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| A dwarf crocodile (Osteolaemus tetraspis) at the Wildlife Adventure Zoo, Emmen, Netherlands. By Eric de Redelijkheid, retrieved via Wikimedia Commons. CC BY-SA 2.0 |
In the winter of 2014, I was lucky enough to make a getaway from the cold clutches of an Ohio February to visit northeastern Florida. While there, we visited the historic city of St. Augustine. We did visit the famous lighthouse, but I had eyes for only one attraction: the St. Augustine Alligator Farm. Spanning more than 7 acres, the park hosts many animal species, including all 24 of the world's crocodilians! Tantalized by the opportunity to not only indulge in my love for crocodilians but to check them all off my list in one go, I paid the park a visit.
It was there that I first saw the dwarf crocodile, although my recollection is dim; I think I was more interested in seeing the gharials. It is easy to see how Osteolaemus, the world's smallest crocodilian, could escape the eye. If I'd had some idea of just how fascinating this animal really is, I might have paid it more mind. Although I did not know it at the time, it also doomed my ambition (at least in the short term) to see every crocodilian species. Even though this goal seems achievable in comparison to that other archosaur group (birds, whose literal thousands of representatives one would need several lifetimes to really check off), nothing is ever that simple. More on that further below; let's discuss the intriguing lifestyles of these animals first.
Watch the Eyeshine
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| O. tetraspis (illustrated as "Crocodylus frontatus", a junior synonym) by G. H. Ford for the Zoological Society of London, 1862. Retrieved from Wikimedia Commons. In the public domain. |
Before discussing what we know about these animals, it behooves us to clarify that there are two recognized species of Osteolaemus. The type is O. tetraspis (described in 1861 by Edward Drinker Cope, with whom dinosaur buffs will be familiar), best known from Cameroon, Gabon, and Nigeria, and the other is O. osborni (named in 1918 after Henry Fairfield Osborn, with whom dinosaur buffs will also be familiar), which is native to the Congo Basin. For a long time, there was considerable doubt over whether the latter was distinct from the former - for the better part of a century, in fact. Today, some workers still treat them as subspecies. The common name of O. tetraspis is simply the dwarf crocodile, though a few alternatives exist. The latter is sometimes called Osborn's dwarf crocodile. I'd personally prefer to use the name dwarf crocodile for the genus collectively, but such are the cards I have been dealt.
Because of this taxonomic history, I must go into this section with an apology beforehand. O. osborni is comparatively less well studied, and so most of the information we have about the lifestyles and ecology of dwarf crocodiles comes exclusively from O. tetraspis. Although they are very similar animals, we should not assume that every detail below is actually the same for both, but alas, it is the best that we can do for now.
Visually, both species are difficult to distinguish. They're both small, large-eyed animals, and, dare I say, quite cute. O. osborni is smaller on average, never exceeding 4 ft (1.2 m) in length, while O. tetraspis can be up to 6.2 ft (1.9 m). As with all crocodilians, they are well-protected by bony scutes which cover most of their body. Among all the crocodilians, the dwarf crocodiles are among the most terrestrial. They are often found moving at night through the wet forests they call home, opportunistically hunting prey including crustaceans (their favorites), fish, frogs, and various invertebrates. In a few instances, plant matter has been found in dwarf crocodile stomachs, but it is probably a result of accidental ingestion in most cases. However, it has been suggested that dwarf crocodiles may ingest palm nuts on purpose, using their hard shells as gastroliths. During the dry season, food is scarce throughout most of their range. Fortunately for the dwarf crocodiles, they, like most crocodilians, can generally endure long periods of poor food accessibility until the rains return.
The historical range of this genus is a wide one, spanning most of Western and Central Africa from the Gambia to northern Angola, and east across the Congo Basin to the border with Uganda. Anecdotal evidence from hunters suggests that dwarf crocodiles do not all nest at the same time. At any old time of the year, they will heap dead leaves and other plant matter against a tree trunk and lay between 10 and 14 eggs in the resulting mound.
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| A dead dwarf crocodile and monkey by a roadside in Gabon. Photographed by Wikimedia Commons user Barada-nikto. CC BY-SA 4.0 |
Osteolaemus was last assessed by the IUCN (as a single species) in 1996, and badly needs another assessment. The IUCN found the genus to be Vulnerable in its range due to habitat loss and human persecution. Sadly, the story is the same as with many other African animals: although dwarf crocodiles are docile creatures and pose little threat to humans, they are hunted extensively for the global bushmeat trade, as well as for leather. While eating these animals is not an evil unto itself, the current rate of growth in this trade can only be regarded as unsustainable and dangerous for the survival of these animals.
Ironically, both the scientists trying to save them and the hunters trying to kill them search for the dwarf crocodiles by using the same method: using a spotlight to search for their eyeshine in the water. One comes with notepads and radios, desperately trying to get accurate numbers for these shy animals; the other comes with guns and machetes to make that job all the harder.
The Curious Cave Crocs
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| Two O. tetraspis with very different lifestyles. Left: a cave-dweller. Right: a forest-dweller. By Olivier Testa, first published in Shirley et al. (2016) and uploaded to Wikimedia Commons by the photographer. CC BY-SA 3.0 |
The nocturnal, forest-stalking behavior I described above is interesting enough, but there is an even stranger lifestyle practiced by a few dwarf crocodiles in Gabon. There, one finds the Abanda cave system, where complex karst galleries and caverns have been carved into the Cretaceous limestone by plentiful tropical rain. The dwarf crocodiles calling these caves home not only have a very different ecology to their forest-dwelling counterparts, but sometimes even look strikingly different.
