The Dodo . . . and Moa

The Dodo of Mauritius (Raphus cucullatus), believed extinct since 1681 (Vanished Species)

The Dodo of Mauritius (Raphus cucullatus), extinct since c. 1681 (Vanished Species)

A new discovery reported in today’s Science Daily suggests that the Araliaceae tree of New Zealand has developed an elaborate camouflage defense that varies depending upon the age of the tree. Most intriguing is the credible hypothesis that this camouflage protection evolved specifically to discourage predation by the Moa, the large flightless bird driven to extinction by Maori hunting 750 years ago.

The discovery brings to mind the marvelous tale of the dodo and the tambalacoque tree related by Stephen Jay Gould in his exquisite book of natural history essays, The Panda’s Thumb, nearly thirty years ago:

“. . . the dodo stands alone, the first recorded extinction of our era. The dodo, a giant flightless pigeon (twenty-five pounds or more in weight), lived in fair abundance on the island of Mauritius. Within 200 years of its discovery in the fifteenth century, it had been wiped out – by men who prized its tasty eggs and by the hogs that early sailors had transported to Mauritius. No living dodos have been seen since 1681.

“In August, 1977, Stanley A. Temple, a wildlife ecologist at the University of Wisconsin, reported the following remarkable story (but see postscript for a subsequent challenge). He, and others before him, had noted that a large tree, Calvaria major, seemed to be near the verge of extinction on Mauritius. In 1973, he could find only thirteen ‘old, overmature, and dying trees’ in the remnant native forests. Experienced Mauritian foresters estimated the trees’ ages at more than 300 years. These trees produce well-formed, apparently fertile seeds each year, but none germinate and no young plants are known. Attempts to induce germination in the controlled and favorable climate of a nursery have failed. Yet Calvaria was once common on Mauritius: old forestry records indicate that it had been lumbered extensively.

Calvaria’s large fruits, about two inches in diameter, consist of a seed enclosed in a hard pit nearly half an inch thick. This pit is surrounded by a layer of pulpy, succulent material covered by a thin outer skin. Temple concluded that Calvaria seeds fail to germinate because the thick pit ‘mechanically resists the expansion of the embryo within.’ How, then, did it germinate in previous centuries?

“Temple put two facts together. Early explorers reported that the dodo fed on fruits and seeds of large forest trees; in fact, fossil Calvaria pits have been found among skeletal remains of the dodo. The dodo had a strong gizzard filled with large stones that could crush tough bits of food. Secondly, the age of surviving Calvaria trees matches the demise of the dodo. None has sprouted since the dodo disappeared almost 300 years ago.

“Temple therefore argues that Calvaria evolved its unusually thick pit as an adaptation to resist destruction by crushing in a dodo’s gizzard. But, in doing so, they became dependent upon dodos for their own reproduction. Tit for tat. A pit thick enough to survive in a dodo’s gizzard is a pit too thick for an embryo to burst by its own resources. Thus, the gizzard that once threatened the seed had become its necessary accomplice. The thick pit must be abraded and scratched before it can germinate.

“Several small animals eat the fruit of Calvaria today, but they merely nibble away the succulent middle and leave the internal pit untouched. The dodo was big enough to swallow the fruit whole. After consuming the middle, dodos would have abraded the pit in their gizzards before regurgitating it or passing it in their feces. Temple cites many analogous cases of greatly increased germination rates for seeds after passage through the digestive tracts of various animals.

“Temple then tried to estimate the crushing force of a dodo’s gizzard by making a plot of body weight versus force generated by the gizzard in several modern birds. Extrapolating the curve up to a dodo’s size, he estimates that Calvaria pits were thick enough to resist crushing; in fact, the thickest pits could not be crushed until they had been reduced nearly 30 percent by abrasion. Dodos might well have regurgitated the pits or passed them along before subjecting them to such an extended treatment. Temple took turkeys – the closest modern analogue to dodos – and force-fed them Calvaria pits, one at a time. Seven of the seventeen pits were crushed by the turkey’s gizzard, but the other ten were regurgitated or passed in feces after considerable abrasion. Temple planted these seeds and three of them germinated. He writes: ‘These may well have been the first Calvaria seeds to germinate in more than 300 years.’ Calvaria can probably be saved from the brink of extinction by the propagation of artificially abraded seeds. For once, an astute observation, combined with imaginative thought and experiment, may lead to preservation rather than destruction.”

It’s a truly wonderful story. But note that Gould adds a postscript which elaborates upon the ensuing controversy – see The Panda’s Thumb for more details – and then concludes: “The debate between Owadally and Temple is too close to call at the moment. I’m rooting for Temple, but if Owadally’s fourth point is correct, then the dodo hypothesis will become, in Thomas Henry huxley’s inimitable words, ‘a beautiful theory, killed by a nasty, ugly little fact.’”

Published in: on July 23, 2009 at 2:34 pm  Leave a Comment  

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