Biodiversity of the Rewa Head B Zoological Society of London ...

Chapter 3. Reptile Species Diversity
No specific herpetological studies took place, however several observations made
during drift transects or through chance encounters are worth recording.
Firstly the complete absence of black caiman (Melanosuchus niger) and spectacled
caiman (Caiman yacare) in the Rewa Head is interesting and is likely to be due to Corona
Falls acting as a dispersal barrier. The contrastingly high numbers of Schneider’s dwarf
caiman (Paleosuchus trigonatus) above the falls may be due to their preference for cool,
fast flowing forest streams rather than ecological release from competition with other
crocodilians. Magnussun et al (1987) considered there to be resource partitioning between
dwarf and spectacled caiman as a result of differing habitats and prey selectivity which
precludes competition from occurring. However, Medem (1971) reported that following
hunting for skins of black and spectacled caiman from rivers in Colombia, the un-hunted
dwarf caiman (due to the low value of its skin) expanded its range, so perhaps competition
does play a role in explaining the pattern seen. Below Corona Falls we observed both black
and spectacled caiman but no dwarf caiman. The species is known to exist in the Lower
Rewa, but at much lower densities (Holland unpublished data) and is rare downstream of
the Kwitaro mouth.
Figure 33. Schneider’s dwarf caiman (Paleosuchus trigonatus) in typical head-up resting pose.
With regards to Testudines, most conspicuous in its absence was the giant Amazonian
Turtle (Podocnemis expansa), classified as ‘Lower risk/conservation dependent’ and
yellow-spotted sidenecked turtle (Podocnemis unifilis), classified as ‘Vulnerable’, both
of which were recorded in the Lower Rewa up to Corona Falls, yet as with the spectacled
and black caiman, absent from the Rewa Head. Another Testudine of particular note
encountered on a hill in dense forest in the Rewa Head was the yellow-footed tortoise
(Geochelone denticulata) described as ‘Vulnerable’ in IUCN Red List reports.
We encountered 6 green anacondas (Eunectes murinus) in the Rewa Head during the
expedition. Five of them were basking on the river bank while the sixth was actively
swimming. In order to ascertain our size estimates were correct, we captured a large
anaconda and measured it, taking three measurements from nose tip to tail tip using a
length of string and assumed the mean measurement to be accurate. Our estimated length
of 17ft proved close, but short by 1’2”. The snake’s total length was 18’2”. Unfortunately no
weight was taken, but the animal’s girth at its thickest point was 27”. The other anacondas
seen were estimated as 8ft, 15ft, 16ft and two at 17ft. The presence of these very large
snakes in the area is extremely interesting as in the longest-running study on anacondas in
the world, in the Venezuelan Llanos, Jesus Rivas captured over 900 animals yet recorded
few specimens growing larger than 16’ long. There is undoubtedly a good prey base for
the anacondas in the Rewa Head, with capybara and dwarf caiman plentiful.
24 Biodiversity of the Rewa Head