Zoo History: Empire of the Brown Tree Snake

"But behavior, even in a pea-brained reptile, is surprisingly flexible.  On Guam, the snake forages as easily on the ground as in the trees, and it will consume anything that smells of blood... What scientists thought was one serpent is in effect two: the native and the colonist, the preinvasive and the postlapsarian."

- Alan Burdick, Out of Eden: An Odyssey of Ecological Invasion

By the 1960s, ornithologists studying the Pacific island of Guam knew that something was wrong – they just didn’t know what.  Before the startled eyes of scientists, the unique bird life of Guam – many of them species birds found nowhere else on earth – began to vanish, one species flickering out after another.  No one could agree on what the problem was.  Some said it was remnants of the pesticide DDT, still active in the environment years after its use.  Others suggested habitat loss, or maybe an introduced disease. 

The only known was that the southern third of the island was virtually devoid of bird life, and the line on “birdlessness” was steadily advancing.

In the 1980’s, biologist Julie Savidge was studying the extinction crises when she came across something she didn’t expect to find – a snake.  Guam had snakes, but they were tiny, worm-like things, not the large, active, muscular creature she found.  The species was identified as Boiga irregularis, the brown tree snake, known throughout Indonesia, New Guinea, and the Solomon Islands… but not on Guam.  Intrigued, she shifted her attention to the snakes and noticed a disturbing pattern – where the snakes appeared, the predator-naïve island birds disappeared.  She summed up her findings in a 1987 paper, “Extinction of an island forest avifauna by an introduced snake.”

In its native haunts, the brown tree snake is a modestly-sized, fairly cryptic species, a tree dweller that preys on arboreal birds (birds which have coevolved in the presence of the snake and are equipped to evade it) and is in turned preyed upon by monitor lizards and birds of prey.  On Guam, the snake found paradise – no predators, and prey that had never seen a snake as a threat before.  The damage was devastating – three of Guam’s endemic birds were eaten out of existence, whereas others now only exist in zoos and breeding facilities.  The brown tree snakes of Guam became different from those of Indonesia or New Guinea – bigger, more adaptable, more aggressive.  They eat the birds, yes, but have also learned to scavenge, everything from raw meat to canned dog food to bloody tissues.  Up to 40 snakes may be found on an acre of forest.  Here on Guam, Dr. Jekyll became Mr. Hyde.

How did the brown tree snake get to Guam – and, recently, onto other Pacific islands – in the first place?  It’s suspected that the brindled white-eye, Guam broad-bill, and other island birds are the last casualties of World War II.  During the war, American warplanes island-hopped around the Pacific, and it is likely that when they did, they carried stowaways.  Brown tree snakes can go a long time without food or water, allowing them to comfortably wait out a ride.  A female can store sperm for months, allowing a single gravid female to colonize an island.

Birds, lizards, and bats aren’t the only victims of the brown tree snake’s expansion.  It’s destroyed local poultry industries.  It’s been known to enter houses and bite people as it forages for food – the species is moderately venomous.  Most irritatingly, it has caused thousands of power outages.  Highly arboreal the snakes climb power poles, and their combined weight can bring the wires down.

There has been little success in controlling the brown tree snake.  Introduced diseases, poisoned baits, physical removal, snake-proof barriers – all tried with limited success.  Some scientists have quietly written Guam and other snake-infested islands off, focusing more on what can be done to prevent the expansion of the snake’s growing range.  Of all the islands in the Pacific, the most heavily guarded is Oahu, in Hawaii.  Hawaii is the crossroads of the Pacific in terms of commerce and transportation – from here, the snakes could get anywhere.  So far, a tiny handful of brown tree snakes have been captured on Hawaii (often with the help of trained dogs), but they are not believed to be established… yet. 

The US government uses posters like this to help people identify and report spreading brown tree snakes

The fight to limit its spread continues, but it is still to be seen what island is the next to fall to the brown tree snake.

So, how does this all count as “Zoo History?”  Later this month, we’ll be showing how zoos have been involved in the struggle to save Guam’s remaining bird life from extinction.