Estuarine Crocodile Crocodylus Porosus Research Articles

Crocodile social environments dictated by male philopatry

Abstract Examining the social behaviors of solitary species can be challenging due to the rarity in which interactions occur and the large and often inaccessible areas which these animals inhabit. As shared space-use is a prerequisite for the expression of social behaviors, we can gain insights into the social environments of solitary species by examining the degree of spatial overlap between individuals. Over a 10-year period, we examined how spatial overlap amongst 105 estuarine crocodiles Crocodylus porosus was influenced by season, sex, and movement tactic. We discovered that crocodiles displayed highly consistent spatial overlaps with conspecifics between months and across years. Furthermore, male crocodiles that exhibited a greater degree of site fidelity displayed more stable social environments, while females and males that were less site-attached had more dynamic social environments with spatial overlaps between conspecifics peaking during the mating season. Our results demonstrate how long-term tracking of multiple individuals within the same population can be used to quantify the spatial structure and social environment of cryptic and solitary species.

Salt-water acclimation of the estuarine crocodile Crocodylus porosus involves enhanced ion transport properties of the urodaeum and rectum.

Estuarine crocodiles, Crocodylus porosus, inhabit freshwater, estuarine and marine environments. Despite being known to undertake extensive movements throughout and between hypo-osmotic and hyperosmotic environments, little is known about the role of the cloaca in coping with changes in salinity. We report here that, in addition to the well-documented functional plasticity of the lingual salt glands, the middle of the three cloacal segments (i.e. the urodaeum) responds to increased ambient salinity to enhance solute-coupled water absorption. This post-renal modification of urine serves to conserve water when exposed to hyperosmotic environments and, in conjunction with lingual salt gland secretions, enables C. porosus to maintain salt and water balance and thereby thrive in hyperosmotic environments. Isolated epithelia from the urodaeum of 70% seawater-acclimated C. porosus had a strongly enhanced short-circuit current (an indicator of active ion transport) compared with freshwater-acclimated crocodiles. This enhanced active ion absorption was driven by increased Na+/K+-ATPase activity, and possibly enhanced proton pump activity, and was facilitated by the apical epithelial Na+ channel (ENaC) and/or the apical Na+/H+ exchanger (NHE2), both of which are expressed in the urodaeum. NHE3 was expressed at very low levels in the urodaeum and probably does not contribute to solute-coupled water absorption in this cloacal segment. As C. porosus does not appear to drink water of salinities above 18ppt, observations of elevated short-circuit current in the rectum as well as a trend for increased NHE2 expression in the oesophagus, the anterior intestine and the rectum suggest that dietary salt intake may stimulate salt and possibly water absorption by the gastrointestinal tract of C. porosus living in hyperosmotic environments.

Ontogenetic shifts in the nesting behaviour of female crocodiles.

Body size and age are crucial factors influencing reproductive capacity and success. As females grow, their reproductive investment and success often increase due to improved overall physiological condition and experience gained through successive reproductive events. While much of this work has been conducted on birds and mammals, surprisingly little is known on how body size affects nesting decisions in other long-lived vertebrates. We monitored the movements and nesting behaviour of 57 wild female estuarine crocodiles Crocodylus porosus over a 10-year period (and across consecutive nesting seasons) using externally mounted satellite tags, implanted acoustic transmitters and a network of submerged acoustic receivers. Applying Hidden Markov models to the telemetry-derived location data revealed that female nesting behaviours could be split into three distinct states: (i) ranging movements within home ranges and at nesting sites; (ii) migrations to and from nesting sites; (iii) and nesting/nest guarding. We found that during migration events, larger females migrated further and remained away from dry season territories for longer periods than smaller individuals. Furthermore, not only were migratory movements stimulated by increases in rainfall, larger females migrated to nest sites at lower rainfall thresholds than smaller females. We provide some of the first evidence of body size influencing nesting decisions in an ectothermic vertebrate, with shifts likely resulting from an increased willingness to invest in nest protection among larger and more experienced females.

