Boiga irregularisBrown catsnake, Brown Tree Snake

Ge­o­graphic Range

Boiga ir­reg­u­laris, brown tree snakes, are na­tive to north­ern Aus­tralia (New South Wales, North­ern Ter­ri­tory, and Queens­land), Papua New Guinea, and east­ern In­done­sia. They have also been ac­ci­den­tally in­tro­duced to the Pa­cific is­land of Guam (Savidge et al., 2007). (Amand, 2000; Savidge, et al., 2007)

Habi­tat

The Brown Tree Snake prefers the low­land wood­lands and coastal forests of North­ern Aus­tralia as well as the more trop­i­cal rain­forests of Papua New Guinea and In­done­sia. How­ever, they are not re­stricted to forests and have been ob­served to live in caves and human made struc­tures (i.e. at­tics, upper por­tions of build­ings, etc.) as well. This snake is often noc­tur­nal and gen­er­ally takes shel­ter in hol­low logs, tree crowns, caves, or mis­cel­la­neous crevices dur­ing the day. (Amand, 2000; O'Shea and Hal­l­i­day, 2001)

Phys­i­cal De­scrip­tion

The Brown Tree Snake has a very slen­der and agile body co­in­cid­ing with its ar­bo­real lifestyle. They gen­er­ally are a light brown with ei­ther darker dor­sal mark­ings or a uni­form col­oration, how­ever, these snakes have also been ob­served to be red, yel­low, pink, or a creamy white. It has a dis­tinc­tively large head with opistho­glyp­hous (rear­ward) fangs and big bul­bous eyes char­ac­ter­is­tic of the genus Boiga (and in keep­ing with noc­tur­nal habits). They are long, lat­er­ally com­pressed snakes (2 to 2.5 m long) with large ver­te­bral scales and a long tail. Ma­ture Brown Tree Snakes have vari­able masses rang­ing any­where from 60 to 2300 g. (O'Shea and Hal­l­i­day, 2001; Savidge, et al., 2007)

  • Sexual Dimorphism
  • male larger
  • Range mass
    60 to 2300 g
    2.11 to 81.06 oz
  • Average mass
    523 g
    18.43 oz
  • Range length
    1.03 to 2.91 m
    3.38 to 9.55 ft
  • Average length
    1.7 m
    5.58 ft

De­vel­op­ment

As with all squa­mates, fer­til­iza­tion is in­ter­nal. Shelled am­ni­otic eggs are laid; the hatch­ling snakes are es­sen­tially minia­tures of the adults and grow in­cre­men­tally. Sex is prob­a­bly de­ter­mined ge­net­i­cally, as ap­pears true for all snakes. It is un­known whether Brown Tree Snakes have in­de­ter­mi­nate growth. (O'Shea and Hal­l­i­day, 2001)

Re­pro­duc­tion

These snakes ap­pear to re­pro­duce year-round, but more re­search is needed on Brown Tree Snake mat­ing sys­tems. (Math­ies, et al., 2010; Savidge, et al., 2007)

Male Brown Tree Snakes have been ob­served to store sperm year-round sug­gest­ing asea­sonal sper­mato­gen­sis; how­ever, this is under de­bate due to the fact that the ma­jor­ity of trop­i­cal snakes have sea­sonal sper­mato­gen­sis (Math­ies et al., 2010). Fe­males have been ob­served to lay clutches of 3-12 leath­ery-shelled eggs bian­nu­ally. Gen­er­ally, fe­males lay their eggs in hol­low logs, rock crevices, or caves in order to pro­tect them from des­ic­ca­tion (Fritts & Rodda, 1998). Eggs hatch after ap­prox­i­mately 90 days and off­spring are around 50 cm long and take around 3 or 4 years to reach sex­ual ma­tu­rity (Savidge et al., 2007). (Fritts and Rodda, 1998; Math­ies, et al., 2010; Savidge, et al., 2007)

  • Breeding interval
    Females lay 2 clutches per year
  • Breeding season
    Aseasonal breeding
  • Range number of offspring
    3 to 12
  • Average number of offspring
    6
  • Range age at sexual or reproductive maturity (female)
    3 to 4 years
  • Range age at sexual or reproductive maturity (male)
    3 to 4 years

Brown Tree Snakes aban­don their eggs after lay­ing and there is no parental care of the young. (O'Shea and Hal­l­i­day, 2001; Savidge, et al., 2007)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
    • protecting
      • female

Lifes­pan/Longevity

Brown Tree Snakes are re­ported to have a lifes­pan rang­ing from 10-15 years (Amand, 2000). (Amand, 2000)

  • Range lifespan
    Status: wild
    10 to 15 years
  • Range lifespan
    Status: captivity
    9.25 (high) years

Be­hav­ior

Brown Tree Snakes are largely ar­bo­real but have been ob­served liv­ing in non-ar­bo­real sit­u­a­tions. They are noc­tur­nal, spend­ing the day­time shel­tered in trees, caves, or cliffs and the night­time hunt­ing and for­ag­ing (Camp­bell et al., 2008). They are also known to be very ag­gre­sive to­wards any po­ten­tial preda­tor that comes within close prox­im­ity (O'Shea & Hal­l­i­day, 2001). (Camp­bell, et al., 2008; O'Shea and Hal­l­i­day, 2001)

Com­mu­ni­ca­tion and Per­cep­tion

These snakes un­doubt­edly have fairly good eye­sight, but will pre­dictably de­pend on de­tect­ing chem­i­cal cues (odors) via the vomero-nasal ("Ja­cob­son's") organ as in other snakes. Per­cep­tion of sound has not been stud­ied in this species. More re­search is needed specif­i­cally on Brown Tree Snake com­mu­ni­ca­tion and per­cep­tion. (O'Shea and Hal­l­i­day, 2001)

