Calculating the energy requirement

The exact energy requirements vary drastically based on an individual’s age, health status, activity, and housing environment. In most cases, typical gibbon diets need to provide about 50-100 kCal/body weight kg/day energy, depending on the individual circumstances.

To determine a baseline more precisely, we can estimate the Basal Metabolic Rate (BMR) using the Kleiber equation:

For example, the BMR of a 10 kg siamang could be calculated as follows:

BMR = 70 × 10⁰·⁷⁵ kCal/day
BMR = 70 × 5.6234 kCal/day
BMR = 393.64 kCal/day

While BMR is a good estimation of the minimum energy necessary for survival, animals tend to use more energy when they are active. Different activity levels require different amounts of energy, leading to a higher daily energy requirement than the BMR indicates. This is called Total Daily Energy Expenditure, or TDEE. Sedentary or older, generally inactive animals, or those living in very small enclosures may require 1.2 times their BMR daily. A very active animal in a large forest enclosure may require as much as 1.9-2 times their BMR. Therefore the above 10kg siamang may require a wide range of energy, depending on their activity level.

If our animal is largely inactive, they would need:

TDEE = BMR × 1.2 kcal/day
TDEE = 393.64 × 1.2 kcal/day
TDEE = 473 kcal/day

If our animal is very active, they would need:

TDEE (active) = BMR × 1.9 kcal/day
TDEE (active) = 393.64 × 1.9 kcal/day
TDEE (active) = 748 kcal/day

As we can see, this is a wide range, therefore, estimating the animal’s activity level is crucial to ensure they don’t get neither too much nor too little food. Continuously monitoring their weight and/or body condition after an initial diet is formulated is also important. It is possible, that we will need to adjust this initial calculation, based on our observations.

From the adult calculation, we can estimate the energy need for a sub-adult as 50% of an adult portion, and a juvenile from weaning as 25%. The housing environment plays a massive role in caloric expenditure. For gibbons housed in smaller enclosures with limited space for brachiation, daily energy requirements are lower. Conversely, gibbons undergoing rehabilitation in semi-free-range, forested enclosures with considerable canopy space require significantly more fuel to support their active lifestyle.

You can use the below calculator to try out how the animal’s weight and activity level influences the daily energy requirement.