A study of these crocodiles revealed what seemed to be a separate population, most likely descended from founding individuals who either fell into the caves, or entered by way of tunnels that were later sealed off. There, in a microclimate dominated by the activities of several species of African bats, the crocodiles have seemingly made a home for themselves away from the light of the sun by subsisting almost entirely on bats and cave crickets.
Although we tend to think of caves as rather cold places, they actually retain temperature very well, particularly in the tropics where there is little seasonal variation. The body heat of the bats, as well as, nauseatingly enough, the exothermic heat from truly epic quantities of guano decomposing, both work to keep the Abanda caves at a comfortable temperature for the dwarf crocodiles that call them home.
As for the bright orange coloration seen in certain adult crocodiles in these caves, it doesn't appear that the color occurs naturally at all. Rather, after years, even decades, of pickling away in the bat guano covering the floors of their home, the alkalines simply erode the thick skin of the crocodiles. In a few instances, some were even noted to have bits of the skull visible through the skin.
This may seem like a pretty rotten life for any animal to lead, but the cave crocodiles actually seem to be doing comparatively well for themselves. Although not enough studies have been made to determine the size of their population, the ones surveyed so far seemed to be eating far better than the forest-dwelling crocodiles above, simply by virtue of having a far more abundant, and regularly available, food supply. It just goes to show that no matter how bizarre (or disgusting), all of the lifestyles that we see in nature really do serve some purpose.
The Species Problem
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| O. osborni in the Republic of the Congo. Photographed by Marius Burger and retrieved via Wikimedia Commons. CC0 |
In the middle of Osteolaemus' western range, there is a region called the Dahomey Gap. This expanse of forest-savanna mosaic habitat stretches to the Atlantic, splitting the great Guinean forests into two halves: the Upper in the west, and the Lower in the east. Although dwarf crocodiles look much the same on both sides of this gap, genetics tell us that the two separated populations are much more different than they appear. Part of the reason why I'm so reluctant to call O. tetraspis (which is found east of this gap) simply the "dwarf crocodile", as if it was the definitive article, is that there are not actually two species of Osteolaemus. There are, at minimum, three, and very possibly even four. As of this writing, only O. tetraspis and O. osborni have official names, but the presence of a third species, living west of this gap, has been known to science since 2009. What's going on here?
Cryptic species are animal species which, while appearing outwardly very similar or identical to their relatives, are genetically distinct. Crocodiles seem to be particularly prone to hosting cryptic species, perhaps due to the fact that they are, as a rule, quite morphologically conservative. It appears that this has been fooling zoos for decades as well; despite all being housed, displayed, and even bred together, surveys of European and American zoos suggest that as many as four different Osteolaemus species are actually represented. Most are O. tetraspis, one individual at a zoo in Santillana, Spain is an O. osborni, and the Upper Guinea forest species (Osteolaemus sp. nov. if you'd like, since it still has not been named) is especially prominent in American zoos. Bizarrely, four individuals in various European zoos do not seem to belong to any of these three species, forming a genetically distinct group that seems to be most closely related to the Upper Guinea forest species. No wild individual or museum specimen has ever been found to belong to this genetic group, and since no solid information for the provenance of these four animals exists, it is simply a head-scratching mystery as to where this apparent fourth Osteolaemus species might actually live in the wild.
Although this situation is probably one of the most complicated, dwarf crocodiles are far from the only animal to have one of these cryptic species complexes. More than anything else, this shows us just how little we still know about the wild populations of many animals, particularly in remote habitats like the forests of Western Africa. This poses issues for conservation on a few different levels. In zoos, Osteolaemus of different species have unwittingly been allowed to breed for decades, producing a number of hybrids who cannot contribute to repopulation efforts in the wild. In the field, these revelations mean that we know even less than we thought about their numbers, which species of dwarf crocodile are most threatened, and even where to find many of them. We have gone from having one Vulnerable species to, very possibly, having four Endangered ones.
I don't view this as being all negative. We are learning more about these animals all the time, and at least the scope of the problem has become obvious to us. It is my hope that further work on the dwarf crocodiles will tell us more about what makes these species different, more about their unique ecology, and even more things to love about them.
If you are interested in learning more about the cave-dwellers of Gabon, Shirley et al.'s 2016 paper "Diet and body condition of cave-dwelling dwarf crocodiles (Osteolaemus tetraspis, Cope 1861) in Gabon" has much more information. There is also a documentary featuring the scientists themselves at work, "Cave Crocs of Gabon", streaming on Amazon Prime, which is pretty solid, if that's more your speed. As for seeing crocodilians in the flesh, the St. Augustine Alligator Farm, mentioned at the beginning of this article, is an excellent place to do that.
My previously-threatened Jurassic World: Dominion review is still due for whenever I happen to watch the movie. Unless that happens first, the next article will be about Maastrichtian pterosaur diversity. Until next time!
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| A resting Osteolaemus tetraspis (probably...) photographed at the St. Augustine Alligator Farm, 2014, by the author. |
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