Exotic companion mammal herbivores have specific nutritional needs, and veterinary technicians and nurses may often find themselves providing consultations to pet owners on nutrition for wellness. As with other companion animals, the patient's species, age and reproductive status must be considered. A nutritional assessment includes body condition scoring as well as muscle condition scoring (Carpenter et al, 2012). Patient activity level and the presence of ongoing illnesses are also important considerations. To make a nutritional assessment and plan for a patient, a modified Nutritional Assessment Checklist is effective, as recommended in the toolkit of the World Small Animal Veterinary Association (WSAVA) Nutritional Assessment Guidelines Task Force (2011). Patient behaviour, the presence of healthy integument, normal stool and urine production, and the presence of diet-related illnesses help determine the suitability of the patient's current diet.
General nutrient requirements
A brief discussion of nutrients and their role in the herbivore diet will provide the basis for understanding nutritional assessment.
Fibre is defined by the digestive capabilities of other vertebrate species, such as humans, and includes indigestible carbohydrates of plant origin (predominantly plant cell wall components) such as cellulose, hemicellulose, pectin, cutin, and lignin (Boden, 2005). As discussed in part 1 (Miller, 2022), these compounds are variably ‘digestible’ to hindgut fermenting herbivores as a result of microbial fermentation in the caecum. The ability of microbial flora to break these chemicals down depends on the fibre's molecular structure. The features of indigestible vs. digestible (fermentable) fibre and their importance to the rabbit gut are summarised in Table 1.
Table 1. Features and mechanisms of fermentable and indigestible fibres in the rabbit gut
| Indigestible fibre | ‘Digestible’ or fermentable fibre |
|---|---|
| Particle size >0.5 mm | Particle size <0.3–0.5 mm |
| Stimulates gut motility | Growth substrate for caecal microbes, contributes to caecal pH |
| Provides forage for food-based environmental enrichment | Fermentation product includes VFAs, a calorie source for microbes and the host animal |
| Wears enamel of hypselodont teeth | Contributes to firm structure of caecotrophs |
| Stimulates appetite for food and caecotrophs | Prevents overgrowth of “bad bacteria” in caecums |
Reprinted from The Textbook of Rabbit Medicine, second edition, Varga M, Rabbit Basic Science, pp28, Copyright 2014, with permission from Elsevier
Carbohydrates that do not fall under the category of ‘fibre’ are digestible, including monosaccharides, disaccharides and starch. Monosaccharides are digested and absorbed directly from the stomach or small intestines to be used as energy. They include glucose, fructose and galactose. Starches are degraded into these monosaccharides by amylase activity (salivary and pancreatic, as well as inside caecotrophs). Undigested starches serve as a substrate for caecal fermentation (Varga, 2014).
Protein serves similar functions in herbivores as in other vertebrates: tissue building and repair, hormone and enzyme production, and as an energy source. Rabbits have 13 essential amino acids. Some are not obtained through the natural diet but via caecal fermentation products that are absorbed from caecotrophs (Table 2).
Table 2. The rabbit's essential amino acids
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Fat serves similar functions as in other species. It is a source of calories, a structural component of cells, a means to absorb fat-soluble vitamins from the diet, and an ingredient in the production of chemicals such as hormones.
Fat-soluble vitamins A, D, E and K, and water-soluble B vitamins serve similar purposes in the herbivore diet as in the diets of other vertebrates. Species-specific idiosyncrasies will be discussed below, notably guinea pigs' nutritional need of vitamin C.
Minerals are thought to serve similar functions in herbivores as in other vertebrates. However, there is a lack of data for the mineral requirements of herbivore mammals' diets.
Water is an essential nutrient obtained by drinking and drawing moisture from foods. Fresh, clean water should always be available. Water serves similar physiological purposes in herbivores as in other species. It is a structural component of cells, a solvent and carrier of nutrients and waste products, and a reaction medium as well as a reactant and reaction product (Jéquier and Constant, 2009). Hindgut fermenters' water intake generally varies depending on diet composition (e.g. if there is a higher proportion of dried grasses and sedges in the diet, the animal will drink more to hydrate the ingesta (saliva is continuously secreted orally, and water into the stomach) (Fekete, 1989). The digestive process of the hindgut fermenter necessitates continuous water exchange along the gastrointestinal system.
