How have crocodiles managed to stay alive despite the various mass extinctions that have occurred since they set foot on Earth? This is the question that a team of researchers at the Milner Center for Evolution at the University of Bath tried to answer.
Crocodiles are one of the oldest surviving lineages to date. They have survived two extinctions in particular: the late Cretaceous extinction, the most notorious one that wiped dinosaurs off the globe 66 million years ago, and a lesser extinction than the Eocene, which occurred 33.9 million years ago, the extinction that wiped out so many aquatic species.
This is not the first study trying to answer this riddle. Previous studies have suggested that diet may have helped these animals cope with these difficult conditions as well as their semi-aquatic behaviour (in essence they are neither marine nor terrestrial animals).
However, a new study, which appeared in the Biological Journal of the Linnean Society, suggests that the answer lies in their particular reproductive biology.
The sex of crocodile cubs, in fact, is determined by the temperature at which they are incubated since these animals do not have sex chromosomes. The higher the temperature during the incubation phase, the more likely it is that males are born. It is a little the discourse of the turtles only that for these last ones the contrary is true.
Precisely for this reason the climate changes, which are seeing an ongoing global warming, are partly beginning to influence also the reproductive biology of this type of animals. For example, in some turtle populations 80% of turtles are born female, something that could lead to devastating consequences for these animals in the near future.
Going back to crocodiles, the scientists have analyzed 20 different species, coming from all over the world, discovering that the smaller species tend to live closer to the equator while the bigger species tend to live at higher latitudes and in more temperate climates. They also discovered that incubation temperatures are not latitude related.
This means that crocodiles may not be threatened in the future, as are turtles, by climate change in relation to birth levels. It should also be added that crocodiles themselves, unlike turtles, take care of their young (turtles lay their eggs on the beach and then leave the young alone).
And it is precisely this more practical approach to parenting that has evidently allowed crocodiles to survive the aforementioned mass extinctions.
This does not mean that crocodiles are not globally threatened: there are human activities that, for these animals, could be considered a more threatening factor than mass extinction.