Human–crocodile conflict in the Indian Sundarban: an analysis of spatio-temporal incidences in relation to people's livelihood

AbstractThe incidence of human–crocodile conflict is increasing, and fear of injury and loss of life is affecting public and political support for crocodile conservation. We studied conflicts between people and estuarine crocodiles Crocodylus porosus across socio-economic dimensions, using a spatio-temporal database. We collected data on 127 crocodile attacks that occurred during 2000–2013, through questionnaires including open- and close-ended questions, administered in 30 villages of five blocks of the Indian Sundarban. Most of the attacks (42%) occurred during winter (December–February), followed by the early monsoon (May–July; 27%). Almost 80% of victims were prawn seed collectors and were 11–50 years old, and 61.16% of victims died as a result of the attacks. Female victims accounted for a higher percentage of deaths (55.12%) than male victims (44.88%). Crocodile attacks were more common in the daytime than at night, with 76.35% of the killings occurring during 08.00–17.00. Most of the cases were recorded from Gosaba (34%), followed by Patharpratima (25.24%) and Namkhana (18.45%) blocks. The mean number of incidents per year was 9.07, with vulnerability and mortality rates of 0.07 and 0.04, respectively, per 10,000 persons. Existing management practices are insufficient to eliminate the risk of crocodile attacks and ensure the conservation of the Sundarban ecosystem. A comprehensive management plan for reducing dependency on forest resources is needed to minimize human–crocodile conflict.

Predicting the probability of large carnivore occurrence: a strategy to promote crocodile and human coexistence

AbstractPreserving large carnivores that perceive humans as prey brings conservation values into direct conflict with human security. Informing when and where humans and large carnivores occupy the same space may reduce attack frequency and promote coexistence. Here, we demonstrate a methodology to better understand the spatiotemporal relationship between a population of large carnivores and humans. The carnivore of study was the estuarine crocodile Crocodylus porosus, a large semi‐aquatic predator responsible for 705 recorded human attacks over the last 20 years. Crocodiles were captured every August over 3 years and individuals greater than 2.5 m in length were implanted with an acoustic transmitter (n = 84). The transmitter emitted a coded pulse detected when in proximity to underwater hydrophones deployed throughout the river. The telemetry data informed which previously captured crocodiles were present during subsequent trapping episodes and adult population size was estimated using a closed‐population model. Over 3 years, 24 of the tagged crocodiles were detected 269 times moving through a shallow‐water area where humans frequently entered the water. The tagged crocodile presence was extrapolated to the population level to provide a probability of adult crocodile presence across a range of temporal scales. The results showed that between September and December, the probability of crocodile presence within the human entry zone was 0.97 ± 0.01 during darkness but decreased to 0.07 ± 0.01 during daylight, except around periods of high tide when it increased to 0.71 ± 0.02. Human visitors confined their activity to shallow water during daylight hours, but no consideration was given to the significant rise in crocodile presence with season and tide. The observed patterns in crocodile and human behaviour, around this shallow‐water river crossing, exhibited parallels with historical incidences of crocodile attack.

Dual mechanisms for nitric oxide control of large arteries in the estuarine crocodile Crocodylus porosus

In reptiles, accumulating evidence suggests that nitric oxide (NO) induces a potent relaxation in the systemic vasculature. However, very few studies have examined the source from which NO is derived. Therefore, the present study used both anatomical and physiological approaches to establish whether NO-mediated vasodilation is via an endothelial or neural NO pathway in the large arteries of the estuarine crocodile Crocodylus porosus. Specific endothelial nitric oxide synthase (NOS) staining was observed in aortic endothelial cells following nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and endothelial NOS immunohistochemistry (IHC), suggesting that an endothelial NO pathway is involved in vascular control. This finding was supported by in vitro organ bath physiology, which demonstrated that the relaxation induced by acetylcholine (10(-5) mol l(-1)) was abolished in the presence of the NOS inhibitor, N-omega-nitro-l-arginine (l-NNA; 10(-4) mol l(-1)), the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10(-5) mol l(-1)), or when the endothelium was removed. Interestingly, evidence for a neural NO pathway was also identified in large arteries of the crocodile. Neural NOS was located in perivascular nerves of the major blood vessels following NADPH-d histochemistry and neural NOS IHC and in isolated aortic rings, l-NNA and ODQ, but not the removal of the endothelium, abolished the relaxation effect of the neural NOS agonist, nicotine (3 x 10(-4) mol l(-1)). Thus, we conclude that the large arteries of C. porosus are potentially regulated by NO-derived from both endothelial and neural NOS.

