Abstract

To date, empirical relationships have been observed for the distributions and variations of stable nitrogen and carbon isotope ratios (δ15N and δ13C, respectively) in ecosystems as well as in living organisms. Here, we investigated δ15N values of internal organs of various animal species, including insects, fish, amphibians, reptiles, birds, and mammals. The δ15N differences of these organs from the corresponding muscles vary within a range of ± 4‰. In general, the δ15N values of the brain and heart are higher than those of the corresponding muscles, except in birds. This may indicate that excreted ammonia is recycled in their cecum. We propose a kinetic descriptive schemas for a nitrogen isotope model of deamination that maintains a TDF of 8.0‰ for glutamate in muscle mitochondria. Differences in the δ15N in skeletal muscle, heart, and brain are evident across species because the cells that make up the organ differ greatly in shape and organization, and the amount and shape of mitochondria distributed in these organs change dynamically. These facts may be the first step to clarify the significance of organ δ15N variation for animal growth physiology and possibly for animal biochemical evolution. These evidences of δ15N variations at intact and organ level in biota may provide a new broad avenue to investigate paleoecology and its environmental conditions.