6
Nutrient Requirements of the Gerbil

The Mongolian gerbil, Meriones unguiculatus, used in laboratories in the United States, originates from animals captured in the Amur River basin in eastern Mongolia in 1935 (Rich, 1968; Thiessen and Yahr, 1977). The Mongolian gerbil is one of 13 similar species of gerbils and jirds of the genus Meriones distributed in North Africa, the Middle East, and central and eastern Asia (Corbet, 1980). The name gerbil is also applied to other genera in the subfamily Gerbillinae. The gerbils as a group are typically arid-adapted inhabitants of deserts and dry steppes (Corbet, 1980). As arid-adapted species they produce concentrated urine and have low water turnover rates (Burns, 1956; Holleman and Dieterich, 1973). In the remainder of this report the name gerbil will be reserved for the Mongolian gerbil, unless otherwise indicated.

REPRODUCTION AND DEVELOPMENT

Some aspects of the reproduction and development of gerbils are summarized in Table 6-1. Adult gerbils weigh about 70 to 135 g. Pups at birth weigh about 2.5 g and are normally weaned at 14 to 18 g. An acceptable weight gain for the postweaning period from 3.5 to 7 weeks is 1 g/day. After reaching sexual maturity at 65 to 85 days, females are polyestrus, have a gestation period of 24 to 26 days, and exhibit postpartum mating (Marston and Chang, 1965; Loew, 1968; Rich, 1968; Gulotta, 1971; McManus, 1971; Schwentker, 1971; Norris and Adams, 1972; Thiessen and Yahr, 1977).

The stomach of the gerbil is simple and the cecum and colon are not especially well developed, suggestive of a species that in nature consumes mostly low-fiber foods such as seeds (Gulotta, 1971; Vorontsov, 1979). Gerbils generally have had acceptable growth and reproduction when fed pelleted natural-ingredient diets formulated for other rodent species such as rats, mice, and guinea pigs. Sometimes supplementary cereals and/or seeds have been used but these are not necessary (Marston and Chang, 1965; Arrington, 1968; Loew, 1968; Rich, 1968; McManus, 1971, 1972; McManus and Zurich, 1972; Norris and Adams, 1972).

ENERGY AND WATER

Growing gerbils consume about 5 to 6 g dry diet/day or 8 to 10 g diet/100 g BW. Dietary energy intake averaged 36 to 40 kcal gross energy/100 g BW/day (150 to 170 kJ/100 g BW/day) (Harriman, 1969; McManus and Zurich, 1972; Mele, 1972). Digestibility of energy was 93 to 94 percent when ambient temperature was 0° to 15° C, and both intake and digestibility decreased at temperatures of 20° to 35° C (Mele, 1972).

Although gerbils are arid-adapted and able to subsist on relatively low water intakes, Rich (1968) observed high

TABLE 6-1 Reproductive and Developmental Indices for the Mongolian Gerbil

Variable

Unit

Amount

Minimum breeding age

Week

10–12

Estrous cycle

Day

4–6

Gestation period

Day

24–26

Litter size

Pups

4.5 (range 1–12)

Birth weight

g

2.5

Incisors erupt

Day

10–16

Eyes open

Day

16–20

Weaning age

Day

21–24

Weaning weight

g

14–18

Litters per lifetime

Number

≥6

Life span

Year

>4

 

SOURCES: Gulotta (1971), Thiessen and Yahr (1977), and references cited in text.



