Abstract 122 – Assessing polar bear body condition via histological image analysis of adipose tissue

Gregory Thiemann, Melissa P. Galicia, Alyssa M. Bohart, Amélie Roberto-Charron,
Evan S. Richardson

Polar bear body condition (i.e., stored energy) is a crucial link between ecological conditions
and population demography. Reliable estimates of polar bear body condition are thus central
to understanding and predicting the consequences of climate warming. Many condition metrics
(e.g., isotopic dilution, body condition index (BCI), energy density) require live-capture and
immobilization. A subjective fatness index (FI, 1-5 scale) can be applied non-invasively, but
yields relatively coarse data. Adipose lipid content (ALC) is a condition metric based on the
understanding that fat cells (adipocytes) expand as fat is stored, increasing the proportion of
lipid in adipose tissue. It can be applied to live-captured and harvested bears and thus provide
data across large scales. However, ALC requires specialized equipment, expertise, and
potentially harmful chemicals. We developed and tested an image analysis protocol to directly
measure the size of adipocytes in polar bear adipose tissue. The process used standard
histological preparation and a widely available software platform. We compared histological
measurements of adipocyte size with other metrics of body condition for both live-captured
(tissue biopsied) and harvested bears. For all bears, adipocyte size was a better predictor of
body condition (assessed by FI) than was ALC. For captured bears, adipocyte size was a better
predictor of BCI than was ALC. Our results indicate that histological measurements of adipocyte
size provide a useful and reliable indicator of overall body condition in polar bears. Given that
samples can be prepared and analyzed relatively rapidly and inexpensively in commercial
laboratories, this approach offers a cost-effective method of monitoring body condition in polar
bears and likely other ursids. In jurisdictions where live-capture work is limited, harvest-based
sampling may offer the best means of detecting ecological changes in polar bears prior to
population decline.