General diet plans for adult maintenance
There are commonalities in feeding plans for all of the species to be discussed, as well as specific requirements for each species. It should be noted that most of the scientific literature dedicated to nutrition in small mammal herbivores is representative of laboratory or production animal science, and may not be indicative of the nutritional needs of companion animals.
Many feeding guidelines exist for small mammal herbivores, which vary in the recommended proportions of dietary components (Figure 1). It is important to note that, despite specifying percentages, none of the guidelines describe whether these proportions should be fed based on weight, volume or caloric contribution to the diet. This helps to emphasise that these guidelines are not precise, perhaps adding to the pet owner's confusion when trying to provide balanced nutrition. Instead of prescribing exact dietary proportions, the focus of this article is to educate pet owners to restrict some dietary components while allowing free access to others. General guidelines for the proportions of different dietary components are as follows.
Grass hays
About 75–90% of the diet should be made up of grass hays, which should be offered freely. Grasses are monocotyledonous angiosperm (flowering) plants in the families Poaceae, Cyperaceae, and Juncaceae, and are often referred to collectively as graminoids. Members of Poaceae such as Timothy hay (Phleum pratense), oat hay (Avena sativa), orchard grass (Dactylis glomerata) are all readily available in the pet trade and have similar nutritional profiles. Meadow and botanical hays are a mixture of grasses and other herbaceous plants to provide food-based enrichment, and have a similar macronutrient distribution as the above staple hays (Varga, 2014).
Grass hays are not only the most important source of dietary fibre, but also provide enamel wear to the ever-growing hypselodont teeth of these herbivores. Poaceae contain microscopic silicate deposits, called phytoliths, which are suspected to be major contributors to enamel wear (Baker et al, 1959; Schulz-Kornas et al, 2017).
Hay should have a fresh and grassy scent, and not be musty or mouldy. Hay should be a mixture of all above-ground components of the plant, including stems, leaves and seed heads. Hay that is predominantly made up of stems or looks ‘straw-like’ is less palatable and less nutritious. Hay varies naturally by scent, texture and (probably) flavour, depending on growing and processing conditions. The nutritional content of hay is also affected by growing and processing: vitamin A (β-carotene) content diminishes with prolonged drying, yet vitamin D₂ content increases with curing time in the sun (Kalač, 2012; Varga, 2014). Fresh-cut grasses of many species are also safe to offer; however, lawnmower clippings are known to ferment quickly after collection, and may cause significant digestive upset (Varga, 2014).
Alfalfa or lucerne (Medicago sativa) and clover (Trifolium spp.) are not suitable staple hays for most adults of the target species, owing to their relatively high protein and calcium content. These are leguminous plants, and not graminoids like the previously noted species of Poaceae. Leguminous hays may be included in the daily diet of growing juveniles and lactating females. Regular inclusion of these high-protein and high-calcium hays in the diet of adult animals may increase the risk of urolithiasis, obesity and caecotroph overproduction (Smith, 2021) (Table 3).
Table 3. Nutritional content of select graminoid and leguminous hays
| Crude protein (% DMB) | Calcium (% DMB) | Phosphorus (% DMB) | |
|---|---|---|---|
| Alfalfa (Medicago sativa), full bloom | 17.0 | 1.19 | 0.24 |
| Timothy (Phleum pratense) | 7.8-10.8 | 0.38-0.51 | 0.15-0.29 |
| Oat (Avena sativa) | 9.5 | 0.32 | 0.25 |
| Orchardgrass (Dactylis glomerata) | 8.4-12.8 | 0.26-0.27 | 0.30-0.34 |
Fresh leafy green vegetables
About 10–20% of the diet may be made up of various lettuces, herbs and leafy cruciferous vegetables. All greens should be thoroughly washed to remove fertiliser and pesticide residue, as well as pathogens (bacteria, parasites). High calcium, high oxalate and cruciferous vegetables can be fed in moderation, offered perhaps once a week only (Varga, 2014) to avoid adverse effects. A variety of greens can be offered, although some should be fed in moderation (Table 4). The author typically recommends to pet owners to include at least three different greens in the pet's daily meal, and to change the rotation of greens every 7–14 days.
