Crocodilians, Which Have Walked Earth for Nearly 100 Million Years, Are Survivors of Mass Extinctions and May Be Able to Adapt to Climate Change

Crocodilians have walked the Earth for nearly 100 million years, and among the main reasons for their extraordinary longevity could be their unique reproductive biology and hands-on parenting skills—traits that may also help them adapt to climate change. That's according to a new study.

Since their emergence, crocodilians have survived two mass extinction events: one that took place 66 million years ago following a massive asteroid strike—during which the dinosaurs were wiped out—and another that occurred around 33 million years ago, decimating life in the oceans.

Furthermore, crocodilians are the last surviving members of a group known as archosaurs that were once found on every continent and have persisted for more than 200 million years.

Previous research has indicated that several factors —including their ability to go without food for months or enter states of "suspended animation" during harsh winters—have contributed to their evolutionary success. However, there is still much we don't know about the crocodilians' knack for survival.

For a study published in the Biological Journal of the Linnean Society, a team of researchers, led by Rebecca Lakin from the University of Bath in the U.K., investigated several areas of crocodilian biology by analyzing 20 different species from around the world, casting new light on their evolution and adaptive abilities.

"We wanted to look at the relationship between latitude and size in crocodilians," Lakin told Newsweek. "This is a relationship that has been well-established in warm-blooded animals since the 1820s, but within cold-blooded animals, it is less clear. In mammals and birds, animals tend to get bigger the further they live from the equator, but this relationship has both been proven and disproven in amphibians, lizards and turtles. Surprisingly, nobody had ever looked at crocodilians, and this was the main focus for our study."

"Secondly, we wanted to look at the relationship between the biology of crocodilians (their body size, egg size, hatchling size, et cetera) and their environment (meaning the latitude at which they live, their incubation temperature, et cetera.) This would help us fill in some interesting gaps in our knowledge of crocodile evolution, and how they might respond to changes in climate," she said.

The researchers made a number of key findings. The first is that, in general, smaller species of crocodilians tend to live at attitudes around the equator, while larger species are more prevalent in higher latitudes.

"We identified a relationship between body size and latitude in crocodilians that we didn't know about before, which fits in with what we know about fossil crocodilians and helps solidify our understanding of the global distribution of crocodilians," Lakin said.

Secondly, they found that there is no relationship between crocodilian egg size and the number of eggs per clutch.

"This was very surprising, as most animals have to balance the size of their eggs against the number of eggs, since the mother only has so much energy she can invest in reproducing," Lakin said. "This was a very interesting revelation and we're still looking into what this could mean for crocodilian biology and reproduction."

Finally, they concluded that there was no significant relationship between latitude and the incubation temperature for crocodilian eggs—the opposite of what is seen in their turtle relatives.

"In turtles, the ideal incubation temperature of the eggs correlates to the position on the globe at which each species lives, which makes them vulnerable to changes in climate," Lakin said. "If the average yearly temperature in a turtle habitat changes, their eggs could die from poor incubation. On the other hand, crocodiles seem to have some kind of buffer to this effect, and it's possible that small changes in temperature won't affect them as badly as turtles."

baby crocodile
A baby crocodile (Crocodylus acutus) on the banks of a stream protected by authorities in Barra de Santiago, El Salvador on June 23, 2015. Marvin RECINOS/AFP via Getty Images

"We know that crocodilians and their kin have been around for hundreds of millions of years, and have survived some of the worst the world has to offer, including the dinosaur extinction. These results suggest that their good parenting skills—which include building a nest in the perfect spot, guarding their eggs, helping their babies hatch and even feeding their newborns—might be one reason they have survived so long," she said.

Behaviors like these could put crocodilians at an advantage over animals such as turtles, which do not take as much care of their young.

"Mother crocodilians will change their nest sites from year to year, choosing spots that cast the right amount of shade while keeping the eggs warm, spots that aren't likely to be flooded if the water level rises, and accounting for many other factors," Lakin said. "Turtles and other reptiles may not be as careful in choosing their nest sites, which makes them more dependent on consistent climatic patterns to continue to reproduce successfully."

Nevertheless, Lakin notes that despite their durability, crocodilians are still vulnerable to several man-made threats, such as pollution, persecution and the damming of rivers. Furthermore, the diversity of crocodilians today is far lower than it was at the time of the dinosaurs, with only about 20-30 species remaining.

"This diversity was squeezed out during the last extinction, leaving only the slow-moving, aquatic ambush predators we see today," Lakin said. "The diversity of crocs that used to exist, to me, is like looking at the diversity of mammals or birds that exist today. I think the main lesson that crocs can teach us is that extinction really is forever, and really destroys so much."