OVERVEIW
Brown tree snake is a mildly poisonous, exotic snake that
threatens to invade North America. It is native to northern
and eastern coastal Australia, New Guinea, and other
islands of northwestern Melanesia. It was found outside
this area on Guam in 1953. The exact date it reached the
island is uncertain, but it probably arrived on military
cargo transport vessels after World War II. In the years
that followed, the brown tree snake population on Guam
exploded. It has now driven many native animals to
extinction or to endangered species lists, attacked pets and
poultry, caused power outages, and even has bitten
humans. From Guam, this snake has used ships and
aircraft to invade other islands in the Indo-Pacific,
including Hawaii, and has even reached Texas.
ECOLOGICAL IMPACT ON GUAM
Guam had only one native snake, a small blind snake
(Ramphotyphlops braminus), which is a little bigger than
an earthworm. As a result, native animals did not
recognize snakes as dangerous predators. By 1968, brown
tree snakes invaded the entire island. Now, due to snake
predation, Guam has lost all breeding populations of native
seabirds, 10 of 13 native forest birds, 2 of 3 native
mammals, and 6 of the 10-12 native lizards. Those that
have survived are considered uncommon to rare. Some
introduced birds have been better able to resist
depredation. Brown tree snakes have virtually eliminated
the poultry industry
from Guam and consumed or simply killed pets and small
mammals. Introduced pigs and monitor lizards do eat
brown tree snakes, but do not control their number. The
full ecological consequence of brown tree snake damage
on Guam remains uncertain. The loss of insectivorous
birds and lizards could open the door for insect pest
problems. Exotic insects reaching Guam may now have an
increased chance of becoming established. Native birds
and bats no longer disperse seeds to maintain natural
forests on Guam. Spider numbers have increased with the
demise of most birds while butterfly numbers have fallen.
HUMAN IMPACTS ON GUAM
Snakes climbing on power lines have caused extensive
power outages (one every three days from 1978 through
1997). New procedures and techniques have helped
reduce electrical problems. Still, economic impacts have
touched virtually every industry on the island including
delays in shipping and transport as well as agricultural
damage and losses. Humans have been hospitalized due to
brown tree snake bites.
INVASION THREAT AND DISPERSAL EVENTS:
Many have expressed concerns that Brown tree snake may
reach the continental U.S. and cause ecological problems
here. Brown tree snake behavior makes it a prime
candidate to invade new areas. It is active at night, lives in
close proximity to man, readily hides among man-made
items, and can survive for long periods of time without
food. Once in a new environment, it readily accepts a
wide array of food items. Guam experiences a large
amount of both military and civilian traffic by which the
snakes have escaped the island. Brown tree snakes from
Guam have already reached Cocos Island off Guam, Diego
Garcia (Indian Ocean), Kawajalein (Marshall Islands),
Photo courtesy of USGS
Oahu in Hawaii (on at least six occasions), Okinawa,
Pohnpei, (Japan), Saipan, Rota (near Guam), Tinian
(Micronesia), Wake Island, Singapore, Taiwan, Australia
(on a U.S. bomber that landed at Darwin), Rota (Spain), as
well as Texas. Texas Records: On May 12, 1993, a
brown tree snake was found and killed at the Naval Station
at Ingleside in a crate of household goods shipped from
Guam. It may have survived in the crate for as long as
seven months. At least two other reports from Texas could
not be confirmed. Survival Potential: Experts believe
brown tree snakes could survive in southern California,
Florida, and probably southern Texas. Hawaii is
especially concerned, with good cause.
CONTROL PROGRAMS:
In an effort to prevent brown tree snakes from leaving
Guam, the U.S. Military (Department of Defense), U.S.
Department of Agriculture, U.S. Fish and Wildlife Service,
U.S. Geological Survey, and local authorities on Guam
have initiated brown tree snake programs. Shipments
leaving the island are inspected for snakes. Many of these
same agencies as well as other local authorities on Hawaii,
other Indo-Pacific islands, and the U.S. mainland are alert
to shipments arriving from Guam that could contain
hidden brown tree snakes. Because this is so important,
the Nonindigenous Aquatic Nuisance Species Prevention
and Control Act of 1990 authorized control of brown tree
snake outside its native range. This material is part of that
effort.
