Chapter 10 Snakes on Islands
10. Snakes on Islands
When not foraging for food, sea kraits
come ashore…, often in large numbers on small offshore islands sequestered from
terrestrial predators….
Harold Heatwole et al., 2005
The
sun was just coming up over the South China Sea one Sunday morning when my two
companions and I set out at low tide to walk a half-mile stretch of mud flat along
Sabah’s coastline. Our destination was a pile of rocks emerging from the sea,
and our plan was to search the rock pile for snakes, photograph them, and
return to the main island before the tide returned. At high tide most of the
rock island was covered with water, but even the highest tide did not cover the
tiny island completely. We knew this because the top of the island was covered
with green plants. Soon our boots were caked with mud and walking became more
difficult, but thoughts of finding a sea krait, one of the most unusual snakes
on the planet, kept us moving forward.
Large
boulders were strewn around the plant-topped rock pile, composing a small
island habitat no more than 10 square meters. We climbed the rocks and, when
our heads were even with the vegetation, we peered into the grasses and sea
grape vines. Within moments, the black- and-blue bands of a sea krait were
visible, and then another, and another. This highly venomous, but very gentle
snake was present in good numbers on this rock; unfortunately, we did not have
time to count them [Figure 10–1].
INSERT
FIGURE 10-1.
Figure 10–1. The Yellow-lipped Sea Krait
(Laticauda colubrina): Top: A small islet off the coast of Sabah, Malaysia.
Middle: A sea krait coiled in a rock crevice. Bottom: A view of the head of
Laticauda colubrina. Sea kraits fill a
unique coastal niche that involves using both terrestrial and marine
microhabitats.
We
carefully lifted a large snake out of the vegetation and lowered it onto a
ledge so it could be examined and photographed. On a similar islet off the
coast of New Caledonia, Hubert Saint Girons found about five sea kraits per
square meter.
Sea
kraits have a remarkable paddle-shaped tail for swimming, and the ability to
compress their body from side to side so that the snakes turns into living oars.
But they also have large ventral scales arranged in a single row along their
entire belly, a characteristic of snakes that live on land. The snakes on this
island had ticks, similar in appearance to those I have removed from my dogs
after walking though Midwestern fields, though I knew these had to be a
different species. The ticks had inserted their heads under the snakes’ scales
and, attached as they were, went with the snakes as they left the island to go
hunting in the sea. Sea kraits are unusual because they show a unique set of
characters that adapt them to a double life. They specialize in finding and
eating eels underwater, but digest their food, bask in the sun, mate, and lay
their eggs on land. And, as in this case, the land is often a small offshore
island. Our time on the rock pile was limited by the incoming tide, so we
finished our photography session, returned the snakes to the vegetation on the
top of the rock, and walked back to the mainland with the incoming tide lapping
at our heels.
The
quote from Heatwole et al. suggests what we found on that small rock off the
coast of Sabah was not unusual. In fact, it is difficult to find a sea krait on
the actual coastline of a large island or continent, but go to offshore rocks
and small islets and they are often plentiful. The reason may be small islands provide
habitat secure from larger predators, as Heatwole suggests. However, safety is
not the only reason snakes live on islands. In this chapter, we explore how
snakes get to islands and disperse into new habitats.
Excluding
Antarctica and the Arctic islands, islands compose about 3% of the Earth’s land
surface, and they support many species of vertebrates. In a survey of
terrestrial vertebrates on islands, Ted Case found island snakes tend to be significantly
smaller than their mainland counterparts. Exceptions included snake populations
adapted to feed on sea bird chicks; these island snakes tend to be larger in
size when compared to mainland populations. Dwarfs or giants, island snake
populations are the result of the reduced competition, limited resources, and
the available ecological niches.
Snakes
are excellent colonists. Their scale covered bodies are often resistant to
water loss. They can survive with very little food; many can go months without
food. Female snakes often mate with several males and store their sperm for
future use. Thus, a single litter of sibling snakes is likely to have a greater
diversity of genes than if they would have all had the same father. Under the
right circumstances, female snakes will even produce offspring without sperm
(see Chapter 2). All of these traits help snakes colonize new environments.
The Sea Kraits of Rennell’s Island
Sea
kraits use small islands for shelter but spend half their time in the water,
foraging for eels on reefs and swimming between islands. Islands used for
resting and mating are close to their food source, and islands supply secure
locations for females to lay their eggs. Sea kraits are the only marine snakes known
to lay eggs; all other coastal and marine species have live birth ― at least we
think they do. Rock piles like the one we explored have crevices and cavities which
are difficult for predators to access and, therefore they are secure locations
for depositing eggs.
Eight
species of sea kraits (Laticauda) are currently recognized and several have
adapted to fresh or brackish waters. Rennell’s Island is one of the Solomon
Islands due east of Queensland, Australia. It is a raised coral atoll 14 km wide,
80 km long, and about 110 m above sea level. At the center of the island is a
large, brackish water lagoon, Lake Te-Nggano, with an average depth of 40 m.
Two species of sea kraits live in the lake. The common and widespread
Yellow-lipped Sea Krait (Laticauda colubrina) is large, reaching 1.4 m and a
weight of one kilogram. Crocker’s Sea Krait (L. crockeri) occurs only on
Rennell’s Island in Lake Te-Nggano; it is smaller reaching 0.7 m and weighs about
280 g. Field observations suggest these two species forage for food in the same
places and often together. Interestingly, there is evidence that the
Yellow-lipped Sea Krait will forage in cooperative groups, so the observation
that it will forage with Crocker’s Sea Krait is not that surprising.
The
principle of competitive exclusion or Gause’s Law states that two species cannot
compete for the same resources and co-exist, at least not for long. Of course,
the Lake Te-Nggano sea kraits don’t compete for the same resources. The
Yellow-lipped Sea Krait feeds only on the native eel (Anguilla obscura) and
Crocker’s Sea Snake apparently feeds only on a native goby (Eleotris fusca).
