Appendix B. Calculations and estimates for measuring the contribution of plant secondary metabolites (PSM), fiber and metabolizable energy to the energetic costs of toxins (ECT, kJ/d).
PSM.
The maximum contribution of energy from PSMs was determined by estimating the
maximum amount of energy that could be excreted as PSMs from juniper. It was
assumed that the energy content of the juniper diet above control was primarily
due to PSMs (Appendix A), which is consistent with
the energetic values and quantities of PSMs present in juniper. For example,
based on the composition of monoterpenes in a 50% juniper diet (3–5% by
dry weight; Adams et al. 1981, Dearing et
al. 2000) and a conservative energetic value for monoterpenes (45 kJ/g;
Hawkins and Eriksen 1954), we calculate that monoterpenes
alone could explain nearly half the elevated energy on a juniper diet. The maximum
intake of energy from juniper PSMs was estimated from:
[Energy content of juniper diet
(kJ/g) – Energy content of control diet (kJ/g)] × intake
of juniper diet (g dry weight/d)]
Juniper also contains large quantities
of phenolics (~5% dry weight juniper) that could contribute to the elevated
energy content of juniper, but the chemical structure of these compounds, and
therefore the energy content, has not been determined (Holchek
et al. 1990).
Fiber. The contribution of energy from fiber was based on the proportion of fiber in the dry mass of feces of specialists and generalists (56%, Sorensen, unpublished data), fecal output (Table 1) and the energy content of fiber (13.23 kJ/g; Dietenberger 2002).
Metabolizable Energy.
The contribution of metabolizable energy (kJ/d) was estimated using the method
of subtraction from:
Metabolizable Energy = ECT – Energy from PSMs – Energy from Fiber
Adams, R. P., T. A. Zanoni, E. V. Rudloff, and L. Hogge. 1981. The south-western USA and northern Mexico one-seeded junipers: their volatile oils and evolution. Biochemical Systematics and Ecology 9:93–96.
Dearing, M. D., A. M. Mangione, and W. H. Karasov. 2000. Diet breadth of mammalian herbivores: tests of the nutrient constraints and detoxification-limitations hypotheses. Oecologia 123:397–405.
Dietenberger, M. 2002. Update for combustion properties of wood components. Fire and Materials 26:255–267.
Hawkins, J. E., and W. T. Eriksen. 1954. Physical and thermodynamic properties of terpenes. II. The heats of combustion of some terpene hydrocarbons. Journal of the American Chemical Society 76:2669–2676.
Holchek, J. L., A. V. Munshikpu, G. Nunez-Hernandez, R. Valdez, J. D. Wallace, and M. Cardenas. 1990. Influences of six shrub diets varying in phenol content on intake and nitrogen retention by goats. Tropical grasslands 24:93–98.