Thermal and Spatial Niches of Sympatric Species of Caimans

CAMPELO, P.H.; SIMONCINI, M.; FACHI, M.; COLLI, G.

Santa Fe

Congreso; XXV Working Meeting Crocodiles Specialist Group - UICN.; 2018

Crocodile Specialist Group/IUCN

Living crocodilians share many morphological andecological traits, potentially competing when in sympatry. In Amazonia, up tofour species of caimans may occur in sympatry, but little is known about theirinteractions, especially those involving Paleosuchus palpebrosus, one ofthe smallest and least studied caimans. Here we compare the thermal and spatialniches of a crocodilian assembly in the Culuene river, the main tributary ofthe Xingu river, in Gaúcha do Norte, Mato Grosso, Brazil. Between 22?25 July2015, we visually sampled caiman populations along a 10 km section of the Culueneriver and four oxbow lakes. From each individual we recorded the species, bodytemperature, habitat, water and air temperatures, and geographicalcoordinates of the location of first sight. Body temperatures were recordedwith an Amprobe IR-750 infrared thermometer gun with a laser pointer; water andair temperatures were recorded with thermocouples connected to the IR-750. Habitatsat river margins were classified as riparian forest, shrubs, sandy bar, or riverbank. We recorded 203 caimans: 24 P. palpebrosus, 133 Caiman crocodilus,and 46 unidentified individuals. Almost all individuals of P. palpebrosuswere found in the river (22 individuals, 91.66%), whereas individuals of C.crocodilus occurred both in the river and oxbow lakes. During dry season, oxbowlakes present a lower water level, leaving the fish denser, what could explainthe domain of the larger species. Caiman crocodilus was seen morefrequently in river sections bordered by riparian forest (76, 57.14%), while P.palpebrosus in sections bordered by shrubs (16, 66.66%). The observed nicheoverlap (pianka = 0.68) falls between 95% of null distribution of simulatedvalues of niche overlap, indicating that there is no spatial niche segregationbetween the two species. Mean body temperature of C. crocodilus (26.15 ±1.57 C) was significantly higher (t =4.04, df = 72, P < 0.05) than P. palpebrosus (24.43 ± 1.53 C). There was no difference betweenthe two species in water (t = 1.26,df = 23.41, P = 0.21) and airtemperature (W = 653.50, P = 0.05). We propose that C.crocodilus has greater thermal inertia, which causes it to take longer tolose heat, maintaining higher average temperature than P. palpebrosus,regardless of air and water temperature. Differences in body size largelyaccount for thermal differences between these two species, but are notcorrelated with habitat use, thus allowing their coexistence.