Photo courtesy of USGS
DESCRIPTION:
Brown tree snake is but one of several genera and species
of tree snakes found in Southeast Asia and the adjacent
Indo-Pacific and Australia regions. Most brown tree
snakes reach lengths of 3-6 feet, but some have reached 11
feet in length. Specimens from Guam are often larger than
in their native range. They are long and slender, with
broad heads, narrow necks, and large protruding eyes with
elliptical cat-like pupils. Coloration and scale patterns
vary throughout its range and color can vary within a
population. Most on Guam are primarily camouflaged in
brown and olive green with a faint banding pattern. Others
in Australia may be brown to reddish brown with irregular
darker bands; red and white in tropical Australia; and pale
scales may be black edged. The belly is cream to salmon
color. Many specimens display a distinct bar through the
eye and along the side of the head. The snakes now on
Guam are most similar to those on the Admiralty Islands.
Snakes entering the U.S. in cargo will likely be the Guam
form. However, others illegally imported via the pet trade
could originate elsewhere and be different in appearance.
BIOLOGY
As their name implies, brown tree snakes are good
climbers. They have a keen sense of smell and excellent
night vision. They are frequently found in and around
homes and commercial buildings in close association to
people. Brown tree snakes are not restricted to forests or
even trees. They also occur in grasslands and among stone
cliffs. They can slip through very small spaces. They
avoid bright light and high temperatures, but may be active
during the day. Prey includes small mammals, birds and
their eggs, frogs, and lizards. Brown tree snakes are rear-
Photo courtesy of USGS
Photo courtesy of USGS
fanged with the last two teeth in the upper jaw grooved to
deliver the poison by chewing and capillary flow of
venom. They wrap their bodies around prey and constrict
as well. Larger individuals prey most heavily on larger
birds and mammals. Females lay 1-12 eggs about 42-47
mm long with leather shells that incubate about 90 days.
Young are about 14-18 inches at hatching. Females often
produce two clutches each year, depending on seasonal
conditions. Recent surveys estimate 12,000-13,000
snakes/square mile on Guam.
Photo courtesy of USGS
SNAKE BITE CONSIDERATIONS:
Brown tree snakes can be aggressive when threatened and
may bite repeatedly. The mouth must be opened widely to
involve grooved teeth and chewing is required to introduce
venom. Teeth often break off during bites and cause
bleeding and local infection, swelling, and itching. Their
bites have been equated to a bee or wasp sting. Though no
human deaths have occurred from brown tree snakebites,
elderly and infants are at particular risk as are those with
allergic reactions. Cases with respiratory problems have
occurred in people bitten by brown tree snakes. Most
problem bite cases relate to small children and typically
involve large snakes. Curiously, brown tree snakes
reportedly seek out small children, being known to crawl
between two sleeping adults to repetitively bite an infant.
Very small children and large snakes suggest the
possibility of suffocation by constriction. No specific
antivenin for brown tree snakes exists.
SIMILAR SPECIES:
Within its native range, brown tree snakes are most similar
to other tree snakes in the genera Boiga and Dendralaphis.
Worldwide, there are many species that might be confused
with the brown tree snake; however, it is unlikely these
snakes would be encountered in North America (other than
in pet trade or zoo situations). Among the native snakes in
the U.S., two in particular are somewhat similar to the
brown tree snake. Lyre snake (Trimorphodon biscutatus),
from the southwestern U.S. and adjacent Mexico, can
reach nearly 4 feet in length, has a head wider than its
body and protruding eyes with elliptical pupils. Color
patterns vary throughout its range and some can be similar
to brown tree snakes, but many display a V-shaped pattern
on the head. Lyre snakes are legally protected as
threatened species in Texas. Coachwhip snakes
(Masticophis flagellum), from the southern U.S., may also
have somewhat similar color patterns, but their heads are
not dramatically wider than their necks and their eyes have
round pupils and do no protrude. Several other American
snakes also have elliptical pupils like brown tree snake, but
they either do not have wide heads or have distinctly
different color patterns. Rattlesnakes, cottonmouths, and
copperheads have elliptical pupils and wide heads, but
have very thick, heavy bodies.
IF YOU THINK YOU HAVE FOUND A BROWN TREE SNAKE?
Snake Handling: It is difficult to provide a blanket
recommendation for what to do. On one hand, it would be
terrible if a brown tree snake entered the U.S. and escaped.
On the other hand, it would be unfortunate to kill a
misidentified and legally protected lyre snake or to be
bitten by a brown tree snake while trying to capture it. If
you think you have found a brown tree snake, do not
panic…this might agitate the snake. Stay calm and in
control. Remember, its bite is not lethal. Also, note that
snakes can be easily captured or killed with commonly
available items (a shovel, rock, broom handle, etc.).
