Think most venomous snakes don’t climb? Think again

2022-06-05

There are a lot of misconceptions about Australian snakes that can lead to potentially dangerous false identifications.

One myth that still gets brought up from time to time is that most venomous snakes don’t climb — ie. if you see a snake in a tree or on a roof, it’s either a harmless tree snake or a python.

But a study conducted by three snake experts has conclusively dismissed that idea.

Their research, published in Herpetology Notes late last year, found instances of climbing in a huge range of venomous Australian snakes (elapids), including king browns (Pseudechis australis), tiger snakes (Notechis scutatus), and the spectacularly patterned Collett’s snake (Pseudechis colletti).

In total, their research, which was based on literature studies, surveys, and personal observations, found instances of 20 different species of venomous snakes engaging in arboreal behaviour — tree climbing.

When their observations were expanded to include venomous snakes climbing things other than trees, or snakes under extreme stress, such as when fleeing predators or rising floodwaters, the number of climbing species rose to 31.

“Even though we have 31 species recorded, which is just over a quarter of all [Australian] elapids, that number is likely to be higher,” said Matt Sleeth, ecologist and lead author on the paper.

“Due to the difficulty of surveying this behaviour in these animals, it is likely that there are more [venomous elapid] snakes that do climb.”

A sliver of truth?

So, where does the misconception come from?

Like most rumours, it is grounded in some truth.

In Australia, there are five families of land snake — the pythons, file snakes, blind snakes, the rear-fanged snakes or colubrids, and the front-fanged snakes or elapids.

Blind snakes are small, shy and rarely seen, file snakes are aquatic and live in northern swamps and billabongs, and pythons are widespread, non-venomous and are prolific climbers.

Then there are the other two: elapids and colubrids.

Almost all our venomous snakes belong to the elapidae family.

Globally, elapids include the cobras of Asia and Africa, the mambas of Africa, as well as coral snakes, sea snakes, and in Australia, over 130 species of land and sea snake.

While many elapids have evolved venom, Australia’s elapids are singular in their potency. The world’s most venomous land snake, the inland taipan, is capable of killing around 250,000 mice with the venom from a single bite, according to the LD50 parameter.

Elapids are thought to have arrived in Australia many millions of years ago as a sea snake, according to evolutionary ecologist Rick Shine from the University of Sydney.

“In the case of the elapids, the ancestor that has come from Asia to Australia looks to be of a modern-day krait — a sea snake species,” Professor Shine said.

Today, sea kraits are among the most venomous snakes in the world, meaning Australia’s elapids likely had a headstart in their development of potent venom.

It also means that Australia’s elapids started out as ground snakes.

Though some may have moved into the trees, Australia then underwent a process of aridification — much of the vast forest that once covered the continent converted to deserts and grasslands.

If any of those early Australian elapids had moved into the trees, the aridification of Australia would have counted against them, according to Matt Sleeth.

“Compared to places like Asia and Africa where there is plenty of forested habitat, Australia is much drier, so the potential for [tree dwelling] to be beneficial is limited,” he said.

Selecting for non-venomous climbers

Which brings us to the colubrids or rear-fanged snakes like the common tree snake, brown tree-snake, and keelback.

The colubrids are the world’s most successful family of snakes, and Australia is unique in being the only continent where elapids outnumber colubrids — only 10 species of colubrids occur here, according to the Queensland Museum.

The colubrids are thought to have arrived in Australia after the elapids, but there is still some debate as to exactly when.

And the path that they took to get here probably explains why many of Australia’s colubrids are tree specialists.

“The colubrids came a lot more recently down through Asia and New Guinea,” he said.

What that means is that their migration path was a heavily forested one that favoured tree-dwelling species.

“There’s probably a connection there where you’re likely to get more tree-dwelling species,” Professor Shine said.

That migration pathway is the likely explanation for why many of Australia’s non-venomous colubrids are climbers.

Combined with the presence of non-venomous pythons, which also tend to climb, it’s easy to see where the myth that climbing snakes are non-venomous came from.

But while it might be a trend, it shouldn’t be mistaken for a rule, according to Matt Sleeth.

“Myths among the public are pretty rampant, especially among non-charismatic species like snakes,” he said.

“Generally speaking, if you do see a snake in a tree it’s probably going to be a non-venomous python or a tree snake … but it doesn’t have to be that way.”

ABC News, 5 June 2022
; https://abc.net.au