Very high concentrations of DDE and toxaphene residues in crocodiles from the Ord River, Western Australia: an investigation into possible endocrine disruption

Organochlorine pesticide concentrations, particularly those of the DDT family and of toxaphene, were measured by gas chromatography in samples of liver and body fat taken from Australian freshwater crocodiles Crocodylus johnstoni at three locations along the Ord River in Western Australia. The three sampling sites were the irrigation area, downstream of the irrigation area, and well upstream of the irrigation area; the last site serving as the control. DDT and toxaphene were applied in large and known quantities to cotton grown in the Ord Irrigation Area from 1964 to 1974. Thus the residues in the crocodile tissues are representative of the situation almost thirty years after the use of DDT and toxaphene ceased in the area. Very high concentrations of p,p'-DDE and toxaphene were found in the lipid-rich tissues that were examined. Livers and body fat from estuarine crocodiles Crocodylus porosus from the downstream site were also analysed. As p,p'-DDE and toxaphene are both known to be disruptive of endocrine systems, a range of blood parameters, including estradiol and testesterone concentrations, were also measured for all the animals studied. The ovaries and testes of the freshwater crocodiles were also examined histologically. There were no obvious effects on blood chemistry or gonad histology of the large burden of pesticides and their metabolites carried by exposed animals, although the limited number of samples and the variability of the breeding state of the animals examined may have masked possible effects. The isolation of the area, the accurately known applications of DDT and toxaphene, and the simplicity of the drainage system make the lower Ord River a unique natural laboratory for studying the long term breakdown and effects of pesticides applied in a tropical environment.

A population survey of the estuarine crocodile in the Ayeyarwady Delta, Myanmar

The estuarine crocodile Crocodylus porosus was formerly abundant in coastal regions of Myanmar, but is now known principally from the lower Ayeyarwady Delta. We conducted a survey during January 1999 to assess quantitatively the status of crocodile populations in protected areas of the lower delta. Spotlight counts were conducted along 275.4 km of waterways. Sixty-one crocodiles (50 juveniles, 4 subadults, 2 adults and 5 eyeshines) were observed in the Meinmahla Kyun Wildlife Sanctuary (MKWS), and the total population was estimated to be less than 100. The Kadônkani and Pyindaye Reserved Forests (RF) apparently no longer support viable populations. Although crocodiles remain critically endangered in the region, a comparison with survey data from 1980 suggests localized recovery is occurring in MKWS. Population declines are attributed to a combination of commercial skin hunting, habitat loss, drowning in fishing nets and over-collection of living animals to supply crocodile farms. A programme of law enforcement, habitat protection and head-starting juveniles is necessary to ensure the continued survival of estuarine crocodiles in the Ayeyarwady Delta.

A population survey of the estuarine crocodile in the Ayeyarwady Delta, Myanmar

AbstractThe estuarine crocodile Crocodylus porosus was formerly abundant in coastal regions of Myanmar, but is now known principally from the lower Ayeyarwady Delta. We conducted a survey during January 1999 to assess quantitatively the status of crocodile populations in protected areas of the lower delta. Spotlight counts were conducted along 275.4 km of waterways. Sixty-one crocodiles (50 juveniles, 4 subadults, 2 adults and 5 eyeshines) were observed in the Meinmahla Kyun Wildlife Sanctuary (MKWS), and the total population was estimated to be less than 100. The Kadônkani and Pyindaye Reserved Forests (RF) apparently no longer support viable populations. Although crocodiles remain critically endangered in the region, a comparison with survey data from 1980 suggests localized recovery is occurring in MKWS. Population declines are attributed to a combination of commercial skin hunting, habitat loss, drowning in fishing nets and over-collection of living animals to supply crocodile farms. A programme of law enforcement, habitat protection and head-starting juveniles is necessary to ensure the continued survival of estuarine crocodiles in the Ayeyarwady Delta.

