Geographic variation in growth of polar bears (Ursus maritimus)

The growth patterns of body length and head size in female and male polar bears (Ursus maritimus) in western Hudson Bay were compared with those of five other populations using the von Bertalanffy equation. Our objective was to determine if differences in growth patterns could account for higher reproductive rates reported in female polar bears from western Hudson Bay compared to the other populations. Significant differences in asymptotic size and growth rate were found in body length and head size. At age of first breeding in western Hudson Bay (4.1 years old), females reached 97% of asymptotic body length, while females in other populations took between 4.5 and 5.5 years to attain the same proportion. Body length of three- and four-year-old females in western Hudson Bay declined between the 1960s and 1990s and temporal variation in growth rates makes cross-population studies uncertain. Zygomatic breadth and head length may provide the most useful measures to compare populations because they are simple to measure, non-varying, and growth in head length is almost complete (97-98%) by the age of first breeding in females. We conclude that higher reproductive rates in western Hudson Bay were associated with higher growth rates. However, the reason for the higher growth rates remain unknown.