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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995 6 Nutrient Requirements of the Gerbil The Mongolian gerbil, Meriones unguiculatus, used in laboratories in the United States, originates from animals captured in the Amur River basin in eastern Mongolia in 1935 (Rich, 1968; Thiessen and Yahr, 1977). The Mongolian gerbil is one of 13 similar species of gerbils and jirds of the genus Meriones distributed in North Africa, the Middle East, and central and eastern Asia (Corbet, 1980). The name gerbil is also applied to other genera in the subfamily Gerbillinae. The gerbils as a group are typically arid-adapted inhabitants of deserts and dry steppes (Corbet, 1980). As arid-adapted species they produce concentrated urine and have low water turnover rates (Burns, 1956; Holleman and Dieterich, 1973). In the remainder of this report the name gerbil will be reserved for the Mongolian gerbil, unless otherwise indicated. REPRODUCTION AND DEVELOPMENT Some aspects of the reproduction and development of gerbils are summarized in Table 6-1. Adult gerbils weigh about 70 to 135 g. Pups at birth weigh about 2.5 g and are normally weaned at 14 to 18 g. An acceptable weight gain for the postweaning period from 3.5 to 7 weeks is 1 g/day. After reaching sexual maturity at 65 to 85 days, females are polyestrus, have a gestation period of 24 to 26 days, and exhibit postpartum mating (Marston and Chang, 1965; Loew, 1968; Rich, 1968; Gulotta, 1971; McManus, 1971; Schwentker, 1971; Norris and Adams, 1972; Thiessen and Yahr, 1977). The stomach of the gerbil is simple and the cecum and colon are not especially well developed, suggestive of a species that in nature consumes mostly low-fiber foods such as seeds (Gulotta, 1971; Vorontsov, 1979). Gerbils generally have had acceptable growth and reproduction when fed pelleted natural-ingredient diets formulated for other rodent species such as rats, mice, and guinea pigs. Sometimes supplementary cereals and/or seeds have been used but these are not necessary (Marston and Chang, 1965; Arrington, 1968; Loew, 1968; Rich, 1968; McManus, 1971, 1972; McManus and Zurich, 1972; Norris and Adams, 1972). ENERGY AND WATER Growing gerbils consume about 5 to 6 g dry diet/day or 8 to 10 g diet/100 g BW. Dietary energy intake averaged 36 to 40 kcal gross energy/100 g BW/day (150 to 170 kJ/100 g BW/day) (Harriman, 1969; McManus and Zurich, 1972; Mele, 1972). Digestibility of energy was 93 to 94 percent when ambient temperature was 0° to 15° C, and both intake and digestibility decreased at temperatures of 20° to 35° C (Mele, 1972). Although gerbils are arid-adapted and able to subsist on relatively low water intakes, Rich (1968) observed high TABLE 6-1 Reproductive and Developmental Indices for the Mongolian Gerbil Variable Unit Amount Minimum breeding age Week 10–12 Estrous cycle Day 4–6 Gestation period Day 24–26 Litter size Pups 4.5 (range 1–12) Birth weight g 2.5 Incisors erupt Day 10–16 Eyes open Day 16–20 Weaning age Day 21–24 Weaning weight g 14–18 Litters per lifetime Number ≥6 Life span Year >4   SOURCES: Gulotta (1971), Thiessen and Yahr (1977), and references cited in text.

