Monday, December 15, 2008

There are no longer 23 species of crocodilians

In fact, there are 25 of them. Possibly more. While we are all extremely concerned about the handful of critically endangered species that are in serious danger of disappearing from the wild forever, it would seem that there are a few crocodilian species that we know nothing about. It's understandable. For a start, most crocodilians look very similar to each other. You have to be very familiar with each species before you can reliably tell them apart, and even then it can leave experts guessing without a very detailed examination of the skin. To complicate matters further, some variation in appearance is normal - a scale here, and colour change there. There are many, many sugggested species, sub-species and regional variations of crocodilians, but up until recently there were only 23 commonly-accepted species.

Enter genetic analysis. This is a tool that boils all those visible and invisible differences between species into a sequence of base pairs - a genetic formula for a species. So if you compare two individuals you'll always find differences (unless they are identical clones). The problem is, which of those differences are important in determining whether they are genetically compatible? In other words, whether they are the same species or different species?

To complicate matters even further, hybridisation is possible between quite a lot of crocodilian species. You remember the part during biology lessons where the teacher told you that only if two individuals could breed and produce fertile offspring they were of the same species? Well, it was wrong. Crocodilians demonstrate this very well, and it makes it even more difficult to say for certain which species is which.

So, genetic analysis should be able to solve this dilemma... if we knew what to look for each time. The main problem with some of these crocodilians is getting enough genetic samples covering their entire distribution to enable meaningful comparison. Without it, you end up with uncertainty, but with sufficient samples you can say with more confidence whether the species you're testing comprises one or more species. This is essentially what Mitchell Eaton and his team from the American Museum of Natural History have done with the African Dwarf Crocodile (Osteolaemus tetraspis). It was always suspected that there were two subspecies of dwarf crocodile: O. tetraspis tetrapsis and O. tetraspis osbornii based purely on morphological differences. Comparing photographs of the two candidates, the differences in the shape of the skull and the scalation were quite noticeable, but this alone was never sufficient evidence to separate them. But now that Eaton's team has compared the genetics across a sufficient wide range it's clear that there are major genetic differences there. The only thing is, there are enough differences to indicate three separate species, not just the two that were previously suspected. Perhaps the presence of this third species was sufficient to confuse any meaningful morphological comparisons earlier? Whatever the reason, we'll all have to revise our websites and our textbooks!

The three species? The third is as yet unnamed, but the first two are O. tetraspis and O. osbornii. It will be interesting to see what they name the third. Suggestions welcome!

Thursday, December 04, 2008

The largest saltwater crocodile in the world

[Updated July 2012]
When this story was written back in 2008, Cassius was indeed the largest saltwater crocodile in the world that we knew of. But in November 2011 we measured a much larger one called Lolong, and in June 2012 he was officially declared by Guinness to be the largest crocodile in captivity. The full story is in my 23 June 2012 blog post which is here.

[Original post]
How many times I have been asked this question: which is the largest crocodile in the world? Of course I always say that it's a saltwater crocodile (Crocodylus porosus) because the evidence we have supports that. But there's not always a lot of actual evidence of large crocodile sizes, just plenty of "big croc" stories. Still, the biggest crocodile ever measured (with a tape measure) was around 6.3 metres long (20.7 feet) from Papua New Guinea.

But what about the largest living crocodile? There may be several huge crocs living in the wild that we don't know about, but the largest living crocodile in captivity has been living on Green Island near Cairns for over 20 years. His caretaker is George Craig, a former crocodile hunter who now runs Marineland Melanesia, a shrine dedicated to crocodiles on the island. It is quite a remarkable place, which is entirely reflective of how remarkable George himself is. You see, George loves crocodiles with the kind of passion that you rarely encounter. He respects them enormously. This is why we were very keen to meet and talk with him on a recent trip to Cairns. The truth is we could have talked for weeks about crocodiles, but we only had a few hours. George was very keen to show us his pride and joy, the largest living saltwater crocodile in captivity. His name is Cassius, and he originally came from the Northern Territory. At the time he was a little under 18 feet (5.5 metres) long, but he's now around 18.5 feet (5.6 metres). Like many extremely large crocodiles, Cassius is as gentle as a lamb around his keeper, unless some food is dangled in front of him. Then, looking remarkably like a dinosaur from a forgotten age, he explodes into action and grabs the food from George. The word "grab" doesn't really do it justice, as the item of food often explodes under the pressure of those jaws. It's a remarkable and sobering sight, and if you ever need reminding how awesome crocodiles really are then a trip to Green Island to see Cassius is essential. Don't forget to talk to George Craig - rarely will you find anyone who knows more about crocodiles and understands them like he does.