The Yellow-lipped Sea Krait frequently leaves the water to bask and mate and
can stay submerged for five minutes, while Crocker’s Sea Krait does not appear
to leave the water at all and can stay submerged for 11 minutes. The
Yellow-lipped Sea Krait lays eggs, as do all other known sea kraits, but the
people on Rennell’s Island say Crocker’s Sea Krait has live birth. Hard
evidence for this has yet to be found.
There
is another difference between the two species. The Yellow-lipped Sea Krait has
alternating black and blue-sliver bands on its upper body, while Crocker’s Sea
Krait is almost jet black. The smaller, melanistic species may be able to more
rapidly warm its body than the larger, more traditionally patterned sea krait.
Sea
kraits, with their paddle-shaped tails are good swimmers, but most snakes are
terrestrial; they live on land and only occasionally enter the water. Land-dwelling
snakes on islands are trapped by the surrounding water. And water, particularly
deep water, is a barrier not only to most land living snakes, but for many
semi-aquatic and shallow water species as well. The question here is, how do
land dwelling snakes get to islands? There are several ways.
Many
islands sit along the continental shelf and as sea levels have risen and fallen
through time; some snakes crawled overland, swam through shallow water, or
floated on mats of vegetation to reach existing islands. Other islands are fragments
of continents shattered by tectonic movements, and snakes that survived the
ride from deep time to the present day represent ancient lineages still living
today. Of course many, if not all, snake lineages used a combination of these
methods over very long periods of time to get to where they are today. However,
snakes also reach islands with the aid of human transport, accidentally or on
purpose. Once on an island, a snake population is usually isolated; what
happens to them depends upon the environment, their genome, and chance events.
Snakes of the Remote Pacific
Islands
isolated by hundreds of kilometers of water might be expected to be snake-free,
or at least free of terrestrial snakes. The ancestors to the Rennell’s Island
sea krait populations were semi-aquatic snakes that could have swam or been
swept by currents from Australia, or from other nearby islands. At least four terrestrial snake species have
dispersed over water to the island groups of Vanuatu, Fiji, Tonga, and Samoa,
however. Two of these snakes are blind snakes in the genus Ramphotyphlops, tiny
burrowing species that feed on ants and termites. These are species most likely
transported by humans carrying potted plants with snake stowaways in the soil.
But, Pacific island boas of the genus Candoia occur on New Guinea and are
widespread in the South Pacific, ranging eastward to the Samoan Islands. The
origin of these snakes has been, and continues to be, something of a puzzle. Their
ancestor was hypothesized to have rafted from the Western Hemisphere across the
Pacific, based upon the assumption that they are most closely related to
Madagascar’s boa-like species. However, a recent molecular study suggests they
are sister to the sand boas of the Middle East, Asia, and Africa. This would
still require them to make the trip across the Pacific.
Elapids have also found their way to the
Pacific Islands, the Solomon Islands and Fiji. How snakes reached these small, remote islands
is not known for sure, but all have close relatives in New Guinea. Traveling
eastward from New Guinea, reptile species diversity decreases and geological
evidence hints that series of island arcs formed as the boundaries of Pacific
and Indo-Australian plates altered the direction of their movements. These
island arcs are thought to have acted as stepping-stones, allowing snake
populations to disperse over relatively short distances at any given time. Over
millions of years, however, elapids have dispersed more than 3300 km east of
New Guinea to the Fiji Islands, while those Pacific Island boas dispersed more
than 7000 km from New Guinea to Samoa.
Of
particular interest are the anatomical and molecular clues that support a close
relationship between the New Guinea Small-eyed Snake, the Solomon Island
elapids, and the sea kraits. Mark O’Shea described the habitat and habits of
the New Guinea Small-eyed Snake (Micropechis ikaheka) as swamps and monsoon
forest creeks. O’Shea found the local dialect name ikaheka means “land eel,” a
reference to its semi-aquatic habitat preferences. The other Pacific Island
elapids existing today live in leaf litter or are burrowers. When DNA from the
New Guinea Small-eyed Snake, the Solomon Island Coral Snake (Salmonelaps par),
or the Solomon Island Small-eyed Snake (Loveridgelaps elapsoides) are included
in studies with sea krait DNA, all three
appear to be related to the sea kraits and are estimated to be about 11.5 MY
old. The ancestor of the Small-eyed Snake and the sea kraits may have been
semi-aquatic or aquatic and it diversified into terrestrial Pacific Island
elapids, the semi-aquatic marine sea kraits, and the land-dwelling Australasian
elapids of today.
Sail
eastward from Sydney, Australia south of the Coral Sea, and through the
southern Polynesian Islands to 27º10’S 109ºE 26W. After a trip of more than 9200 km, you’ll reach Easter Island, an 88 km2
volcanic island. The only snake reported in the vicinity of this island is the
pelagic, Yellow-bellied Sea Snake, Pelamis platura, Samuel Garman wrote,
This sea serpent was taken in Lat. 26º
34’S; Long. 108º 57’W…about fifty miles northeast of Easter Island. It has been
directly compared with numerous specimens… without discovery of characters on
which to base so much as a variety.
There is but one snake species truly
adapted for open water swimming and its has been able to disperse into the very
remote Pacific; terrestrial snakes have apparently been unable to colonize
Easter Island.
Island Snakes and Body Size
Coastal
southern Australia and the island of Tasmania are home to the highly venomous
snake known to Australians as the Tiger Snake, and to science as Notechis
scutatus. Tiger Snakes are so called because some populations are black with
yellow bands, although other populations may be uniform yellow, solid black, or
two-toned. The two toned snakes fade from yellow or red at the front of the
body to black or gray on the posterior end of the body. Some populations of Tigers
are small with an average size of about 0.7 m. However, other populations have
body sizes that exceed 1.6 m. It seems likely that Tiger Snakes are the souther-most
living venomous snakes. Go south off the southern coast of Tasmania and the
next landfall is Antarctica.
