NOVA: Why did crocodilians survive when the dinosaurs went extinct?
James Perran Ross: The short answer is we don't know. But we can look at what crocodiles do now and how they work and speculate on some things that may be involved. Crocodile design has lasted an awfully long time. A great many of the fossils of crocodiles are virtually identical to the crocodiles we see today. They seem to have successfully adapted to their environment and have undergone few changes. That's not universally true, because crocodiles have occasionally veered off into some other quite interesting evolutionary lines. There is, for instance, a secondary return to terrestrial life among crocodiles. There's even a crocodile that had hooves, and one speculates it must have been a crocodile behaving like a small deer or something.
I've heard of a fossil croc in Australia that was a tree-climber.
Conceivable. So they've periodically left the mainstream, but all those little branches didn't go anywhere. I think we have to look at their basic design and concede that it's a good way of being an amphibious predator. It really works. I think one of the aspects that may play to their survival is that they are extremely tough and robust. We're learning now that the immune systems of crocodiles, for instance, are just incredible. They can sustain the most frightful injuries.
In territorial fights they commonly tear each other's legs off. They go away and sulk for awhile and seem to heal up. You often find animals in the wild with missing limbs, missing tails—what must have been very serious injuries. I found one in the wild with the whole of its lower jaw torn off, all healed up and swimming around. It was a bit skinny but had obviously survived that very traumatic event. So I think their inherent toughness is one aspect. They are also long-lived. They routinely live for decades.
"This crocodile liked to come into the house and lie in front of the fire on cool winter evenings."
The adaptability of their behavior is also something that may play into their survival. It certainly has in modern times. We haven't lost a species of crocodile to extinction since humans have been dominant on the planet, even in the last few hundred years when our impact has been appalling. The reason appears to be in large part because crocodiles learn quickly and adapt to changes in their situation. They particularly learn to avoid dangerous situations very quickly. For research purposes, we find that we often have to change capture techniques, because it's very hard to catch them with the same trick twice.
Would you call that intelligence?
Well, it's certainly rapid learning. Whether they sit and ponder or whether they just have neural synapses that fire quickly is a moot point. There are some people who keep crocodiles who claim that they are truly intelligent. I know some people whose opinions I respect who very sincerely believe that crocodiles know individual people and that they learn simple routines very readily, such as when the man with the bucket has food and when he doesn't.
They also become tame quite readily. There are alligator shows all over the country in which people routinely handle quite large alligators, which have become used to being handled and take it in their stride. There are a couple of well documented stories of truly tame crocodiles. The famous crocodile biologist Frederico Medem described a doctor who lived in Villavicencio, Colombia who had a large Crocodylus intermedius, the Orinoco crocodile. He had raised it from a hatchling. It was a female about 10 feet long, and it lived in his house. It played with his children, it played with the family dog. Villavicencio is up in the hills, and this crocodile liked to come into the house and lie in front of the fire on cool winter evenings. And it was housetrained.
Can you speculate on what else might have helped them outlast the dinosaurs?
Well, there is their association with water. Whatever devastated the terrestrial environments 65 million years ago—and the strong suspicion now is that it was an asteroid impact causing a long-term climatic change through dust in the atmosphere—appears to have had a less intense impact on freshwater environments.
You can speculate on why crocodiles may have been able to survive that. For one thing, you can look at how crocodiles are kept in commercial captivity. In the less favorable situations, they're kept in complete darkness, under the most appalling conditions of bad hygiene. People fail to clean the water and toss them stinky dead stuff for food. Yet they go on thriving in these conditions.
A nuclear-winter, meteor-impact scenario would be similar. It would be dark all the time—that apparently doesn't bother alligators. Dead stuff would be falling in the water as the rest of the fauna succumbed to the collapse of the food chain. The crocodiles conceivably were big enough to survive through that and under conditions that most other organisms couldn't tolerate.
And crocs can go for long periods without eating, right?
Yes, they have an awesome capacity to deal with starvation. There are numerous examples of animals not feeding for an entire year. They become desperately thin, but they're still active and are perfectly capable of feeding when food appears. So again, if the demise of the dinosaurs was caused by an asteroid-winter type of scenario, then crocodiles may well have been able to survive that.
Well, there is another element. Crocodiles, for all their ability to get their body temperature up when they have sunlight, do very well at low body temperature in the darkness. All their systems continue to work adequately well. There is some speculation that at least all the big dinosaurs were probably homeothermic, or warm-blooded. That is, they had developed the circulatory and metabolic changes that enable one to maintain a body temperature independently of the surrounding temperature.
"When food is available they can eat a lot of it, turn it into new crocodile, and grow quickly."
