Estimating Energy Requirements


Exotic animal veterinarians are well aware of the differences between energy expenditure in small and large species. Larger animals use more energy overall, however when energy expenditure is divided by body weight (kcal/kg/day), large animals use relatively less energy than small animals (Fascetti 2012) (Fig 1).

Ostrich and marsh wren

Figure 1. Proportionally the energy requirements of a large species like the ostrich (Struthio camelus) (left) (Photo by S. Nygaard) are much lower than that of a tiny species like the marsh wren (Cistothorus palustris) (right) (Photo by USFWS)

Basal metabolic rate (BMR)


The basal metabolic rate or BMR, also known as basal energy expenditure or basal energy requirement (BER), is defined as the amount energy expended by a healthy animal under thermoneutral conditions 12 hours after a meal. The animal is conscious but not exercising.

Practical applications

The basal metabolic rate applies primarily to human nutrition because the conditions needed to measure BMR are so stringent that it is virtually impossible to do so in animals.

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Resting energy rate (RER)


The resting energy rate or resting metabolic rate is defined as the number of calories required for an animal to maintain homeostasis while resting quietly. The resting metabolic rate is similar to BMR but includes energy expended for recovery from physical activity and feeding.

Practical applications

The RER is a parameter commonly used in animal nutrition.

There are multiple ways to calculate RER, but each method is based on the following equation in which a pre-determined K value is multiplied by body weight in kilograms: K (Wkg)0.75 = kcal/day

In dogs and cats, one method is based on body weight (Table 1).

Table 1. Formulae used to calculate resting energy requirements in dogs and cats based on body weight
Body weight Formulas used to calculate recommended kcal/day
< 2 kg 70 (BWkg)0.75
2-45 kg 30 (BWkg) + 70

Faster metabolic rates translate into higher K factors for exotic animal patients (Table 2 2).

BER = K (Wkg)0.75 = kcal/day

Table 2. K values used in select birds
Taxonomic group K value
Psittacine birds 175
Passerine birds 175
Aquatic non-passerine birds 150
Terrestrial non-passerine birds 125

Alternatively, RER can also be calculated as 1.25 x BER. Traditionally RER was then multiplied by an “illness factor” between 1.0 and 1.5 to account for increases in metabolism associated with different conditions and injuries however this practice has fallen out of favor (see overfeeding below). HOWEVER current recommendations place much less emphasis on these subjective “illness factors”. Instead RER is used as an initial estimate or starting point of a critically ill animal’s energy requirements. The patient’s response is then monitored closely, and adjustments in nutritional support are made as needed.


Use of “illness factors” (see above) put the patient at risk for overfeeding. Potential sequelae to overfeeding include vomiting, diarrhea, cramping, nausea, electrolyte disturbances, hyperglycemia, hepatic dysfunction, and even respiratory distress.

Consequences of overfeeding

  • Vomiting, diarrhea, cramping, nausea
  • Electrolyte disturbances
  • Hyperglycemia
  • Hepatic dysfunction
  • Dyspnea

Maintenance energy requirement

The maintenance energy requirement is defined as the energy required for a moderately active adult in a thermoneutral environment. This includes the amount of energy needed to maintain an animal at its current weight and body condition.

Activities such as flying and swimming require large amounts of energy. These activities generally burn up five to ten times the energy of BER.


Caloric content of select diets

There are a number of reputable products available for enteral feeding of exotic animal patients (Box 3).

Box 3. Caloric content of select diets

Manufacturer Product kcal per gram kcal per ml*
EmerAid LLC EmerAid Intensive Care Carnivore 5.14 1.67
EmerAid LLC EmerAid IC Herbivore 2.95 1.32
Emeraid LLC EmerAid IC Omnivore 4.06 2.39
Harrison’s Bird Foods Recovery Formula 3.92 n/a
Kaytee Exact Hand-Feeding Formula 3.840 n/a
Kaytee Exact Macaw Hand-Feeding Formula 3.980 n/a
Roudybush Formula AA 3.450 n/a
Kilocalories (kcal) milliliters (ml) information not available (n/a).
*When prepared as directed.




Ackerman N. Companion Animal Nutrition. Butterworth Heinemann Elsevier: New York; 2008

Battaglia AM. Small Animal Emergency and Critical Care for Veterinary Technicians, 2nd ed. Saunders Elsevier: St. Louis, MO ;2007

Earle KE. Calculations of energy requirements of dogs, cats, and small psittacine birds. J Small Animal Pract 34:163-173, 1993.

Fascetti AJ, Delaney SJ (eds). Applied Veterinary Clinical Nutrition. Wiley-Blackwell: Ames, Iowa, 2012.

Jenkins JR. Hospital techniques and supportive care. In: Altman RB, Clubb SL, Dorrestein K, Quesenberry K (eds). Avian Medicine and Surgery. Philadelphia, PA. WB Saunders. 1997:246-248.

Klasing K. Comparative Avian Nutrition. CAB International; New York, NY, 1998. Pp. 210-233.

Macintire DK, Drobatz KJ, Haskins SC, Saxon WD (eds). Manual of Small Animal Emergency and Critical Care Medicine. Ames, Iowa: Blackwell; 2006.

To cite this page:

Pollock C. Estimating energy requirements. January 13, 2013. LafeberVet Web site. Available at