Validation and application of stable isotope analysis of dugong tusks to determine long-term shifts in foraging patterns

Understanding the capacity of animals to shift diet in times of food shortage is crucial in assessing their resilience to climate change. Dugongs are specialist herbivores that live in genetically discrete populations and forage locally. This study validated the use of stable isotope analysis of collagen from the permanent, continuously growing tusks of dugongs to assess potential trophic and/or spatial shifts in foraging. Tusks were accessed from museum collections and included five locations spanning the dugong's Australian subtropical to tropical distribution. Collagen from tusk growth layers deposited during three life stages (nursing calf, subadult, and adult) and two seasons (winter, summer) were analyzed for δ¹⁵N and δ¹³C. In tropical Australia, mean δ¹⁵N values decreased significantly from calves to adults reflecting the ontogenetic dietary shift from milk to seagrass. In contrast, δ¹⁵N was significantly enriched in adult dugongs in subtropical Moreton Bay suggesting omnivory. δ¹³C values in tusks did not change with geographic location. Season did not appear to have a significant impact on δ¹⁵N or δ¹³C values in either tropical or subtropical locations, but dietary variation was greatest in the subtropics. Stable isotope analysis of tusks appears to be a useful method of investigating dietary/trophic shifts over a dugong's lifetime.