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Has bubble burst over exploding toad tale?

April 29, 2005 By Michael Hopkin This article courtesy of Nature News.

Evidence points to bloated toads and hungry birds, but not explosions.

A mystery of exploding toads has turned many people into armchair zoologists this week. Amphibians in a previously obscure German pond have reportedly been blowing up in their thousands, leaving a grisly trail of innards stretching several feet in their wake - and observers desperately trying to work out why.

The bizarre phenomenon has prompted a full-scale environmental investigation of the pond by researchers at the nearby Institute for Hygiene and Environment in Hamburg. The list of suspected culprits has grown to include bacteria, fungi, ozone and vehicle exhausts; or simply the pecking of hungry birds.

In the wake of the confusion, reports have also emerged of toads meeting a similarly gruesome fate in Denmark.

Despite much puzzling, experts have yet to find any reason for the amphibians to balloon to three times their size before literally exploding, as eyewitnesses to the unfortunate incidents have claimed.

Various theories independently explaining observations of bloated toads and messy remains lead many biologists to think that observers have been leaping to conclusions, and that the toads are not really exploding.

Body count

I really think someone needs to go back and check the primary source.
Barry Clarke
Natural History Museum, London
The toads first came to prominence when walkers noticed an unusually large number of corpses splattered on the ground near what has now been dubbed the 'Pond of Death'. The current body count in Hamburg has reached some 1,300.

Experts, having sampled the pond water, say they have ruled out the possibility of a killer pathogen. "We don't know what happened but we know there's no infective problem," says veterinarian Anke Himmelreich of the Institute for Hygiene and Environment.

The water from the pond and from the nearby River Elbe contains no pathogen or pesticide that is known to be lethal to wildlife. And when Himmelreich and her colleagues carried out a biotest on the water (they put fish and shrimps in to see if they could survive) it came out clean.

Frank Mutschmann, a Berlin-based veterinarian, has examined some of the corpses and says that they bear the scars of a predator's attack. He thinks birds may simply have made a very messy job of eating their favourite parts of the toads, such as the liver.

April and May are the months when toads migrate to ponds to spawn, Himmelreich points out, which means that this season could represent easy pickings for birds. Perhaps the walkers let their imaginations run wild when they chanced upon the victims, she proposes. Himmelreich says she has never seen a toad explode.

There are some symptoms that might lead an observer to think that a toad was on the verge of blowing up, Himmelreich adds, particularly if a wounded toad wandered into a pond. "Maybe they were full of water, and in their agony they were also trying to suck in air," Himmelreich says. People watching bloated, rasping toads might well think an explosion was imminent, she says.

Some toads are also known to puff themselves up as a defence reaction, perhaps as a means of warding off attack by snakes aiming to swallow them whole. But zoologists doubt that a toad could swell to three times its usual size.

"I really think someone needs to go back and check the primary source," comments Barry Clarke, a herpetologist at the Natural History Museum in London. "I've learnt never to say with animals that anything is impossible. But the idea of exploding toads - well let's face it, it's pythonesque."


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