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Land Iguana
Darwin was not much impressed with the land iguana:
"...they are ugly animals, of a yellowish
orange beneath, and of a brownish-red colour above: from their
low facial angle they have a singularly stupid appearance."
The land iguanas are typically divided into two
species, Conolophus subcristatus and Conolophus pallidus,
although there are some data to suggest that the later is merely
a local variant, a subspecies, of the former. C. subcristatus
is more widespread throughout the archipelago while C. pallidus
is restrected Santa Fe. C. subcristatus
is pretty much as Darwin described it, but since he had not visited
Santa Fe he never saw C. pallidus
, which is much more uniform and paler in color, and possesses
a more pronounced ridge of spines.
C. subcristatus once
once had a much broader distribution and higher population numbers
than it does today. In 1835 Darwin was impressed with their numbers,
remarking that:
"...when we
were left at James [Santiago],
we could not for some time find a spot free from their burrows
on which to pitch our single tent."
Today, there is not a single land iguana to be
seen on Santiago. They have become extinct
at the hands of man, or man's introduced animals. C. subcristatus
can be found on Santa Cruz, Isabela,
Fernandina and, most dramatically, on Plaza.
Plaza is a very tiny islet that surprisingly supports a large
population of iguanas. Visitors are often disappointed by the
size before disembarking but are always surprised by the richness
of the islet, especially the iguanas. There is also an introduced
population on N. Seymour. In the early
1930's William Randolph Hearst visited the Galapagos and was surprised
to find no land iguanas on N. Seymour while
just a few hundred yards acoss a small channel, Baltra
supported a large population. Since he could see no difference
between the two islands, he tried the experiment of transplanting
(gasp!) some Baltra iguanas to N. Seymour. The disappearance
of the Baltra population after World War
II is generally blamed on US airforce and navy personnel who were
stationed there to protect the Panama Canal. According to the
standard story, the sailors were desperately bored, and would
shoot at iguanas for ammusement. Reports from the Darwin Station,
however, suggests that the Baltra iguanas
were already dying out for unknown reasons. The Baltra
iguanas still thrive on N. Seymour, where
they can still occasionally be seen if you're lucky. Recently,
conservationists at the Darwin Station have reintroduced iguanas
back to Baltra.
Land iguanas, like all iguanas, are vegetarians,
subsisting mostly on the fruit and pads of Opuntia cactus.
It is not unusual to see them sitting under a cactus, waiting
for pieces to fall. They normally use their front feet to scrape
the larger thorns from the pads, but they don't seem to mind the
smaller thorns. Usually they will gulp down a cactus fruit in
just a few swallows. I once watched an iguana dislodge a thorn
from its tongue by sticking the tongue out and dragging it back
across its teeth several times. In addition to cactus, they also
eat a variety of other plants including Alternanthera, Tiquilia,
and Portulaca..Portulaca. is a low-growing plant
that is found in abundance on the western part of Plaza.
After a wet period, they blanket the island in yellow flowers.
The flowers, however, last only a few days because they are eaten
by the iguanas. Occasionally land iguans will dine on carrion
or arthropods that crawl in their burrow. Again like other iguanas,
the juveniles feed primarily on insects.
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Land iguanas are territorial
and signal their aggression with head-nodding. During mating a
male literally catches the female and copulates with her. Following
copulation, the female flees and finds a spot to dig a nest in
which to bury a clutch of up to 25 eggs. On Plaza
land iguanas occasionally hybridize with marine
iguanas. My Galapagos guide, Walter Campoverde, has reported
that male marine iguanas, in a mating frenzy, have occasionally
been seen attacking and raping female land iguanas. Molecular
studies on the hybrids would seem to bear this out. While offspring
get half of their nuclear DNA from each parent, their mitochondrial
DNA, found in the cytoplasm, comes only from the mother. The mitochondrial
DNA in the hybrids matches the land iguana. In general form, the
hybrids, such as the one below, resemble the land iguana and seem
to live the land iguana lifestyle. Their coloration, however,
is very dark, like the marine iguana, and they exhibit dorsal
striping which is characteristic of juvenile marine iguanas, but
not juvenile land iguanas. There are only a few hybrids and nobody
knows if they are fertile.
The fact that there is crossbreeding between the
two genera of iguanas, Conolophus and Amblyrhynchus, the marine iguana, raises interesting questions about the evolutionary relationship between
the two. Standard definitions of the term "species"
include the presence of a fertility barrier. Some closely related
species can hybridize to form viable, fertile offspring, as is
the situation among the Darwin Finches. Less closely related species can interbreed,
such as horses and donkeys, but their offspring are infertile.
It appears that Ctenosaura, the black or spiny-tailed iguana
of Central America, is ancestral to both Galapagos iguanas. It
has always been assumed that the marine iguana
evolved from the land iguana, and there are some
data to support that assumption. On the other hand, other data
sets suggest that the land and marine iguanas are no more closely
related to each other than to any of the other iguana genera.
Molecular clock studies of the marine iguana suggest that these
iguanas diverged from other iguanas around 8 million years ago.
The oldest Galapagos island, however, is only around 4 million
years ago. Either the marine iguana originally evolved on the
mainland and colonized the Galapagos in their present form, or
there have been older Galapagos Islands that have now sunk beneath
the waves. Indeed, there does seem to be some evidence
for this later possibility.
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