Thursday, 1 April 2010

Blown away: some weird and wonderful animal defence mechanisms

At a time when environmentalists are calling for farmers to swap cattle for non-ruminant species such as kangaroos in an effort to stem bovine methane emission, a recent report by a leading Argentinean palaeontologist reminds me of Karl Marx's popular axiom "History repeats itself first as tragedy, second as farce".

The report's theme concerns animal defensive mechanisms, a classic example of truth being infinitely stranger than fiction. Consider for instance the bombardier beetle, an innocuous enough looking insect that when endangered can squirt a boiling liquid from its rear abdomen. Okay, that's only mildly weird. Well what about the several species of frogs and newts that when threatened extrude internal claws or spines by puncturing their own skin? Or the Asian carpenter ants whose soldiers literally self-destruct in the defence of their colony, in the process spraying a sticky poison over their attackers? Surely if anyone needed a good argument against Creationism then this panoply of the bizarre would suit admirably, since it postulates an equally bizarre, not to say warped, sense of humour on behalf of a Creator.

But the news from Argentina may well outshine (if that is the right word) all of the above, not least from the sheer scale of the animals involved. The main players are those undisputed giants of the dinosaur world, the South American titanosauria sauropods of the mid- to late-Cretaceous. Partial remains found over the past twenty years imply species such as Argentinosaurus may have reached lengths of 40 metres, thereby exceeding their better-known Jurassic relatives such as Diplodocus by around 20 per cent.

In 2002 Fernando Calvo, Professor of Natural Sciences at La Salta University in Argentina, became intrigued by sauropod growth patterns and nutrition. Although coprolites (fossilised poo) have not been found for any species of Argentinean titanosaur, the study of microscopic phytoliths, silicified plant fragments, suggest these animals enjoyed a broad plant diet. The notion that Mesozoic vegetation consisted primarily of conifers, cycads, horsetails and ferns has been overturned by recent discoveries of palms and even tall, primitive grasses. Since modern grazers such as cattle can survive solely on such unpromising material, how about titanosaurs?

Calvo and his team began a study to go where no scientists had gone before and assess the potential digestive systems of Argentinosaurus and its relatives. One of the luxuries of an enormous bulk is being able to subsist on nutritionally-poor foodstuffs, a case of sheer quantity over quality. The La Salta group hypothesised that their native sauropods were amongst the most efficient of digesters just because of their size: by the time plant material had worked its way through such a large digestive tract most of the nutrients would be absorbed, no doubt aided by gastroliths, literally stomach stones deliberately swallowed to help churn the material.

The preliminary report was published in March last year and quickly became notorious in palaeontological circles. For there was no delicate way of describing the findings: the titanosaurs would easily top the Guinness Book of Records' list of “World's Greatest Farters”. Whilst sauropods did not have the multiple stomach arrangements of modern ruminants the hypothesis was clear: titanosaur herds would have been surrounded by an omnipresent cloud of methane.

For Calvo, the next step came several months later when a tip-off from a farmer in Chubut led to an astonishing series of finds. The site, whose exact location remains secret, revealed the semi-articulated fragments from a tight-knit group of three predatory Giganotosaurus and approximately 15 per cent of the skeleton of a single, adult Argentinosaurus. Team member Jose Chiappe led the extraction work on the latter colossus and postulated that it had died slowly, perhaps due to blood loss following an attack.

What were far more intriguing were the positions of the attackers: all three had a slumped, head-down attitude, implying sudden collapse and virtually instantaneous death. Calvo found himself asking the obvious: how could they have died? Whereas a Diplodocus tail was well-formed for use as a whip, it was a much more gracile animal than its Cretaceous counterparts. The larger bulk of Argentinosaurus didn't bode well for a fast reaction: by the time a titanosaur had noticed the approach of a Giganotosaurus it would have had precious few seconds to position its tail for a whiplash response. Then Chiappe remembered an Early Cretaceous site in Liaoning Province, China, where animals had died of suffocation due to volcanic gases.

The resemblance in the post-mortem postures of the Giganotosaurus led to an incredible but as yet unpublished hypothesis: if correctly positioned, a frightened titanosaur could have defended itself by the simple expedient of raising its tail and expelling gaseous waste directly into the conveniently-placed head of an oncoming predator. An initial calculation based on scaling up from modern animals suggested an adult titanosaur could have produced about one tonne of methane per week. Computer simulations suggest a sustained five-second burst at close range would have K-O'd an eight-ton Giganotosaurus, and with a brain barely half that of Tyrannosaurus, it's unlikely the predators had the wherewithal to avoid their fate. If only the late Michael Crichton had known this, perhaps he would have written a scene involving an ignominious demise at the rear end of a sauropod for some of the characters in Jurassic Park (Jurassic Fart, anyone?) Or since this occurred in the Cretaceous, in the name of scientific accuracy perhaps that should that be Gone with the Wind?

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