Tiger
Snakes have confused those who study them for some time. The differences in
body size and coloration from population to population suggested multiple
species may have gone unrecognized. Current thought by those who work with them
is that science should recognize them as one species undergoing evolution in
many different environments. Tiger Snakes are in the process of evolving into
multiple species.
About
12,000 years ago the Antarctic ice sheet was at its maximum, global ocean
levels were exceptionally low, and a large portion of Australia’s southern
coastline sat exposed, linked by dry land, to what is now the island of
Tasmania. Australia and Tasmania are presently separated by a body of water
known as the Bass Strait containing small islands, many of which have Tiger Snake
populations. In the mild climate of the Holocene (about 11,700 YBP to today),
Tiger Snakes could have easily colonized land masses that would become islands
once the ice sheet melted and the sea rose. The islands are all less than
10,000 years old, and most are only 5,000–7,000 year old. Mainland Tiger Snakes colonized new
environments where freshwater was scarce, prey was small, or prey was large,
and they did so recently; thus, providing an opportunity to study evolution in
action.
On
mainland Australia and the island of Tasmania Tiger Snakes eat mostly
frogs. On offshore islands it is a different story,
however. On offshore islands they eat lizards, birds, and mammals. On the tiny
island of Roxby in the Bass Strait, Terry Schwaner found dwarf Tiger Snakes and
three species of lizards living together. The Roxby Tiger Snakes have the
smallest average body lengths of all examined populations. Roxby lizards are
small prey, and it seems likely that small snakes were favored over snakes with
large bodies because they could find enough food to sustain themselves. However,
some Bass Strait islands have large-bodied snakes. The largest individuals come
from Chappell Island, and they are twice the length of the Roxby Island snakes.
Hypotheses
to explain body size differences are not in short supply. Male Tiger Snakes are
not known to display male combat behavior in competition for mates, and tiger
snakes are the top predator on the islands they inhabit. Therefore, hypotheses
about competition, predation, and sexual selection can probably be ruled out as
explanations for dwarf and giant Tiger Snakes on islands.
Mainland
snakes eat mostly frogs but also take other prey (up to about 70 g), and have
adult body sizes between 0.78 and 0.92 m. Dwarf populations have adult body
sizes less than 0.7 m. Diet data were available only from the Roxby Island
population, and they were feeding on lizards that were 10 g or less in body
weight. The dwarf mountain population is thought to feed only on seasonally available
tadpoles. The giant Tiger Snakes have body sizes that average about 1.20 m, but
may exceed 1.60 m, and they feed on mammals or birds between 300 and 350 g.
Snake populations trapped on islands adapt their body to the appropriate size
for available prey. But how rapidly can these changes occur?
The
capacity for the environment to alter a population’s physical characters, such as
body size, is known as phenotypic plasticity. Faben Aubret and Richard Shine
collected gravid female Tiger Snakes from Carnac Island, a population that had
been established by humans 80 years before. The island population feeds on sea
bird chicks, but also on lizards. Aubret and Shine also collected gravid
females from a mainland population at Herdsman Lake, a population that was
known to feed on frogs. Snakes from the island population tended to be 13%
longer and weigh 86% more than snakes from the mainland population. Offspring
from each population were selected at random and fed a diet of mice and chicks;
the snakes were then measured after nine months. The Herdsman Lake snakes were
larger than those found on Carnac Island. Thus, phenotypic plasticity was
responsible, at least initially, for the differences in body size seen in
island and mainland Tiger Snakes.
The Smallest Snakes
Worm snakes (family Leptotyphlopidae) are
small, usually less than 350 mm, and exceptionally slender―often just a few millimeters
thick. All are short-tailed burrowing species specialized in feeding on ants
and termites and have greatly degenerated eyes and teeth. Worm snakes use
chemical trails left by the insects and follow them into their nests where the
snakes use a raking motion of the lower jaw to collect insect pupae for
ingestion (See Chapter 13). Bites from solider ants and termites are deterred
by the highly polished scales that don’t allow the insects to puncture or gain
hold of the snake. Worm snakes are of interest because they are the smallest of
snakes, and the tiniest one lives on Barbados in the Lesser Antilles.
Body
size is important to snakes because it determines the prey size, number of prey
needed, how quickly or slowly the snake will gain or lose heat, how many
offspring (or eggs) a female can produce, and which predators can consume it.
S. Blair Hedges found six species of worm snakes that are known to have total
lengths less than 105 mm. Of the six, four are known from only one specimen and
one of these is based on a juvenile specimen. In Kenya Leptotyphlops tanae is
known from a series of specimens, nine of which are males and one of which is
an immature female. The largest is a 103 mm male. Sex is difficult to determine
in these snakes, but male worm snakes are usually smaller than females. Thus,
it is likely that the 103 mm male is not representative of the species’ maximum
size. However, Carla’s Worm Snake, Tetracheilostoma [formerly Leptotyphlops] carlae,
is known from five specimens from Barbados, the largest of which is a 104 mm
female containing one greatly elongated egg. Clutch size is closely correlated
with body size in snakes, and it is not surprising that the world’s smallest
known snake has a clutch size of one.
The
world’s smallest snake made headlines twice in the fall of 2008; first, when the
snake’s discovery was announced and then, again, when Barbados residents
protested Professor Hedges naming the new species after his wife instead of
something related to the island. Some Barbadians
claimed their parents had shown them the snake. Unfortunately, neither the
protestors’ parents, nor the current generation of Barbadians, were zoologists.
Island Pitvipers
Thirteen
kilometers off the coast of northeast China’s Liaodong Peninsula is Shedao
Island. Shedao literally means “snake island.” Although the island is in
Chinese waters, the presence of a pitviper on Shedao was first reported to the
scientific world by the Japanese when they occupied Manchuria in the 1930’s.
