Turnover of carbon isotopes in tail hair and breath CO₂ of horses fed an isotopically varied diet

Abstract

Temporal stable isotope records derived from animal tissues are increasingly studied to determine dietary and climatic histories. Despite this, the turnover times governing rates of isotope equilibration in specific tissues following a dietary isotope change are poorly known. The dietary isotope changes recorded in the hair and blood bicarbonate of two adult horses in this study are found to be successfully described by a model having three exponential isotope pools. For horse tail hair, the carbon isotope response observed following a dietary change from a C₃ to a C₄ grass was consistent with a pool having a very fast turnover rate (t _1/2~0.5 days) that made up ~41% of the isotope signal, a pool with an intermediate turnover rate (t _1/2 ~4 days) that comprised ~15% of the isotope signal, and a pool with very slow turnover rate (t _1/2 ~140 days) that made up ~44% of the total isotope signal. The carbon isotope signature of horse blood bicarbonate, in contrast, had a different isotopic composition, with ~67% of the isotope signal coming from a fast turnover pool (t _1/2 0.2 days), ~17% from a pool with an intermediate turnover rate (t _1/2 ~3 days) and ~16% from a pool with a slow turnover rate (t _1/2 ~50 days). The constituent isotope pools probably correspond to one exogenous and two endogenous sources. The exogenous source equates to our fast turnover pool, and the pools with intermediate and slow turnover rates are thought to derive from the turnover of metabolically active tissues and relatively inactive tissues within the body, respectively. It seems that a greater proportion of the amino acids available for hair synthesis come from endogenous sources compared to the compounds undergoing cellular catabolism in the body. Consequently, the isotope composition of blood bicarbonate appears to be much more responsive to dietary isotope changes, whereas the amino acids in the blood exhibit considerable isotopic inertia.