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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995 mortality when gerbils were restricted for long periods to dry-type diets without water or succulent feeds such as carrots or lettuce. Thus free access to water and/or succulent feeds should be provided when dry diets are used (Marston and Chang, 1965; Rich, 1968; McManus, 1971; Mele, 1972; Norris and Adams, 1972). Gerbils will voluntarily consume 4 to 10 mL water/100 g BW/day (Winkelman and Getz, 1962; Harriman, 1969; McManus, 1972). Total daily water intake (including free water in food and metabolic water) has been estimated as 8 to 13 percent of body weight (McManus, 1972; Holleman and Dieterich, 1973). LIPIDS Purified diets fed to gerbils have contained 2 to 20 percent fat (Zeman, 1967; Arrington, 1968; Harriman, 1969; Arrington et al., 1973; Hegsted et al., 1973; Kroes et al., 1973; Hegsted et al., 1974), but the growth response to variation in dietary fat per se has not been quantitated. No minimum requirements for fat and essential fatty acids have yet been determined, although gerbils have been maintained for prolonged periods on purified diets containing as little as 1 to 2 percent of metabolizable energy from 18:2 [Pronczuk et al., 1994 (in press)]. Although the gerbil can convert linoleic acid to arachidonic acid, arachidonic acid is minimally present in plasma cholesterol esters and comparatively low in the body fat of the gerbil. The body fat of gerbils is higher in oleic and palmitic acid than is the body fat of rats (Gordon and Mead, 1964). The gerbil responds to high-fat, high-cholesterol diets with increased HDL- and LDL-cholesterol concentrations, especially when diets contain casein, and may prove to be a useful model for the study of cholesterol metabolism (Nicolosi et al., 1976; Forsythe, 1986; DiFrancesco and Mercer, 1990). High dietary cholesterol leads to excess deposits in several body organs but not in arteries. However, older breeder animals fed natural-ingredient diets show spontaneous arteriosclerosis (Gordon and Cekleniak, 1961; Wexler et al., 1971; D'Elia et al., 1972). PROTEIN Weight gains of 1 g/day were obtained when weanling gerbils (18 g) were fed purified diets containing 16 percent or more protein, but weight gains were lower (0.6 to 0.8 g/day) when the gerbils received diets containing 12 to 14 percent protein (Arrington et al., 1973). Young gerbils (38 g) fed purified diets with 13 percent protein as casein gained only 0.69 g/day as compared to gains of 0.81 to 0.88 g/day when fed 17 to 25 percent protein (Hall and Zeman, 1968). Based on these studies, the protein requirement of growing gerbils seems to be about 16 percent when dietary fat is 2 to 5 percent. Little specific information is available on amino acid requirements of gerbils. However, purified diets based on amino acids have been fed to gerbils with mixed success (Otken and Garza, 1983). Gerbils fed an amino acid-based purified diet had greatly improved growth when taurine was added to the diet at a concentration of 4.5 g/kg (36 mmol/kg) (Otken et al., 1985). Taurine added at concentrations of 7 g/kg diet (60 mmol/kg diet) resulted in lower growth rates. MINERALS MACROMINERALS Calcium and Phosphorus The amounts of minerals in natural-ingredient rodent diets commonly fed to gerbils (e.g., Table 2-3) are apparently sufficient to meet the needs of gerbils. In the absence of specific data on the requirements of gerbils, the recommended dietary concentrations of calcium (5.0 g Ca/kg diet) and phosphorus (3.0 g P/kg diet) are the same as for the rat (Table 2-2). Magnesium A low incidence (≤2 percent) of convulsive seizures, especially in response to handling, environmental change, or other stimulation, has been noted in many gerbil colonies (Marston and Chang, 1965; Zeman, 1967; Thiessen et al., 1968; Harriman, 1974; Loskota et al., 1974; McCarty, 1975). Gerbils fed a low-magnesium, purified diet had an elevated susceptibility to seizure in a novel environment; the seizures were eliminated when magnesium was added to the diet at 1.39 g/kg (Harriman, 1974). Gerbils fed purified diets low in calcium, sodium, or vitamin B6 did not have seizures. Gerbils develop some degree of alopecia when fed purified diets containing ≤1.0 g Mg/kg, with the severity related to the extent of magnesium deprivation. Alopecia became noticeable after 14 days when dietary magnesium was less than 0.12 g/kg, and a mortality rate of 70 to 83 percent occurred within 40 days when magnesium was 0.06 to 0.12 g/kg diet. A dietary concentration of 0.25 g Mg/kg prevented weight loss and death (A. E. Harriman, 1976, Oklahoma State University, personal communication). These results indicate a dietary magnesium requirement of ≥1.0 g/kg diet. A dietary concentration of 1.5 g/kg is recommended. This is higher than the requirement of the rat.

