Okapi digestive physiology and metabolism
As previously highlighted, relatively little is known about okapi digestive physiology and metabolism. In order for an effective captive diet to be designed and implemented, it is important for keepers to have an appreciation of the various factors of okapi digestion that are known and understood, and how these factors influence the dietary requirements of this species.
The basics
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Okapi are mid-large sized strictly browsing ruminants (Cerling et al. 2004; Hummel et al. 2005a). Ruminant animals have a complex, compartmentalised stomach (see Figure 1), in which the plant matter consumed by the animal is fermented by micro-organisms in one of the compartments; the rumen (Cheeke and Dierenfeld 2010a).
Browsing ruminants, also known as concentrate selectors, feed in a selective manner, consuming the soft and succulent parts of plants, such as the leaves (Cheeke and Dierenfeld 2010a). To find out more about the diet of wild okapi, click here.
Browsing ruminants, also known as concentrate selectors, feed in a selective manner, consuming the soft and succulent parts of plants, such as the leaves (Cheeke and Dierenfeld 2010a). To find out more about the diet of wild okapi, click here.
Digestive physiology
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Mouth and muzzle: The muzzle is elongated and tubular, with a long, muscular and prehensile tongue, allowing for selection of particular plant parts (Hart 2013). The free part of the tongue is 42.5% of the total length, which is within the average range for a browsing species (Clauss et al. 2006). Vast quantities of saliva is produced, which is believed to help counteract toxins produced by the plants the animals consume (Hart 2013). The sizes of the parotid and mandibular glands (both salivary glands) are more similar in weight to typical grazer species values, rather than the values normally observed in browser species (Clauss et al. 2006). The sizes of these glands are suggested to limit okapi to their natural habitat, browsing a wide range of plants to reduce the intake of any one secondary compound (Clauss et al. 2006).
Foregut: See Fig. 1 for a diagram of a typical ruminant foregut, clearly showing the different compartments (image taken from sheep101.info). The capacity of the rumen and reticulum is of typical browser size (Clauss et al. 2006). The honeycomb structure of the reticulum is relatively shallow, which is suggested to prevent complete closure of the reticular lumen (Clauss et al. 2006), which allows for suitable separation of food particles (Clauss et al. 2010a). The omasum is small (Clauss et al. 2006). The reticular crest and omasal laminae are similar to that of other browser species, being lower and less absorptive than that of grazers (Clauss et al. 2006). The microbial community in the rumen is dominated by Entodinium spp. (Clauss et al. 2006).
Hindgut and other organs: The caecum and the colon are large, assisting in the microbial digestion of ingesta (Hart 2013). The length of the intestine is slightly larger than that usually found in browsers, but is far smaller than that typically observed in grazers (Clauss et al. 2006). As in giraffe, the okapi does not have a gall bladder (Burne 1971). The liver is large, as is typical in browsers, which is suggested to be an adaptation to facilitate the detoxification of secondary plant compounds (Foley et al. 1995).
Like in other browser species, short food retention times have been reported in okapi (Hummel et al. 2005a). This is an adaptation to the okapis' natural feedstuff - browse - in which energy is released from quickly, requiring less fermentation time (Hummel et al. 2005a). This is in contrast to the feedstuff of grazers - grasses - from which energy is released from more slowly, hence slower food retention times are observed (Hummel et al. 2006a). As a consequence of the length of food retention time in the gut, browsers such as okapi tend to eat little and often, whereas grazers eat in less frequent bouts (Hummel et al. 2006a). Fast food retention times, and therefore eating little and often, directly influences the feeding behaviour of okapi, which should be supported in a captive diet; not facilitating this behaviour can result in the development of abnormal behaviour in okapi, such as stereotypies (Bashaw et al. 2001).
Foregut: See Fig. 1 for a diagram of a typical ruminant foregut, clearly showing the different compartments (image taken from sheep101.info). The capacity of the rumen and reticulum is of typical browser size (Clauss et al. 2006). The honeycomb structure of the reticulum is relatively shallow, which is suggested to prevent complete closure of the reticular lumen (Clauss et al. 2006), which allows for suitable separation of food particles (Clauss et al. 2010a). The omasum is small (Clauss et al. 2006). The reticular crest and omasal laminae are similar to that of other browser species, being lower and less absorptive than that of grazers (Clauss et al. 2006). The microbial community in the rumen is dominated by Entodinium spp. (Clauss et al. 2006).
Hindgut and other organs: The caecum and the colon are large, assisting in the microbial digestion of ingesta (Hart 2013). The length of the intestine is slightly larger than that usually found in browsers, but is far smaller than that typically observed in grazers (Clauss et al. 2006). As in giraffe, the okapi does not have a gall bladder (Burne 1971). The liver is large, as is typical in browsers, which is suggested to be an adaptation to facilitate the detoxification of secondary plant compounds (Foley et al. 1995).