Table 4. Leafy green vegetables in the diet
Vegetables that are high in oxalates or oxalic acid (≥10 mg of oxalates per 1 cup of food) (Holmes and Kennedy 2000)
¤Cruciferous vegetables
Fortified food
About 5–10% of the diet may be made up of a concentrate such as fortified pellets or extruded formulae. These foods are a source of protein and micronutrients. Fortified foods should be grass hay based and uniform to prevent selective feeding. Restricting access to concentrates is important to encourage hay consumption, as the target herbivore species will selectively eat nutrient-dense food, resulting in poor fibre intake and dental wear (Varga, 2014).
Succulent vegetables, root vegetables, legumes, and fruits
These should make up a minimal part of the diet, ≤5% is a common estimate. They do not need to be included in the daily diet. Many of these fruits and vegetables are rich in simple carbohydrates and are very palatable. They can be a source of food-based enrichment and some micronutrients (Table 5).
Table 5. Succulent vegetables, root vegetables, legumes, and fruits in the diet
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Vegetables that are high in oxalates or oxalic acid (≥10 mg of oxalates per 1 cup of food) (Holmes and Kennedy 2000)
¤Cruciferous vegetables
Commercial treats
Commercial treats should be restricted to ≤5% of the diet (Figure 2). While treats do not contribute much to the diet in the way of nutrition, aside from carbohydrates and fats, they can be extremely useful for positive reinforcement and to strengthen the human-animal bond. Arguably, fresh produce can fulfil the role of treats as a high-value reward for companion animals. Treats that contain ‘gratuitous’ sugar such as honey or corn syrup should be avoided, as should dairy-based treats, since adult mammals cannot digest lactose.
Domestic rabbit, Oryctolagus cuniculus
The diet of wild rabbits is predominantly grasses (graminoids). However, it can vary seasonally and geographically to include various dicotyledonous woody plants' low-growing leaves, shoots and sometimes fruits (Martins et al, 2002; Ferreira and Alves, 2009; Marques and Mathias, 2009).
Rabbits' energy needs depend by the life stage (Table 6). Dietary energy is obtained mostly from carbohydrates. Caecotrophs make up an important dietary component (protein 25–30% dry matter basis, B-complex vitamins, volatile fatty acids) (Varga, 2014). The presence of numerous uneaten caecotrophs in the animal's environment is a cause for concern.
Table 6. Energy needs of rabbit life stages
| Lifestage | Daily calorie needs (kcal/day) |
|---|---|
| Adult | 100(BWkg)0.75 |
| Growth (4-12 weeks) | 190(BWkg)0.75 to 210(BWkg)0.75 |
| Early gestation | 135(BWkg)0.75 |
| Late gestation | 200(BWkg)0.75 |
| Lactation | 300(BWkg)0.75 |
A rabbit's daily water intake is estimated at 50–150 ml/kg/day (Cizek, 1961). It varies depending on the water content of their food. Rabbits may prefer drinking from bowls rather than sipper bottles (Varga, 2014; Mayer, 2021). Obese or mature animals with a large dewlap may experience a moist dermatitis from skin contact with their water dish (personal observation).
It is important to note that diets intended for meat and fur production are designed for rapid growth, and continued use of these diets in adult rabbits may predispose the animals to obesity, urolithiasis and caecotroph overproduction (Smith, 2021). A suggested dietary analysis for adult companion rabbits has been detailed by Varga (2014) (Table 7). The angora breeds of rabbit, which are characterised by exaggerated wool production, may require a higher protein diet of 16–18% (dry matter basis) to sustain wool production (Villamide et al, 2010) (Table 8).
