Paleosuchus trigonatus distribution map
The two species are sympatric, in that their distributions overlap. However, they are rarely syntopic - which means that although a certain area may contain both species, only rarely are both found together at the same locality. The separation of habitats is next...
The ranges, particularly of palpebrosus, are large enough to encompass differing habitats, seasonal influences, and competing species, so generalizing is difficult.
Having said that, let's generalize: P trigonatus seems to prefer closed-canopy rainforest streams, and P palpebrosus appears to have some connection with rivers draining savanna areas - though they are not found in the savannas themselves.
Medem described both as occurring in small, swift streams in Colombia. In Venezuala, trigonatus seems restricted to the southern forested regions, whilst palpebrosus occurs in the northern llanos area of the Orinoco drainage. In Surinam, trigonatus inhabits small to large rivers of the interior, and palpebrosus inhabits rivers that drain the savannah areas closer toward the coast. In Amazonia, again palpebrosus occurs in savana areas, or inundation forests surrounding the major rivers and lakes, and trigonatus in small rainforest streams.
Perhaps competition explains the differences. In Colombia, there are 6 species of crocodilian: the 2 Paleosuchus, C crocodilus, Melanosuchus niger, Crocodylus acutus, and C intermedius. Such competition could restrict the smaller Paleosuchus species to the rainforest brooks and streams. Indeed, some authors (notably Medem) report the extending of Paleosuchus ranges after the more commercially valuable species were shot out.
In Surinam, the 2 Paleosuchus and Caiman crocodilus are the only crocs. In this country, P trigonatus can be found in the larger rivers. Where it is syntopic with the spectacled caiman, it tends towards the swifter sections of the rivers, eg rapids. In similar types of rivers, where only P trigonatus is found, none were found near the rapids. So the seeming preference for swifter waters could very well be forced by competition.
Most authors report only adult males and wondering juvenile P trigonatus in larger rivers, never females. In Surinam, however, juveniles and adults of both sexes can be found in these larger rivers - although hatchlings were only found in smaller forest streams. It seems that the nesting habitat of P trigonatus is fairly uniform throughout its range - closed-canopy forest.
Both Paleosuchus, even those in larger rivers, seek out shelter from shoreline vegetation. P trigonatus is known to spend much time in terrestrial 'retreats' - rarely seen basking. It seeks out hollow logs, fallen trees at the river edge, or burrows - which may be quite a way from the nearest body of water. In Colombia, Medem reported that when water levels dropped in the dry season, many burrows were revealed in the Rio Pacoa.
P palpebrosus likewise is known to inhabit burrows. Writers as early as the 1800's recorded palpebrosus as inhabiting burrows. These burrows are 1.5 - 3.5 metres long, widening at the end to provide an area where the caiman can rest and also have enough space to turn around. Not much has been written about these burrows, but I assume they are similar to those built by the American alligator - where snout, feet and tail are used to dig out the burrow, or 'customize' existing caves beneath the shoreline. They don't appear to be as complex as the Chinese alligator, whose burrows may incorporate several chambers, as well as air holes.
In Bolivia, Paleosuchus are found in both large and small streams. They are found at night along stretches of shore that are devoid of floating and emergent vegetation. They sit exposed on bare sand, gravel etc, or shelter behind piles of dead tree trunks. Not one was found among living plant matter, where Caiman crocodilus were found. In fact, as soon as the researchers could determine whether the eye shine came from a patch of green vegetation or from a bare bank, they could predict whether it was Paleosuchus or Caiman. Paleosuchus are generally found singly or in pairs. They are not gregarious, like other caiman species. It is interesting that where the two Paleosuchus are syntopic, one or the other is quite rare. Nowhere are they both abundant within the same locality.
Palpebrosus, in contrast, can often be seen basking to some extent. Usually, it is on rocks, or in shallow water with its back exposed and head held high. It is not the basker that most crocodilians are. Perhaps this reflects its preference for cover of some sort. Since the osteoderms on the backs of crocodilians are known to be used as little 'power cells' by heating the blood that flows over them, perhaps palpebrosus doesn't need to bask as much as other species since it has more extensive osteoderms ?
As for the growth of algae on P trigonatus, this could be explained by it's cooler habitat. In the closed-canopy rainforest, it would remain moist for longer periods even when out of the water. Since it doesn't bask, it doesn't thoroughly dry out.
Ouboter found P palpebrosus in temperatures similar to those of the spectacled caiman - between 24 - 28°C. He also recorded temperatures as low as 22°C. He also noted that palpebrosus never sought direct insolation as did C crocodilus. P trigonatus was found generally at lower temperature - mid-20's - although in some areas, it was found at temperatures similar to the other species. Interesting to note that trigonatus was found ashore more often when the water was warmer, than it was in areas where water temperatures were lower.
Whilst Medem's earlier conclusions about palpebrosus occurring at lower temperatures than trigonatus may not be stricly correct, he never-the-less shows that palpebrosus can withstand quite low temperatures - as low as 6°C - and still thrive. Indeed, a specimen in captivity sought out cooler termperatures (13 - 19°c) - though this could have been due to wariness.