Like in other browser species, short food retention times have been reported in okapi (Hummel et al. 2005a). This is an adaptation to the okapis' natural feedstuff - browse - in which energy is released from quickly, requiring less fermentation time (Hummel et al. 2005a). This is in contrast to the feedstuff of grazers - grasses - from which energy is released from more slowly, hence slower food retention times are observed (Hummel et al. 2006a). As a consequence of the length of food retention time in the gut, browsers such as okapi tend to eat little and often, whereas grazers eat in less frequent bouts (Hummel et al. 2006a). Fast food retention times, and therefore eating little and often, directly influences the feeding behaviour of okapi, which should be supported in a captive diet; not facilitating this behaviour can result in the development of abnormal behaviour in okapi, such as stereotypies (Bashaw et al. 2001).
Metabolism
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Unfortunately, even less is known about metabolism in okapi than is known about digestive physiology. Therefore, it is necessary to take a very broad overview of generic ruminant metabolism. However, given the wide variation found in the digestive physiology of different ruminants, it is likely that the information given here is inaccurate in some way. All information described below is from Cheeke and Dierenfeld (2010a).
Protein: Basic units are amino acids, important in the structure of many different animal tissues, such as muscle, and for metabolic reactions: proteins (enzymes) are catalysts. Dietary and non-dietary protein (NDP) is digested in the rumen by microbes. The okapi then digests microbial protein in the small intestine, along with any dietary protein that may have not been digested by gut microbes. Ammonia is produced as a bacterial end-product, metabolised by the liver, and converted by urea cycle enzymes to urea. Urea is carried in the blood to the salivary glands and recycled to the rumen via the saliva, where rumen microbes use it for microbial protein. Nitrogen produced during amino acid degradation is excreted in the urine.
Carbohydrate: Basic units are monosaccharides, essential for energy production. Dietary carbohydrates are fermented in the rumen by gut microbes, producing three major (others in small quantities) volatile fatty acids (VFAs): acetic; propionic; and butyric acids. Okapi then use the VFAs as an energy source. Proportion of each type of VFA depends upon the diet that the animal consumes.
Lipids: Basic units are fatty acids (FAs). Unsaturated FAs are partially/fully hydrogenated in the rumen. Most FAs reach the small intestine as free FAs. As browsers have a less developed rumen than other ruminants, there is reduced hydrogenation of unsaturated FAs, leading to higher polyunsaturated body adipose tissue than grazers. As dietary derived glucose is scarce in ruminants, they have evolved mechanisms for FA synthesis by converting acetyl CoA from acetate rather than from glucose or glucose precursors.
Water: As urea excreted in urine is the end product of nitrogen metabolism, mammals such as okapi have a higher requirement for water than other taxa. Ruminants have a higher requirement than non-ruminants in order to form a suspension of ingesta in the rumen. Ruminants from wet tropical areas, such as okapi, tend to have high rates of water usage and dilute urine.
Protein: Basic units are amino acids, important in the structure of many different animal tissues, such as muscle, and for metabolic reactions: proteins (enzymes) are catalysts. Dietary and non-dietary protein (NDP) is digested in the rumen by microbes. The okapi then digests microbial protein in the small intestine, along with any dietary protein that may have not been digested by gut microbes. Ammonia is produced as a bacterial end-product, metabolised by the liver, and converted by urea cycle enzymes to urea. Urea is carried in the blood to the salivary glands and recycled to the rumen via the saliva, where rumen microbes use it for microbial protein. Nitrogen produced during amino acid degradation is excreted in the urine.
Carbohydrate: Basic units are monosaccharides, essential for energy production. Dietary carbohydrates are fermented in the rumen by gut microbes, producing three major (others in small quantities) volatile fatty acids (VFAs): acetic; propionic; and butyric acids. Okapi then use the VFAs as an energy source. Proportion of each type of VFA depends upon the diet that the animal consumes.
Lipids: Basic units are fatty acids (FAs). Unsaturated FAs are partially/fully hydrogenated in the rumen. Most FAs reach the small intestine as free FAs. As browsers have a less developed rumen than other ruminants, there is reduced hydrogenation of unsaturated FAs, leading to higher polyunsaturated body adipose tissue than grazers. As dietary derived glucose is scarce in ruminants, they have evolved mechanisms for FA synthesis by converting acetyl CoA from acetate rather than from glucose or glucose precursors.
Water: As urea excreted in urine is the end product of nitrogen metabolism, mammals such as okapi have a higher requirement for water than other taxa. Ruminants have a higher requirement than non-ruminants in order to form a suspension of ingesta in the rumen. Ruminants from wet tropical areas, such as okapi, tend to have high rates of water usage and dilute urine.