Proteins can be tracked by the Nitrogen (N) they contain because dietary components, metabolites and body tissues vary in the ratios of heavy (15N) to light (14N) isotopes of N. Diets of caribou are often low in N when females endure the most demanding phases of reproduction in late pregnancy and early lactation. Maternal stores of N in body protein are therefore crucial for the production of calves.

The principal focus of this work is to evaluate body protein loss of animals by comparing measures of isotopic N (15N/14N) in urea (an excretory product of protein catabolism) with those of body protein and dietary protein. We calculate the proportion of urea derived from the body protein vs. the diet.

Females that derive more that 47% of the N in urea from body protein are losing body protein stores at more than 200mg N•kg -0.75• d -1. That rate can compromise the deposition of protein in the growing fetus or in milk .