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

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.


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Introduction 161 162 Growth curve of body length 163 To estimate the age at which the growth of body length was completed, growth pattern in 164 body length was examined using a von Bertalanffy curve as previously described in bears 165 (Kingsley, Nagy & Reynolds, 1988;Derocher & Stirling, 1998;Derocher & Wiig, 2002; 166 Bartareau, Cluff & Larter, 2011). The von Bertalanffy size-at-age equation was used in the form 167 A t = A ∞ (1 -e -K(t -T) ), where A t is body length (in cm) at age t, A ∞ is asymptotic body length (in 168 cm), K is a size growth rate constant (year -1 ), and T is a fitting constant (extrapolated age at zero 169 size; in years). We conducted F tests to determine whether the parameters of the von Bertalanffy 172 2015) in R (R Core Team, 2019). According to the age reaching 95% of the asymptotic body 173 lengths obtained from this analysis (detailed below), bears were assigned to three age classes for 174 each sex: cubs (0-1 years old), subadults (age 1-4 years and 1-7 years for females and males, 175 respectively), and adults (age ≥5 years and ≥8 years for females and males, respectively).
176 177 BCI of killed or captured bears 178 We calculated BCI as previously described in Cattet et al. (2002 S1): photographs that had a score of 1 for all attributes were assigned to "Good", 237 those with a score of 2 for body straightness only were assigned to "BS", those with a score of 2 238 for neck flexing only were assigned to "NF", and those with a score of 2 for neck lateral bending 239 only were assigned to "NB". Photographs that were not assigned to any category were excluded 240 from these analyses.   Map of the Shiretoko Peninsula, Hokkaido, Japan.
1 Manuscript to be reviewed

Measurement precision within photographs of an adult female brown bear (bear-ID: HC)
in the Rusha area of the Shiretoko Peninsula, Hokkaido, Japan.
The standard error (SE) in the ratio of torso height to body/torso length at a certain number of measurements was calculated by considering the standard deviation (SD) obtained from 50 times measurements as the population standard deviation. CV means coefficient of variation.
1 Manuscript to be reviewed Mean (± SD) ratio of torso height to body/torso length obtained from photographs of an adult female brown bear (bear-ID: HC) in the Rusha area of the Shiretoko Peninsula, Hokkaido, Japan.
P values are based on comparisons of mean ratios from the "Good" category versus other categories for each measurement method with Tukey multiple comparisons. Bold characters indicate significant differences. The "Good" category contained photographs with a score of 1 for all attributes, "BS" had a score of 2 for body straightness only, "NF" had a score of 2 for neck flexing only, and "NB" had a score of 2 for neck lateral bending only.
1 Table 4: 2 Mean (± SD) ratio of torso height to body/torso length obtained from photographs of an adult female brown bear (bear-ID: HC) 3 in the Rusha area of the Shiretoko Peninsula, Hokkaido, Japan.
4 P values are based on comparisons of mean ratios from the "Good" category versus other categories for each measurement method with 5 Tukey multiple comparisons. Bold characters indicate significant differences. The "Good" category contained photographs with a score 6 of 1 for all attributes, "BS" had a score of 2 for body straightness only, "NF" had a score of 2 for neck flexing only, and "NB" had a 7 score of 2 for neck lateral bending only.