An actively controlled heart valve.

Vertebrate hearts typically have cardiac valves that are thin and leaf-like and which work passively, allowing blood to move forward during systole and preventing it from flowing back during diastole. Crocodilian hearts have nodules of connective tissue, resembling opposing knuckles, or cog-teeth1,2,3, in the subpulmonary conus just proximal to the pulmonary valves. Here we show that these cog-teeth act in the estuarine crocodile Crocodylus porosus (Fig. 1) as a valve that regulates the flow of blood between the lungs and the systemic circulation in response to a β-adrenergic mechanism. To our knowledge, this is the first report of an actively controlled intra-cardiac valve in a vertebrate.

Field test of a paradigm: hysteresis of heart rate in thermoregulation by a free-ranging lizard (Pogona barbata).

The discovery that changes in heart rate and blood flow allow some reptiles to heat faster than they cool has become a central paradigm in our understanding of reptilian thermoregulation. However, this hysteresis in heart rate has been demonstrated only in simplistic laboratory heating and cooling trials, leaving its functional significance in free-ranging animals unproven. To test the validity of this paradigm, we measured heart rate and body temperature (Tb) in undisturbed, free-ranging bearded dragons (Pogona barbata), the species in which this phenomenon was first described. Our field data confirmed the paradigm and we found that heart rate during heating usually exceeded heart rate during cooling at any Tb. Importantly, however, we discovered that heart rate was proportionally faster in cool lizards whose Tb was still well below the 'preferred Tb range' compared to lizards whose Tb was already close to it. Similarly, heart rate during cooling was proportionally slower the warmer the lizard and the greater its cooling potential compared to lizards whose Tb was already near minimum operative temperature. Further, we predicted that, if heart rate hysteresis has functional significance, a 'reverse hysteresis' pattern should be observable when lizards risked overheating. This was indeed the case and, during heating on those occasions when Tb reached very high levels (> 40 degrees C), heart rate was significantly lower than heart rate during the immediately following cooling phase. These results demonstrate that physiological control of thermoregulation in reptiles is more complex than has been previously recognized.

Osmoregulatory mechanisms of the Australian freshwater crocodile, Crocodylus johnstoni , in freshwater and estuarine habitats

The estuary of the Limmen Bight River in Australia's Northern Territory is home to an unusual salt water-adapted population of the Australian `freshwater' crocodile, Crocodylus johnstoni. Crocodiles were captured from tidal reaches of the estuary ranging in salinity from 0.5–24‰ and from several permanent fresh water reaches more or less remote from saline waters. C. johnstoni is an effective osmoregulator in moderately saline waters and has osmoregulatory mechanisms very similar to its more marine-adapted relative, the estuarine crocodile Crocodylus porosus. Fasted C. johnstoni in brackish water appear to lose little sodium in cloacal urine, relying on their lingual salt glands for excretion of excess sodium chloride. The lingual glands show clear evidence of short-term and long-term acclimation to salt water. Like estuarine crocodiles, C. johnstoni drinks fresh water and will not drink sea water. Gross sodium and water fluxes in brackish water are very similar to those in other crocodilians, suggesting differences in integumental permeability are not a major influence on osmoregulatory differences between crocodilians. The data reinforce the hypothesis that crocodylids differ fundamentally from alligatorids in the structure and function of the renal-cloacal-salt gland complex and are of interest in current debate over the evolutionary and zoogeographical history of the eusuchian crocodilians.