A salt water crocodile jumps for food near Darwin, Australia.
Credit: J. Patrick Fischer
Most of the world's crocodilians, an order of reptiles that includes crocodiles and alligators, are threatened or endangered. Australia's two species of crocodiles, however, have enjoyed consistent population growth since a ban was placed on hunting in the 1970s.
But now these poorly understood animals face new challenges to their survival, including the effects of climate change and invasive species such as cane toads. The most effective way to help their survival may be to understand them – and, in particular, how they eat.
That's the aim of four scientists, from Charles Darwin University, conservation group Wildlife Management International and Crocodylus Park, all based in Darwin in Australia's Northern Territory. The scientists are investigating the digestive physiology of crocodiles with a long-term goal of improving growth rates of captive animals in the crocodile farming industry.
"With Crocodiles you only have something like 50 years of experience of taking these animals from the wild to domestication," says Grahame Webb, from Wildlife Management International. "But with other animals, such as chickens, there are thousands of years of history.
"When it comes to digestive systems, there is something odd going on. In the wild, crocodiles have a high conversion rate," explains Webb, referring to the amount of food that is absorbed and converted into energy. "In captivity the food conversion rate seems to go way down."
This is a problem for the crocodile farming industry, which exports crocodile skin to be made into leather goods, usually in Europe, and also supplies a growing domestic market for meat.
"Quite probably, crocodiles are not designed to have a full stomach everyday like a cow or a pig," says Webb. "Maybe that is what is causing fundamental differences in how they digest food."
Using recently developed techniques that measure the way nutrients pass through the intestines, the researchers have discovered that crocodiles have a surprising ability to absorb nutrients passively across the intestines. Passive absorption (as opposed to active transport) requires no energy to occur.
The extent of passive transport in crocodiles is comparable to that found in birds and bats – animals that have very high energy requirements. This unexpected result may be due to the fact that crocodiles and birds share a common ancestry – but there's a chance the results were skewed, as the measurement were all taken fast-growing juvenile animals.
Next, the researcher will measure larger and therefore slower-growing crocodiles in an attempt to further understand the pattern revealed by the initial experiments. Either way, the results will provide new insight into the inner workings of crocodiles, and ultimately these insights may be able to be exploited to produce an improved diet for captive animals.
A second project, which will begin mid-year will examine the effects of cane toads on populations of freshwater crocodiles. Freshwater crocodiles are smaller and less deadly than their infamous saltwater counterparts.
It is well established that crocodiles can die after eating a cane toad, and since both crocs and toads inhabit the edges of waterways, it seems a forgone conclusion that toads will decimate freshwater crocodile populations.
There are, however, several complicating factors. One factor relates to the main predators of freshwater crocodile eggs, goannas, being poisoned by toads. With fewer goannas around, scientists expect many more hatchlings to be produced. Determining the balanced population – between more hatchlings but fewer larger crocodiles – is one of the scientists' main aims.
Another complicating factor is related to apparent differences in the susceptibility of freshwater crocodile populations to cane toads – at least based on preliminary observations.
Jonathan Webb and Mike Letnic, of the University of Sydney, recently documented high mortality among freshwater crocodiles soon after the invasion of toads in the NT's Victoria River. Similar observations have been made in other rivers, including Katherine River. But preliminary observations in McKinlay River suggest much less mortality.
Although a thorough survey of McKinlay River has not been carried out since the arrival of toads, a survey of nests found no reduction after the arrival of the toads, suggesting that female numbers have not declined.
Variability among populations in various river systems may be caused by any number of factors, such as the ability of individual crocs to learn not to eat toads; the speed at which populations may evolve to learn to not eat toads; or pre-existing differences among populations in their propensity to eat frogs and toads.
In some river systems there are native frogs that are also toxic, raising the possibility that crocodiles in those areas may have a long-established aversion to eating frogs. These are among the various possibilities that researchers hope to investigate over the next few years.
The freshwater crocodiles in McKinlay River will be an important part of the study because of the long-term population data that have been collected since the 1970s. Analyses of the population structure, size-dependant survivorship, and reproduction before and after the cane toad invasion will result in one of the most comprehensive population studies of any long-lived reptile.
In a sense, the invasion of cane toads and their effect on crocodile populations is an evolutionary experiment on a grand scale – the sort that crocodiles have, so far, endured over the millennia.
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Richie Hodgson is a writer for the magazine Origins.
This feature is adapted from a story in Origins magazine, published by Charles Darwin University.
