearly always twist in the air so as to land
facing a different direction. As the next leap is often at 90oor even 180o
to the direction expected they frequently elude capture.
I recently returned from a wonderful visit to Fiji. During
this trip I made the drive to Namosi. More in hope than expectation I visited
a
small grove? of giant swamp taro by the side of the road. To my delight the
eight or so plants contained over a dozen small tree frogs.
All were in the water-filled leaf axils and all initially were oriented head
upwards but turned and swam further into the leaf axil when disturbed. I suspect
they were "young of the year" from eggs laid in December or January. It is
encouraging that such a casual investigation revealed an apparently thriving
population.
The future of the tree frog is probably secure, but life is much more
difficult for the ground frog (Cornufer vitianus formerly Platymantis vitianus). I suspect
that the giant toad and the ground frog compete for food. The ground frog
does not always hunt prey in the "normal" frog manner. That is, it does not always
flick out a long sticky tongue to catch prey. Instead, ground frogs sometimes
leap at flying or crawling insects and catch them in the mouth. The hands
may assist in subduing struggling food. During the day they hide under logs or
lie immobile pressed flat against the ground in a small depression, while
at night they come out and hunt.
According to most authorities the ground frog is a rain forest inhabitant.
Gorham has found them along the banks of streams and states that they are
expert swimmers and that their speed in the water “reminds one of a
fish darting”. However in Seemann’s time they were apparently
more widespread and as he said (1862): “A large frog, Boto or Dreli
(Playmantis Vitianus) (sic) is common about the swamps.” Perhaps they
were more widespread until the introduction of the giant toad or the mongoose
and have been steadily eliminated from their former range until only the
rain forest is left as a habitat.
Whatever the true situation, my preconceptions of ground frog distribution
received a rude shock after a visit to Viwa Island in Bau Water. My generous
and gracious Fijian hosts assured me that ground frogs were found along the
beach at night. I was highly sceptical but nonetheless ventured forth when
it got dark. I was all set to give up after an hour’s fruitless search,
when we suddenly secured our first specimen. A very large bare-footed Fijian
trod on the poor animal and let out a blood-curdling scream. I duly measured,
weighed and toe-clipped this animal and by the end of the evening seven frogs
had received this treatment and I had become a believer. This evening marked
the start of regular visits to Viwa to collect data, and to really rub salt
into the wound, so to speak, one of the frogs produced a large faecal pellet,
which contained the remains of a crab. Why are the ground frogs on Viwa so
unusual? I suspect that they are still present on Viwa because there are
no mongoose there. Whatever the reason, this is an unusual habitat for a
frog, reminiscent of Cornufer papuensis in New Britain or even the crab
eating frog Rana cancrivora in South East Asia. Perhaps one of these
Asian salt tolerant frogs made the journey to Fiji via the Solomon Islands.
Although supposedly closely related to the tree frog, the ground frog
is much larger, growing to 106 mm snout-vent length, compared with 54 mm
for the tree frog, and does not have adhesive suckers on the fingers. Despite
this it can still climb and occasionally can be found up low bushes. On Viwa
they must be capable of negotiating almost sheer banks unless they hide under
beach stones during the day. They are marvellously athletic and can cover
up to 1m during one leap. Unlike most frogs they can put together a series
of 10 to 20 giant leaps in a row. During the weighing operations on Viwa
one disgruntled individual got away from us. We had to run 40m down the beach
after it and were lucky to recapture it.
Cornufer vitianus , Fiji ground frog, on rocks on beach at Viwa Island
When threatened they will puff themselves up with air and also release
copious quantities of bladder water. The air makes them look bigger and the
loss of bladder water distracts a predator and makes the frog considerably
lighter and thus capable of greater leaps. When under duress they will occasionally
vocalise with a series of short, almost birdlike croaks.
Unlike the tree frog, P. vitianus does not exhibit a huge
range of colour pattern. Usually they are a dull grey-brown but reddish and
light tan individuals do occur. Some possess a thin mid-dorsal line and nearly
all have a white spot or blotch behind the eardrums. Ground frog eggs have
not been reported from the wild but captive specimens have laid eggs. Development
is similar to that of the tree frog.
The two native frog species differ from the introduced giant toad
(Rhinella marina) in a number of ways. The most important is their
lack of a free-swimming tadpole stage. The large (7mm) eggs contain sufficient
yolk for the larva to undergo complete development in the egg. I have studied
a number of tree frog clutches and have reared them through to froglets. The
metamorphosis is fascinating. The baby frog starts off life as a fertilised
cell lying on top of a large pale yellow, almost white, yolk. The eggs are crystal clear
with the yolk filling approximately half of the volume. They look very similar
to small glass marbles. Over the next two to three days a streak develops
along the yolk as the cells divide. It soon curls itself to form a tube.
By five days a head and tail can be distinguished and limb buds have formed.
The embryo becomes progressively more frog-like and often rotates rapidly
inside its self-contained swimming pool. The developing frog lies on top
of the yolk until around the second week when its weight causes it and the
yolk to rotate and lie upside down. The froglet stays this way until near
the end of the fourth week, when it once more rotates to lie on top of the
yolk. It remains in this position until it hatches a few days later. The
young frogs don’t feed for at least five days after hatching for they
continue to use up the still substantial yolk deposits.
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Fiji tree frog eggs
The phenomenon of direct development eggs is of particular advantage
to amphibians living in, or around, fast-flowing streams, as it enables eggs
to be laid away from water, thereby avoiding the possibility of eggs or tadpoles
being swept away. It also helps enormously in colonising islands that have
little free water.
Opinions as to how the frogs arrived here differ, but there are two
possibilities. Either they were introduced by man, deliberately as food or
accidentally in vegetation, or they made their own way here by rafting or
on ancient land bridges. I tend to the view that they made their own way
here. If man introduced them, they could be expected to be found on all inhabited
islands with suitable habitat, but they are not.
I believe there may be two species of ground frog in Fiji. The Taveuni ground frogs are considerably more gracile than their Viwa relatives and tend to sit more upright when out at night. Genetic analysis will answer this.
Ground frog from Viwa (left) and Lavena (right)
If you make it Fiji it might be worth going out at night into rainforest to look for these fascinating beasties
For more photos of these interesting animals check out this link. For A Field Guide to the Herpetofauna of Fiji by Clare Morrison click here. For Fiji's Natural Heritage by Paddy Ryan click here.