Energy Requirements

Indian Flying Fox

The basal metabolic rate (BMR) of a homeothermic animal is defined as the energy used while the animal is at rest, in a post-absorptive state, in a thermoneutral environment, and without physical or psychological stress.^[22] In order to calculate the BMR of the Indian flying fox, we will be using McNab’s equation for calculating the BMR of eutherian mammals based off of their body weight.^[17] Although there is an equation available for flying frugivores, the weight range of this equation is 0.010-0.598 kg.^[17] The average weight of the Indian flying fox exceeds this, so we will use the general mammal equation instead.

BMR calculation:

• Equation for mammals: BMR = 57.2X^{0.716 [17]}
• The weight of the flying fox is between 0.6-1.6 kg
  • The average weight is 1.1 kg, which we will use for the calculation.
• BMR = 57.2(1.1)^0.716 = 61.2 kcal/day

Large flying frugivores tend to have a slightly higher BMR than insectivores and large nectivores of the same size.^[16]

The daily energy expenditure (DEE) of an animal includes its BMR, in addition to the energy expended in activity, thermoregulation, and food metabolism.^[22] Calculating daily energy expenditure of flying foxes is also somewhat difficult, as there is no equation available for flying mammals. The two closest available equations are the equations for calculating DEE of terrestrial eutherian mammals and non-passerine birds.^[22] DEE includes activity; as flying animals, the activity requirement of bats is more likely to resemble that of birds than of terrestrial animals. Therefore, we will use the equation for non-passerine birds as a tentative approximation of bat DEE, acknowledging that there may be some differences in food metabolism between the two groups.

• Equation for non-passerine birds: DEE = 2.5 x BMR^[22]
• DEE = 2.3 x 61.2 kcal/day = 153 kcal/day

Flying is an energetically taxing mode of locomotion.^[22] Here is a calculation of the energy expenditure associated with flying in the Indian flying fox:

• Flying = 14 x BMR^[22]
  • Note: This is the equation for “active flight” by birds.^[22] Studies have indicated that this equation is also applicable to bats.^[9]
• 14 x 61.2 kcal/day = 856.8 kcal/day

A growing animal has a higher BMR than an adult animal.^[22] Here is a calculation of the BMR of a rapidly growing young Indian flying fox:

• BMR of rapidly growing animals = 2 x BMR^[22]
• 2 x 61.2 kcal/day= 122.4 kcal/day

Female mammals must expend a great deal of energy on reproduction.^[22] Here is a calculation of the daily energy expenditure of a pregnant female Indian flying fox:

• DEE of pregnant female mammal = 2.7-3.1 x BMR^[22]
  • The average of 2.7-3.1 is 2.9, therefore we will use the value of 2.9.
• 2.9 x 61.2 kcal/day = 177.48 kcal/day

Here is a calculation of the daily energy expenditure of lactating female Indian flying fox:

• DEE of lactating female mammal = 4-7 x BMR^[22]
  • The average of 4-7 is 5.5, therefore we will use the value of 5.5.
• 5.5 x 61.2 = 336.6 kcal/day

Animals meet their energy needs via the consumption of food.^[22] Although Indian flying foxes have a highly varied diet consisting of fruits, leaves, flowers, and seeds, here we will calculate the food consumption requirements to meet their energy needs if they were only consuming one species of fruit. For this calculation, we will be using the papaya (Carica papaya), a large fruit common throughout the range of the Indian flying fox.^[25]

• Papayas weigh about 450 g, or 0.45 kg
• From our ME calculation above, we know that the papaya contains approximately 307.29 kcal/kg.
• 307.29 x 0.45 kg = 138.28 kcal per papaya
• To determine how many papayas the Indian flying fox needs to eat, we can divide the DEE by the amount of kcal contained within a papaya.
• (153 kcal/day)/(138.28 kcal) = 1.11 papaya per day
  • However, when looking at the nutrition of fruit bats, it is important to note that bats only extract and absorb about 25-30% of the metabolizable calories contained within the whole fruit, because they typically only consume the juice of the fruit.^[15]
• 138.28 kcal (0.25) = 34.57 kcal
  • This is the amount of kcal that the bats are actually getting if they only consume the juice.
• (153 kcal/day)/(34.57 kcal) = 4.43 papayas per day (if only consuming the juice)

Grey-Headed Flying Fox

In order to calculate the BMR of the grey-headed flying fox, we will be using McNab’s equation for calculating the BMR of eutherian mammals based off of their body weight.^[17] Although there is an equation available for flying frugivores, the weight range of this equation is 0.010-0.598 kg.^[17] The average weight of the grey-headed flying fox exceeds this, so we will use the general mammal equation instead.

BMR calculation:

• Equation for mammals: BMR = 57.2X^{0.716 [17]}
• The weight of the flying fox is between 0.55-1.1 kg
  • The average weight is 0.825 kg, which we will use for the calculation.
• BMR = 57.2(0.825)^0.716 = 49.8 kcal/day

As with the Indian flying fox above, we will use the equation for non-passerine birds as a tentative approximation of bat DEE, again acknowledging that there may be some differences in food metabolism between the two groups.

• Equation for non-passerine birds: DEE = 2.5 x BMR^[22]
• DEE = 2.3 x 49.8 kcal/day = 124.5 kcal/day

Here is a calculation of the energy expenditure associated with flying in the grey-headed flying fox:

• Flying = 14 x BMR^[22]
• 14 x 49.8 kcal/day = 697.2 kcal/day

Here is a calculation of the BMR of a rapidly growing young grey-headed flying fox:

• BMR of rapidly growing animals = 2 x BMR^[22]
• 2 x 49.8 kcal/day= 99.6 kcal/day

Here is a calculation of the daily energy expenditure of a pregnant female grey-headed flying fox:

• DEE of pregnant female mammal = 2.7-3.1 x BMR^[22]
  • The average of 2.7-3.1 is 2.9, therefore we will use the value of 2.9.
• 2.9 x 49.8 kcal/day = 144.42 kcal/day

Here is a calculation of the daily energy expenditure of lactating female grey-headed flying fox:

• DEE of lactating female mammal = 4-7 x BMR^[22]
  • The average of 4-7 is 5.5, therefore we will use the value of 5.5.
• 5.5 x 49.8 = 273.9 kcal/day

Like the Indian flying fox, the grey-headed flying fox has a highly varied diet consisting of fruits, leaves, flowers, and seeds. Once again, we will calculate the food consumption requirements to meet their energy needs if they were only consuming one species of fruit. For this calculation, we will be using the Moreton Bay fig (Ficus macrophylla) a dietary staple of grey-headed flying foxes.^[20,23]

• Figs weigh about 40 g, or 0.04 kg
• From our ME calculation above, we know that the fig contains approximately 288.5 kcal/kg.
• 288.5 kcal/kg x 0.04 kg = 11.54 kcal per fig
• To determine how many figs the grey-headed flying fox needs to eat, we can divide the DEE by the amount of kcal contained within a fig.
• (124.5 kcal/day)/(11.54 kcal) = 10.79 figs per day
  • However, when looking at the nutrition of fruit bats, it is important to note that bats only extract and absorb about 25-30% of the metabolizable calories contained within the whole fruit, because they typically only consume the juice of the fruit.^[15]
• 11.54 kcal (0.25) = 2.89 kcal
  • This is the amount of kcal that the bats are actually getting if they only consume the juice.
• (124.5 kcal/day)/(2.89 kcal) = 43.1 figs per day (if only consuming the juice)