Plant wax components: a new approach to estimating intake and diet composition in herbivores.

The nutrient status of the herbivore depends on the nutritive value of the plants available, the botanical composition of the consumed diet and the intake of the animal. It has always been difficult to quantify these last two. At present, intake is usually calculated from separate estimates of fecal output and diet digestibility. In this review we discuss the errors inherent in this approach, especially those associated with the determination and application of digestibility in vitro. We then critically evaluate a new approach to the estimation of intake, based on the use of plant cuticular wax alkanes as markers. Plant alkanes are predominantly odd-chain and substantially indigestible. They can be used, in combination with orally dosed even-chain alkanes, to obtain an intake estimate which is essentially independent of marker recovery in feces and which is more truly "individual" because it accommodates the level of digestibility occurring in individual animals. We present published data which indicate that the method is accurate and can be extended to measure diet composition as well. Previous approaches to estimating diet composition have been based on the laborious microscopic examination of esophageal extrusa, stomach contents or feces. However, most plant species have a characteristic pattern of alkane concentrations in their cuticular wax. This permits the estimation of diet composition from the pattern of alkanes in the feces and in the plants available. We present data to show that this approach can provide accurate estimates of diet composition in terms of either plant species or plant parts. A major advantage of the approach is that, if the animals are also dosed with even-chain alkanes, estimates of total intake and diet composition can be obtained simultaneously. The method is equally applicable to domestic and wild herbivores and to animals receiving supplementary feeds. In future work, the method will be extended to the simultaneous estimation of plant species and plant parts in the diet, and to the use of other wax components as markers.