Aspects of milk composition and lactation in polar bears

Abstract

Milk samples were collected on 135 occasions from fasting polar bears (Ursus maritimus) on land and from 26 feeding polar bears on the sea ice. Milk specimens were analyzed for fat, protein, carbohydrates, total solids, and gross energy. Fat content of milk from mothers with cubs captured on land declined from 35.8% when they were emerging from dens in spring to 27.5% the following autumn, and to 20.6% one year later when they had yearlings. No changes in milk fat were found for bears captured on the sea ice as offspring age increased. Milk protein levels increased while carbohydrate levels decreased through lactation for both feeding and fasting bears. Total solids ranged between 21.6 and 51.0%. For bears on land, gross energy declined from 16.9 kJ/g for females with 3-month-old cubs to 9.7 kJ/g when offspring were 34 months old. The proportion of females with cubs and yearlings that were lactating declined through the autumn. We suggest that when female polar bears do not have access to food, those with older offspring cease lactating earlier in the autumn and cessation of lactation results in lower milk fat and energy contents. Changes in milk composition may act to conserve maternal energy stores to ensure survival.