This tiny island (about 0.73 km2) has a highly seasonal climate with cold, dry
winters and warm, rainy summers, and it has a pitviper population estimated at
9100–11,500 (about 1 snake per square meter). The snake, the Shedao Mamushi
(Gloydius shedoaensis), is similar to the Rock Mamushi (G. saxatilis), which
also inhabits the Chinese mainland as well as the Liaodong Peninsula, North and
South Korea, and eastern Russia. Chinese
scientists discovered adults feed almost exclusively on small birds (warblers
and buntings) that make a migratory stop on this little island on their way to
and from Siberia. The Shedao Mamushi climbs into bushes, lies along a branch,
and waits for a bird to land within striking distance. Or, they lay on the
ground concealed by leaves, waiting for a migrating bird to land nearby.
Richard Shine and colleagues visited the island and found smaller snakes used
ambush sites in bushes and trees, and larger snakes used ambush sites on the
ground. Experiments showed that
smaller, and presumably less experienced snakes, had a lower rate of success
capturing birds than did adults. Adults often struck the bird’s head while
juveniles often struck the wing or body and had a less secure grip. Shine and
colleagues hypothesized that the arboreal ambush sites increased the snakes’ hunting
success because a pearching bird’s position was more predictable. Smaller birds
used the perches while larger birds landed on the ground.
Despite
the large size of newborn Shedao Mamushi, they are apparently too small to feed
on birds immediately. The few juvenile specimens examined with stomach contents
contained centipedes and pill bugs (isopods). The neonates weighed twice as
much as their mainland relatives at birth, and it seems likely this allows them
a head start in being able to eat small birds after just a little growth. Prey
is rare on the island and the survival of Shedao Mamushi depends upon the
supply of migrating birds. Females give birth just before the fall bird migration,
providing them the opportunity to replace the body weight lost during
reproduction.
How
the Shedao Mamushi arrived on this island is not known, but the island is on
the continental shelf in shallow water and at glacial maximum, an overland
connection was undoubtedly present. Dispersal over water cannot be ruled out,
however.
Two
tiny islands off the coast of southern Brazil support two species of endemic
pitvipers. In many respects, the snake population on the Ilha da Queimada
Grande is similar to China’s Shedao Island. The Golden Lancehead (Bothrops
insularis) is found only on this 0.43 km2 island. Bothrops insularis occurs in
a dense population, the snakes feed mostly on migrating birds, and it uses both
the ground and low vegetation for ambush sites. The Golden Lancehead is perhaps
best known for its exceptionally toxic venom that liquefies tissue, causes intense
pain, and prevents blood from clotting. Local fishermen tell stories about the
Lancehead to rival bad snake sci-fi cinema and the mamba story which began this
book. The last lighthouse keeper and his family to live on the island were
reportedly “attacked.” Snakes poured into the building through the windows
forcing the family to flee through the vegetation only to be bitten by snakes
hanging from trees and shrubs. The crew from a Navy supply ship supposedly found
the bodies scattered around the island. And then there is the more believable
story of a fisherman who went ashore to pick bananas and was bitten, but made
it back to his boat. He was found dead, sprawled across the blood-soaked deck.
Golden
Lanceheads are also known for their unusual reproductive organs; many
individuals are intersexual. Intersex snakes have reproductive organs of males
and females, a condition also reported in other lancehead (Bothrops) species
from mainland Brazil. Some intersex snakes with reduced male genital and
ovaries have even been found to contain well- developed embryos. The Golden Lancehead
reaches a total length of about 1.18 m, but most adults are about 0.7 m in
total length.
Ilha
da Queimada Grande is free of rodents and, in an interesting experiment using
free-ranging Golden Lanceheads, Otavio Marques and Ivan Sazima presented the
island snakes with house mice to see how the snakes would react. Much to their
surprise, snakes naïve to mice responded in the way their mainland counterparts
do; they struck and released the mice. These are the same snakes that are
familiar only with birds as prey, and they strike and retain the bird in their
jaws until it has been immobilized.
About
35 km off coastal Sao Paulo, Brazil is Alcatrazes Island, a 1.35 km2 island
covered with the remnants of Brazil’s Atlantic Forest. The Alcatraze Lancehead
(Bothrops alcatraz) is a dwarf species, with males reaching 0.46 m, and slightly
larger females growing to lengths of roughly 0.52 m. Like the Bass Strait tiger
snakes, diet has been the major factor contributing to the dwarfism in this
species. Litter sizes in this snake appear to be reduced, averaging only one or
two offspring per litter with newborn snakes measuring only 150 mm at birth.
Perhaps most interesting is the food chain on Alcatrazes Island. Snakes were
found lurking under sea bird nests and were often white-washed with uric acid
from the bird droppings. Cockroaches are abundant and feed on the birds’ guano;
the roaches are eaten by centipedes, and the Alcatraze Lancehead feeds on the
centipedes as well as the occasional lizard. This is currently the only species
of Bothrops known to feed exclusively on ectothermic prey. The Golden Lancehead
appears to be related to the Caatinga Lancehead (Bothrops erythromelas) and the
Chaco Lancehead (B. diporus).
In
a recent survey, the Jararaca (Bothrops jararaca), a large species (1.6 m
maximum size) that inhabits southeastern Brazil, was found on 11 of 18 islands.
Based on this distribution, it appears that the Jararaca is continuing to disperse
in the coastal environments. The Brazilian islands are on the continental shelf
and it seems likely that the snakes arrived on the islands during times of
lower sea levels. But again, over-water rafting cannot be ruled out,
particularly for island populations of the Jararaca.
Convergence
between the Shedao Mamushi and the Golden Lancehead was studied by Wolfgang
Wüster and colleagues by comparing fang length, head length, and other
anatomical features in both island species and their mainland relatives. The
authors found both island species had elongated heads when compared to their
mainland ancestors. The Golden Lancehead, with shorter fangs and a longer tail
than its mainland relative, probably diverged from the ancestral mainland
population about 2 MYA. The Shedao Mamushi has a fang length and tail length
similar to those found in mainland populations; however, the fang length was
already short in the ancestor. Wüster and colleagues proposed that the reduced
fang lengths may be the result of a hunting technique that involves striking
and holding the bird in the mouth until it can be swallowed. Mainland Jararaca
strike and release mammals and then track the mammal until it is immobilized. The
shorter fangs of island snakes may not break as easily when holding struggling
prey.
