An individual who consumes primarily marine food will have a carbon isotope ratio in their bone collagen that is measurably different from an individual who consumes primarily terrestrial food.

The difference is large enough — several parts per thousand in the standard measurement scale — to be readily measured and interpreted.

The nitrogen isotope ratio in bone collagen reflects the individual’s position in the food chain.

Each step up the food chain — from plants to herbivores to carnivores to top predators — adds a predictable increment to the nitrogen isotope ratio.

A person who consumes primarily plant foods will have a lower nitrogen isotope ratio than a person who consumes primarily terrestrial herbivore meat, and a person who consumes primarily marine carnivore flesh — large fish, sea mammals — will have a higher ratio than either.

Shuká Káa’s bone collagen showed both the carbon isotope signature characteristic of marine food consumption and a high nitrogen isotope ratio consistent with consumption of high-trophic-level marine species — large fish and marine mammals rather than simply shellfish and intertidal organisms.