Visually estimating body mass of wild pigs

Body mass of wild pigs (Sus scrofa) can provide useful information regarding reproductive capacity of a population, and population health and resilience for this highly-destructive invasive species. Body mass of females is an indicator of whether they reproduce before 1 year of age, which could have substantial impacts on reproductive capacity of a population. Measuring body mass can be difficult because large wild pigs may require > 1 person to weigh, are often located in remote areas making equipment difficult to transport, or are often culled without access to the carcass (e.g., shooting from aircraft). We evaluated our ability to accurately estimate the body mass of wild pigs by visual inspection, and identified which factors (i.e., morphometrics and demographics) influenced the accuracy of our estimates. We visually estimated the body mass of wild pigs and then collected actual weights of 1,210 wild pigs across 5 regions (Alabama, Texas, Hawaii, Guam, Queensland). We also collected morphometric measurements and age to evaluate how these factors impacted our estimates. On average we found our estimates were accurate, averaging only -0.14 kg underestimated weights across all wild pigs weighed. However, our estimates were most severely underestimated (e.g., up to -20 kg) for younger wild pigs (i.e., < 1-3 years) that were heavier (i.e., > 30 kg). We also confirmed that although growth rates slowed after 1 year of age, wild pigs continued to grow in body length, head length, height, and girth as they aged, which explained why the age of an animal influenced our ability to generate accurate estimates. We surmised that young-yet-heavy wild pigs were disproportionally stouter than older animals, thus were underestimated due to their shortened appearance. Underestimating the body mass for young-yet-heavy females could misinform management plans, because these animals may have substantial influence on the reproductive capacity of a population. For visually estimating body mass of wild pigs, we recommend considering indicators of age (e.g., morphological proportions) to avoid underestimating young-yet-heavy animals with stout appearances. We also recommend calibrating observers regularly using known weights and morphometrics.