Early
explorers found Caribbean islands relatively free of dangerous animals, with
two exceptions; the volcanic islands of Martinique and St Lucia. Both islands
supported populations of large, dangerous pitvipers. On Saint Lucia is the lancehead,
Bothrops caribbaeus, commonly named for its home island. The Martinique pitviper
has been called the Vipère Jaune, the Fer-de-lance, and the Martinique Lancehead
(Bothrops laceolatus). The name Fer-de-lance has been used indiscriminately for
many of the snakes in the genus Bothrops, but was original intended for the
Martinique species. Unlike the Brazilian islands with pitvipers, Martinique and
Saint Lucia have well-established human populations and these two snakes have
had a long and turbulent relationship with humans.
One
19th century author wrote that the total absence of idyllic and pastoral
elements in Martinique literature resulted from the presence of the Fer-de-lance.
The author attributed the fact that the flora and fauna of the island remained
unknown because the Fer-de-lance “…renders all serious research dangerous in
the extreme.”
With
the arrival of the Europeans in the 16th century, the Lesser Antillean forests
were cut and crops were planted. This habitat alteration allowed the rat
population and their predators to dramatically increase in number. In the 19th
century, Saint Lucia’s George William Des Voeux estimated the annual snakebite fatality
rate at 1.2 per 1000, or about 25–30 deaths per year (accurate records were not
taken). He passed legislation to pay a bounty on venomous snakes. Within seven
months, bounty was paid on 12,000 snakes and the allocated funds were exhausted.
Continuation of Des Voeux’s plan was not without opposition; merchants living
in towns were opposed because they were not in danger of being bitten, and the
plantation owners were opposed because they recognized the snakes’ beneficial
role in rat control, thus neither group wanted to pay for controlling the
snakebite problem. Despite this, Des Voeux suggested the bounty worked and the
number of deaths was steadily reduced: 22 in 1869, 16 in 1870, and 12 in 1871.
Determined
to solve the Saint Lucian snakebite problem, Des Voeux obtained several
mongooses from Philip Scalter at the London Zoo and set-up a fight-to-the death
match between a snake and one of the viverrids on the lawn of the Government
Building. The mongoose killed the snake, and Des Voeux proceeded to introduce
the species to Saint Lucia. Despite evidence that mongooses ate chickens, Des
Voeux maintained that they were controlling the snakes, citing the reduced
number of deaths, which was one by 1898. Of course, we now know the mongoose is
a diurnal predator, prefers to eat birds, and rarely encounters the nocturnal
Saint Lucian Lancehead.
For
the first half of the 20th century, the two island pitviper populations were
considered the same species as the widespread South American common lancehead,
the Barba Amarilla (Bothrops atrox). Herndon Dowling proposed Carib Indians had
transported the snakes via canoe to the islands so they could be used as a
biological weapon against the Arawak people. If true, this would make the
island pitviper populations only a few hundred years old. Des Voeux also
suggested a historical hypothesis about the origin of island pitvipers, stating
that 16th century planters had imported them to prevent their slaves from
hiding in the forests.
However,
detailed examination of specimens from known localities, as well as ecological
and behavioral data, suggested the island species were distinct from the South
American species and distinct from each other. Saint Lucian snakes had 25–29
scale rows at mid body and 198–212 ventral scales, while the Martinique animals
had 29–33 rows of dorsal scales and 218–237 ventral scales. Both species also
had uniform coloration on their belly while mainland populations had pigmented
bellies. While both species climb, Saint Lucian pitvipers spend much of their
time on the ground, whereas the Martinque snakes spend considerable time in the
trees. There were also differences in the defense behavior of the two species.
Despite the evidence that each island had its own endemic species, other
commentators suggested that the islands were constantly being supplied with new
genetic material from snakes that were floating out to the islands from the
mainland.
DNA
sequences from the two populations settled the question. Wolfgang Wüster and
colleagues used 1,600 base pairs from two genes and found the Lesser Antillean
snakes were distinct from mainland South American species, and distinct from
each other. The study suggested the mainland Barba Amarilla–Terciopelo species
complex (B. atrox-asper) had a shared ancestor with the island species in the
late Miocene or early Pliocene (8.9–4.2 MYA), and that the two island species
had shared an ancestor 6.5–3.1 MYA. Thus, the Caribbean pitvipers are
relatively old species, diverging from each other about 6–5MYA, not the few
hundred to a few thousand years previously suggested.
Rumors
about pitvipers on other islands such as Tobago, Guadeloupe, and Dominica
persist. These islands are relatively well known and none are currently known
to support populations of pitvipers. Stories about Tobago pitvipers come from
mislabeled specimens that originated on Saint Lucia. The British Museum of
Natural History houses two specimens of Bothrops lanceolatus said to be from
Guadeloupe, mislabeling is a possible explanation. But, Lafcadio Heam stated
that the species was introduced into Guadeloupe to control a plague of rats. No
date is given, but the introduction was likely to have occurred in the 19th
century. If true, it could explain the locality given for the museum specimens.
The Dominica locality remains a puzzle, but an unsuccessful human introduction
cannot be ruled out.
Sky Island Snakes
The
trail was steep and covered with Ponderosa Pine needles and cones shed by the
huge conifers. As the ground leveled out, the forest opened, revealing the base
of the mountain’s peak and a huge jumble of eroded rocks. Within minutes I
heard a low buzzing sound, and found the source to be a small, grey-spotted
rattlesnake coiled at the base of a boulder. Twin-spotted Rattlesnakes
(Crotalus pricei) are easily recognized by the double row of spots on the
dorsum and the orange tipped tail at the base of the rattle [Figure 10–2]. It
is one of several species of dwarf sky island rattlesnakes inhabiting the pine-oak
forests of the southwestern USA and northern Mexico. Like snakes trapped on
oceanic islands, snakes found in isolated islands of habitat on mountain tops
tend to evolve small body sizes or large body sizes depending on available
prey.
