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First published online April 17, 2009
Journal of Experimental Biology 212, i (2009)
Copyright © 2009 The Company of Biologists Limited
doi: 10.1242/jeb.032078
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ALLIGATORS HINT AT WHAT LIFE MAY HAVE BEEN LIKE FOR DINOSAURS
kathryn{at}biologists.com
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During the last 540 million years, the earth's oxygen levels have fluctuated wildly, probably peaking at 30% about 300 million years ago and bottoming out at 12% 100 million years later. Knowing that the dinosaurs appeared around the time when oxygen levels were at their lowest, Tomasz Owerkowicz, Ruth Elsey and James Hicks wondered how these monsters coped at such low oxygen levels. But without a ready supply of dinosaurs to test their ideas on, Owerkowicz and Hicks turned to a modern relative: the alligator. `We knew testing the effects of different oxygen levels would work with alligators,' Owerkowicz explains, `because crocodilians have survived in their basic shape and form for 220 million years. They must be doing something right to have survived the oxygen fluctuations.' Choosing to start at the beginning of alligator development, the trio decided to try incubating alligator eggs at different oxygen levels, to find out how the youngsters grew and developed (p. 1237).
Receiving newly laid alligator eggs from Elsey at the Rockefeller Wildlife Refuge, Owerkowicz divided the eggs into groups incubated at 12% oxygen (hypoxia), 21% oxygen (normoxia) and 30% oxygen (hyperoxia), and waited to see what would happen. After almost 10 weeks of waiting, the eggs began hatching and Owerkowicz could see that there were no obvious differences between the normoxic and hyperoxic alligators.
But he was in for a shock when the hypoxic hatchlings began to emerge. The tiny alligators' bellies were enormously swollen. They had failed to absorb all of the egg yolk food supply, leaving them with huge yolk-distended bellies. In some cases the bellies were so big that the animals' legs could not reach the ground, and the alligators had to sit around until they had burned off the yolk and could begin moving. Owerkowicz suspects that there was not enough oxygen for the developing embryos to consume the yolk. The hypoxic youngsters' organs were much smaller too, all except the heart, which was relatively large; presumably to maximise use of the youngsters' limited oxygen supplies. Owerkowicz admits that he had thought that the hypoxic newborns' lungs would also be enlarged, to compensate for the low oxygen levels, but they were not, probably because the incubating youngsters do not use their lungs and instead obtain their oxygen supply from blood vessels in the egg's membrane.
Next Owerkowicz was curious to see how the alligators performed after 3 months in their respective atmospheres. Checking the reptiles' breathing and metabolic rates, it was clear that the animals in the hyperoxic atmosphere were breathing much less than the normoxic and hypoxic animals, probably because they breathe in more oxygen per lungful, translating into a significant energy saving, which the reptiles could invest in growth. And when Owerkowicz checked the size of the 3 month old hypoxic youngsters' lungs, he could see that they had caught up with his expectations and were larger than those of the normoxic alligators. The alligators' lungs were enlarged to compensate for the low oxygen supply, allowing the alligators to increase their metabolic rates, but not as much as the normoxic or hyperoxic alligators.
Owerkowicz admits that although his results can't tell us what life was like for his alligators' prehistoric predecessors, it is clear that `their growth and metabolic patterns would have been significantly different,' he says.
References
Owerkowicz, T., Elsey, R. M. and Hicks, J. W.
(2009). Atmospheric oxygen level affects growth trajectory,
cardiopulmonary allometry and metabolic rate in the American alligator
(Alligator mississippiensis). J. Exp. Biol.
212,1237
-1247.
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