However, handling a snake increases chances of being
bitten. Snakes are very vulnerable to injury and a sharp
blow to the spine (except on the tail) can kill or immobilize
it. However, do not allow an injured snake to crawl away
to die where it cannot be retrieved. Living, injured, and
dead snakes can often be lifted with a stick or hoe, then,
placed in a covered garbage can to contain them. Covering
hands with cloth or gloves can be helpful. Possible brown
tree snake sightings in the U.S. should be immediately
reported to 1-877-STOP-ANS.
NEVER totally destroy the snake (enough needs to remain for identification purposes, especially the head). If possible
preserve the specimen (freeze, refrigerate). It is important for researchers to confirm its identity and determine its
origin.
WHAT CAN BE DONE?
Extensive research on trapping, fumigation, barriers,
repellants, toxicants, parasites, and diseases is underway.
Shipments leaving and (sometimes) arriving from Guam are
inspected. Still, some snakes may be overlooked.
Watch cargo arrival areas accepting shipments (directly and
indirectly) from Guam. Support regulatory agency and other
governmental control efforts. Also, watch petrade sources
that deal in exotic snakes. Report any sighting immediately.
REFERENCES
http://www.mesc.usgs.gov/resources.education/bts
http://www.hawaii.gov/dlnr/Snake.html
http://www.abi.org/publications/leastwanted.snake.html
http://159.189.24.10/btreesnk.htm
http://www.invasivespecies.gov/profiles/bts.shtml
http://www.pwrc.usgs.gov/btdisp2.htm
http://www.anstaskforce.gov/BTS%20Control%20Plan.htm
http://www.dfw.gov.mp/newpage11.htm
BROWN TREE SNAKE: DESCRIPTION DETAILS
General shape: Brown tree snakes are long and slender, with broad heads, very narrow necks, and large protruding eyes with
elliptical cat-like pupils.
Shape and length: Most brown tree snakes are 3-6 feet in length, but they can reach up to 11 feet. Specimens from Guam are
often larger than others within their original native range.
Coloration: Coloration and scale patterns vary throughout its range and color can vary within a population. Most on Guam
are primarily camouflaged in brown and olive green with a faint banding pattern. Others in Australia may be brown to
reddish brown with irregular darker bands; red and white in tropical Australia; and pale scales may be black edged.
The belly is cream to salmon color. Many display a distinct bar through the eye and along the side of the head. The
snakes now on Guam are most similar to those on the Admiralty Islands. Snakes entering the U.S. in cargo will likely
be the Guam form. However, others imported via the pet trade could originate elsewhere and be different in
appearance.
Scale counts: Brown tree snakes have 10-23 scale rows along the midbody, 225-265 ventral scutes (belly scales), a divided
anal plate, and 85-130 postanal or subcaudal scutes (ventral tail scales) that are divided.
Long and slender
Large eyes with elliptical pupils
Divided Anal Plate
Bulging eyes
Broad head
Narrow neck
SNAKE DENTATION: FANGS AND TEETH
Often, poisonous snakes are considered to have two, large,
hollow, hypodermic-like fangs followed by rows of typical
teeth. Non-poisonous species would lack such fangs.
Snake dental patterns are actually much more varied.
Many non-poisonous snakes do have roughly equal-sized
teeth. Some that feed on fish or birds may have somewhat
longer teeth. Among poisonous snakes with hollow fangs,
cobras and their relatives (sea snakes, coral snakes, kraits)
have enlarged, front fangs that are fixed in place and do
not fold down when not in use. Vipers and pit vipers have
particularly large fangs that are movable and are folded
down when the mouth is closed. Other tooth patterns exist
in other snakes. In brown tree snakes, the last two, top rear
teeth are slightly longer than the preceding teeth and are
grooved to allow venom to pass into a wound when the
snake bites. This method of delivering venom is
somewhat inefficient and requires the snake to bite, hold,
and chew to deliver significant amounts of poison.
African boomslang (Dispholidus typus ) is also considered
to be a rear-fanged snake, but has its enlarged, grooved
teeth more centrally-located in the upper jaw and has other
teeth behind them.
Non-poisonous snake skull with fixed
teeth of near equal sizes.
Cobra skull with forward, fixed, moderate-size fangs.
Brown tree snake skull with slightly enlarged, rear fangs.
Rattlesnake skull with forward, movable, long fangs.
Boomslang skull with slightly enlarged fangs more
centrally-located in the upper jaw.