INSERT
FIGURE 10-2.
Figure 10–2. The Twin-spotted Rattlesnake
(Crotalus pricei) a small Madrean sky island species in southern Arizona.
Islands
are cauldrons for evolution and it does not matter whether it is land isolated
by water or a terrestrial habitat isolated by another habitat. Islands produce
endemic species because over time they separate gene pools. About 22 sky island
complexes can be found around the world and the Twin-spotted Rattlesnake
inhabits the Madrean Sky Island Archipelago from southeastern Arizona southward
through Mexico’s Sierra Madre Occidental. Many of these populations are
isolated in pockets of woodland or forest with rock outcrops that are between 1,850
to 3,200 m in elevation and separated by valleys with dry savanna and desert.
The drier, more open habitats are alien landscapes to the Twin-spotted Rattlers.
Thus, they move only when their habitat is more widely distributed into the
lower elevations of the valleys during times of higher rainfall.
Africa
has a sky island complex that is particularly rich in endemic species,
including snakes. The Dagger-toothed Vine Snake (Xyelodontophis uluguruensis)
is endemic to the montane evergreen forests of East Africa’s Eastern Arc
Mountain sky island complex. This species is a relative of the Boomslang and African
vine snakes previously discussed in Chapter 4. In fact, the Usambara Vine Snake
(Thelotornis usambaricus) is also an Eastern Arc Mountain sky island
inhabitant, but uses coastal forests at lower elevations and has a relict
population on the Kenya coast. Unlike the Twin-spotted Rattlesnakes, however,
these species are not dwarfed, they are average size for their clade and
slightly exceed a meter in length.
Several
of the small, broad-headed bush vipers of the Atheris complex are also Eastern
Arc Mountain species. Bush vipers have stout bodies, prehensile tails, and
heavily keeled overlapping scales. They are usually associated with hill
forests that are over 1,200 m in elevation and their distributions seem to
reflect changes in the vegetation during Pleistocene. Habitats in this sky
island chain include rock escarpments, grasslands isolated by savanna, and dry
and wet forests. Various species of bush vipers have adapted to different
habitats and while some species occur throughout the Eastern Arc, others are
restricted to a single mountain top.
William
Branch and Julian Bayliss recently described the remarkable Mount Mabu Viper
(Atheris mabuensis) from mid level elevations (1,000 to 1,550 m) on Mount Mabu
and Mount Namuli in northern Mozambique. The snake is a dwarf, reaching a
maximum total length of 380 mm. Adults retain the brown and gold coloration
seen only in juveniles of other species, and which camouflages the snake in
forest leaf litter. This little viper will climb onto small twigs lying on the
floor of its cage; it presumably does the same in the forest. It caudal lures
to attract frogs and lizards. The discovery of this viper in 2008 emphasizes
both the unique biodiversity of Africa’s sky islands and the fact that much
remains to be learned about snake biodiversity.
The
Pantepuis sky island complex of northern South America has flat-topped
mountains commonly known as tepuis that were made famous by Sir Arthur Conyan
Dolye’s novel the Lost World. The tepui complex has few endemic snakes that
have been discovered to date, but snakes can be notoriously difficult to find.
In 2002, an endemic coral snake (Micrurus pacaraimae) was described from this
area. It was collected in shrubby vegetation with rock outcroppings at the edge
of a small river dissecting a valley. Not much can be said about it since it is
known from a single specimen. However, more endemic snakes are likely to be
found in this unusual sky island complex since there are a number of known
endemic frogs and lizards from the flat topped mountains.
Arboreal Snake Dispersal Over Water
The
Natuna Islands form an archipelago of about 272 islands located midway between
Borneo and peninsular Malaysia in the South China Sea. While these islands were
connected to Borneo, Sumatra, and Malaysia during the Pleistocene, the Natuna
snake fauna does not appear to be the result of simple overland migrations.
An
analysis done by Robert Inger and Harold Voris found 11 arboreal species, five
terrestrial species, one aquatic species, and one fossorial species on Natuna.
About 77 widespread snake species occur in Borneo, Malaysia, and Sumatra and,
based on their ecological types, the Natuna Islands would be predicted to have
five to six arboreal species and nine to ten terrestrial species.
West
of the Natuna Islands and much closer to the coast of peninsular Malaysia, is
the Seribuat Archipelago, a cluster of 62 islands organized into three arcs. The
inner arc is close to the mainland, and sits in water less than 10 m deep. The
middle arc is about 12 km west of the first arc, in 10–19.5 m of water. The
outer arc is about 20 km further east and in water that is 20–60 m deep. Lee
Grismer and colleagues surveyed the herpetofauna of the archipelago and found
44 species of snakes, four of which are endemics. Their findings mirror the
work done by Inger and Voris in the Natunas. Most of the snakes were arboreal,
shrub and a tree trunk specialist, and only two were fossorial species. One
species, the Reticulated Python (Broghammerus reticulatus), is a very good
swimmer and undoubtedly capable of traveling between the mainland and many of
these islands, but it also climbs trees.
The
absence of many of the terrestrial and fossorial species from both of these
archipelagos is striking since they were connected by land as recently as
10,000 years ago at the last glacial maximum.
Inger
and Voris suggest that at least some of Natuna’s snakes dispersed not overland
at times of lower sea levels, but by island hopping on trees swept out to sea
as the large rivers reached flood stage. Borneo’s largest river, the Kapuas, empties
into the South China Sea about 75 kilometers from the Natuna Islands. None of
the known Natuna snakes are endemic to the islands and it would seem likely
that the exaggerated arboreal snake fauna is the result of over water
dispersal.
