Habitat use and densities of two sympatric foxes in the Mburucuyá National Park (Iberá Wetlands ecoregion, Argentina) during the winter season

In Argentina, two fox species —Lycalopex gymnocercus and Cerdocyon thous— are generally considered abundant and coexist across various natural, transitional and anthropogenic environments. However, studies on habitat use and population size estimation in sympatric areas remain scarce. Their low densities, elusive behavior and morphological similarities make them particularly difficult to study. The Mburucuyá National Park (Corrientes, Argentina), which conserves terrestrial ecosystems within the Iberá wetlands region, served as the study site for assessing habitat use based on density estimates. Surveys were conducted between May and August 2015, during both diurnal and nocturnal periods, along 12 linear transects spanning three habitat types: mesophilous forest, mesophilous grassland and Butia yatay savanna. Fox detections were analyzed using the hierarchical distance sampling (HDS) method, applying a probability function to model detection distances perpendicular to the transect centerline. A total of 63 fox detections were recorded over a sampling effort of 103.8 km. Of these, 37 detections were identified as L. gymnocercus (51.4% in savanna, 29.7% in forest and 18.9% in grassland), while 26 detections corresponded to C. thous (73.1% in savanna). The mean population density was estimated at 0.27 individuals/km² for L. gymnocercus and 0.50 individuals/km² for C. thous. No statistically significant differences were found between species or across habitat types within each species. The findings of this study will contribute to understanding species coexistence, enable comparisons with other sympatric populations and help assess carnivore responses to varying anthropogenic pressures.