Yesterday I wrote about crocodile evolution, and some of their amazing extinct relatives (here). I wrote about them partly because prehistoric crocodylomorphs are amazing, and that’s as good a reason as any. But it was also to prove a point: modern crocodilians, 23 species all with similar appearances, might seem like ancient members of a group that has hardly changed at all. This is not so. Crocodilians are instead the only survivors of a vast and hugely diverse lineage of animals, most of which looked nothing like the crocodilians alive today.
Like the finned sea-crocodiles of the Jurassic, modern crocodilians are an off-shoot, just one branch in a massive crocodylomorph tree. Unlike the sea-crocodiles, by some combination of chance and adaptation, modern crocodilians have managed to avoid extinction (so far). Unlike the sea-crocodiles, crocodilians are still evolving.
Dr. Adam Britton, a world-renowned crocodilian biologist, points out that his favorite animals are far more advanced than they look. “I do see them as highly refined survivors of their ancient lineage. The analogy I use when talking about croc evolution is to compare modern crocodiles to Ferraris: they might superficially look and function similarly to a Model T Ford, but they are so much more refined.”
One of Dr. Britton’s favorite species is the Australian freshwater crocodile (Crocodylus johnstoni), the smaller of Australia’s two native crocodiles. Freshwater crocodiles typically reach 7-10 feet in length — impressive, but dwarfed by the saltwater crocodile (Crocodylus porosus), males of which can grow to over 20 feet. The two species share Australia, but because saltwater crocodiles are larger and fiercely territorial, freshwater crocodiles are often relegated to sub-optimal habitats, such as smaller rivers and ponds.
Such habitats include the remote, rocky upstream gorges of the Victoria and Liverpool Rivers (Webb 1985). Here, and at a few other sites in northern Australia, unique freshwater crocodiles live in relative isolation from humans and saltwater crocodiles. Unfortunately, they also live without much food — small streams mean few fish, which make for malnourished crocodiles.
So they evolved. Over time, the crocodiles became smaller to make up for a poor diet, and now they are truly tiny, with the largest reaching 5 feet in length. They became the pygmy crocodiles, small enough that you could (unadvisedly) pick one up and carry it around with you.
Stunted growth is one thing — any crocodile, fed a poor diet, will fail to reach its maximum size potential. Pygmy crocodiles are different. They have been growing this way for enough time that they are now genetically predisposed to small size. If you took a pygmy croc from the wild and fed it the same diet as a normal freshwater crocodile, the former would still be much smaller than its cousin.
Pygmy crocodiles aren’t quite distinct enough to be classified as their own species — yet. They haven’t been isolated for very long, so for now they are still considered an unusual population of freshwater crocodiles. They may interbreed with larger crocodiles, in which case they will never fully separate. A more exciting possibility is that they may continue to evolve and diverge in isolation, in which case they may someday become genetically unique enough to constitute a new species.
There is another possibility: pygmy crocodiles may go extinct before they get a chance to evolve any further. Although their habitat is isolated and relatively secure, they are threatened by invasive cane toads, introduced to Australia in 1935. A hungry crocodile will happily snap up a toad, but because cane toads are extremely toxic, they are often the crocodile’s last meal.
Pygmy crocodiles, because they are so small, are especially vulnerable to the poison. Although some populations have been unaffected (Doody et al. 2014). others have declined in abundance by more than 60% since the toads’ introduction (Britton et al. 2013). Why some populations are more vulnerable than others is one of many crucial questions that remain unanswered (Somaweera et al. 2012).
Dr. Britton is leading an effort to study pygmy crocodiles in their natural habitat. The goals of his research are two-fold. First, he means to assess their wild populations to determine if the crocodiles might be endangered. Second, Britton and his team wish to collect DNA from the pygmy crocodiles, to better understand their evolutionary history, and their genetic relationship with larger freshwater crocodiles.
This is an achievable and worthy project, but an ambitious one. Field work is always costly, but pygmy crocodiles live in isolated, hard-to-reach places, and getting there requires use of a helicopter. Dr. Britton and his team have started a crowd-funding effort to raise funds to support pygmy crocodile research — I’ve donated, and if you think pygmy crocodiles are amazing, I strongly encourage you to do so as well. There are some great prizes for donors, including crocodile-themed artwork and jewelry!
You can learn more about the project at its crowd-funding site, here. On the website is a short video in which Dr. Britton discusses and handles pygmy crocodiles. They are positively adorable.*
If crocodiles are Ferraris, then pygmy crocodiles are Smart Cars — tiny and vulnerable, but awesome in an enticingly bizarre sort of way. Pygmy crocodiles are an evolutionary quirk, just like the prehistoric pelican-snouted crocodile Stomatosuchus, or the armadillo-backed Armadillosuchus, or the shark-tailed … you get the picture. There’s just one important difference: pygmy crocodiles are alive. We, as residents of a special time in the history of life, get to appreciate them for the amazing creatures that they are. Let’s try and keep it that way.
*The crocodiles, I mean. Although if you like listening to British scientists get super-duper excited about wildlife, it’ll be a happy three minutes for you.
A big thank you is owed to Dr. Britton, who graciously allowed me to use his images for this article. Once again, I encourage you to donate to his effort to study these amazing crocodiles. You can learn more about crocodilians at Dr. Britton’s encyclopedic and lavishly illustrated website here.
Britton A.R.C., E.K. Britton, and C.R. McMahon. 2013. Impact of a toxic invasive species on freshwater crocodile (Crocodylus johnstoni) populations in upstream escarpments. Wildlife Research 40: 312-317.
Doody J.S., P. Mayes, S. Clulow, D. Rhind, B. Green, C.M. Castellano, D. D’Amore, and C. Mchenry. 2014. Impacts of the invasive cane toad on aquatic reptiles in a highly modified ecosystem: the importance of replicating impact studies. Biological Invasions 16(11): 2303-2309.
Somaweera R., R. Shine, J. Webb, T. Dempster, and M. Letnic. 2012. Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles? Animal Conservation 16(1): 86-96.
Webb G.J.W. 1985. Survey of a pristine population of freshwater crocodiles in the Liverpool River, Arnhem Land, Australia. National Geographic Society Research Report 1979: 841-852