Development of a noninvasive photograph-based method for the evaluation of body condition in free-ranging brown bears

Abstract

Body condition is an important determinant of health, and its evaluation has practical applications for the conservation and management of mammals. We developed a noninvasive method that uses photographs to assess the body condition of free-ranging brown bears (Ursus arctos) in the Shiretoko Peninsula, Hokkaido, Japan. First, we weighed and measured 476 bears captured during 1998–2017 and calculated their body condition index (BCI) based on residuals from the regression of body mass against body length. BCI showed seasonal changes and was lower in spring and summer than in autumn. The torso height:body length ratio was strongly correlated with BCI, which suggests that it can be used as an indicator of body condition. Second, we examined the precision of photograph-based measurements using an identifiable bear in the Rusha area, a special wildlife protection area on the peninsula. A total of 220 lateral photographs of this bear were taken September 24–26, 2017, and classified according to bear posture. The torso height:body/torso length ratio was calculated with four measurement methods and compared among bear postures in the photographs. The results showed torso height:horizontal torso length (TH:HTL) to be the indicator that could be applied to photographs of the most diverse postures, and its coefficient of variation for measurements was <5%. In addition, when analyzing photographs of this bear taken from June to October during 2016–2018, TH:HTL was significantly higher in autumn than in spring/summer, which indicates that this ratio reflects seasonal changes in body condition in wild bears. Third, we calculated BCI from actual measurements of seven females captured in the Rusha area and TH:HTL from photographs of the same individuals. We found a significant positive relationship between TH:HTL and BCI, which suggests that the body condition of brown bears can be estimated with high accuracy based on photographs. Our simple and accurate method is useful for monitoring bear body condition repeatedly over the years and contributes to further investigation of the relationships among body condition, food habits, and reproductive success.

Dining from the coast to the summit: Salmon and pine nuts determine the summer body condition of female brown bears on the Shiretoko Peninsula

“…Previous studies of bears have shown cyclical annual patterns in body condition: declining through spring to summer, beginning to increase concomitantly with the onset of high-energy food consumption, and then peaking before hibernation (Hilderbrand et al 2000;McLellan, 2011;Schwartz et al 2014). In our study area, although data were available for only one individual, a seasonal fluctuation in body condition was reported from late June to early October, with the lowest condition in late August and the highest in early October (Shirane et al 2020).…”

“…Following Shirane et al (2020), we assessed the body condition of adult female brown bears using morphometric measurements from photographs. Shirane et al (2020) confirmed that this method accurately reflects true body condition (i.e., the body condition index obtained from the regression of body mass against body length) and has high measurement precision between photographs. However, this photograph-based method is limited by its requirement for photographs with sufficient quality for morphometric measurements and bear posture that does not affect the evaluation.…”

“…However, this photograph-based method is limited by its requirement for photographs with sufficient quality for morphometric measurements and bear posture that does not affect the evaluation. To overcome this, we first graded lateral photographs of each individual bear based on several attributes for photograph condition and bear posture (see Appendix B for details; Shirane et al 2020). Photographs were scored 1 (good quality), 2 (medium quality), or 3 (poor quality) for each attribute, and those with a score of 3 for any attribute were removed from subsequent analyses.…”

“…Photographs were scored 1 (good quality), 2 (medium quality), or 3 (poor quality) for each attribute, and those with a score of 3 for any attribute were removed from subsequent analyses. Then, we used ImageJ version 1.52a (Schneider et al 2012) to extract morphometric measurements from lateral photographs of bears following the protocols described in Shirane et al (2020). Specifically, first we adjusted the angle of all the photographs according to the ground surface; then, we measured the torso height (TH) and the horizontal straight-line torso length (HTL) in pixels: TH was the distance perpendicular to the ground from the lowest point of the abdomen to the highest point of the waist, and HTL was the straight-line distance from the base of the tail to the highest part of the shoulder parallel to the ground.…”

“…Male-biased dispersal and female philopatry have been previously reported for brown bears on the Shiretoko Peninsula (Shirane et al2018(Shirane et al , 2019; thus, we investigated the contents of scat collected in the Rusha area to reflect the typical diet of female brown bears inhabiting the area. Using photographic evaluation to assess the body condition of free-ranging brown bears (Shirane et al 2020), we noninvasively monitored the body condition of identifiable bears throughout the study period. This study focused on mechanisms driving the timing of the inflection point at which the body condition turns from deterioration to recovery.…”