Abstract
During carcass decomposition, tissues undergo biochemical changes: Cells autolyze, enteric microbes ferment cellular products, and tissues degrade. Ultimately, decomposition fluids are released as an ephemeral nitrogen (N) and carbon source to the surrounding environment. However, decomposition fluids are δ15N-enriched relative to body tissues, leading to a disconnect between starting tissue composition and ending fluid composition. It remains largely unknown when or if tissues exhibit δ15N enrichment postmortem despite the importance of tissue stable isotopes to ecologists. To test our hypothesis that tissues would become progressively δ15N-enriched during decay, soft tissues and bone were collected from beaver carcasses at five time points. All soft tissues, including muscle, were significantly δ15N-enriched compared to fresh tissues, but were not as enriched as decomposition fluids. Tissue breakdown is initially dominated by anaerobic autolysis and later by microbe and insect infiltration, and partly explains decay fluid isotopic enrichment. We speculate that after rupture, preferential volatilization of δ15N-depleted compounds (especially ammonia) contributes to further enrichment. These results constrain the timing, rate, and potential mechanisms driving carcass isotopic enrichment during decay, and suggest that found carcasses (e.g., road kill) should be used with caution for inferring trophic ecology as decay can result in significant postmortem δ15N enrichment.
Highlights
Vertebrate tissue isotopic composition is widely used by ecologists to address questions related to broader ecosystem functioning, including population dynamics, animal migrations or ranges, provenance, age, and diet, which can be used to infer trophic-level interactions[1,2,3,4,5]
Gut contents were isotopically-depleted compared to soft tissues and bone from the same animal (Fig. 3), and mean tissue δ15N enrichment above gut contents ranged from 1.3 to 4.1‰
Tissue stable isotopic composition of one individual (Fig. 3, Beaver 2) deviated from typical enrichment, with abnormally enriched gut contents compared to the three other individuals and relative to other tissue stable isotopic compositions
Summary
On shorter timescales (5 days), fish muscle increased by 1.3‰ when allowed to air dry[30], and after three days at > 20 °C, whale skin exhibited 6.4‰ enrichment[31] While these controlled laboratory studies on tissue subsamples provide a critical first look at vertebrate tissue decomposition, it is unclear how applicable these results are to whole carcasses in natural ecosystems. As most animals die and decay in natural ecosystems, and decomposition processes involve multiple tissues simultaneously, observing decay of whole carcasses in nature is critical for better understanding decomposition These observations are essential for assessing the suitability of naturally-decomposed tissues for ecology and forensics-based studies
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