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Nutrient Requirements of Laboratory Animals: Fourth Revised Edition, 1995 Sodium Chloride A purified diet that did not include added sodium chloride produced alopecia within 30 days, but no weight loss. Recovery was dramatic when NaCl was provided (Cullen and Harriman, 1973; A. E. Harriman, 1976, Oklahoma State University, personal communication). Gerbils are able to tolerate relatively high sodium intakes because of their ability to produce concentrated urine. Gerbils that receive a 0.75 M sodium chloride solution as the only liquid maintained body weight, but food intake declined progressively as sodium chloride solutions increased from 0.5 to 1.5 M (McManus, 1972). However, Rowland and Fregley (1988) found that gerbils are reluctant to ingest NaCl either spontaneously or after treatment with several of the natriogexigenic stimuli that are effective in rats. As there is no reason to expect that gerbils require more sodium or chloride than rats, the recommended minimal dietary concentrations for both sodium and chloride are 0.5 g/kg (see Table 2-2). By analogy to the rat, the estimated potassium requirement for gerbils is 3.6 g/kg. TRACE MINERALS No studies could be located that specifically addressed the iron, copper, zinc, or manganese requirements of gerbils. Patt et al. (1990) reported that adult gerbils fed a low-iron diet (concentration not stated) for 8 weeks developed low brain and serum iron concentrations compared to controls. The observed reductions in tissue iron were apparently functionally significant in that they were associated with a decreased risk of brain reperfusion injury. Until data specific to the gerbil are available, the recommended dietary concentrations for iron, copper, zinc, and manganese are the same as for the rat (Table 2-2). The recommended concentration of iron is 35 mg/kg diet for growing and adult animals and 75 mg/kg diet for pregnant and lactating dams. The recommended copper concentration is 5 mg/kg diet for growth and maturity and 8 mg/kg for reproduction. Based on the use of soybean protein-based diets by some investigators (DiFrancesco et al., 1990a,b), the recommended zinc concentration is 25 mg/kg diet for all stages of life. The recommended concentration for manganese is 10 mg/kg diet. By analogy to the rat, it is assumed that the gerbil requires about 150 µg I/kg diet, 150 to 400 µg Se/kg diet, and 150 µg Mo/kg diet (Table 2-2). Other potentially beneficial mineral constituents are discussed in Chapter 2, and in the absence of information on gerbils, it is assumed that the same conclusions apply to this species. VITAMINS FAT-SOLUBLE VITAMINS The concentrations of fat-soluble vitamins provided in natural-ingredient diets developed for rats and mice (e.g., Table 2-3) appear to be adequate for gerbils, although the fat-soluble vitamin requirements of gerbils have not been studied. WATER-SOLUBLE VITAMINS Relatively little is known about the requirements of gerbils for water-soluble vitamins. Hall and Zeman (1968) reported growth retardation and urinary riboflavin excretion when gerbils were fed diets containing riboflavin at 0.46 to 0.70 mg/kg diet as compared to 3.5 to 7.7 mg/kg. This is consistent with the estimated requirement of 3.0 mg riboflavin/kg diet for growth in rats. For the rat, the recommended riboflavin concentration for reproduction is somewhat higher (4.0 mg/kg diet); it is not known if this is true for the gerbil. The gerbil appears to have a definite requirement for choline that can not be replaced by a high intake of methionine (Otken and Garza, 1983). The best growth was obtained with diets containing 2.3 g choline chloride/kg (16.5 mmol/kg) as well as 11.7 g methionine/kg and 5 g cystine/kg (Otken, 1984; Otken and Garza, 1983). Male gerbils fed diets containing about 2 percent fat do not have a dietary requirement for myo-inositol because of intestinal synthesis of that vitamin. Females require more than 20 mg/kg diet. This requirement increases to 70 mg/kg when diets contain 20 percent saturated fat. The male requires myo-inositol when fed this same amount of saturated fat, but the amount has not been determined. Addition of sufficient myo-inositol to these diets prevented weight loss or decreased weight gain, hyperkeratosis of the skin and accumulation of fat in the intestinal tissue, and increased myo-inositol content of intestinal tissue (Hegsted et al., 1973; Kroes et al., 1973; Hegsted et al., 1974). Cholesterol added to the diet apparently increased the need for dietary myo-inositol. Until further information becomes available, it is recommended that the concentrations of other water-soluble vitamins in gerbil diets meet or exceed the concentrations recommended for the rat (Table 2-2). In the previous edition of this report, concentrations of vitamins and minerals that had been used in purified diets for gerbils were summarized; but as some of these concentrations may have been suboptimal, they do not provide a useful model and are omitted from this edition.

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