Serum testosterone concentrations, testicular biometry, and testicular tone in wild male lowland tapirs (Tapirus terrestris) in the Pantanal and Cerrado biomes, Brazil

This study aimed to contribute to the knowledge about the reproductive physiology of wild lowland tapirs (Tapirus terrestris) by describing the serum testosterone concentrations, testicular biometry, and testicular tone of individuals living in the Pantanal and Cerrado biomes of Brazil, and to check for reproductive seasonality for the species, and to investigate correlations among these reproductive parameters and individual age and weight. Meteorological data (air temperature, wind speed, relative humidity, and global radiation) of the study sites were characterized through automated stations of the National Institute of Meteorology (INMET). From 2015–2020, field data were collected from 22 and 11 wild male lowland tapirs from the Pantanal and Cerrado biomes, respectively. After capture and chemical restraint, the age and weight of the animals were estimated. Blood samples were collected to determine serum testosterone concentrations using radioimmunoassay. Testicular biometry measurements included width, length, height, and volume, while testicular tone was assigned using scores from 3 (firm) to 1 (soft). Animal age, weight, and testosterone concentrations did not differ between the two biomes (P > 0.05). Positive correlations were found between age and weight (ρ = 0.86; P < 0.001), and serum testosterone levels with both estimated age (ρ = 0.45; P = 0.0047) and weight (ρ = 0.51; P = 0.0011). Testicular volume was positively correlated with age (ρ = 0.75; P < 0.0001), weight (ρ = 0.65; P = 0.0011), and serum testosterone levels (ρ = 0.42; P = 0.0081). In addition, testicular tone in adult subjects (>48 months) was significantly higher than in younger subjects (P < 0.05). Testicular consistency was positively correlated with weight (ρ = 0.36; P = 0.0241) and serum testosterone levels (ρ = 0.38; P = 0.0174). Seasonal effect on tapir reproduction was not identified, but testicular tone was influenced by wind speed (ρ = 0.37; P = 0.0211) and air humidity (ρ = 0.37; P = 0.0262). This study is the first scientific description of reproductive parameters of male lowland tapirs.