Table 7. Suggested nutritional analysis for adult companion rabbit diet
| Nutrient | Dietary content |
|---|---|
| Crude fibre | >18% |
| Indigestible fibre | >12.5% |
| Crude protein | 12–16% |
| Fat | 1–4% |
| Calcium | 0.6–1.0% |
| Phosphorus | 0.4–0.8% |
| Vitamin A | 6000–10 000 IU/kg |
| Vitamin D | 800–1200 IU/kg |
| Vitamin E | 40–70 mg/kg |
| Magnesium | 0.3% |
| Zinc | 0.5% |
| Potassium | 0.6–0.7% |
Reprinted from The Textbook of Rabbit Medicine, second edition, Varga M, Rabbit Basic Science, pp44, Copyright 2014, with permission from Elsevier
Table 8. Macronutrient requirements for other life stages of rabbits
| Total (and digestible) protein, % DMB | Fat, % DMB | Fibre, % DMB | Digestible carbohydrates, % DMB | |
|---|---|---|---|---|
| Growth | 16 (12) | 2-4 | 14-16 | 45-50 |
| Gestation | 15 (11) | 2-3 | 14-16 | 45-50 |
| Lactation | 17 (13) | 2.5-3.5 | 12-14 | 45-50 |
| Orchardgrass (Dactylis glomerata) | 8.4–12.8 | 0.26–0.27 | 0.30–0.34 |
Mayer, 2021. DMB = dry matter basis
Rabbits express a unique mode of calcium metabolism compared with most vertebrate mammals, who regulate dietary calcium uptake via an endocrine feedback loop based on the individual's need, e.g. hypocalcaemia results in endocrine signals to upregulate calcium absorption from the diet (Goltzman et al, 2018). In contrast, rabbits absorb a large proportion of dietary calcium independently of serum calcium levels, mostly via a passive diffusion gradient. This results in serum ionised and total calcium concentrations that can vary widely depending on dietary calcium content (Chapin and Smith, 1967; Eckermann-Ross, 2008), unlike in most mammals where serum calcium concentration is tightly controlled. This may be an adaptation to ensure that there is adequate calcium to constantly generate new enamel for the hypselodont teeth (Eckermann-Ross, 2008). Rabbits appear to tolerate a relative, mild hypercalcaemia; prolonged hypercalcaemia may result in soft tissue calcification (Redrobe, 2002). Excess dietary calcium that has been absorbed is excreted into the urine in rabbits in the form of calcium carbonate (Figure 3) (Cheeke and Amberg, 1973; Kennedy, 1965), resulting in an opaque, white to off-white character to the urine.
When dietary calcium concentration is low, the rabbit relies on the vitamin D pathway for absorption (Redrobe, 2002). Rabbits will produce endogenous vitamin D when exposed to natural sunlight or artificial broad spectrum light (Emerson et al, 2014), specifically ultraviolet B (UVB) radiation (wavelength range of 290–315 nm) (Holick, 2007). Rabbits can absorb dietary vitamin D in the form of cholecalciferol or vitamin D3, as well as ergocalciferol or vitamin D2. Both forms are hydroxylated in the liver to 25-hydroxy-cholecalciferol, which is then converted to its active form (1,25-dihydroxycholecalciferol) in the kidney in response to parathyroid hormone (PTH) (Redrobe, 2002).
Guinea pig, Cavia porcellus
As Cavia porcellus is a domesticated species, no ‘natural’ or ‘wild’ diet has been described. Other closely-related members of genus Cavia are described as eating predominantly grasses and herbs (Eisenberg and Redford, 2000; Salvador and Fernandez, 2008).
Guinea pigs are assumed to have similar nutrient requirements and dietary needs as rabbits, with a few exceptions. There are few data describing their nutrient requirements. The US National Research Council (NRC) Subcommittee on Laboratory Animal Nutrition (1995) identifies ‘estimated nutrients for growth’ in laboratory guinea pigs, and Navia and Hunt (1976) also describe nutrient requirements for growth. Pignon and Mayer (2021) recommend a desired macronutrient distribution in a fortified pellet as crude protein 18–20% and crude fibre 10–16% (dry matter basis).
The daily maintenance calorie needs of an adult guinea pig (with a body weight (BW) between 400–600 g) may be met by 136(BWkg)3/4 kcal/day (Argenzio et al, 1988; Berger et al, 1989).
Guinea pigs prefer sipper bottles or ‘nipple drinkers’ (Balsiger et al, 2017). Their daily water intake has been reported between 7.5–21.7 ml/100 g BW/day (Liu, 1988; Tsao and Young, 1989). The author recommends offering more than one sipper bottle in the guinea pig's living environment and to check patency daily, as these bottles may become obstructed by food remnants in the animals' oral cavity.
Like rabbits, guinea pigs appear to absorb most dietary calcium via a passive diffusion gradient, independent of the PTH endocrine pathway unless the concentration of dietary calcium is low (O'Dell et al, 1957). It has been demonstrated that guinea pigs have the capacity for endogenous vitamin D production when exposed to natural sunlight or artificial lighting with UVB radiation (Watson et al, 2014, 2019). Guinea pigs appear to be able to absorb both cholecalciferol and ergocalciferol from their diet (NRC Subcommittee on Vitamin Tolerance, 1987).
