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Isotopic fractionation in goats: effects of diet, diet quality, and intake

Final Report Summary - I-GOAT (Isotopic fractionation in goats: effects of diet, diet quality, and intake)

The aim of this research was to investigate how different plant food types (with different biochemical compositions) influence the stable isotope composition of body tissues and excreta (faeces) of herbivorous mammals. We would then apply the knowledge gained to existing datasets of free-ranging herbivores from southern African savanna habitats, and reconstruct their diets to link these with presumed diet-adapted morphophysiological and behavioural traits.

Stable isotope compositions of consumer body tissues are known to reflect the stable isotope composition of the foods they eat. This principle can be applied to reconstruct the diets of free-ranging animals, which has several benefits over traditional approaches including:
i) cryptic diets (e.g. nocturnal feeding) and cryptic species (e.g. very small species) are fairly easy to study through stable isotope analysis;
ii) the method is fast and easily repeatable, therefore animal diets can be compared over various geographic regions and time periods;
iii) different body tissues have different metabolic rates; comparisons of data from multiple tissues can hence be used to address questions about dietary behaviour over multiple temporal scales (e.g. weekly, seasonally, annually); and
iv) fossil bones also preserve lifetime diet isotopic signatures, such that analysis of ‘old’ bones can be used to reconstruct dietary histories of lineages in historical and palaeontological time.

The basic tenet for the approach to animal diets is “you are what you eat”. However, numerous intrinsic (physiological and biochemical) and extrinsic (phenological shifts in baseline isotope composition, climate, ecological interactions) combine to influence the consumer isotope signature. Thus, resolving dietary behaviour through stable isotope analysis depends, in a large way, on our ability to control for confounding factors. Numerous experimental studies of laboratory animals on controlled diets have been carried out to address the problem, but these are too few (there are far less of these than applied field-based studies) and we are only beginning to understand how the multiple factors combine to drive biological isotope systems.

In this project, we setup long-term experiments using a medium-sized ruminant, domestic goats (Capra hirca), to investigate key aspects of herbivore-diet isotope systems. We were primarily interested in differences in consumer signal across different types of diets. For example, a common assumption of applied studies is that tissue isotope compositions are offset from those of their diets by some amount of fixed magnitude (at least for the same species and same body tissue). We hypothesized that this offset would vary depending on the composition of the diet, because of differences in biochemical composition and hence digestibility. Simply, an easily digested food should show higher isotopic recovery rates in animal tissues compared with less digestible foods. Experimental animals were kept on controlled diets for 3 months, and thereafter switched to a new diet for a further 6 months in order to track the rate of isotope exchange in their tissues. Diets were either a single plant food type (grass or lucerne) or various mixed hays.