SIMILAR U.S. SPECIES
Lyre snake (Trimorphodon biscutatus) occurs from
southern and western Texas westward into southern New
Mexico, Arizona, and California, as well as in northern
Mexico. It reaches lengths of nearly 4 feet and has a head
that is wider than its neck with protruding eyes and
elliptical pupils. Scale counts include 21-24 scale rows;
220-244 ventral scutes, 58-86 caudal scutes that are
divided, and anal plate that is divided. Coloration varies
throughout its range with some forms showing bold
banding patterns of dark brown saddles (17-24) that are
Coachwhip snakes (Masticophis flagellum) occur across the
southern half of the U.S. from coast to coast. Lengths average
42-60 inches, but can exceed 100 inches. Heads are only slightly
wider than their necks, eyes do not protrude boldly, and pupils
are round. Coloration varies with subspecies and region. In the
east, coachwhips are typically uniformly dark anteriorly. In the
west, they may be unicolored, narrow banded, or wide banded in
shades of yellowish-brown to darker brown (reddish in some
populations). Tails appear braided. Juveniles are banded. They
are typical of grasslands and savannahs. Scale counts include 13
or fewer scale rows, 173-212 ventral scutes, 91-129 caudal scutes,
and anal plate is divided. They lack both fangs and venom.
Coachwhips are fast, active, and often nervous species that
may fight and bite if cornered or restrained.
widest above and narrow on the sides on lighter brown or
gray, but others have only vague markings. Many
specimens display a V-shaped lyre pattern on top of the
head. Cat-eyed snake (Leptodeira septentrionalis) from
the lower Rio Grande Valley also has a wide head, narrow
neck, and vertical pupils, but has distinctive bold dark
brown saddles. Both species are rear-fanged and mildly
venomous. Lyre snake is found in rocks and crevices in
arid regions and is protected as a threatened species in
Texas.
OTHER SNAKES WITH REAR FANGS
OR VERTICLE PUPILS
OTHER REAR-FANGED SNAKES:
In addition to lyre snake (Trimorphodon biscutatus) there
are also a number of other rear-fanged snakes that are
native to the U.S. and Mexico, with numerous other
species throughout the world. Those species found in the
U.S. are generally small and rarely recognized as “fanged”
snakes.
Black-striped snake (Coniophanes imperialis) – The
U.S. distribution of this species is restricted to the southern
tip of Texas. It has round pupils and has a definite
longitudinal striped pattern unlike brown tree snake.
Black-striped snake reaches about 20 inches in length.
Night snake (Hypsiglena torquata) – This species ranges
widely through the southwestern two-thirds of the U.S. and
into Mexico. It has rear fangs, vertical pupils, and
sometimes displays a gray and tan blotched pattern similar
to brown tree snakes. However, it only reaches about 26
inches in length and its head is not dramatically wider than
its neck.
Cat-eyed snake (Leptodeira septentrionalis) – This
species too is restricted in the U.S. to a limited area in
extreme southern Texas. It has rear fangs, vertical pupils,
and a head that is wider than its neck. However, cat-eyed
snakes have a bold pattern of dark brown saddle marks that
are distinctively different than brown tree snake.
Other native rear-fanged snakes – Pine woods snake
(Rhadinaea flavilata) is a small, 15-inch long, brown snake
that is widely distributed in the southeastern U.S. coastal
areas; though technically a rear-fanged snake, would probably
never be confused with brown tree snake. Native hognose
snakes (Heterodon spp.) also have enlarged rear teeth
(perhaps for deflating toads, their primary food), but have
heavy bodies and upturned snouts.
Exotic rear-fanged snakes - There are many species of
rear-fanged snakes throughout the world. Only a few are
likely to appear in zoos or the pet trade. However, one
species that is closely related to brown tree snake is
regularly imported and bred in the U.S. for the pet trade.
Mangrove snake (Boiga dendrophyla) is boldly patterned
in black and yellow, making them attractive pets.
Nonetheless, they are poisonous and can bite. Mangrove
snakes reach 6 to 7 feet in length and originate in coastal
areas of Indonesia and Malaysia. Reports suggest their
bites are extremely painful and victims may become very
ill. Deaths may have occurred from mangrove snake bites.
Still, at this time, federal regulations only prohibit
importation, sale, and possession of brown tree snake.
OTHER SNAKES WITH VERTICAL PUPILS:
Pit vipers and vipers: Rattlesnakes (Crotalus spp.), pigmy
rattlesnakes and massaugas (Sistrurus spp.), and copperheads
and water moccasins (Ankistrodon spp.) also have eyes with
vertical pupils. Although their heads are triangular and wider
than their necks, they have heavy bodies, keeled scales, and
small scales on top of the head (except in Sistrurus). None
would likely be confused with brown tree snakes.
Keeled scales (above left) have a ridge running from the front of the scale to the back. Unkeeled scales (above right)
lack this feature. Snakes with keeled scales have a generally rough or dull appearance. Those with unkeeled scales
often appear smooth and glossy.