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!


Wakefield Tolbert said...

Dr. Britton, do you accept the generally accepted taxon difference where C. yacare is not longer deemed a subset of c. yacare?

So a mix of these is true hybridization?

The reason I ask is in the pet trade, these mixes are common.

Adam Britton said...

Do you mean, no longer a subspecies of C. crocodilus yacare? If so then yes, it has been generally accepted that C. yacare has replaced C. c. yacare and not just in the pet trade. That would indeed suggest that interbreeding them with C. crocodilus would produce hybrids.

Hybridization is quite common amongst closely-related crocodilian species, both in the pet trade and in the wild, although the former tends to hybridize species that would never meet in the wild.

Wakefield Tolbert said...


My bad.


I meant to ask if C. c. yacare was previously under the "umbrella", so to speak, of what was termed the C. crocodilus complex.

In other words, just another version of caiman crocodilus?

And why the change in taxon?

DNA confirmation?

I've seen the young of both, and to me if you see one you've seen them all.

And then again what is the "cutoff" line to determine a true species. Is it merely some minute physical differences or some significant DNA arrangement showing some kind of lineage?

With some animals (but maybe more for mammals, and perhaps in reptiles this can be different?) even among the same genus, there is no interbreeding possible even with the attempts.

Hope like the blazes the one I have does not get nasty and big.

Like holding a t-bone steak that wiggles and has teeth.

Was told "ah--she'll be nice and sweet."


Wakefield Tolbert said...

I'm assuming by now, of course, you've seen or heard of this video, Dr. Britton; seems a man in Hiroshima has quite a pet. My experience both handling true spectacles and owning one is that they are far from this sluggish and would not hesitate to have a go at some fingers!

Any insights on why this particular specimen is so laid back.

In some states here in the US we have some jokers who use them to "guard" (using a mote, like mini-medievel castle!?) drug stashes and other illegal operations.

Kinda funny.

But a general search on the Net, other than the picture you posted about a young woman in the UK getting bitten, shows no true attack mode caimans of any serious consequence.

Thought the video was cute.

Shane said...

I was wondering Adam, if there is a scientific paper out on this yet and if so what journal?! Thanks a lot


Unknown said...

I believe the new species should be named Osteolaemus eatoni or Eaton's Dwarf Crocodile considering he is (or was considering my revision of the time stamp) pioneering this genetic investigation.