Anna Salleh
ABC Science Online
Tuesday, 30 November 2004
The red-bellied black snake has adapted the size of its head and body since exposure to cane toads
The toxic cane toad has placed evolutionary pressure on snakes to adapt their body shape, Australian researchers say.
Evolutionary biologist Ben Phillips and PhD supervisor Professor Richard Shine of the University of Sydney say some snakes have grown longer, an adaptation that makes them better able to survive their toxic meal.
The changes in body length, which have arisen in the past 70 or so years since the cane toad was introduced to Australia, were reported today online ahead of print publication in the journal Proceedings of the National Academy of Sciences.
"I'm particularly interested in evolutionary biology and from that perspective, cane toads represent a really strong selective force," Phillips told ABC Science Online.
Cane toads (Bufo marinus) were first introduced to Australia in 1935 to eat sugar cane beetles.
But Phillips said little was known about how the toads affected native animals that tried to eat them.
A large gland on the toad's shoulder is packed with potent cardiac toxins and most species of Australian snake die after trying to eat the toad, he said.
In previous research, Phillips sampled snakes from around Australia and found the cane toad toxin did not affect all snakes the same way.
He found that larger snakes were less affected by toad toxin. In particular, the snakes with the larger ratio of body-length to head-size were less affected.
The bigger a snake, the smaller its head is compared to its overall body length. And since the size of a snake's head limits the size of its prey, this means a larger snake is less likely to eat a large enough toad to poison it, said Phillips.
The cane toad is not so deadly for some clever snakes (Image: Ben Phillips)
Phillips thought snakes might actually have evolved their body shape in response to exposure to cane toads.
To test his theory he looked at how four species of snake in Queensland changed over time, before and after cane toads were introduced into their area.
He studied 600 preserved specimens dating back to the beginning of the last century from the Queensland Museum.
He predicted that two of the four species, Pseudechis porphyriacus (red-bellied black snake) and Dendrelaphis punctulatus (green tree snake), were highly sensitive to toad toxin.
He thought the other two, Hemiaspis signata (swamp snake) and Tropidonophis mairii (keelback snake), would not be.
Over the 70 odd years since cane toads invaded, Phillips found the sensitive snakes got longer by around 3-5%, while the other two snakes didn't change length.
Phillips said the fact that morphology, such as arm, leg and toe length, is generally inherited, and the finding that only the sensitive snakes got longer, supported his theory.
"It probably means the impact of toads has been slightly reduced through time as a consequence of snakes evolving in response to them."
He said researchers did not often see such rapid evolution.
"I think part of that reason is that there's an idea amongst biologists that evolution takes a long time and so people tend not to look for it in short time-scales," he said.
But he suspected rapid evolutionary changes were more common than scientists thought.
"It may be a force that's ameliorating conservation concern impacts like invasive species."
Phillips said cane toads were still marching across the north of Australia and will have reached the Western Australia border by the end of 2005.
http://abc.net.au/science/news/stories/s1250708.htm
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