Similarly,
the east coast of Malaysia opposite the Seribuat Archipelago has about seven
major rivers draining into the South China Sea. It would seem probable that the
archipelago species would be the result of over water dispersal from peninsular
Malaysia. However, molecular studies to support the origins of the Natuna and
Seribuat faunas have yet to be done, and surprises are always possible.
Treeboas
(Corallus) are widespread in the West Indies, Central America, and South
America. When Robert Henderson and S. Blair Hedges compared West Indian
populations to mainland populations using mitochondrial DNA sequences, the West
Indies populations turned out to be most closely related to populations from Guyana
and the Amazon. This was unexpected, given that Trinidadian and Venezuelan
populations were much closer to the Grenada Bank islands and St. Vincent.
However, the South Equatorial current appears to be responsible for the Grenada
Bank-St. Vincent populations of treeboas. Given the DNA results, it is more
likely that trees washed into the Caribbean from a river in the Guianas or the
Amazon carried the snakes into the West Indies, rather than vegetation mats from
the Orinoco River or the island of Trinidad.
Treeboas
have been found swimming in the Caribbean 1.5–3.5 km from shore. Robert
Henderson and Richard Sajdak reported the observations of Larry Maul and his
wife who twice encountered treeboas during the summer of 2001. The snakes were
not associated with floating debris. Henderson and Sajdak suggested that in
most cases, the snakes would likely die from dehydration, drowning, or
predation. However, should one survive to make landfall, it could colonize a new
location.
Invasive Snakes on Islands
On
a rainy tropical morning sometime after World War II, possibly in 1948, a cargo
plane loaded with wooden crates and scrap metal took off from a base in the
Admiralty Islands and landed on the island of Guam. Guam is the southernmost
island in the Mariana Island group and a United States Territory in the
southwestern Pacific. The plane was carrying used war equipment and metal for
recycling, but it was also carrying a stowaway.
A
slender Brown Treesnake (Boiga irregularis), about 1.5 m long, had been living
on the Solomon Island airbase hunting roosting birds and lizards. This female
was very successful at capturing food, and she converted her prey’s energy and
molecules into stored fat and body mass. The lipids altered her odor, making
her more attractive to males. She mated with several males while on the air
base and stored their sperm in her reproductive tract.
In
time she grew restless, her body had started to convert the fat into eggs. In a
few more weeks she would fertilize her eggs with the stored sperm and she
needed a safe place to deposit the eggs. A wooden crate provided refuge; the
female crawled inside and found a secure hiding placing in the packing
materials.
As
the cargo was unloaded, so was the female snake. After dark, she crawled across
the runway into the nearby forest. The female snake did not realize it, but she
had just won reproductive success for her genes and offspring at a level few
animals ever experience. The safe landing of the plane carrying the Brown
Treesnake corresponds with the start of an ecological disaster. The disaster
was a quite one, and went unnoticed for more than 30 years.
Guam is an oceanic island with a mixture
of grasslands and forests. It had 18 species of birds; 10 were forest dwelling
species, and five were endemic to Guam. Fifteen of these eighteen bird species
have now disappeared. Guam also had lizards. Some like the house gecko, and the
oceanic geckos were species that probably stowed away on ships and traveled with
humans to the island. But others had been on Guam long enough, like some of the
birds, to become distinct species, endemics found nowhere else. Many of Guam’s
lizards, particularly the geckos, now appear to be extinct or greatly reduced
in numbers. Only the introduced House Gecko (Hemidactylus frenatus) has
increased its numbers on the island, and this may be due to the reduced
competition from other species, but it may also have something to do with its
ability to live on walls of buildings that are inaccessible to the Brown Treesnake.
Guam had only three native mammals, all
species of bats. One was a small species of fruit bat and it has not been seen
since 1968. While the snake is suspected of causing its extinction, scientific
evidence does not exist to support the suspicion. The Marians Fruit Bat is a
large species, its numbers had been reduced to about 50 animals in 1978, but
the species rebounded to more than 1000 animals by 1982. Since that time the
species has gone into decline, possibly due to the snake feeding on the young
bats when they cannot be protected by the parent. The Pacific Sheath-tailed Bat
(Emballonura semicaudata) has not been recorded on Guam since 1972 and the Guam
population is believed extinct. Again the Brown Treesnake is the suspect.
Not
only has the Brown Treesnake been a disaster for the Guam fauna, but it has
caused numerous problems for humans living there as well. Perhaps most unusual
are the power outages it causes. Between 1978 and 1994, there were almost 1,200
power outages attributed to treesnakes. The snakes would ascend guy wires used
for support and gain access to a set of conducting wires that were surrounded
by ionized air. When the snake reached these wires it caused a flashover fault
resulting in a power outage.
The
Brown Treesnake is a rear-fanged species with grooved fangs, venom, and an
attitude that results in attempts to swallow food items much larger than it can
actually deal with, including human infants. A survey of 284 Guam residents
asked them about finding tree snakes in their houses and yards, 79% said they
had seen the snakes in or around their home. These snakes are quite aggressive
and are not deterred from entering an area when humans are present. Between
1989 and 1993, 166 bites were reported, 11 of these were considered serious. All
serious bites were sustained by sleeping infants less than a year old. Most
occurred between dusk and dawn when Brown Treesnakes are hunting and it seems
likely the bites were attempts to consume the newborn humans. The snakes
involved in the bites were large, usually more than 1.3 m long, and in seven
cases the snakes tried to constrict the infants. The Brown Treesnake is now
estimated at population densities of more than 5000 per km2 in some forested
areas and it seems unlikely that control is possible.
