The Black Caiman is opportunistic

Three scientific expeditions led by an international team of researchers and involving Nicolas STURARO and Gilles LEPOINT (FOCUS Research Unit / Faculty of Sciences)  enable to learn more about the behaviour and habits of the largest predator in the Amazon basin, which was still recognized as an endangered species a few years ago but is now tending to stabilize. The data collected by the researchers were published in the scientific journal PLoS ONE(1).

T

he black caiman (Melanosuchus niger), which can reach a size of 5 metres and weigh one tonne, is one of the largest crocodiles on the planet and one of the largest predators in the Amazon basin. Once widespread in this region, the black caiman  lost more than 80% of its population during the 20th century due to intensive hunting and habitat destruction. Despite the fact that it is protected by the World Conservation Union, which has classified it as an endangered species, the status of some populations is still poorly assessed and the survival of the Black Caiman is still highly dependent on conservation efforts.

In French Guiana, the Kaw Marshes - a nature reserve covering nearly 100,000 ha - is a wetland of ecological importance for the extraordinary biodiversity it contains, including many individuals of Black Caiman. A godsend in a way for a team of scientists who wanted to know more about the animal, but basically, we knew little about it in this country. The research team, led by Stéphane Caut, a researcher at the CSIC ( The Spanish National Research Council in Spain ) and who carried out a post-doctoral fellowship at ULiège, was able to benefit from a floating platform placed in the marshes, and only accessible by helicopter for conducting a series of observations and surveys.

"This remote habitat, accessible only by helicopter, has made it possible to protect the species from human disturbances in this region," explains Nicolas Sturaro, a researcher at ULiège's oceanology laboratory. The floating scientific platform (6 x 4 meters), was built in 2001 in the Agami Pond in the heart of this marsh. Given the inherent difficulties in studying this species, our knowledge of its environment, its ecological requirements and its role in structuring the food web has remained very fragmentary. »

The three expeditions carried out from 2013 to 2015 - at seasons with different hydrological and ecological conditions - provided a little more information on the feeding ecology and movements of the black caiman in the Agami Pond through the use of different approaches such as behavioural observations, stable isotope analysis, tagging and telemetry, which made it possible to characterize the state of this population, intra- and interannual variations in individual movements and the feeding habits of this species.

A total of 76 caimans were captured by a classic "lasso" method during the night and marked. Various samples were taken from individuals: a biopsy of subcutaneous tissues at the tail, blood samples taken from the tail or cranial sinus, implantation of a subcutaneous electronic chip at the base of the tail, biometric measurements (height, weight). "Argos beacons have also been placed on some individuals to track their movements, says Nicolas Sturaro. In addition, we collected day samples from the most common and dense species in the ecosystem (fish, birds, crustaceans, amphibians, plankton, insects and plants) in order to study the food web and establish the diet of the caiman".

Initial findings

The researchers have already been able to draw some conclusions from the data acquired on site. The first is that environmental conditions have a strong influence on the caiman's diet and more particularly the seasonal variation in conditions that influences the availability of prey. During the dry season, the caiman feeds mainly on fish, the marsh being the rare place to contain water. However, in the wet season , when migratory birds are breeding, the caiman exhibits a particular behaviour preying on  birds and chicks that have fallen from the nest. This leads researchers to conclude that the black caiman is ultimately an opportunistic predator. The researchers also observed a difference in the use of food resources according to the size of individuals. "During development, a food shift occurs, from small invertebrate prey to vertebrates," explains Nicolas Sturaro. This would  likely reduce intra-specific competition between youth and adults. "Other data such as telemetry and markings show that movements differ according to age, time of year and individuals. The data reveal the existence of different types of behaviour; some individuals are very nomadic while others are very attached to the marsh.  

The new information gathered by the research team should help to better predict the species' responses to potential ecosystem disturbances (e. g. water pollution and habitat destruction), which is essential for the development of an effective management plan for their conservation.

This research was supported by the Consejo Superior de Investigaciones Cientıficas (CSIC) and the Estacion Biologica de Donana; Paris Zoological Park; Fonds de Dotation pour la Biodiversity and the private company Lacoste; the National Geographic Society/Waitt Grants Program (#W269-13); and La Ferme aux Crocodiles Zoological Park

PHOTOS : Stephane Caut / MANTIX

Scientific reference

Caut S, Francois V, Bacques M, Guiral D, Lemaire J, Lepoint G, Marquis O, Sturaro N (2019) The dark side of the black caiman: Shedding light on species dietary ecology and movement in Agami Pond, French Guiana. PLoS ONE 14 (6): e0217239

Contacts

Nicolas STURARO – Email

Gilles LEPOINT