A common belief about crocodilians is that they stop growing only when they die, but new research, the longest known alligator study in the world, has found at least one outlier among the group. By tracking American alligator growth in a South Carolina nature sanctuary, scientists have discovered that the creatures do in fact stop growing after reaching sexual maturity, a finding they claim turns conventional scientific understanding on its head.
With the appropriate licensing, hunting of the estimated 100,000 alligators is legal throughout the waters of South Carolina, but not so at the Tom Yawkey Wildlife Center on the Atlantic coast. The center was handed over to the state in 1976, but has provided a sanctuary for American alligators long before that and now offers researchers a living laboratory to learn more about the long-term behaviors of the animals.
"Yawkey alligators have been unhunted for almost 100 years," says wildlife biologist Thomas Rainwater, from Clemson University. "No other place that I know of has a dataset this robust and this long term. You can't really know how long wild alligators live and grow unless you have an animal that was caught 20 or 30 years ago and you catch it and measure it again."
For the landmark alligator study, Rainwater worked with retired wildlife biologist Phil Wilkinson, who began investigating alligators in Yawkey in 1979 for population and nesting research. As Wilkinson did so, he noticed that a lot of the alligators he saw swanning about had been captured and marked before, which presented a good opportunity to check in with them and see how much they were growing.
Between 1981 and 2015, Wilkinson, Rainwater and their research team captured hundreds of the alligators. Of those, 50 were adults snapped up five to 33 years after their previous captures, 19 male and 31 female. Seven of the 19 males showed no appreciable growth, while 19 of the 31 females had done the same.
"The dogma for years has been that crocodilians and reptiles in general grow in length until they die," Rainwater said. "Our study shows that, at least at Yawkey, adult alligators stop growing before they die. We have animals that Phil first caught and tagged back in 1981 that we recaptured in 2016 and they are still exactly the same length they were 35 years ago."
The researchers estimate that on average, the male alligators stopped growing sometime between 25 and 30 years of age.
Another useful finding from the study relates to the reproductive behavior of the alligators, with the researchers discovering that females can continue producing fertile eggs for much longer that previously thought. Some of the females are still nesting at around 70 years of age, and one is still reproducing more than 46 years after reaching sexual maturity.
The results of the study may indeed go against the grain of conventional scientific theory, but it is not the first time so-called determinant growth has been documented in alligators, or indeed the American variant. A 2011 study uncovered evidence of determinant growth in captive American alligators, represented by bone microstructures that develop as the creatures reach skeletal maturity.
The researchers did note the need for further study to establish how widespread this phenomenon really is, but their work can provide researchers managing alligator populations in the region with useful data.
"The Yawkey study provides a unique opportunity to estimate baseline survival and growth rates in a protected population," says Abby Lawson, a doctoral candidate at Clemson University who studies alligator numbers in the area. "We can compare our findings to hunted areas to better evaluate how harvest may impact alligator populations through changes in survival, reproduction and length differences."
The team published their research in the journal Copeia, and you can hear from the scientists in video below.
Source: Clemson University
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