A dietary source of vitamin C is required, and vitamin C deficient diets result in the development of scurvy. Guinea pigs lack a functional form of L-gulonolactone oxidase, an enzyme involved in converting monosaccharide glucose to vitamin C (Nishikimi et al, 1992). Non-breeding adult guinea pigs require 10–25 mg/kg/day, and growing, pregnant or lactating animals require 30 mg/kg/day (Pignon and Mayer, 2021). Vitamin C that is not in the stabilised form of L-ascorbyl-2-polyphosphate rapidly degrades when exposed to air, light and heat starting at 30°C, according to Igwemmar et al (2013). Stabilised vitamin C kept in dark and dry conditions at 21°C remains viable for up to 6 months (Pignon and Mayer, 2021). Liquid vitamin C supplements intended to be mixed with the guinea pigs' water supply are not recommended, as they often discourage animals from drinking because of the aversive taste. In addition, their stability is poor, requiring frequent water changes (Rhody, 2020; Pignon and Mayer, 2021).
The author recommends offering guinea pigs a variety of vitamin C sources in the event of storage failure, including:
- A fortified pellet diet containing vitamin C in the form of L-ascorbyl-2-polyphosphate
- Fresh vegetables rich in vitamin C, with a focus on leafy greens that are appropriate for this species
- A hay-based vitamin C tablet intended for use with guinea pigs.
Long-tailed chinchilla, Chinchilla lanigera
Chinchillas are opportunistic or generalist herbivores whose dietary intake varies seasonally, although graminoids make up a large portion of their wild diet (Cortés et al, 2002). Chinchillas' dietary requirements are assumed to be similar to those of rabbits, although as for guinea pigs, hard data are generally lacking.
The energy requirements for companion chinchillas have been described as 114.7 kcal/kg/day (Wolf et al, 2003).
The chinchilla's daily water intake has been reported as 45–90 ml/kg/day (Wolf et al, 2003). Water bowls are preferred to sipper bottles (Hagen et al, 2014).
Commercial chinchilla pellets are elongated to allow for these animals to easily pick them up with their hands for ingestion. Mans and Donnelly (2021) recommend a pellet diet with the following macronutrient distribution (on a dry matter basis): protein 16–20%, fat 2–4%, crude fibre 15–20%.
Although chinchillas appear to be capable of endogenous vitamin C production (Hughes, 1957), many commercial diets are supplemented with vitamin C.
Little work has been done to examine the calcium metabolism of chinchillas; however, they appear to be dependent on the vitamin D pathway for uptake of dietary calcium. The minimum recommended feed concentration of vitamin D3 (cholecalciferol) has been reported as 3.5–9 IU/g of feed (Wallach and Hoff, 1982). Chinchillas appear to be able to synthesise endogenous vitamin D3 when exposed to UVB radiation (Rivas et al, 2014), raising the question as to whether broad-spectrum lighting should be provided to captive individuals.
The inclusion of fresh vegetables in the captive chinchilla diet is represented by a strong dichotomy in the literature. While wild chinchillas are reported to eat grasses, they are also described as a generalist herbivore whose dietary preferences may change seasonally with available vegetation (Cortés et al, 2002). Many resources recommend including small volumes of fresh greens or herbs that are low in sugar in the diet (Prebble, 2011; Pollock and Parmentier, 2019; North Carolina State Veterinary Hospital Animal Medicine Department, 2022). Others claim that fresh vegetation is not part of the natural diet (Mans and Donnelly, 2021), and that succulent produce can be upsetting to the gastrointestinal tract (Mans and Donnelly, 2021). The author personally recommends adding small amounts of suitable fresh leafy greens or herbs to the diet for nutritional variety and food-based enrichment. New food items should be introduced one at a time and in restricted quantities while the individual is monitored for digestive upset.
Degu, Octodon degus
The wild diet of the degu varies seasonally yet is predominantly grass-based. Other vegetation selected includes the foliage, flowers, and seeds of forb plants, e.g. non-graminoid herbaceous plants (Meserve, 1981; Simonetti and Montenegro, 1981; Meserve et al, 1983, 1984).