In Surinam, the highest densities of palpebrosus occur in black water rivers. Again, this may be explained by these rivers flowing through the preferred habitat rather than a distinct preference for black water. No other author has reported a preference for black water in other parts of palpebrosus' range.
One specimen found on a beach in Guyana would appear to have been either washed down-river, or have strayed from its normal habitat.
Medem reports of a captive palpebrosus that died after entering a saltwater tank. Ouboter proved that palpebrosus can withstand moderate salinity levels, but high levels resulted again in the death of two specimens. P trigonatus is found nowhere near the coast of Surinam, so none were tested for their resistance to saline conditions.
The experiments on palpebrosus showed that the integument (ie. skin) of palpebrosus has a higher permeability for water than does the spectacled caiman. Tests showed that palpebrosus loses more bodyweight when kept dry than does C crocodilus. When water was added, palpebrosus increased weight quicker than did C crocodilus.
Salinity levels up to about 4,250 mg/l can be withstood by palpebrosus, but owing to the higer permeability of the skin, it can be concuded that palpebrosus is NOT adapted to saline conditions.
Both Paleosuchus species are associated with fallen trees or branches near the water's edge or on the bank.
Palpebrosus is often reported near steeper banks - but again this could be a relfection of the inundation forest habitats where at certain water levels, the shore will be steeper than at other times. Ouboter notes that palpebrosus seems to prefer shallower water - which explains it being nearer the shoreline. It also explains the migration of palpebrosus during inundation times, where it again seeks out higher ground.
In the section on habitat, the use of burrows was detailed. Researchers in Amazonia that were after P trigonatus specimens regularly caught them by diving into the rivers by day and removing the caimans from their burrows. Both species apparently spend much of their time during the day in these burrows, emerging late afternoon and evening to hunt. They are not, however, strictly nocturnal. Remember, there is very little known about Paleosuchus in comparison to other species, and much more needs to be done. Some populations of the spectacled caiman are known to be addicted to basking, whilst others emerge at night, and are primarily nocturnal in their hunting behaviour. The same could apply to Paleosuchus. Very little mention is made of burrows in Surinam.
Magnusson noted the early dispersal of young trigonatus. He found that trigonatus hatchlings stayed under the watchful eye of their mother for only about 7 days. After this, the young began dispersing. This dispersal seems to be a feature of the Paleosuchus species, and continues into adulthood. Both are known to migrate overland many kilometres. This migration perhaps aids survival of the species - allowing it to inhabit many localities, and keeping local populations low which makes hunting them more difficult.
Male trigonatus select home territories of about 5 kms. No other males are found in this area, though females will be.
It is generally considered that trigonatus is more aggressive than palpebrosus. The reasons for this are not explained, and I have no personal knowledge of this. Perhaps the smaller size of palpebrosus causes it to be more wary? If cornered, both will react like any wild animal - inflating their body and hissing loudly. Tail slaps, and 'biting' the water are also reported for Paleosuchus, as they are for other species. Defensive behaviour when nesting or around the young is detailed in the Reproduction section.
Paleosuchus, when in the water, will often hold their bodies at a steep angle - almost vertical. In this position, only the head and neck are visible.
They are both shy species, and difficult to locate. Their coloration allows them to remain motionless on the bottom of streams, where they are easily overlooked - blending into the background of rocks and leaf-litter.
Several authors report group behaviour. I don't know the details of this in regard to other species. I have noted that my 2 palpebrosus exhibit such behaviour. When one caiman was allowed to wander around the living room (not really recommended), it became distressed, and hissed at me when I went near it. As I did this, a hiss behind me alerted me to the fact that the other caiman had jumped to edge of its enclosure and was preparing to jump to the floor. I took this to be a response to the distress of the other caiman (they are both males, by the way). Upon returning the caiman to its enclosure, both disappeared beneath a shelf over the water of the cage.
P trigonatus males are known to occupy territories, but I don't know the extent of their territoriality. I have found nothing in the literature describing territorial behaviour in palpebrosus.
P trigonatus eats more vertebrate prey than fish compared to other crocs in Amazonia. This reflects the habitat of rainforest in Amazonia. In the larger rivers that trigonatus inhabits in Surinam, one would expect the diet to consist of more fish.
One thing noted about Paleosuchus is the large number of gastroliths. These are the 'stomach stones' that all crocodilians swallow. In some specimens of Paleosuchus, up to 67 stones have been found. Whilst the function of these stones is not 100% determined, it seems that they aid digestion. Studies in spectacled caiman used x-ray technology to view the digestion of food. The gastroliths could be seen in the stomach of the caimans. After a period, "all hell broke loose", as the stones churned within the stomach, breaking apart the food. Since digestion is reliant to a great degree on temperature (at low temperatures, food could literally rot in a croc's stomach), perhaps Paleosuchus consume more gastroliths to counteract the lower temperatures they are often associated with.