A little something poop can tell you
Abstract
Diet selection in herbivores is essentially the most important factor which influences niche separation and consequently high biodiversity in the savannah biome. The experimental procedure was undertaken to determine the fibre content in the dung of eight herbivores which were: impala, waterbuck, wildebeest, zebra, cattle, buffalo, giraffe and elephant. High fibre content signifying a low quality diet whilst low fibre content signified high quality diet. 1g of the dried and milled dung was added to a glass tube with 5ml of kerosene and shaken thoroughly. An additional 2ml was added to wash down sediments sticking on the glass tube sides and samples were allowed to sediment for 2 hours. The volume of each sediment was read off which was used to calculate the bulk value. The bulk value generally increased with the body mass of the herbivore as the impala had the lowest bulk value and mass whilst the elephant had the largest bulk value and body mass. This relationship was attributed to the variation in the digestive tract anatomy of the herbivores as the larger herbivores were ruminants whilst the smaller herbivores were non-ruminants, a feature which dictated the quality of herbage that was to be consumed so as to gain nutritional value. The increase in the gut fill with size and well as high metabolic requirements in relation to body size also prompted the smaller herbivores to consume high quality herbage in a bid to gain more nutrients after little consumption.
Keywords: herbivores, diet selection, digestive tract, metabolic requirements, herbage, quality.
Introduction
Mammalian herbivore dietary requirements differ from species to species according to two main factors which are: the metabolic requirements and the functional anatomy and physiology of the digestive tract. Herbivores in the savannah biome are thought to allow inter-specific niche separation by the Bell-Jarman principle (Bell, 1971; Jarman, 1974) which suggests that larger herbivores are able to feed on herbage of lesser quality (high fibre content) with less nutrients when resources are limited during the dry season. However, this principle was proposed on the basis that body mass dictates the digestive tract capacity thus, an increase in body mass would consequently lead to an increase in digesta retention time as well as the gut fill (Illius & Gordon, 1992; Clauss et al., 2007a).
The metabolic rates of smaller herbivores are large relative to their body size in comparison to larger herbivores (Geist, 1974; Schmidt-Nielsen, 1990), a feature which has led to their preference of highly nutritious forage (concentrate feeders) even during the dry season which is highly digestible. Larger herbivores on the other hand have adaptive techniques of the digestive tract which allows efficiency in digestion of low quality food material (bulk and roughage feeders) thus fibre content in the dung would increase. Longer digesta retention may be a trade-off of low chewing efficiency in larger herbivores noted between foregut fermenters (ruminants) (Schwarm et al., 2009) and hindgut fermenters (non-ruminants) (Clauss et al., 2005).
The aim of this experimental procedure was to estimate the fibre content of dung in different herbivores of variant body sizes, giving an indication of the differences in diet quality among the species.
Materials and methods
Eight dung samples of herbivores (buffalo; cattle; giraffe; impala; elephant; wildebeest; zebra and water buck) of variant masses and diet were dried at 50oC to 60oC and milled in a coffee grinder. 1g of each milled sample was measured and placed in a glass tube. 5ml of kerosene was added to cover the sample which was shaken thoroughly. An additional 2ml was added to wash down all sediments left sticking to the sides. Samples were allowed to sediment for 2 hours and each sample was replicated twice. After 2 hours the volume of the sediment of each sample was read off and used to calculate their respective bulk values.
Results
The data show the bulk values of herbivores of variant masses ranging from 10.01kg/l to 715kg/l with the Impala recording the lowest bulk value and the elephant with the highest bulk value. The data also show the increase in the mass of the herbivores is directly proportional to the increase in the bulk value (Table 1).
Discussion
The bulk value increased with animal body mass thus the larger animals showed more fibre in their dung in comparison to the smaller herbivores (Table 1). This may be attributed to the diet selection of the herbivores mainly determined by the metabolic requirements as well as the anatomy of the digestive tract.
Herbivores of smaller body masses generally have a smaller gut fill compared to their larger counterparts, high metabolic rates relative to body size, are prone to predation as well as shorter digesta retention. These factors compel these organisms to select highly nutritious herbage so as to cater for their metabolic requirements regardless of the limitations of their gut fill. The impala, water buck, wildebeest and the zebra had low bulk values which translated to less fibre in their diets hence they are concentrate feeders. This also suggests that these organisms use hindgut fermentation as a digestion method hence they have to prioritise feeding and high digestion as they have a four chambered stomach but do not regurgitate ingested material (non-ruminants). Requirements of essential digestion would be ingestion of high nutritious herbage.
The cattle, buffalo, giraffe and elephant showed high bulk values which would consequently mean long digesta retention time and high fibre in their diets. This is a sign of low quality herbage ingestion suggesting a highly adapted digestive tract which undergoes foregut fermentation thus allowing rumination. Attempts to correlate digestive efficiency with body mass have however been argued as no systematic direct proportionality of the two parameters has been demonstrated across herbivorous species (Justice & Smith, 1992; Clauss & Hummel, 2005; Clauss et al., 2007b); differences in digestive tract efficiency between sexes of dimorphic species are either low or absent (Gross et al., 1996) and the differences in the digestive tract anatomy of herbivores (ruminants and non-ruminants) prove it difficult to unify all herbivores as posing the same characteristics of digestion efficiency and mass (Illius & Gordon, 1992).
In conclusion the diet selection in herbivores is not only limited to the herbage availability and plant defences to herbivory but also depends on the digestive tract anatomy and the metabolic requirements of the herbivores.
References
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Written by: Lorraine K Sal