Invasive
species are out of place, they are in environments where they did not evolve,
and their presence can have long term, cascading consequences that are
difficult to predict. Hanne Mortensen and colleagues examined the impact of the
Brown Treesnake on vertebrate pollinators visiting the Black Mangrove Tree
(Bruguiera gymnorrhiza) and Tiger Claw Tree (Erythrina variegate). The study
compares tree pollinator behavior in Guam with its high snake population to the
behavior of the same or similar pollinators in Saipan where there are few
snakes. Both species of trees are pollinated by birds. On Guam, no birds
visited either of the tree species under observation, while on Saipan bird
visits per day to both tree species approached 120. Insects and lizards also
visited the flowers, but only the lizards were considered potential pollinators
of these flowers. As expected, Saipan trees produced more seeds than did the
trees on Guam. Treesnakes have had a significant impact on the Guam ecosystem, affecting
organisms well beyond their prey species.
Cozumel
Island is a well known point of debarkation for tourists in México, but it
became the destination for a group of common Boa Constrictors (Boa constrictor)
in 1971. Someone working for a film production company released Boa
Constrictors near Palancar Beach after the filming was complete. While the Boa
Constrictor is native to México, it was absent from Cozumel Island until the
release of the snakes, and it very quickly increased its numbers. A wildlife
survey 25 years later reported 1.8 boas for every 100 km of forest surveyed. Road
surveys done by Irene Romero-Nájera and co-workers in 2001 and 2002 found 0.03
boas for every 10 km of roads driven and they found the snake widespread and
relatively abundant. Boas were present in virtually all habitats, including the
city. Romero-Nájera and colleagues recognized the paradox, the snake is
considered threatened on the Mexican mainland, but has become invasive on
Cozumel.
A
similar situation seems to be occurring on the Dutch West Indies island of
Aruba. The Boa Constrictor was first discovered in 1999 with five specimens
captured that year. In 2001, forty snakes were captured, and by 2003 the annual
capture rate was 273 snakes. The boas ranged in size from neonates to 2.8 m
adults and included two pregnant females. John Quick and colleagues documented
the snakes island wide and examined the stomach contents of 47 specimens. They
were eating lizards, birds, and mammals. The origin of the invasion remains
unknown. Quick and colleagues point out that a major difference between the
Aruba Boa Constrictor invasion and the
Brown Treesnake invasion on Guam is that Aruba does not have many endemic
species. Most of the island’s vertebrates are derived from nearby mainland
populations where the Boa Constrictor occurs naturally. Therefore, the native
vertebrates may be better equipped to respond to the sanke.
Rare Island Snakes
The
isolation of islands and reduced numbers of species makes them more sensitive
to environmental disruptions. The Lesser Antillean island of Antigua and its
satellites have a land surface area of about 282 km2 and are home to the Antiguan
Racer (Alsophis antiguae). The Antiguan Racer was once found over most of this
area. By 1936, H. W. Parker at the British Museum declared the species extinct.
But in 1991 Rick Sajdak and Robert Henderson reported the species restricted to
Great Bird Island, an area about 0.1% of the original distribution of the
snake. Populations elsewhere had been destroyed by the introduced Asian
mongoose. Jenny Daltry and colleagues surveyed the last population in 1995 and
estimated 51 snakes remained, but reported the snakes were being seriously
injured by the introduced Black Rat (Rattus rattus). An attempt to breed the
snakes in captivity was made in 1996 at the Jersey Zoo, but failed because the
snakes were highly sensitive to snake mites (Ophinonssus natricis), a common
parasite in captive snake collections. In 1999, a trial re-location was attempted,
10 snakes were moved to Rabbit Island where the Black Rat had been eradicated.
The colony took hold. By 2002 the snakes were reproducing, and by 2006 the Rabbit
Island population was estimated to be 40 to 50 snakes. A rat eradication
program on Great Bird Island was successful, and soon after the Antiguan Racer
populations increased dramatically; it doubled in the first 18 months and
increased by 300% over the next nine years.
The
Mascarene Islands of the Indian Ocean are remote. They are volcanic in origin
and were created by the Reunion hot spot, with the first island formed about 35
MYA. Round Island is a small (169 hectares) member of this island group and is
located about 20 km northeast of Mauritius. It is home for two related, ancient
snakes placed in the family Bolyeriidae. The DNA clock suggested these snakes
evolved in the Cretaceous, about 97 MYA. The Round Island snakes are sometimes
called “boas” but they are not; they are more ancient than boas and are of
interest because together they form the sister to shield-tailed snakes
(Uropeltidae) and the booid snakes (Boidae, Pythonidae, Xenopeltidae,
Loxocemidae, and the Pythonidae). Bolyerids have an unusual jaw divided into
front and rear sections, which may be an adaptation to feeding on skinks but
little is known of their natural history.
Goats
and rabbits introduced to Round Island in the 19th century subsequently
destroyed the native vegetation. Nine species of reptiles were known to inhabit
the island and several of them are thought to have passed into extinction with
the help of humans and the introduced herbivores. The Round Island Burrowing Snake
(Bolyeria multicarinata) has not been seen since 1975, but the Round Island
Keeled-scaled Snake (Casarea dussumieri) is still present, though probably in
low numbers. The Jersey Zoo’s captive breeding attempt with Casarea met with
some early success, but ended in the early 1990’s. The Zoo was again successful
after the discovery that the snakes need vitamin D3 to reproduce. In 2005, eggs
were successfully hatched. Casarea was also found on the island of Mauritius
but its population on that island appears to have become extinct shortly after
it was discovered. The Round Island goat population was eradicated by 1978, and
the rabbits were gone by 1986. The lizard and snake populations were predicted
to recover, and while some of the lizard populations have increased their
numbers, Casarea numbers have not increased significantly.
Evolution
of island species may increase after the founding members find themselves in
isolation. The island may be free of both competitors and predators and
resources may be severely limited or very abundant. Island invaders encounter new
food resources and a new community of organisms to interact with. While island
populations may lead to a dead end for many species, the possibility of species
dispersal from islands back to the mainland or dispersal off a mountain top
into lowland environments cannot be ruled out. Understanding species survival
on islands will become increasingly important as humans fragment virtually all habitats
into islands across the globe. Fragmented habitats can be expected to
accelerate extinction for many species, but the species that do survive will be
following the rules of island biogeography.
Comments
Post a Comment