Population demographics of Nilgai in southern Texas

Nilgai antelope (Boselaphus tragocamelus) are bovids that are endemic to India and portions of Pakistan and Nepal. They were introduced into South Texas in the 1920’s and now have a free-roaming population of approximately 37,000 individuals. There is little known about nilgai population demographics, and their populations appear to be continuing to rise across its introduced range. For this reason, our objectives for this study were to (1) determine where nilgai are found throughout the landscape and quantify the landscape structure around nilgai locations, (2) determine a reliable aging method for aging nilgai in the field, and (3) determine nilgai reproductive capabilities in southern Texas. We collected nilgai point-locations from aerial surveys to assess the landscape structure around observed nilgai locations. We then performed a fine scale landscape analysis that quantified the scale of effect of landscape structures surrounding the nilgai locations and then performed a broad scale analysis that determined nilgai habitat use versus availability. Nilgai harvests were conducted during the summers of 2018-2021. From each harvested nilgai, we took complete dentition photos, collected the central incisors and the right mandible, and assessed the pregnancy and lactation status of each female. If a fetus was present, it was sexed, and the crown-rump length was measured. Our results revealed that, during the wintertime, nilgai were commonly observed on shrublands and woodlands. We were able to determine 13 tooth eruption stages and 6 age classes for female nilgai which can be used to age nilgai in a field-based setting. Overall, out of the sexually mature nilgai cows, 79% were pregnant and 55.5% of those pregnancies were carrying twins. Our results will provide land managers with the tools necessary to properly assess nilgai populations which will be essential in managing and controlling their populations in the future