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Stench Of Death Alerts Animals To Danger

September 10, 2009

Every dead animal gives off a distinct odor, referred to as the “stench of death”, according to new research by Canadian scientists.

Animal corpses ranging from insects to crustaceans all exude the same scent, which is produced by a special blend of fatty acids.

The scientists explained that the smell alerts other animals to avoid those that have been overcome by disease or predators, a “Ëœdeath recognition system’ that probably evolved over 400 million years ago.

The team of researchers who made this discovery were from McMaster University, near Hamilton, Ontario, Canada, and is published in the journal Evolutionary Biology.

Headed by Professor David Rollo, the team incidentally found the phenomenon during a study of live cockroaches.

While examining the aggregation behavior of cockroaches, they found that cockroaches emit pheromones when they find a safe place to shelter that draws more of its kind.

In order to find the exact chemicals involved, the team took fluids from dead cockroaches to see what effect they would have.

“We were astonished to find that nearly 100% of cockroaches avoid shelters treated with whole body extracts. Something in the extract was overriding any attractive chemicals,” Prof Rollo told BBC News.

“We initiated extensive work to figure out what could be so important to make all these insects go away.”

They were able to throw out other theories, such as cockroaches producing alarm signals and they considered the idea that a specific chemical is released by the insects upon death.

“A search of the literature turned up a very old article by famous sociologist and ecologist E. O. Wilson,” says Prof Rollo.

“Wilson found that ants removed the dead from their nest and dumped them in a cemetery. Moreover, he identified the active signal as oleic acid.”

“The famous story goes that Wilson found that a drop of oleic acid on a perfectly healthy ant resulted in her being carried kicking and screaming to the cemetery. Ants can’t scream, but you get the picture.”

With this being the only information on the subject, Rollo’s team made an educated guess that cockroaches possibly use a similar chemical to signal death.

Further analyses of the cockroach extract confirmed their conjecture. The repelling scent contained only simply fatty acids, with oleic and linoleic acids the two main components.

This left a huge question unanswered.

Rollo brought attention to the fact that ants and cockroaches diverged millions of years ago, and are as unrelated as aardvarks are to beavers.

Then, the question is whether they both produce the same chemicals when they die by chance, or does a wide group of animals give off the same “stench of death”.

Another research team carried the work further to show that a very primitive type of insect called a collembola also uses these same fatty acids to recognize dead kin.

Now, Rollo’s team has also confirmed that the phenomenon is indeed used more widely.

They have found the “stench of death” to be present with terrestrial woodlice as well, as they use the ability to recognize their dead to avoid both crushed woodlice and intact corpses.

Also, when they tested both tent moth caterpillars (Malacosoma americanum) and fall webworms (Hyphantria cunea), the team found that they strongly avoided extracts taken from the bodies of other dead caterpillars, along with pure oleic and linoleic acids.

This signifies that various types of distantly-related insects, as well as woodlice, which are a type of crustacean, share a common system for recognizing death.

Since insects and crustaceans diverged more than 400 million years ago, it is likely that most species that follow can recognize their dead in a similar way.

Rollo said that such a skill is incredibly useful, and that “it is one that does not require any posthumous best wishes from the dead.”

“Consider that you enter a shelter but there are members of your species present that have recently died,” says Prof Rollo. “Moreover, perhaps they died of the equivalent of bug bubonic plague.”

“Recognizing and avoiding the dead could reduce the chances of catching the disease, or allow you to get away with just enough exposure to activate your immunity.”
 
“Alternatively, imagine that this particular neighborhood has a local voracious predator that has dismembered several of your relatives,” he continues.

“Avoiding the area where others have been taken by predators could also be very useful”¦in fact, as we describe, fatty acids are reliably and relatively rapidly released or exposed from cells following death,” he added.

“Evolution may have favored recognition of such cues because they are so reliable and exposure to risks of contagion or predation are so important.”

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