If this was so, they constantly had to pay the great cost of becoming warm-blooded, which as we know ourselves is you have to eat all the time. The energetic cost of maintaining your body temperature burns up more than 80 percent of what you eat. So dinosaurs that had made the commitment to becoming warm-blooded had given up their capacity to not eat for long periods. At the time that was a very effective trade-off, but when crunch time came with the asteroid or whatever it was, it was the crocodiles that still retained the primitive ability to continue to function at low body temperature and therefore not require as much to eat.
Just how cold can they take it?
It depends a great deal on the species, but the three species that occur in temperate zones—the American alligator, the Chinese alligator, and the broad-snouted caiman—can tolerate occasional frost or freezing. There are records of alligators surviving beneath ice as long as they can continue to breathe. Of course, alligators normally become quiescent in cold weather anyway. They dig deep burrows into the banks of rivers and lakes and retire to them. They don't hibernate in the strict sense, because they don't reduce their metabolism down close to zero. But they're able to tolerate low temperature quite well.
How long can crocs remain submerged, and how do they manage it?
It's a trade-off between what they're doing and how cold or warm it is. Maximum time seems to be on the order of an hour or two. When the temperature is fairly low and they're relatively inactive, they can remain beneath the surface for a prolonged period. I don't know of anybody who's really looked in detail at their diving physiology.
But the general pattern of all organisms that have this capacity to hold their breath a long time is two-fold. One is they shunt blood away from non-vital tissue, and the anatomy of crocodiles suggests that they can do this. They have a very sophisticated circulatory system; it's one that doesn't really belong in a reptile. If you took the heart, lungs, and major veins and arteries of a crocodile, hung them up and asked 10 physiologists what it was, most of them would say it was a mammal. Crocodilians have a well-developed four-chambered heart and the capacity to separate oxygenated blood and unoxygenated blood, which is quite unique among reptiles.
Crocs are said to use energy in food more efficiently than almost any other animal.
Partially because they're cold blooded, they convert their food to crocodile tissue as well as or better than fish, which also are very efficient, and much, much better than chickens or cows or pigs or people. When food is available they can eat a lot of it, turn it into new crocodile, and grow quickly. But then when food is not available, they appear to be able to shut down and live off their own tissue for a long period of time.
I've heard that crocodilians survive better from birth by orders of magnitude over other reptiles.
They do remarkably well. The hatch rates of their eggs, when you compare them to other reptiles such as freshwater turtles and monitor lizards, are uniformly very high. Eighty or 90 percent of eggs laid hatch. Rates of predation on their nests, though they can be high, are lower than for other reptiles as well and generally are not catastrophic.
Do crocodilians feed cooperatively?
I have a photograph of mass feeding caiman, in which there are animals of quite different sizes all mixed in together at the mouth of a stream. The stream was flowing out into a larger river, and there are 40 or 50 of them all in a crescent array, apparently snapping up something that is washing out of the stream.
"There are no strict rules with crocodiles. Perhaps their flexibility is part of their success."
This kind of behavior was first described by William Bartram back in the 1770s, during his trip through Florida, which he wrote up in his marvelous book The Travels of William Bartram. He describes alligators at a narrow point in the St. John's River being thick enough that he could have walked from one shore to the other on their backs. This was once considered a truly fanciful account. People nodded and winked and wondered what young Bill was smoking out there in the Florida woods. Subsequently, we come to find that, particularly where there are schools of fish running through narrow channels, this sort of phenomenon has been described and even photographed in several species, including alligators. It looks like Bartram just happened upon one of these mass feeding events, during which there is a suspension of what might otherwise be aggressive behavior, a kind of mutual truce.
You also see the opposite, of course. I have observed in the wild very clear dominance relationships related to food sources. A dominant animal will posture and display next to a source of food—a dead animal, say—and feed first on it. The others sit around waiting. Only when the dominant one has had his fill and retired will the others get what's left. There are no strict rules with crocodiles. Perhaps their flexibility is part of their success.
Never knew crocs could be so sociable.
They look a bit like logs, and everybody assumes they behave like logs. But studies have shown crocodilians to have quite complex social behavior. Individuals know other individuals and have long-term relationships with one another in terms of dominance and so forth. These relationships structure crocodile groups, certainly in captivity and probably also in the wild, in order to distribute access to food and even successful reproduction.
Of course, there's been some speculation that the dinosaurs also were more complex than we originally thought, in terms of things like maternal care. So again, I can't find any clear difference between what we know crocodiles do and what we suspect dinosaurs might have done, which is why it's hard to answer the question, Why did crocs survive?