Degus require a similar basic diet plan to rabbits, but indepth research into their nutritional needs is lacking. This species is demonstrated to be a selective feeder in captivity, preferring foods low in fibre and high in nitrogen (e.g. protein) (Gutiérrez and Bozinovic, 1998). Their wild diet is naturally very low in sugars and starches, and this needs to be respected in captive degus (Lee, 2004; Edwards, 2009). Captive degus that are fed foods rich in simple carbohydrates, such as fruits and root vegetables are at risk of developing type 2 diabetes mellitus as well as cataracts as a result of hyperinsulinaemia, followed by pancreatic beta-cell failure, and subsequently, hypoinsulinaemia (Brown and Donnelly, 2001; Edwards, 2009). Caviomorph rodent insulin has been described as having lower physiological activity than other mammalian taxa (Opazo et al, 2005). However, it is uncertain why degus alone appear to be so much more susceptible to developing diabetes than other caviomorph species. The structure of guinea pigs' insulin and glucagon hormones is reportedly similar to that of degus, but guinea pigs do not appear to be predisposed to developing diabetes mellitus (Nishi and Steiner, 1990).
Daily energy needs are poorly described for degus as veterinary patients. There is an abundance of literature describing changes in basal metabolic rate during pregnancy and lactation (Veloso and Bozinovic, 2000), and in response to changes in environmental conditions (Veloso and Bozinovic, 1993; Bozinovic et al, 2009; Nuñez-Villegas et al, 2014).
To estimate the resting energy requirement (RER) of degus, the author uses the modified formula for mammal RER of Gross et al (2010), which accounts for allometric scaling via Kleiber's Law. The formula is RER = 70(BWkg)2/3 kcal/day. This is a simple tool to estimate RER in very small mammal species. It provides a starting point for nutritional support in cases where monitoring of exact calorie intake is needed.
Wild degus experience seasonal variability in available drinking water, and are well adapted to tolerate periods of drought (Bozinovic et al, 2003). Captive degus should be offered water ad libitum (Jekl, 2021); a preference for sipper bottles or open dishes appears to depend on the individual animal (Hagen et al, 2014).
Commercial pellets for non-breeding adult degus are recommended to contain the macronutrient distribution (on a dry matter basis) of 13.5% crude protein and 3% fat. Degus have a known dietary requirement for linoleic acid (1.1%) and lysine (0.6%) (Lee, 2004; Edwards, 2009). Many fortified commercial diets for degus contain vitamin C, although degus have been demonstrated to produce the enzyme gulonolactone oxidase, which is responsible for endogenous vitamin C production (Jenness et al, 1980).
Degus appear to metabolise calcium in a similar way to rabbits and guinea pigs, independent of the vitamin D hormone pathway (Hankel et al, 2018). The author would cautiously assume that this species could benefit from access to natural sunlight or broad spectrum lighting as in the other species discussed.
Conclusion
While the herbivorous companion animal species discussed share a similar basic diet plan, each has unique idiosyncrasies which veterinary professionals must take into careful consideration when attending to patients and educating the pet owner (Table 9).
Table 9. Summary of the dietary needs of selected monogastric hindgut fermenting herbivorous companion mammals
| Species | Diet plan summary | Daily water intake in ml/kg/day | Daily calorie needs (adult) in kcal/day |
|---|---|---|---|
| Domestic Rabbit, Oryctolagus cuniculus |
|
50–150 | 100(BWkg)¾ |
| Guinea pig, Cavia porcellus |
|
7.5–21.7 | 136(BWkg)¾ |
| Long-tailed chinchilla, Chinchilla lanigera |
|
45–90 | 114.7(BWkg) |
| Degu, Octodon degus |
|
Not described | Consider 70(BWkg)2/3 to estimate resting energy requirement |
Part 3 in this series will explore how to take an effective nutritional history, common nutritional diseases and disorders encountered in these species, and how to provide nutritional support for inpatients and outpatients.
KEY POINTS
- Husbandry-related diseases, including nutritional disease, are a common cause of illness in exotic companion animals.
- Providing appropriate nutrition for wellness is a cornerstone in preventative medicine.
- Similar basic diet plans may be followed for the target species, with a few notable exceptions.