Food Habits

The Brown Tree Snake eats a wide va­ri­ety of prey rang­ing from birds, small mam­mals, am­phib­ians,and other rep­tiles. This snake sub­dues its prey ei­ther through the use of its venom or through con­strict­ing its prey with its long body (Amand, 2000). In Guam, the Brown Tree Snake is in­va­sive and is be­lieved to have caused the ex­tinc­tion of sev­eral bird species, and has greatly re­duced pop­u­la­tions of other ver­te­brates (O'Shea & Hal­l­i­day, 2001). (Amand, 2000; Fritts and Rodda, 1998; O'Shea and Hal­l­i­day, 2001)

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles

Pre­da­tion

In their nat­ural range, Brown Tree Snakes are eaten by wild pigs, birds of prey, and cer­tain other snakes and large lizards, such as mon­i­tors. There are few known preda­tors of this snake on the is­land of Guam, which has al­lowed the species there to at­tain ab­nor­mally large pop­u­la­tion den­sity and to grow to larger than av­er­age size (Up to 3 m; O'Shea & Hal­l­i­day, 2001). Brown Tree Snakes are rear-fanged and ven­omous, and are cryp­ti­cally col­ored to hide in veg­e­ta­tion. (Amand, 2000; Fritts and Rodda, 1998; O'Shea and Hal­l­i­day, 2001)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators

Ecosys­tem Roles

The Brown Tree Snake is a preda­tor that has nor­mal con­straints on its pop­u­la­tions in its nat­ural range. It has no nat­ural preda­tors on the is­land of Guam and there­fore has be­come an apex preda­tor. Be­cause of this, Brown Tree Snakes have be­come over­abun­dant there and have se­verely dis­rupted the ecosys­tem pri­mar­ily through its preda­tory im­pacts on na­tive ver­te­brate species (Amand, 2000) (Fritts & Rodda, 1998). (Amand, 2000; Camp­bell, et al., 2008; Fritts and Rodda, 1998; O'Shea and Hal­l­i­day, 2001)

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

The Brown Tree Snake pro­vides no spe­cific ben­e­fit to­wards hu­mans, though in its nat­ural habi­tat it con­tributes to a func­tion­ing ecosys­tem. (Fritts and Rodda, 1998; Savidge, et al., 2007)

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

Brown Tree Snakes are mildly ven­omous to hu­mans; bites can be painful and oc­ca­sion­ally de­bil­i­tat­ing. Bites to chil­dren or in­fants can be more se­ri­ous. Where in­tro­duced, as on Guam, in ad­di­tion to eco­log­i­cal dam­age, this snake can cause power out­ages by hid­ing in trans­form­ers, and de­tracts from tourism. Sig­nif­i­cant eco­nomic in­vest­ments have been made in Guam to erad­i­cate this in­va­sive species from the is­land. (Amand, 2000; Fritts and Rodda, 1998; O'Shea and Hal­l­i­day, 2001; Savidge, et al., 2007)

  • Negative Impacts
  • injures humans
  • household pest

Con­ser­va­tion Sta­tus

Brown tree snakes are not con­sid­ered threat­ened or en­dan­gered. Where they have been in­tro­duced, they pose a sub­stan­tial threat to na­tive fau­nas and erad­i­ca­tion pro­grams are ac­tive.

Con­trib­u­tors

Stephen Kerr (au­thor), Michi­gan State Uni­ver­sity, James Hard­ing (ed­i­tor), Michi­gan State Uni­ver­sity, Tanya Dewey (ed­i­tor), Uni­ver­sity of Michi­gan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

arboreal

Referring to an animal that lives in trees; tree-climbing.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

year-round breeding

breeding takes place throughout the year

Ref­er­ences

Amand, A. 2000. Boiga ir­reg­u­laris (Brown Tree Snakes) on Guam and Its Ef­fect on Fauna. Restora­tion and Recla­ma­tion Re­view, 6/6: 1-6.

Camp­bell, S., S. Mackessy, J. Clarke. 2008. Mi­cro­hab­i­tat Use by Brown Treesnakes (Boiga ir­reg­u­laris) Ef­fects of Moon­light and Prey. Jour­nal of Her­petol­ogy, 42/2: 246-250.

Fritts, T., G. Rodda. 1998. THE ROLE OF IN­TRO­DUCED SPECIES IN THE DEGRA­DA­TION OF IS­LAND ECOSYS­TEMS: A Case His­tory of Guam. Annu. Rev. Ecol. Syst., 29: 113-140.

Math­ies, T., J. Cruz, V. Lance, J. Savidge. 2010. Re­pro­duc­tive Bi­ol­ogy of Male Brown Tree Snakes (Boiga ir­reg­u­laris) on Guam. Jour­nal of Her­petol­ogy, 44/2: 209-221.

O'Shea, M., T. Hal­l­i­day. 2001. Rep­tiles and Am­phib­ians. United States: Dor­ling Kinder­s­ley.

Savidge, J., F. Qualls, G. Rodda. 2007. Re­pro­duc­tive Bi­ol­ogy of the Brown Tree Snake, Boiga ir­reg­u­laris (Rep­tilia: Col­u­bri­dae), dur­ing Col­o­niza­tion of Guam and Com­par­i­son with That in Their Na­tive Range. Pa­cific Sci­ence, 61/2: 191-199.